Liquid fuels for transport

Pro#. Ener#y Combust. Sci. Vol. 8 pp. 233-260.

0360-1285/82/03/0233-28514.000 Copyright © 1982. Pergamon Press Lid

Printed in Great Britain. All rights reserved.

LIQUID FUELS FOR TRANSPORT E. M. GOODOER School of Mechanical Engineering, Cranfield Institute of Technology, Cranfield, Bedford, MK43 OAL, U.K.

I. INTRODUCTION

2. CURRENT TRANSPORT ENGINE TYPES

Some controllable form of mechanical propulsion is virtually essential for the scheduled movements and large payloads of commercial transport, and for the speed and convenience of most leisure travel. Over the years, each branch of the transport sector has developed its individual combination of vehicleengine-fuel, the fuel invariably being carried in the liquid phase in order to meet the constraints of storage and transfer in vehicular systems, and of vaporization and combustion in engines. Most liquid fuels for transport are derived from petroleum which, as a fossil fuel, must inevitably diminish, as foreshadowed recently by repeated price rises and occasional supply shortfalls. In addition to the conventional fuels adopted for transport purposes, therefore, this review also covers the alternatives that appear to offer potential for the future. Some related developments in engine combustors and fuel systems are included. The viewpoint is largely British, but specific references are made to American terminology and practice.

The three main branches of the transport sector comprise aviation, automotive (road and rail) and marine, with propulsion characterized by airstream, land-wheel and waterstream reactions respectively. The engine types suited to these branches are displayed in the central band of Fig. 1, and discussed in turn below. The long-established internal-combustion engine adopted for light aircraft and private car propulsion is the spark-ignition (S-I) version of the reciprocatingpiston unit operating on repeated mechanical cycles incorporating discontinuous combustion. Since combustion occurs within the working fluid, the needs for continual replenishment of oxygen, and rejection of exhaust products, are met by timed valve operation for the working fluid itself. This type of engine therefore works on an open-circuit basis rather than a thermodynamic cycle; nevertheless its pressurevolume curves approximate those comprising the Otto ideal air-standard cycle (Fig. 2) incorporating

LAND

AIR Light

i Jet I aircraft

Cars

I Trucks J

SEA Trains

Naval

Merchant

ships

sh ps

High Medium Low ELEC speed speed speed STEAM S-I PISTON iGAS'IIJRBINE~ C-I PISTON TURBINE \

/

i

Aviation Motor GASOLINE

~r

D,DIESEL

:KEROSINE

GAS OIL

~ FUEL

FUEL OIL

I IL

PETROLEUM = Special requirements for Navies . . . . .

~

= Limited use

FIG. I. Current transport engine and fuel types. 233

234

E.M. GOOtX;ER

( Diesel) ( Diesel ) 2

2 (Ofto)

-Hzl +

~.ql I

1

(Joule) OF

• t,*,-,*0

(Braylonl

2q3

1

{ Rankine )

~ / V ~ l

. li

Nz-"~'----

1 ',, q = heat transfer

3w,

h/= worktransfer

FIG. 2. Representations of Otto, Diesel, Joule and Rankine thermodynamic cycles.

heat release at constant volume, and this thermodynamic criterion is used for performance analysis of S-I engines. F r o m the expression shown in Table 1, thermal efficiency is seen to rise with compression

ratio but, as discussed later, an upper limit is set in practice by the onset of abnormal combustion. Fuel economy is also limited to some extent by the need to operate on the fuel-rich side of the stoichiometric

TABLE 1. Key characteristics of combustion and fuels for transport engines Engine type Ideal thermo. cyclic criterion

S-I

GT

Otto

Joule (Brayton)

Ideal thermal efficiency

1 -(1/r~) ~-t

Ideal combustion type

C-I

ST

l-(1/rp) '~-tg~

Diesel [ ~v-I ] . 1- ~ (l/rv)'-'

Rankine (h~-h,)-(h2-hl)

Discontinuous constant volume for work transfer

Continuous constant pressure for work transfer

Discontinuous constant pressure for work transfer

Continuous constant pressure for heat transfer

Key combustion requirement

Freedom from spark knock

Freedom from corrosive or erosive products

Freedom from diesel knock

High rate of heat transfer

Key fuel requirement

Low ignitability (High ON/PN)

Freedom from contaminants

High ignitability (High CN)

High combustion temperature and radiative flame

Main fuel types

Gasoline

Wide-cut gasoline Kerosine

Gas oil Diesel fuel Fuel oil

Fuel oil Coal

ON = Octane number PN = Performance number CN = Cetane number.

hs-h 2

Liquid fuels for transport mixture to achieve maximum power by approaching complete fuel-air mixing, and to offset the effects of maldistribution of mixture to multiple cylinders. Within these constraints, the S-I piston engine has developed over many years as a reliable unit of high power density and wide flexibility. Whereas improvement in economy results from increased compression ratio, improvement in power follows more effectively from an increased charge throughput derived by rotary supercharging, the drive taken either through mechanical step-up gearing from the crankshaft or, more flexibly, from the controlled flow of exhaust gas through a turbine. The gas turbine engine, particularly in its aero version, can be visualized as a direct descendant of the turbo-supercharged S-I piston engine in which, as the roles of the compressor and turbine became dominant, discontinuous combustion at variable pressure was replaced by continuous combustion at constant pressure, with power offtakes from the turbine and/or exhaust jet rather than a crankshaft. As with the S-I piston engine, it is customary to effect combustion within the stream of working air rather than externally with inward heat transfer; consequently this type of engine also is of open-circuit design, and does not operate on a thermodynamic cycle. However, its pressure-volume curves approximate closely those of the Joule (or Brayton) ideal air-standard cycle, incorporating heat release and rejection at constant pressure, which is therefore used as the performance criterion. The thermal efficiency, as shown in Table 1, is seen to rise with pressure ratio, the upper limit to which is set by the strength and mass of the engine. This expression corresponds to a direct relationship with the maximum temperature of the cycle and this, in turn, is limited by the thermal strength of the turbine blades which are already under the centrifugal stress of high rotational engine speed. Reheating the airstream increases the work output, but at the expense of thermal efficiency since the reheat combustion takes place at a lower level of pressure. The electrical propulsion included in Table 1 is discussed briefly in Section 11.2. The compression-ignition (C-I) version of the reciprocating-piston engine bears a close resemblance

235

to its S-I counterpart with regard to general layout, repeated mechanical cycles and discontinuous combustion. However, for the lower and medium speed versions, the pressure-volume curves approximate more closely those comprising the Diesel ideal air-standard cycle incorporating heat release at constant pressure. As shown from the expressions in Table 1, the thermal efficiency of the Diesel cycle is lower than that of the Otto cycle at the s a m e compression ratio. Nevertheless, relatively high values of compression ratio are essential in C-I engine practice to achieve spontaneous ignition when the fuel is injected into the compression-heated air (Fig. 2 has been drawn in a manner to emphasize this point). Furthermore, the higher the speed of the C-1 engine the more its pressure-volume curves approach those of the Otto, rather than the Diesel. cycle. As a result, C-I engined vehicles generally show better fuel economy (up to 25 °o) than their S-I engined counterparts. The upper limit to compression ratio is set by the mass, and corresponding expense, of the engine. Too low a compression ratio can give rise to a form of abnormal combustion, discussed later. A steam turbine, in general, represents one component of a complete plant which may well be of closed-circuit design incorporating indirect continuous heating and cooling in a boiler and condenser respectively. The criterion appropriate for performance assessment is the Rankine cycle, with the expression for thermal efficiency shown in Table 1. Rankine efficiency is therefore directly dependent upon the overall range of enthalpy (113-hl) across the complete plant. Efficiency can be improved by raising the steam temperature land thus the enthalpy) into the superheat region, by using bleed steam to heat the feed water and/or by using the flue gases to heat the feed water or the combustion air. As with superheating, the reheating of steam between stages of turbine expansion improves the dryness of the steam and so reduces blade erosion caused by water droplets. Representative performance and design data for the above four main types of propulsion engine are shown in Table 2. In such large-scale vehicles as sea-going ships, space can sometimes be made available for dual propulsion plant, optimized for both cruise and full power,

TABLE2. Representative data on transport engines Engine type Speed irev/min) Max r* Power (or Thrust ) Specific fuel consumption kg kW- lh- ~

S-I

GT

C-I

ST

High (5200)

High (12,000)

r,, 9 100 kW

% 30 250 kN (Jet) 4550kW (Prop and shaft)

Low (110-1301 Medium {300-1000) High (1900-3000) r,. 20 200 k W {road ) 2.5 MW (raill 60 MW tmarinel

Medium (3000LP} (6000 HPI % 100 33 M W

0.26

0.36

0.20

0.27

* r, = compression ratio = max. volume/min, volume rp = pressure ratio = max. pressure/rain, pressure. JF:'ECS

8:3

- E

236

E.M. GOODGER

represented by the symbols CODOG, COSAG and C O G O G indicating respectively combined diesel or gas turbine, steam and gas turbine, and two different types of gas turbine (for example, Rolls-Royce Olympus and Tyne engines installed in frigates and destroyers).

3. MAIN C O M B U S T I O N FEATURES OF TRANSPORT ENGINES

The characteristics of combustion within these four types of engine determine the key property requirements of their fuels, as discussed in turn below. In normal combustion within the S-I piston engine, the nucleus of flame generated at the electrode gap of the sparking plug tends to propagate radially (modified to some extent by mixture swirl) throughout the combustible mixture, the resulting rise in temperature generating a rise in pressure. The pressure drives the piston which provides propulsive work, part of the unused heat being taken up by the chamber walls, the remainder being retained in the scavenged exhaust gases. Under severe operating conditions, however, particularly at heavy load and low engine speed, abnormal combustion can arise because the remaining mixture (comprising the end gas) not only reaches a level of temperature sufficient for spontaneous ignition, but remains so for a period of time equivalent to the associated delay. In consequence, instead of awaiting progressive consumption by the advancing flame, the entire end gas ignites spontaneously with a shell-bursting effect, leading to violent high-frequency vibrations of the burnt products, together with a sharp metallic noise from sympathetic vibrations of the engine structure. This explosive form of combustion is termed spark knock, being a high-pressure spontaneous ignition occurring near the end of the combustion period. It arises because the fuel-air mixture has excessive ignitability, in that its levels of spontaneous ignition temperature and/or delay are too low. (It is noteworthy that, with some modern motor gasolines containing olefin components derived by cracking, spark knock may be more prone at the high temperature conditions of high engine speed despite the reduced time available for spontaneous ignition to develop). 1 One additional combustion problem that emerged with automotive engines during the recent trend to high compression ratios was the onset of preignition. This is premature ignition of mixture local to the glowing surfaces of deposits or overheated metal, and may lead to "running on" after the electrical ignition is switched off. Since its timing is uncontrolled, preignition can occur well in advance of the main ignition spark, leading to adverse pressure rise during the compression stroke with consequent overheating, and sometimes an occasional spark knock (wild ping). In severe cases at compression ratios in excess of 9.5/I, the resultant high rates of pressure rise can promote mechanical noise, known as "rumble", due to the

impulsive take-up of the clearances in the power train. This can lead ultimately to battering of the springs of the (still open) valves, and to melting of the piston crown, z

In the aero gas turbine combustor, the air flow from the compressor enters at about 150ms -1, and is diffused rapidly down to about 25ms -1. Modern trends are towards the single annular chamber, although sometimes the former individual tubular chambers are incorporated into it in the form of a tubo-annular design. The fuel is usually inti'oduced by some type of pressure-jet atomizer or vaporizer tube, and the flame stabilized by means of an entry air swirler to promote recirculation and flow reversal, assisted by side-entering air jets. A stable flame front is therefore achieved somewhere within the recirculation zone by a mechanism of velocity balance between the flame and the approaching mixture. About 28 % of the air flow, classed as primary, is used for combustion, the remainder entering the chamber progressively downstream, first as secondary air to reduce the temperature of the combustion products to about 1800 K to offset the effects of dissociation, and then as dilution air to bring the temperature down further to the maximum level of about 1250K permitted by the metallurgy of the turbine blades. Higher turbine entry temperatures of 1600 K and above are possible with internal cooling of the nozzle guide vanes and turbine blades using air bled from the compressor. Despite fuel residence times of a few milliseconds only, combustion efficiencies as high as 99~0 are customary. In reheat combustors, flame stability is generally effected by air recirculation round gutter shaped baffles, with the fuel injected immediately upstream. In normal combustion within the C-I piston engine, the temperature level for spontaneous ignition of the fuel is achieved by high compression of the air, the fuel being injected near the end of the compression stroke. This method of fuel injection generates locally ignitable mixtures, even at part load, and thus tends to give fuel-lean mixtures permitting power control via fuel quantity injected without the need for an air throttle with its attendant pumping losses. The high pressure necessary for fuel injection gives rise to fine atomization with consequent rapid vaporization, the initial droplets of the fuel charge passing rapidly through their precombustion reactions to spontaneous ignition, and acting subsequently as igniting agents for the remainder of the charge which thus burns at the rate at which it is injected, that is, under control of the fuel injection system. However, under conditions of relatively low operating temperature, or with unsuitable fuel, the ignition delay period of the initial droplets is extended, during which the injection system continues to deliver fuel to the chamber. Consequently, when ignition does eventually occur, the flame finds an excessive charge of fuel to burn in the chamber, with no control exercised by the rate of fuel injection. This rapid form of combustion leading to rough running and smoke is termed diesel knock, and arises because the fuel-air

Liquid fuels for transport mixture has insufficient ignitability, in that its levels of spontaneous ignition temperature and/or delay are too high (in contrast to the cause of spark knock). In a steam turbine plant, combustion is continuous, spray stabilized and at constant pressure, as in the gas turbine engine, but the objective in this case is to transfer the fuel chemical energy initially in the form of heat rather than work. The earlier locomotive and marine boilers were of the fire-tube (shell) design operating at pressures below about 25 bar, with transfer of combustion heat effected mainly by forced convection, and partly by radiation. Current marine boilers are of water tube design with conditions ranging from about 60 bar and 510°C non.reheat to 100 bar and 520°C with reheat, the heat transfer again being mainly convective. Problems to be avoided are those of metallic heat-transfer surface corrosion by aggressive combustion products, and of soot deposition. 4. CONVENTIONALTRANSPORT FUELS The above characteristics of combustion within these four combustor types determine the key properties required of their corresponding fuels. Clearly all forms of combustion are controlled by

237

flame initiation and propagation but, as a broad generalization, the primary fuel requirement for piston engines can be considered as an appropriate ignitability, whereas fuels for continuous-flow combustors require wide ranges of flammable mixture strength and high flame speed. Spontaneous ignitability can be determined experimentally at atmospheric" pressure using a standard test procedure in which a single droplet of fuel sample is injected into a heated flask, and the delay measured to spontaneous ignition, repeated tests indicating the minimum temperature to give spontaneous ignition (the SIT), and the corresponding maximum delay. The SIT curve for the hydrocarbon fuels in Fig. 3 shows an overall reduction with increase in fuel density, this being due mainly to the inability of the heavier, more complex molecules to withstand thermal agitation without rupturing, the consequent exposure of broken molecular bonds to atoms of oxygen permitting the chain reactions leading to ignition. The customary laboratory methods used for the measurement of flammable mixture range and flame speeds are based on ignition at one end of a tube rig followed by inspection of subsequent movement of the flame.

700"

s,r

•

Flash point

~H~r0gen

o

-

~n~

500

~2

Z

,= 300

--

E

~

_~

! SIT

.

.-_ u-

100

--Ambient o ~

IFLAHHABLEI -100 0"6

0'.8 ~'.o Fuel density, Kg/I

FIG. 3. Spontaneous ignition temperature and flash point curves for petroleum fuels, and individual values for substitutes (derived from Ref. 3).

238

E. M. GOODGER

TABLE3. Representative properties ofconventional transport fuels (derived from Ref. 3) Property H/C atomic Density (a 20/4°C, kg 1- z B. range, °C Freeze/Cloud point, °C Pour point, °C Sp. energy gross, MJ kg- ~ net, MJ kg- i En. density net, MJ 1-1 Vapn. enthalpy (q bp, kJ kg- 1 Sp. heat, liquid Ca 15°C, kJ kg- IK- 1 (A/F h mass Flamm. range (A/F) mass

Aviation gasoline

Motor gasoline

Kerosine

Gas oil

Diesel fuel

Fuel oil (Class G)

2.10

1.82

1.95

1.82

1.72

1.58

0.72 46-170

0.75 33-190

0.80 152-300

0.84 180-360

0.87 200 +

0.97 -

- 48

- 7 -20

- 65

- 65

0

10

47.4 44.2

45.9 43.0

46.4 43.4

45.7 42.9

44.3 41.8

42.7 40.3

31.8

32.3

34.7

36.0

36.4

39.1

292

279

207

277

270

2.04 14.9

2.00 14.5

1.94 14.7

1.90 14.5

1.85 14.2

1.74 13.9

26-4

25-4

22-4

21-3

20-3

19-3

Flame speed = 0.5 m s- 1 approximately throughout.

4.1. Gasolines It follows from the above discussion that a lowdensity gasoline of high SIT is suited to the S-I engine. The gasolines are colourless blends of volatile hydrocarbons with components ranging from about C4 to C12, but average properties roughly equivalent to C8H15. These fuel blends boil within the temperature range of about 30-190°C, and have densities of about 0.72kg1-1 for aviation gasoline (Avgas), and 0.75 kg 1-1 for motor gasoline (Mogas), as shown in Table 3. It is, of course, true that the spontaneous ignition processes of spark knock occur under engine conditions at much higher pressures and with much shorter delays than obtain in the laboratory heatedflask method discussed above; consequently the more representative tests employ actual engine conditions. For motor gasolines, the appropriate tool is the Cooperative Fuel Research (CFR)single-cylinder sparkignition unit in which the compression ratio can be varied, during engine operation, over a wide range by raising or lowering the complete cylinder-and-head body. Broadly, the test technique involves finding the compression ratio at which the sample exhibits a standard intensity of knock, and then bracketing the samlSle reading with readings from two blends of reference fuels, as outlined in Table 4. The upper reference fuel is 2,2,4-trimethylpentane, one of the isooctanes, i-CsHIs, and the lower reference fuel is nheptane, n-CTH16. (An isomer is a molecule which has been re-arranged structurally without gain or loss of any atoms. The original molecule from which the isomer was derived is described as normal). The rating is then expressed as an octane number, equal to the volumetric proportion of isooctane in the interpolated matching reference blend. For samples superior to isooctane in anti-knock quality, the reference fuels

comprise isooctane with different concentrations of tetraethyllead (TEL) additive. The matching TEL concentration in isooctane can then be converted to an extrapolated octane number up to 120. For motor gasoline, the initial method of octane rating is based on a relatively mild mixture temperature, and gives rise to a Research octane number (RON), whereas a later method with a more severe mixture temperature gives a Motor octane number (MON) which would therefore be a few numbers lower than the RON for a given sample. A number of road, and chassis dynamometer, methods have also been developed for more closely matching the test ratings with likely road performance. Generally, these methods are based on the extent of spark advance required to give just audible (trace, or borderline) knock, and thus they compare fuel performance over a range of engine speeds instead of at one constant level. With aviation gasoline, anti-knock performance at fuel-lean cruise conditions is now determined by conversion from the MON, whereas the fuel-rich takeoff condition is simulated by means of the supercharge method. Levels of RON for Mogas generally range up to about 100, but anti-knock qualities for Avgas often exceed this, particularly from the supercharge test. In such cases, the matching TEL concentration in isooctane may be converted to a performance number which represents the level of knock-limited power for the sample relative to that for unleaded isooctane. Avgas, with a "grade" rating of 100/130, for example, has a lean mixture rating of 100 octane (or performance) number, and a rich mixture rating of 130 performance number. The basis for performance number is thus particularly logical since it represents directly the relative levels of maximum power available, whereas no such linear relationship exists with octane rating. A performance number of 110, for

239

Liquid fuels for transport TABLE4. Test conditions for CFR anti-knock rating (derived from Ref. 4) Method

Research

Motor

Supercharge

Cetane

600

900

1800

900

(Mild) 15.6--51.7related to baro. pressure 100 13

(Severe) 107 191 45

100

Rev/min Inlet °C Coolant °C Ignition (Injection) °btdc Primary reference fuels

t

149 100 14--26 related to r,.

(13J

isooctane, i-Call i sI2,2,4-trimethylpentane)

1

Rating procedure* Rating scale

A Research octane no. (0-120)

Significance

Low speed (urban) driving.

n-heptane, n-CTH16 i-C8H18 +TEL A Motor octane no. (0--120) Aviation octane no. (0-100) Aviation performance no. (100-130) High speed (motorway) driving, Aviation cruising at lean mixture.

66

B Supercharge octane no. (0-100) Supercharge performance no. (100-160) Aviation full power at rich mixture.

n-cetane, C16H34 isocetane, C16H34 (2,2,4,4,6.8,8heptamethylnonane) C Cetane no. (15 100)

Compressiop-ign. characteristics indicating tendency to diesel knock.

* A. (i) At r~ giving standard knock intensity of sample, bracket reading of knockmeter for sample with readings for two reference blends, OR {ii) Convert r~ giving standard knock intensity for sample directly to octane or performance number from data checked frequently with reference fuels. B. At constant r v of 7/1, bracket power curve limited by light knock intensity for sample with curves for two reference blends. Ratings above 100 converted directly from lead content of isooctane. C. At constant ignition delay of 13°, bracket r~.for sample with rvvalues for two reference blends.

example, represents 10 ~o more knock-limited power obtainable from the sample than from isooctane, whereas an octane number of 90 does not necessarily mean 10~o less power obtainable from the sample than from isooctane. With the adoption of gasoline for the S-I engine, comparatively simple metering of the fuel became possible by means of suction at the throat of a carburettor venturi, the high volatility permitting vaporization to occur progressively throughout the intake manifold and finally within the cylinders. In this sense, volatility is also an important property of a gasoline intended for a carburetted S-I engine. However, the intensive development of fuel metering in the closing years of the high-performance aero piston engine demonstrated clearly the following fundamental weaknesses of carburation: (a) (b) (cJ (d)

Blockage of air duct by venturi; Complete loss of fuel flow at low air flow; Fuel enrichment with increase in air flow; Susceptibility to attitude (for example, inverted flight) and acceleration forces due to dependence on float chamber for datum level; le) Icing in mixture stream due to absorption of vaporization enthalpy of fuel; (ft Vapour lock in float chamber if overheated; (g) Momentary weakening ("flat spot") on engine

acceleration due to inertia in the dependence of fuel flow on air flow (h) Fuel enrichment with reduction in air density at altitude; (i) Maldistribution of mixture to individual cylinders. It is true that not all these weaknesses apply to less demanding surface transport, and also that many of them may be relieved by suitable corrective devices. Nevertheless, the erstwhile "simple" carburettor now becomes complex, and some weaknesses still persist. Fuel metering has been improved by replacing the float chamber with diaphragms to deliver the fuel at moderate positive pressure oL more effectively, by applying the Charge Weight Law and using the engine itself as the air meter, with a fuel pump output suitably sensitive to engine speed, air inlet pressure and temperature and also to exhaust back pressure. In supercharged engines, the evaporative cooling effect is utilized by injecting the fuel into the eye of the supercharger. For normally-aspirated engines, the more satisfactory method is the application of timed fuel injection, either under moderate pressure into the inlet port, or under high pressure into the cylinder itself as in diesel practice, since this overcomes the adverse effects of mixture maldistribution in the form of premature spark knock, fuel wastage and excessive

240

E.M. GOODGER

emissions plus fouling and corrosion within the cylinder. Electronic microprocessor systems of "engine management" are already in use in the United States to control precisely both fuel metering and ignition timing, based on a closed loop system sensitive to exhaust emissions and incipient spark knock. Such systems could also match engine output more closely with traction requirements. The exhaust heat needed for improved mixture preparation is provided by diversion of exhaust gases to a heat exchanger, or by installing a heat pipe between inlet and exhaust manifolds. (Charge heating by such means will be essential for some proposed alternative fuels of high vaporization enthalpy). In general, however, the substantial rise in effective volatility by fine atomization makes fuel volatility far less significant when the fuel is injected in the S-I engine. 4.2. Kerosines In the gas turbine combustor, no such characteristics of abnormal combustion arise to set such a firm constraint on fuel quality, the more general requirements being a reasonably low viscosity to permit satisfactory atomization, sufficiently high volatility to permit vaporization, together with an overall freedom from contaminants so that there is no excessive radiation from the flame to the chamber liner, nor generation of either molten or solid contaminant products. The former damage the blading by deposition and corrosion, and the latter by erosion. The compromise fuel to emerge for the aero gas turbine engine is kerosine (Avtur), a colourless blend of petroleum fractions boiling between about 150 and 300°C, with a density of about 0.8kg1-1. A blend of kerosine and gasoline known as wide-cut gasoline (Avtag) was developed in order to ensure strategic supplies of suitable fuel at the beginning of the jet era, and a special lower volatility kerosine of

high flash point (Avcat) is produced for Naval carrierborne gas-turbined aircraft to give improved fire safety. 4.3. Gas Oils and Diesel Fuels With the high speed C-I transport engine, fuel selection depends largely on resistance to diesel knock and, to some extent, on a sufficiently low viscosity and moderate volatility to permit atomization and vaporization without preheating. In both cases, the most suitable fuel to emerge is gas oil (named after its former use to generate carburetted water gas for the enrichment of town gas, but also termed "diesel fuel" in some parts of the world), a brownish petroleum fraction comprising distillates boiling between about 180 and 360°C, with a density of about 0.84kgl -~, equating roughly to C15H2B, and having a moderately low level of SIT (Fig. 3). The heavy medium to low-speed C-I engines used for power generation and some marine applications use diesel fuels. These are darkish brown petroleum fractions usually comprising gas oil and some residual components, the heaviest of which would boil above the upper test limit of 370°C set in order to avoid molecular rupturing (cracking) due to the high level of thermal agitation. The level of density is typically 0.87 kgl - I . As in octane rating, the concepts of variable compression ratio and a fuel scale are used in the rating of fuel resistance to diesel knock. However, in view of the very high levels of compression ratio involved, a precombustion chamber (see later) is used, incorporating a moveable surface to provide the change in volume and thus in compression ratio. Furthermore, since the ignition delay is such a critical factor in the promotion of diesel knock, a constant delay (equivalent to 13 degrees of crank angle)is used as the standard condition for sample and reference blends alike. + The upper reference fuel is a normal paraffin, n-

BO"

M+r --20

~o

6'o

1oo

\ i

ON

F16.4. Representative relationship between octane and cetane numbers Iderived from Ref. 5 and other sources).

Liquid fuels for transport

cetane C16H34, and the anti-knock quality of the sample is expressed as the cetane number (CN) equal to the volume percentage of cetane in the matching reference blend. The lower reference fuel was originally the polycyclic aromatic ct-methylnaphthalene C l l H l o , with CN = 0, but this has now been replaced by i s o c e t a n e (2,2,4,4,6,8,8-heptamethylnonane) with CN -- 15. It follows that, since the spontaneous ignition requirements for the S-I and C-I engines are diametrically opposed, an inverse relationship results between octane and cetane numbers, as shown in Fig. 4. A rough guide to compression-ignition fuel quality is given by the "diesel index" defined as, Diesel index -- 0.01 (aniline point, °FI (API gravity), where aniline point is the lowest temperature at which the sample is completely miscible with an equal volume of aniline, and therefore represents paraffinic concentration, and API gravity is the method adopted by the American Petroleum Institute for expressing fuel density, where Degrees API -

141.5 rel d (a ~60/60 °F

131.5.

Alternatively, an empirical expression based on the 50~',o distillation temperature and the API gravity permits the calculation of the "cetane index". Gas oil has also emerged as the most suitable fuel for the industrial gas turbine. 4.4. Fuel Oils With the larger medium and low-speed C-I engines, and with boilers for steam turbine plant, the space and weight requirements of fuel heating equipment are usually available, consequently neither cetane rating nor viscosity is quite so significant and, in merchant marine diesel engines, the distillate fuels have steadily been repla~zed by petroleum fuel oils of a residual nature. These are brownish-black fluids which have been classified largely on a basis of kinematic viscosity {see Table 3 and Fig. 11 ). The lighter members of the fuel oil group are atmospheric distillation residues blended with sufficient gas oil or other distillate "cutter stock" to meet the viscosity requirements. The heavier fuel oils may be entirely residual in origin, with their viscosities reduced to the required levels by means of a mild form of molecular cracking known as "visbreaking". This trend towards heavy fuel usage at sea occurred first with the low-speed range of engines in the early 19503, and then the medium speed in the 1960's. Currently, high viscosities can generally be handled by preheating, and recent vessels adopt automatic preheat control to give satisfactory atomization. In these heavier fuels, the importance of ignition quality, as indicated by cetane number, is now superseded by c o m b u s t i o n quality, as determined by resistance to carbon formation, and to the effects of such components as asphaltenes, ash, sulphur and vanadium. This arises because, in general,

241

the undesirable components in a parent crude oil naturally gravitate towards, and become concentrated in, the residual fractions during distillation. Sulphur is the exception here since it depends more on the source, rather than the processing, of the crude oil: nevertheless the high sulphur crudes will have to be accepted eventually. One adverse effect of operating marine diesel engines with fuel oil is evident as increased wear rates of cylinder liners and piston rings due to the presence of fuel-borne abrasives, including any particulate carry-over from catalytic cracking treatment. Wear rates can be seriously high, but may be reduced by the use of anti-scuffing additives, and probably further reduced by improved materials and design. One other problem involves the steel surfaces of exhaust valve faces which are susceptible to gas-phase oxidation due to reaction of the steel with the high-temperature oxygen-bearing exhaust gas. This problem is emphasized by the increased concentrations of slowburning asphaltenes in the heavier fuels which raise the exhaust temperature further, leading to softening of the valve seats, and subsequent channelling due to the pressure marks of solid deposits and gasborne particulates. Furthermore, the contaminating presence of vanadium and sodium promotes the formation of low melting-point compounds which act as corrosive liquid fluxes in dissolving the layer of ferric oxide (Fe203) on the valve face, exposing the underlying steel surface to additional corrosion and erosion. These problems are alleviated largely by the use of improved materials, by means of corrosioninhibiting fuel additives, and by temperature control through cooling water, but further difficulties can arise when the temperature of the valve stem falls below the dew point of the sulphuric acid generated from the sulphur content of the fuel oil. The build-up of hard V - N a - S deposits on the valve seats is tackled by fitting devices that rotate the valves during opening. No such problems arise of course, in the case of valveless (i.e. portedl low-speed two-stroke diesel engines.

5. PETROLEUM FUEL CONSUMPTION AND AVAILABILITY As seen above, the conventional liquid fuels for transport have invariably been derived from petroleum, and it is therefore of some concern to be aware of current rates of consumption, and to assess the continuing availability of this resource. Recent estimates for world petroleum fuel consumption by the various branches of the transport sector are given in Fig. 5. These show approximately one third of the total consumption to be attributable to transport, largely for road vehicles. (In the United States, this proportion now exceeds one half). These proportions would appear unlikely to fall during the next few decades: in fact, with a continuing conversion to coal, nuclear and solar resources in the industrial/ domestic sectors, they are more likely to rise. Since

242

E.M. GOODGER 37-

3/*-

31-~oved Reserves

/ Consumption) ratio

28-

f

f

Avgas /

"~

f

f ~

/~?....-

o

J AIR~ I I LAND

J

I

f

f

""

---"

/

OTHER

,¢

1970

19'80

19190

2000

Year

FIG. 5. World oil annual consumption rate and (proved reserves/consumption rate) ratio (derived from Ref. 6 and other sources). Gt = Gigatonne JP = Jet propulsion fuels. petroleum is a fossil-based resource with a formation life measured in millions of years, it is a finite commodity of an irreplaceable nature, and these growing rates of consumption represent an approaching depletion. Resource consumption history is generally illustrated in terms of the familiar bell-shaped curve of the Hubbert model showing, after discovery, an accelerating and then decelerating growth rising to a peak, followed by an approximate mirror image of decline and eventual depletion. It is invariably difficult to predict a fuel lifetime from this model since the right-hand, future, rim of the bell could either drop sharply to an early end or stretch extensively into the long term at a very low rate of consumption depending on a host of technological, economic and political factors. A useful parameter which, although not precise for absolute purposes, has the merit of simplicity and can be used for rapid comparisons, is the ratio (proved reserves/annual consumption rate). It is true that increases (hopefully) in reserves and (almost inevitably) in consumption rate are both ignored, but they do tend to cancel each other out to some extent. It is also true that the above concept holds only for resources which are in quantity production, since lack of exploitation of a resource,

whatever its magnitude, would inflate its reserves/ consumption ratio to meaningless optimistic levels. It is salutary to see from Fig. 5 that this ratio for petroleum is currently no more than about 29 years, and already appears to be in a general decline. Furthermore, consumption of the new reserves of petroleum expected to be found is already planned, to the extent of one third of the world oil demand in year 2000. Also important is the likely deterioration in overall fuel quality from now on. This follows because, in the declining application of petroleum, fuel refiners will have less opportunity to continue selecting the types of crude appropriate to required products, and will have to accept whatever is available. As a result, growing demands for the light distillates will be met only by increasing use of energy-intensive cracking and reforming, both thermal and catalytic, applied to heavy oil feedstock. This will result in greater proportions of the unstable olefin components with, for example, depressed levels of octane quality (from 98-95 RON). This in turn will call for isomerization, alkylation, hydrotreatment and other more severe processing methods with higher refinery energy consumption. The kerosines are likely to tend to higher levels of aromatics which, being carbon rich, can lead

Liquid fuels for transport to smoky combustion with radiant flames. In order to assist research into combustors for aero gas turbines using lower quality fuels in the future, NASA has recommended an Experimental Referee Broad Specification (ERBS) fuel with a mean value of 12.8 % mass of hydrogen corresponding approximately to a maximum aromatics content of 35 % volume 7 (cf 22 ~o in current Avtur--see Section I 0). The gas oils are expected to continue their current fall in volatility level and in octane number (from 50 to 40), with resulting increases in carbon monoxide emissions, cold starting difficulties, piston erosion and engine noise. The remaining stocks of fuel oils, depleted of some components for conversion to distillates, will experience correspondingly higher levels of contaminating asphaltenes and ash. It is pertinent to add that, since the liquid hydrocarbon fuels have proved themselves so well suited to propulsion, pressure may be exerted for an energy policy to reserve these premium fuels for such premium purposes as transport and the manufacture of chemicals and lubricants, with other consumers switching to coal, nuclear and, possibly, solar energy. But the overall fact remains that, for oil-importing nations, the problems of balance-of-payments already demand replacements for imported oils. The world, in the long term, must replace oil itself. 6. SUPPLEMENTAL FUELS

Developments in transport propulsion over the y e a r s have resulted in the above optimal combination

of engines and petroleum fuels; consequently, the growing demands for these selected fuels tend to hasten their premature decline. At the same time, the investments in fuel refining and engine manufacture, as well as in training, experience and expertise, are massive and cannot lightly be discarded for some alternative. Clearly the lifetimes of these conventional fuels can be extended to some extent by all-round

243

improvements in the efficiency of their use. and general gains in fuel economy are evident in all branches of the transport sector by attention to the design of engine components (combustors. compressors, turbines, etc.) and propulsive devices (transmissions, propellers, jet pipes, etc.). Another approach to delaying the adoption of substantially different engines and fuels is to supplement the supplies of conventional fuels with virtually identical materials derived from resources alternative to petroleum, and here the reserves/consumption ratio values can sometimes prove of value as a guide to the most suitable of these resources. The alternative sources with potential in this connection are ott shale, tar sands and coal. On an energy basis, worldwide, reserves of oil shale and tar sands appear to be about the same magnitude as petroleum, but neither as yet is under large-scale production; thus their current reserves/consumption ratios are meaningless. Oil shale contains resinous remains of vegetation (kerogen) which, on heating, decompose to a complex oily liquid containing a relatively high proportion of compounds of sulphur, nitrogen and oxygen, from which a synthetic crude oil (syncrude) may be derived. Shale syncrudes are usually rich in aromatics (35"o), with relatively high concentrations of wax and oilbound nitrogen, tending to smoky combustion, high pour points (35°C), corrosive emissions and thermal instability. Field tests to date include a flight with a shale-derived JP-4 fuel (wide-cut gasoline) in a T-39 aircraft in 1975, and a successful trial with a shalederived marine fuel in a vessel on the Great Lakes in America. Tars (asphalts, pitches, bitumens, etc.) are generally difficult to handle due to cohesion with their reservoir rock or sand, although some tars can be separated by hot water. Asphalts and bitumens are already being exploited as feedstocks, with liquid fuels derived fay solvent extraction and other techniques. Coal, on the other hand, has vast reserves, and has

b,. CHilL)

25

3-

20

.o

.••H GHa( 3 ~0(l) l)

o~

asolines Ker°s&n~LC~s oil Diesel fuet"~ RFO(G) .

io

-15 o~L. -10

~~

~ed.votxoQt

:z:

-5

Anthracite" 0 0"~,

o'.e

1'-z

1.6

20

0

Fuel density Kg/t

FIG. 6. Representative hydrogen/carbon atomic ratios for commercial fuel~ (dcri~cd t~,,,~ :t ~:t x i. RFO = Residual fuel oil.

244

E. M. GOODGER

TABLE5. Coal liquefaction processes Method

Direct

Principle

Hydrocarbonisation (Moderate P)

Pyrolysis (air-free carbonisation) in H 2 gas (derived from steam and 02 over hot coke)

Hydrogenation (High P)

Catalytic depolymerisation by solvent then reaction with H 2 gas

Depolymerisation by H 2 donor solvent e,g. tetralin Gasification to CO +H2 then catalytic synthesis to liquids

Indirect

been developed as a fuel for far longer than petroleum itself, with a current reserves/consumption ratio of some 280 years. Coal is therefore of immediate interest as a source of supplemental transport fuels through some suitable processes of fluidization. Several processes are under development for the liquefaction of coal, the two main stages being as follows: (a) Rupture the very large polynuclear aromatic rings forming the carbon platelets of the coal structure into small molecules with weaker inter-molecular bonds permitting a liquid physical state; (b) Increase the hydrogen/carbon atomic ratio of these resulting molecules from about 0.75 to the comparable liquid fuel values of about 1.5-2.0 by the addition of hydrogen. These two stages are illustrated by a "North-West" displacement within Fig. 6. The molecular rupturing can be achieved by heat or by solvent action and the direct hydrogen addition by gas under pressure or by employing the solvent as a temporary carrier of the hydrogen atoms. As shown in Table 5, an example of the latter approach is that using the "donor solvent" hydroaromatic tetralin (tetrahydronaphthalene) which releases four atoms of hydrogen per molecule as it dissolves the feedstock coal: C10H12 Tetralin

~ CloH 8 + 4 H Naphthalene

Conversion efficiencies range from about 45-65 o,;, giving yields of 15-33 ~ mass, or 200-450 litre/tonne coal. It should be emphasized that the generation of hydrogen for process purposes represents a major proportion of the costs and energy requirements for the production of such liquid fuels. These coal-derived synthetic crudes tend to reflect the aromatic richness of the parent crude molecular groups; consequently the densities are high and the cetane quality low. However, subsequent processing can not only raise the intrinsically high octane

Examples COGAS COED GARRETT COALCON CLEAN COKE OCCIDENTAL NCB(UK) CCL SRC SYNTHOIL H-COAL CSF EXXON LURGI FISCHER-TROPSCH

q ~o

45-65

40-55

number to 108, but also the cetane number to the automotive requirement of 50. In the United Kingdom, the National Coal Board has advised that the price ratio of gasoline derived from coal to those from petroleum has dropped from the 2.5 level of 3 years ago to the current 1.3, and predicted that the economic cross-over point could be reached by about 1985 provided sufficient funds are forthcoming to meet continuing developmental costs. The United States production target of coal-derived gasoline is reported to be I0 megatonne/annum by the late 1990's. In the earlier indirect route adopted by Sasol in South Africa, the complete gasification of the coal to syngas ( C O + H 2 ) destroys the aromatic structure, and subsequent synthesis is controlled to provide fuel molecules of the required type. However, the overall efficiency of such relatively severe processing is lower, although there is promise of some improvement. Problems of contamination of coal are generally solved by ash and sulphur removal during liquefaction, but fuel-bound nitrogen may lead to additional emissions of NOx .9 The feasibility of using coal-derived heavy liquid fuels for marine boilers has been demonstrated by the U.S, Navy in a sea trial using a C O E D (Coal-OilEnergy Development) derived fuel oil with a flash point raised to an acceptable level. Possible sources of supplemental liquid fuels are shown in Fig. 7, but it should be emphasized that substantial benefit would result from designing engines to operate on the as-produced qualities of these alternative-resource fuels rather than attempting to upgrade them to match exactly the existing fuels and engines. Unquestionably, fuels and combustors should be studied together. Since coal can serve as a feedstock for methanol, the development of the Mobil method ~° for dehydrating methanol with a zeolite catalyst to high octane Mogas represents a route alternative to coal liquefaction. The optimal route from coal to vehicle propulsion will depend on the efficiencies and yields of the various

Liquid fuels for transport

245

I

I I

I I

I

, ~ ..............

. . . . .

-4,-

BIONATTER . . . . . . . . . .

ffi L i m i f e d

or

later

- - :

use

FIG. 7. Possible sources of supplemental fuels.

conversion stages in each case, together with the overall costs involved in producing and handling each type of fuel, and in manufacturing the engines to suit. 7. EXTENDERS TO CONVENTIONAL FUELS An additional approach to prolong the availability of conventional fuels, and to delay the need for major modifications to engines and fuel systems, is to incorporate in the fuels moderate quantities of blending agents which are derived from alternative sources, and which do not make significant changes to the key combustion and handling properties. The alcohols, although currently derived largely from fossil sources, could be extracted from biomatter (see later), and are of interest as extenders to conventional fuels. Both methanol and ethanol have been used as extenders for Mogas in S-I engines, the experimental results reported depending largely on test conditions, particularly on any increase of carburettor jet sizes, warming of the mixture, optimization of ignition timing, or raising of compression ratio. The properties and resultant performance of the alcohols are discussed in Section 9, but the reported effects of blending x % by volume of methanol with Mogas may be summarized as an increase in power of about 0.1 x %, and in specific fuel consumption of about 0.3 x %.a.ll Furthermore, emissions of unburnt fuel may be increased. Addition of about 10 % by volume of methanol to Mogas is practicable without modification to engine or fuel system, the resulting improvement in anti-knock quality being about 3.5 Research octane numbers. It is likely that startability, which can be poor under low temperature conditions, may be predicted from the volume percent recovered at the distillation temperature of 70°C. With methanol as a Mogas extender, a negligible power variation has been found, with a slight improvement in fuel consumption, and in CO emissions. Of the higher alcohols, tert-butyl alcohol (t-C,,H9OH) or TBA is also of interest as a gasoline extender. With regard to the other oxygenates, methyl

tert-butyl ether (CHaOC4H9) or MTBE, and also ethyl tert-butyl ether (C2H5OC4H9) or ETBE, have higher specific energies, energy densities and antiknock qualities, with general reduction in emissions, apart from aldehydes.12 Tests at Cranfield show that alcohols also have potential as fuel extenders for the gas oil used in industrial and marine gas turbines. Additions of 20 and 3 0 ~ methanol, for example, showed general improvements in combustion efficiency and in emissions of CO and NOx, whereas temperature traverse improved below an outlet temperature of 650°C, but deteriorated above it. These improvements, which resulted from the chamber not burning its optimal fuel (kerosine), indicate that performance deterioration is unlikely to follow methanol blending with the optimal fuel. 13 Alcohols have also been tested as extenders for gas oil in the C-I engine, and up to about 3 0 ~o of fuel energy has been provided by methanol or ethanol which is either blended with the gas oil, or emulsified with it, usually with an added stabilizer. 14 8. ALTERNATIVEAPPLICATIONSOF CONVENTIONAL FUELS On occasions, certain types of conventional fuel appear in excess of current requirements and/or more attractive in price compared with those normally adopted for a given application. It then becomes profitable to consider using them as replacements for the normal fuels. Recent legislation to restrict the flaring of petroleum gases at well heads, for example, has resulted in a temporary surplus of gaseous propane. Since this can be liq'uefied and stored under moderate pressure at ambient temperature, interest has quickened in the use of propane for S-I engined vehicles such as private cars and light vans. As a liquefied gas, its contaminant content is negligible, and it vaporizes rapidly to give uniform distribution in multi-cylinder engines. These factors, coupled with a high octane quality, give possibilities of lean

246

E.M. GOODGER

operation with reductions in emissions, noise, engine wear and maintenance requirements, but at the expense of power and economy, with problems of cold starting. Significant numbers of propane-fuelled vehicles are already Operating in the Netherlands and the United States, and schemes for converting vehicles either to liquid propane alone or to alternative fuelling with gasoline are available in the United Kingdom. The main problem appears to be one of capital cost, not only of vehicle conversion, but of bulk storage at fuelling stations. However, there could be overall advantages for tied urban fleets such as buses, taxis and delivery vans operating from central supply points, with a payback of about 23,000 miles. In some areas, mixtures of the petroleum gases propane and butane are used in liquefied form as LPG, but butane is in demand also as petrochemical feedstock, and therefore in short supply. Natural gas has been proposed for S-I engines, and methane has given satisfactory test results, 15 but problems of storage as a cryogenic liquid, LNG, or as a compressed gas, CNG, make it unattractive overall. Aircraft use has also been proposed for L N G because of its good combustion characteristics in gas turbines and of the overall economics 16 but, although cryogenic fuel storage is likely to be somewhat less of a problem in aircraft than in road vehicles, its adoption makes possible the use of the higher performance fuel liquid hydrogen, as discussed later. 9. SUBSTITUTE FUELS A fuel is described here as substitute when it not only replaces a conventional fuel but is also largely independent of it in terms of source and properties. The examples given below comprise hydrocarbon oxygenates and hydrogen, together with the hydrides of nitrogen alone and of nitrogen and carbon together.

The biofuels covered may be used either as such, or as sources of alcohols and hydrogen. 9.1.

Hydrocarbon Oxygenates

The monohydric alcohol molecule consists of a hydrocarbon in which one atom of hydrogen has been substituted by a hydroxyl group, and is thus represented by the formula ROH, where R is the remaining hydrocarbon group. Of main interest as substitute fuels are the first two members of the paraffin (alkyl) alcohols, or hydroxyalkanes, in which symbol R is either CH 3 methyl or C2H 5 ethyl. As indicated in Sections 7 and 10, the higher alcohols are more promising in the role of gasoline extenders and blend stabilizers than as substitutes. The main sources for methanol and ethanol are natural gas and ethene (ethylene, C2H4) respectively, but other feedstocks for both materials include the fossil fuels and biomatter generally, including solid municipal waste, with ethanol produced by fermentation. The properties of the alcohols are shown in Table 6, the outstanding features being the high enthalpies of vaporization and low stoichiometric air/fuel ratios, the large content of oxygen depressing the levels of calorific value. In the S-I engine, the high enthalpies of vaporization of alcohols assist the volumetric efficiency but, in conjunction with the constant boiling point, manifold heating may be required for cold starting, and driveability may be impaired. Alcohols burn cleanly and have high octane qualities but, like the aromatics, show poor resistance to preignition. In test engines with unmodified carburettor jets, a change from gasoline to methanol results in a mixture-weakening effect due to the relatively low stoichiometric air/fuel ratio, (A/F)~, of methanol, whereas jet enlargement to give the same equivalence

TABLE6. Representative properties of substitute transport fuels (derived from Refs. 3 and 8) Property Formula Density @ 20/4 °C, kg 1- i B.p., °C M.p., °C Sp. energy gross, MJ kg- ~ net, MJ kg -1 En. density net, MJ 1- i Vapn. enthalpy @ bp, kJ kg- 1 Sp. heat, liquid @ 15°C, kJ kg- tK - 1 (A/F), mass Flamm. range (A/F) mass Flame speed, m s -1

* At boiling point.

Methanol

Ethanol

Liquid hydrogen

Liquid ammonia

Hydrazine

Nitromethane

CHaOH

C2HsOH

H:(1)

NH3(I)

N2H4

CHaNO2

0.791 64.7 - 97.8

0.790 78.5 - 114.9

0.071" - 252.7 - 259.1

0.615" - 33.4 - 77.7

1.008 113.5 1.4

1.120 101.1 - 28.5

22.7 19.9

30.2 27.2

142.4 120.2

22.4 18.6

19.4 16.7

12.0 10.9

15.9

21.6

8.4

11.4

16.9

12,3

1080 2.52 6.5 12.6-1.6 0.55

845 2.43 9.0 18.5-2.7 0.52

450 9.50* 34.2

1370

1254

564

4.68 6.1

3.06 4.3

1.75 1.7

345-5

8.9-4.6

18.3--0

%?

3.5

0.15

'~

0.5

Liquid fuels for transport ratio, (A/F)s/(A/F)actuai,promotes a rise in specific fuel consumption of about 100°o. Nevertheless, the lean misfire limit extends by about 0.13 units of equivalence ratio in comparison with Mogas, and the fuel consumption problem can also be eased by raising compression ratio from 10/1 to 14/1 due to higher octane qualities. The rise in volumetric efficiency and flame speed can increase the power output by 12-20~o. Emissions are generally reduced with the exception of aldehydes. 12 In practice, engines would be designed to suit the particular alcohol fuel envisaged. Alcohols have already shown satisfactory performance as substitutes for gas oil in the industrial versions of gas turbines, together with an increase in power of 8 °o in an experimental gas turbine engine for a passenger car. Reductions are reported in NOx and smoke emissions, in consequence of the lower combustion temperature, but increases in CO and unburnt hydrocarbons (UHC) have been found in the combustion chambers which were initially developed for hydrocarbon fuels. 1~ As fuels for the C-I engine, alcohols pose the basic problem of low ignition quality (CN of methanol = 3), and their use without blending or emulsification necessitates very high compression ratios and, with methanol, the addition of up to 20yo ignition accelerator which tends to be uneconomic. Ignition assistance by electrical means is discussed in Section 11.1. However, there is some promise for both methanol and ethanol by using conventional fuel alone for low load conditions, and for pilot injection at high load with the alcohols introduced on a dualfuel basis, either by carburation in the air stream or by separate injection is (see "hybrid fuels" in Section 9.6). Lower emissions of unburnt fuel and smoke have been reported when dual-fuelling with methanol, whereas the emissions of unburnt fuel and NO~ were raised and lowered respectively with ethanol) 2 When applied to boilers for a steam-turbine plant, alcohols suffer from the fundamentally low level of flame emissivity which inhibits the transfer of combustion energy in the form of heat within the flame zone. However, alcohol fuels have shown some promise as energy sources for land-based steamraising boilers, with reduction in emissions of CO and NOx, and absence of SO 2, aldehydes, organic acids or unburnt fuel. One useful bonus is the burning off of previous soot deposits by the methanol flame) 9 9.2. Hydrogen Currently, gaseous hydrogen is mostly produced by either direct decomposition or steam reforming of natural gas. Other possible feedstocks include the liquid petroleum fractions and coal, in conjunction with water, the hydrogen evolving from both fuel and water, and the fuel carbon oxidising to CO2. Decomposition of water can also be achieved by added energy in the form of electricity (electrolysis), heat (thermochemical) or light [see Section 9.5). Liquefaction for transport storage convenience is achieved

247

by precooling below the inversion level of 205 K, using liquid nitrogen (b.p. 77.2K) so that subsequent expansion will incur a further decrease in temperature to the boiling point of 20.2 K. Alternatively, hydrogen may be stored as compressed gas, or in the form of a metallic hydride (see Section 10). In the S-I piston engine, preignition, backfiring and spark knock all tend to occur with hydrogen fuel, due largely to the very low levels of minimum ignition energy and quenching distance, and partly to the high flame speed and wide range of flammability. Smoother running is found with the hydrogen injected directly into the combustion chamber, particularly after valve closure, with spark ignition timing delayed until about t.d.c., with low jacket temperature, and with elimination of lubricating oil ingress past the top piston-ring groove. Compression ratios as high as 16/1 have been found possible under controlled conditions, and the only emissions involved are HzO, which returns to the hydrological cycle, NOx from the high temperature effect on atmospheric air, and possibly some unburnt fuel. The high diffusivity of the small hydrogen molecule may permit unburnt fuel to enter the crank-case via piston-ring blow-by, consequently inert gas purging may be required in order to avoid explosions initiated by hot spots. Successful road operation of hydrogen-fuelled S-I engined vehicles has been accomplished by Billings Energy Corporation 2° and Daimler Benz 2~ using metallic hydride storage, by UCLA 22 and Musashi Institute of Technology 23 using cryogenic storage, and by UCLA and Cranfield 24 using high-pressure gas storage. In the gas turbine, hydrogen tends to show high combustion performance with good stability and temperature distribution, and minimal tendencies for pollution. Actual engine experience with hydrogen fuel has been gained by Pratt & Whitney, and by NASA. Excellent combustion performance within a J-57 engine was found in 1965, with prevaporized fuel injected via an axial tube system. In a high-altitude P & W 304 engine also tested successfully, the hydrogen was vaporized by exhaust heat, the resulting expansion driving the compressor turbine. The hydrogen was then burnt to provide thrust. The 1957 NASA flight tests further demonstrated the potential of hydrogen as an aviation fuel by using it in one of the two J-65 engines in a B-57 aircraft. :s A recent hypothetical study by NASA-Lockheed-AiResearch concerns Tristar aircraft with fuselage stretched to accommodate two large diameter liquid hydrogen tanks. Four such aircraft are proposed for joint operation by the United States, the Federal Republic of Germany, Saudi Arabia and the United Kingdom, over a four-point network linking Pittsburgh, Frankfurt, Riyadh and Birmingham where facilities could exist to generate about 18 tonne of liquid hydrogen per day from either coal or natural gas. 26 For the C-I engine, hydrogen is not fundamentally attractive due to the very low level of minimum ignition energy since any overheated surface may

248

E.M. GOODGER

promote preignition. With ignition timing controlled by the pilot injection of gas oil, however, up to 97 ~ of the energy input has been supplied by hydrogen in conjunction with very low levels of compression ratio and intake air temperature. 27 For steam-generating boilers, hydrogen again appears to be unpromising, in this case due to its fundamentally low level of flame emissivity. It has certain limited possibilities for some industrial boilers, particularly if refractory brickwork is present to re-radiate the heat, but probably not for transport boilers. 9.3.

NitrogenHydrides

The two major members of this group are ammonia NH 3 and hydrazine N2H 4 which can be considered as carriers of hydrogen, alternative to the carbon hydrides (hydrocarbons), with possibilities of handling convenience greater than that of liquid hydrogen itself. Currently, most ammonia is manufactured by the direct synthesis of nitrogen and hydrogen, and some from the steam treatment of calcium cyanamide. Most hydrazine is produced by the oxidation of ammonia with sodium hypochlorite, although reaction of chlorine with a ketone may be practicable, or the oxidation of urea. Although ammonia exists as a gas at ambient conditions, it would be stored and handled as a liquid for transport applications. Its high enthalpy of vaporization would then make necessary prevaporization and possibly controlled partial decomposition in a heated catalyst chamber. In S-I engine tests, a relatively high jacket temperature (180°C) has been found helpful to assist decomposition. The low flame speed requires advanced spark timing (or multiple plugs), but the high anti-knock quality permits increased compression ratio and thermal efficiency. Emission of NOw tend to be reduced, despite the high nitrogen content of the fuel, due to the low flame temperature, and there are, of course, no carbon containing pollutants. 2s The few engine tests with hydrazine show smooth operation possible but a high rate of fuel consumption. 29 The gas turbine combustor has not yet shown satisfactory combustion of ammonia fuel unless the ignition system remains in continuous operation, or the ammonia is partially decomposed beforehand. Despite a reasonably high combustion efficiency (97~o), the stability loop is found to deteriorate markedly, and the diameter of the chamber may need to be doubled. 3° Hydrazine saw considerable service as a German rocket fuel during World War 2. The most notable application of ammonia to C-I engines appears to be the conversion of bus engines, with compression ratio reduced from 16/1 to 8.5/1, for use in Belgium during World War 2. Ignition was initiated on the dual-fuel basis by means of coal gas. The liquid ammonia was prevaporized by heat from the engine cooling water. 31 Gas oil has also been used as an igniting fuel in tests with a CFR diesel engine at

a compression ratio of 30/1 and with advanced injection timing. Little work, or promise, appears to relate to the use of nitrogen hydrides for steam raising. 9.4.

Nitrohydrocarbons

The nitroparaffins are produced by the nitration of paraffin vapours giving nitromethane CHaNO2, nitroethane C2HsNO2, and so on. Nitromethane has a high anti-knock quality and exhibits a relatively high value (1.28) of the molar ratio of (products/ reactants) of combustion, which augments the pressure resulting from combustion at constant volume. It is therefore used in S-I engines for motor racing, a2 Nitromethane is sometimes used as an additive with methanol for miniature aero diesel engines, but appears to have little use so far in any other type of combustor. 9.5.

Biofuels

Biomatter generated as vegetation, municipal waste or sewage consists largely of carbohydrates, CxH,(H20)z which, as indicated earlier, form a logical source of such hydrocarbon oxygenates as alcohols, and also of methane, which may be further processed for liquid hydrocarbon fuels. Biomatter may also serve directly as a fuel source, as in the case of the turpentine-fudled Saab test car in Finland in which the engine compression ratio is lowered and a preheated dual-fuel system fitted. Mogas is used for starting, idling, warm-up and heavy load, and the turpentine or other liquids from wood for the cruising condition. Extensive road tests during a severe Lapland winter are reported to have confirmed both reliability and emissions control. 33 These fuels are less expensive than either gasoline or gas oil, and the vehicles were judged to have better acceleration and less engine noise than a comparable diesel engine. An automobile gas turbine engine is also reported to have operated successfully on peanut oil. 34 Some interest has been shown in the use of peanut, palm, soya, sunflower, coconut, rapeseed and cotton seed oils for C-I engines in the United States, 3s Japan, Brazil, Australia, China, the United Kingdom and elsewhere (realising one of the predictions of Dr. Diesel), particularly for agricultural engines, the general conclusions being that these oils, with cetane levels ranging from about 35 to 45 and specific energies approximately 8 5 ~ that of gas oil, give power and consumption levels very similar to those for gas oil, but with rather more smoke and some tendencies to nozzle coking. The use of bagasse (crushed sugar cane) and vegetable-based municipal wastes for steam raising appears to be limited to static industrial applications. Some species of vegetation (particularly the Euphorbiaceac) have been found which extend the carbon dioxide-to-cellulose process to produce hydro-

Liquid fuels for transport

249

method of imparting the handling convenience of fluids to solid coal by pulverization and suspension in a stream of fuel oil. Suspension in a stream of air is also possible. This mixture can be pumped, sprayed and burnt in much the same way as a liquid, and is suited to the generation of steam in a boiler. Its use in engines, on the other hand, has not yet been fully successful because of the presence of contaminating ash. However fine the pulverization, the ash is still present and can lead to problems of blade deposition and corrosion in gas turbines, and cylinder wear in diesel engines. Very fine pulverization followed by flotation has been suggested as a means of ash removal, but the grinding costs involved are expected to be high. This group also includes the separate handling of dual fuels as with gas and liquid [e.g. hydrogen and gasoline) or liquid and liquid (e.g. alcohol and gas oil).

carbons in the form of latex sap of relatively low molar mass, and further development may permit a useful oil yield by tapping the main stems, as in the harvesting of rubber, a6 Certain green algae may also be converted to hydrocarbons, to the extent of 75 % of the dry mass. More direct use of solar radiation appears possible for the future production of fuels. Hydrogen may be isolated from water by methods based on photochemistry (using a water-chemical solution), photoelectrochemistry (using a water-based electrolyte in a cell), photobiology (using algae in water) or possibly by some combined biological and chemical technique (using the solar energy in vegetable matter, such as spinach chloroplasts, coupled with water-splitting catalysts). Alternatively, photoelectrochemistry might be employed to produce electricity from a cell, or bacteria used for the photobiological conversion of wastes to fuels. Laboratory experiments are also under way to simulate the process of natural photosynthesis.

9.7. Cofeedstock Fuels These are single fuels derived indirectly from more than one source. A representative example is the scheme to derive charcoal from agricultural wastes using a mobile pyrolytic converter travelling to farms, sawmills and forests, and then mixing with coal of high sulphur content. The augmented mixture could then be gasified, liquefied or converted to substitute fuels as discussed above.

9.6. Hybrid Fuels These are combinations of finished fuels derived directly from more than one source, and may comprise single fuels of emulsified liquid mixes (e.g. waterin-fuel), liquid solutions (e.g. alcohols-in-gasoline), or slurries of solid particulates supported in liquids (e.g. coal-in-oil). The last named example is the established

120. Key • = MJ/kg

-60

,V~0 ~;.,T.

p,, .~

z

1 I

l+O-

2O

(~.~,

6.s Fuel density

kg/t

FIG. 8. Calorific values of transport fuels.

l'.Z

250

E.M. GOODOER

10. MAIN PROPERTIESAND HANDLING CHARACTERISTICSOF LIQUID TRANSPORTFUELS

Initial fuel selection is seen to be determined largely by the combustion requirements of the type of engine suited to the given application. All the combustion characteristics, together with a large number of other physical and chemical properties, determine the techniques and precautions required in fuel storage, distribution and handling generally. Since a fuel, by its very nature, is an energy store, the quantity of energy stored is of fundamental interest but it is, perhaps surprisingly, of more concern in handling than in engine performance, as shown below. The term "calorific value" is still used in a generic sense but, fortunately, the former unwieldy terms "gravimetric calorific value" and "volumetric calorific value" are giving way respectively to the heater "specific energy" and "energy density". The curves in Fig. 8 show the well-known variations in both properties for the whole range of petroleumbased fuels together with one parent element, hydrogen, as liquid. (Comparable values for carbon, as solid graphite, are: density 2.17kg1-1, specific energy 32.8 MJ kg- i and energy density 71.2 MJ l- ~). Also shown are the substitute fuels discussed in Section 9. The specific energy of hydrogen is seen to be some 3.7 times that of carbon; consequently the corresponding values for the petroleum fuels fall with increase in density because of the reduction in hydrogen content. Net values are used throughout here since the product water invariably leaves the

combustion chamber in the vapour phase, in accordance with the constraint of the second law of thermodynamics on heat engines. Values of energy density (MJ 1-1) are obtained directly by multiplication of specific energy (MJ k g - 1) with density (kgl-l), and the curve is seen to rise (from the particularly low value for low-density hydrogen) due to the fact that the rate of increase of density exceeds the rate of reduction of specific energy. Both forms of calorific value are important in vehicles, since a greater proportion of the energy is available for moving the payload if the fuel specific energy is increased (lower density fuel), whereas fuel storage volume and fuel system mass are reduced if the energy density is raised (higher density fuel). In operation, of course, a combustor receives a metered mixture of air and fuel. Taking stoichiometric mixtures as a basis for comparison, the specific energy of a stoichiometric air-fuel mixture can be expressed as follows: Q Q/F (A + F)s = (A/F)s + 1 by dividing throughout by F where A and F are masses of air and fuel respectively, and the subscript "s" represents stoiehiometric. It is of interest to note that the fuel specific energy, Q/F, bears an almost linear relationship with the stoichiometric air/fuel ratio, (A/F)~, and therefore also with (A/F)~ + 1, as shown in Fig. 9. As a result, specific energies of stoichiometric air-fuel mixtures show a remarkable

( A / F ) s +1 21 I

/,,1

I

I

I

120"

/

/

/ / 7-

/

Z

/

/

/

/

/

40 Vegetableoils/) C~W~'troleumfuels Anthracite MTBE ,~m~:~Clgr) NH ~'~ C2HsOH N H "~:FE~OH"3/"

0

V/CH3NO2 o~Blasffurnacegas o

h

StoichiometricairI fuel

mass ratio, ( A / F )s

FIG. 9. Relationship between net specificenergy and stoichiometric air/fuel mass ratio.

Liquid fuels for transport

251

--rV

0

•

,=

Mean 2'B5

z= ~ i ~ --~

"E

E La 5

t.d

-t-

-r"

~

La (~

Petroleum fuels

2"0

1"0 Fuel density, Kg/I

FIG. 10. Net specificenergies of stoichiometric fuel-air mixtures.

constancy, varying little from the average value of 2.85 MJ kg- 1 mixture, over a wide range of fuels, both conventional and alternative (and, in fact, also for gaseous and solid fuels). As shown in Fig. 10, those fuels that do exceed the average include hydrogen and, perhaps surprisingly, nitromethane which has previously been noted as a relatively low-energy fuel in terms of mass of fuel alone. Thus, combustors are virtually independent of fuel specific energy, this parameter acting instead as a controlling variable of the sizes of fuel storage tanks and transfer pumps within vehicles. Fuel consumption rates would be high, for example, with such fuels as the alcohols, ammonia and nitroparaffins. Although methanol is showing promise in a piston-engined Piper PA-18 Super Cub light aircraft, 3~ the fact that 50~o of its mass consists of oxygen, which is freely available in the atmosphere, highlights its unsuitability for mainline jet aircraft with fuel loads in the region of 100 tonnes. The mass and space requirements for storing automotive liquid fuels are compared in Table 7 with those for hydrogen stored as liquid, compressed gas, and as a representative metallic hydride. A tank size of 75.71 (20 US gal) is adopted, together with a standard

Mogas as a datum. Of the alternative fuels shown, minimum mass of fuel-plus-tankage is seen to occur with liquid hydrogen, and minimum volume with ethanol. Compressed gaseous hydrogen is unacceptable on both counts. Another important property related to the storage characteristics of a fuel is the level of volatility which determines the extent of vapour loss from the tank vents, and of vapour lock within the system itself. Of the conventional fuels, normally only gasoline gives rise to such problems, and then only under conditions of high temperature and/or the low pressure of high altitude. For this reason, the vapour pressure of Avgas is specified considerably below that of Mogas which operates only at ground level where atmospheric pressure is highest. Kerosine might just reach a problem at high altitude under the heat-soak conditions of supersonic flight, consequently aircraft fuel tanks are sometimes pressurized up to about 21 kPa (31bfin -2) above ambient. Most of the alternative fuels are less volatile than Mogas, with the exception of ammonia, although the blending of about 3 ~o or more of alcohol with Mogas raises the vapour pressure due to the formation of low boiling point azeotropes, and may necessitate the removal of some

TABLE7. Comparison of automobile fuel storage for 2450 MJ Iderived from Refs. 26, 38-40 and other sources) Fuel Fuel type Mogas H2(I) cryogenic Representative Metallic hydride H2(g) ca 136atm (2000 lbfin -2 ) Methanol Ethanol Ammonia Hydrazine Nitromethane

Fuel + Tankage kg I

kg

I

56 20.4

75.7 288.6

66 100

77 360

208

560

240

1840 155.5 116 233 146 197

1200 145 105

1845 157 117

517 20.4 123 92 132 147 224

252

E.M. GOODGER Temperature "F

3000 lOOO.

-80 ~

.

-t,O

' .

'

.

.

0

' .

'

.

.

100" 5o.

60

' .

=

80

'

120

k,,~ ~ '

.

-~

160 200

'

'

~ ~

"

,

~

, ULP

. . . . . .

~

Residual

"~15°i'

i =;

ULA

Ga o,,o

\

%

\

\ 081

~ ~ ~ ,

? ~ i n e

/ I I

0.~/

\\\

\ "~.

A%onia

,

-60

~ p

,~,-~

-60

.

.

.

-20

.

.

.

.

.

.

.

0 20 60 Temperature"C

60

\ . Hydrazine \\

,'N, .\.

80 100

,

FIG. 11. Variation of kinematic viscosity with temperature for conventional and substitute liquid fuels. BP = boiling point ULP = upper limit for pumpability CP = cloud point ULA = upper limit for atomization.

of the more volatile gasoline components. As a result, the addition of, say, 20 ~o of methanol may realise an energy increase of only 4 ~o. This problem is eased by replacing alcohols with ethers. Fire safety is generally assessed in terms of flammability based on the level of flash point (the temperature at which the air-vapour mixture above the surface of a liquid fuel sample will inflame momentarily on the introduction of a standard-sized test flame) relative to the datum of 32°C. The curves in Fig. 3 show the low-density petroleum fuels (e.g. gasoline) to have low flash points and thus to be relatively flammable, in contrast to the high-density petroleum fuels (e.g. diesel fuels and fuel oils). Of the alternatives, such volatile fuels as ammonia and the light alcohols are considered highly flammable. Most of the fuels considered have high levels of SIT at atmospheric pressure, but hydrogen shows a very low minimum energy for spark ignition, and a low quenching distance.

At the low end of the operating temperature range, problems can arise through the increasing level of viscosity, and eventual freezing. Kinematic viscosity (in units of centistokes, cSt) is of general interest in liquid fuel technology, in connection with pumping and atomization. The variation of viscosity with temperature is shown for a number of fuels in Fig. 11. The cloud point of a non-aviation fuel sample is defined as the temperature at which a cloud of wax crystals first appears in the sample during coolin# without stirring, and this approaches closely to the freezing point of an aviation fuel sample at which the wax crystals disappear during warmin# with stirrin#. At a lower temperature level with non-aviation fuels, the pour point is also determined when solidification of the sample appears imminent. In fact, tests conducted with distillate petroleum fuels in full-scale fuel tanks and pumps have shown minimum temperatures for pumpability to lie 4--16°C below the laboratory measured freezing points, and 1-7°C

Liquid fuels for transport below the pour points. For this reason, a new test method is currently being evaluated. Development of future aircraft fuel systems is being assisted by use of the ERBS fuel (see Section 5) which has a maximum freezing point of - 2 9 ° C (cf. - 4 7 ° C for current Avtur). For such a high level of freezing point, substantial heat addition will be required in order to maintain fluidity in flight, and potential sources of energy include the cabin air-conditioning efflux, main engine tailpipe, oil-fuel cooler and, more attractively, an engine-driven heater system capable of support from auxiliary power on the ground. 41 In the marine world, the pour point should not exceed about 35°C otherwise special heating facilities are required. The melting points of most alternative fuels of the individual material type (as distinct from blends) which represent complete solidification, lie below - 1 0 0 ° C except for the nitrogen hydrides: hydrazine, for example, freezes at 1.5°C, although it contracts in the process and so does not damage storage tanks. Cryogenic liquids bring their own low-temperature problems of storage and handling, although none appears to be insoluble technically. Structural materials of low-temperature strength, in conjunction with lightweight insulants, make the design of vehicular cryotanks appear feasible, and the proposed hydrogen-fuelled Lockheed Tristar aircraft incorporate fuselage tanks which, despite mass and bulk, are stated to give a reduction in aircraft gross mass of 27~ o, in comparison with hydrocarbon fuelling, due to the very low mass of the hydrogen fuel. z6 On occasions, the influence of certain types of hydrocarbon component is so significant that its concentration has to be specified. With aviation kerosine, for example, the volumetric concentration of aromatics is limited to a maximum (22 ,°~o in U.K., and 20 '~o with a waiver to 25 o//oin U.S.) in order to restrict problems of flame radiation, soot deposition and smoke. In addition, the bicyclic aromatics, naphthalenes, are limited to 3 ~ maximum since they also have high freezing points. Since fossil fuels are derived from natural storage in the earth, they invariably contain trace materials derived from their environment, as well as from their organic origins. The majority of these materials are undesirable, and are generally classed as contaminants. Those in gaseous hydrocarbon fuels may be incombustible (COz, N2) , corrosively combustible (SO2) or malodorous (H/S), whereas those in liquid petroleum fuels include asphaltenes, gums, acids, sludge, ash, water, sodium, vanadium, sulphur in various forms, fuel-bound nitrogen, and finely-divided solids promoting sediment, together with traces .of other inorganic materials. Where necessary, concentrations of such contaminants are limited within the various fuel specifications. In practice, fuel treatment is sometimes necessary immediately prior to use in order to meet the contaminant limits demanded by the combustor. This

253

arises particularly with the heavier fuels in which the contaminant concentrations have been raised by the removal of distillates during refining. With marine fuels, for example, the concentration of sediment is limited to 0.3 ° o in order to obviate overloading the centrifuges. It may also be necessary to reduce the concentration of sodium to about 0.5 ppm by water washing, and to inhibit the vanadium with magnesium additives to raise the melting point of the sodiumvanadium oxides and so prevent their deposition on the turbine blades with consequent corrosion. Concentrations of additives themselves must also be specified in order to limit any deleterious side-effects: the phasing down of lead in gasoline is a marked example. Most of the candidate fuels are stable in storage, with the exception of hydrazine and the nitroparaffins. Storage stability is one of the characteristics of conventional fuels, provided the content of the unsaturated olefins is limited. However, partial oxidation is possible with some commercial fuels when subjected to heat, even if the temperature is well below the level of SIT. This is evident with some aviation kerosines when exposed to the kinetic heating of supersonic flight, leading to the formation of gums and solid particulates in the fuel system. As a consequence, the specifications for aviation kerosines include a test for thermal stability, in which the sample is heated and filtered, stability being assessed jointly by the extent of filter blockage, and the nature of the deposits on the heater surface, within a given period of test. The ingress of water to blends of gasoline with methanol, and to a lesser extent with ethanol, gives rise to blend separation as the alcohol and water combine. The standard technique of blending with byproduct tert-butyl alcohol for improved octane quality also serves to reduce water sensitivity. This problem could largely be solved by maintaining the two fuels separate until the point of sale, but at high costs of segregated handling. In conjunction with the effects on vapour pressure, mixture strength and driveability, plus the blockage of filters due to loosened dirt in the fuel system, alcohols may be preferred for use as fuel substitutes, probably in a small self-contained dedicated fleet, rather than extenders. Again, the problem of water separation is less severe with the ethers which blend well with hydrocarbons and are only slightly soluble in water. Explosive peroxides may result from the ethers, but not from MTBE (see Section 7). The materials used in fuel system construction must also be selected with care since each fuel has a number of metals, and some plastics, with which it is incompatible. Health tactors must always be taken into account when handling fuels. Although relatively innocuous, the conventional hydrocarbon fuels can lead to skin irritation on external contact, and to pneumonitis on aspiration, whereas methanol and ammonia have high values of hazard index (= vapour pressure/threshold limit concentrationl, with risk of blindness, and the

254

E. M. GOODGER TABLE8. Representative specifications for liquid transport fuels Specification reference

Number of properties specified

Fuel LPG Aviation Motor Widecut Aviation High flash

Gasoline Kerosine Gas oil Diesel fuel

US

BS 4250 DERD 2485 BS 4040 DERD 2486 DERD 2494 DERD 2498 BS 2869 Class A1 BS 2869

D 1835 ASTM D 910 ASTM D 439 JP4/JET B ~ D JET A/JET A1 J 1655 JP 5 ASTM D 975 ASTM D 2880 ASTM D 975 ASTM D 2880

6 15 6 20 23

ASTM D 396

6

ASTM D 396 ASTM D 396

6 6

BS 2869 Class F Class G Class H

Medium Fuel oil

UK

Heavy Very heavy

11.1. Heat Engine Propulsion In conventional piston engines, the broad principles of combustion chamber design may be grouped under the two headings of open chamber (or single chamber) with a controlled degree of mixture swirl generated by induction, compression or fuel injection, and prechamber (dual, divided or auxiliary chamber) in which combustion is initiated under sheltered conditions, and from which the burning gases discharge as a vigorous jet plume to promote torch ignition of the remainder of the mixture within the main chamber (Fig. 13). Arrangements for local cooling or heating are made to suit the required type

!1. ENGINE DEVELOPMENTS AFFECTING FUEL QUALITY Further developments in transport engine design are likely to affect the quality requirements of the

I

AiR

Light aircraft

SEA

I LAND

Jet aircraft

Cars

Trains

Trucks

High

.=_=

EL~ S-I PISTON

GAS TURBINE ~

Naval Merchanf ships sh ps

Medium Low

speed srmd speed \ C-!

PISTON j

STEAM TURBINE

v

Liquid

Hydride

HYDROGEN

AMHO

Liquid

Sol.id

NIA OXYSEN/ffF.~BIOFUELS COAL/SHALE I

I

~'

9

related fuels, and the following discussion outlines some current advanced designs of propulsive unit which may well find wide adoption in future vehicles.

cryogenic liquids cause cold burns on external contact. Overall consideration of all the above combustion and handling characteristics has led to systems of specification for conventional fuels, devised nationally but in some cases recognized internationally, as referred to in Table 8. Worthy of note is the relatively high number of property limits specified for aviation fuels, reflecting the essential standards of reliability, and the wide ranges of operating conditions. The probable relationships of the alternative fuels with engines of basically conventional design are shown in Fig. 12.

,B

ll

= Specialrequirements

for

Naves

• " = Minor application,: only

FIG, 12. Transport engine and future fuel types.

COAL

Liquid fuels for transport

1 OPEN

PRE

( or Single)

( or Dual or Oivided, or Auxiliary )

FIG. 13. Main groups of piston engine combustion chamber. of ignition, hence the cooling of end-gas zones in open chamber S-I engines, and the use of heater plugs in some prechamber C-I engines. In the S-I engine, the octane requirement could be markedly reduced, and possibly eliminated, by dispensing with the conventional basis of flame propagation towards an end-gas zone. This can be achieved by charge stratification, i.e. local placement

255

of rich mixture in the ignition zone, with subsequent rapid consumption of the remaining weak mixture. Valuable bonuses from this approach include overall lean burning, which aids the much-needed fuel economy, and reduction in emissions due to the more complete burning at lower temperatures. Since charge stratification is an inherent feature of C-I engines due to the need for fuel injection within the combustion chamber, the more efficient diesel also appears attractive for the future, particularly if its mass and cost can be reduced. However, some candidate alternative fuels are of such poor ignition quality that even a prechamber heater plug proves insufficient, and ignition assistance by sparking plug becomes necessary. The various engine types are classified in Table 9, together with some representative examples, and it is clear that the two hybrid types of engine, i.e. the "unthrottled stratified S-I" and the "'spark-ignited diesel" (sometimes described as DISC, or direct injected stratified charge, engines) have very much in common, apart perhaps from the later injection in the diesel. It follows that the very desirable independence from anti-knock quality can be expected, as shown in Table 10, leading to two very significant conclusions for both types of hybrid engine: (a) Several individual fuels ranging from highoctane gasoline to gas oil, together with such substitutes as methanol, are acceptable:

TABLE9. Piston engine combustion chamber classification (Ref. 42) Chamber type Open

Spark-ignition (S-I) Carburettor + throttle, r v8.8 avge. (Conventional)

Compression-ignition (C-l) Direct injection unthrottled, r~ 17 (Conventional DI )

Swirl charge with carburettor + throttle, lean burn, high r~ (e.g. Fireball) Stratified charge with injection, unthrottled, lean burn, high rv 11 (e.g. TCCS, PROCO) Pre

Stratified charge with injection + sparking plug, unthrottled, lean burn, moderate rt, 14.5 (e.g. FM) lndirect injection unthrottled, (Conventional IDly

r,,

Indirect injection + glow plug (e.g. Comet ILl) Stratified charge with carburation, throttled, lean burn, r~ 8 (e.g. CVCC) Stratified charge with injection, unthrottled (?), lean burn, high rv 10 (e.g. SKS) Fireball = Michael May Fireball TCCS = Texaco controlled combustion system PROCO ~=Ford programmed combustion FM = M.A.N. FM CVCC = Honda compound vortex controlled combustion Comet = Ricardo Comet SKS = Porsche SKS.

Stratified charge with injection + sparking plug, unthrottled, lean burn, moderate r t, 12 (e.g. Spark Comet V)

20

256

E. M. GOODGER TABLE 10. Fuels suited to piston engine combustion chambers (Ref. 42) Chamber type Open

S-I

C-I

Conventional-- Mogas 90 to 97 RON

Conventional DI--Gas oil/diesel fuel 40~,5 CN, kerosine, and possibly (gasoline + luboil) with r.. 19 + heat

Fireball-TCCS-- Mogas 100 or 91 (LF) RON, Gas oil, WCD PROCO--Mogas 91 (LF) RON

FM--Mogas high or 83 (LF) RON, Gas oil, methanol Conventional IDI--Gas oil 50 CN, diesel fuel, heavy fuel oil Comet Ill--Gas oil to heavy fuel oils

Pre CVCC--Mogas SKS--Mogas 86 RON, methanol

Spark Comet V--Gasoline, gas oil, WCD

(LF) = Lead free WCD = Wide cut distillate.

(b) A single wide-ranging distillate incorporating the gasoline, kerosine and gas oil fractions could replace these individual fuels, thus saving refinery energy and costs. Equally important is the general reduction in combustion emissions (with some exceptions in the case of unburnt hydrocarbons), as shown in Table 11. The Stirling double-piston engine is attractive since it operates on a closed gas cycle with a theoretical Carnot efficiency, and its continuous external combustion gives multi-fuel capability with low levels of emissions and noise. Small versions ( < 1 0 k W ) w e r e commercially successful during the 19th Century until superseded by internal combustion engines and electric motors. Passenger car installations have been tested recently by KB United Stirling (Sweden) AB & Co., with less gear-changing found necessary due to the favourable torque characteristics, but there appears as yet to be no serious challenge to conventional propulsion systems. The Wankel and comparable designs of engine are also attractive since continuous rotary motion is far more elegant mechanically than the repeated accelerations and decelerations of the masses of pistons and connecting rods. In its S-I version, the Wankel engine

is tolerant to octane requirement, and is reported to operate smoothly with fuels of R O N levels as low as 50. However, it has not yet been able to meet emission limits with respect to unburnt hydrocarbons due largely to the high surface/volume ratio of the combustion space. Although satisfactory for high performance applications, the gas turbine does not lend itself to scaling down in size to any road vehicle smaller than a large truck. With regard to aero gas turbines, one of the most significant developments to improve emissions control is the separation of the combustor into two zones, This can be effected either in series with prevaporization and premixing prior to a fuellean main stage as in the Pratt & Whitney Vorbix JT9D-7 engine, or in parallel with cruise and high power double annular chambers as in the General Electric CF6-50 engine. 43 These chambers also show the liner temperature to be largely independent of fuel hydrogen content, and thus aromatics content. In the marine steam turbine plant, future developments are likely to centre on the fluid-bed combustor with its multifuel capacity, its 4-5 times higher heat transfer rates due to the submerged steam pipes, and the potential for counteracting the corrosive fuel contaminants by chemical absorption within the bed

TABLE 11. Emissions from piston engine combustion chambers (Ref. 42) Chamber type Open

S-I

C-I CO

Conventional Fireball TCCS

PROCO

L L L?

UHC NO~ (L) -

L L L?

Pre CVCC SKS

L -

(L) = Slightly lower than conventional L = Much lower than conventional (H) = Slightly higher than conventional.

L L

L L

CO Conventional DI

-

UHC NO~ -

FM

-

Conventional IDI Comet 1II

-

(H)

-

-

Spark Comet V

L

L

Liquid fuels for transport material itself, using lime or dolomite. Much experience has been gained with fluidized reactors in the chemical industry, and the fluidized bed principle has been tested in a waste-heat boiler fitted in a Shell tanker. Multiple tiers of vertically-disposed beds are envisaged, with movement of the refractory bed particles in heavy seas controlled by subdivision of each bed. Steam conditions of 140 bar and 600°C are expected to reduce the specific fuel consumption to about 0.24kgkW-lh -1, and the system should be able to accept the poorest quality of bunker fuel oils. 4a The fluid bed combustor also lends itself to crushed, rather than pulverized, coal as fuel and, when developed as a pressurized unit, could generate fuel gas for direct feeding to the combustion chamber of a gas turbine engine. Combined plant incorporating both steam and gas turbines are likely to reduce the specific fuel consumption further to about 0.21 kgkW- lh- 1 at the expense of machinery complication. 11.2. Electrical Propulsion

The various systems of electrical propulsion may be grouped on an energy basis as follows: A. In-vehicle storage (a) generating station/battery/electric motor; tb) liquid or gaseous fuel/fuel cell/electric motor; !c) generating station/flywheel/electric motor; B. External supply (a) generating station/conductor/electric motor; tb) solar radiation/photocell/electric motor; (c) generating station/electromagnetic track/induction motor C. Hybrid la) generating station/liquid fuel/heat engine/' battery/electric motor ("Engine/battery"); (b) generating station/conductor/battery/electric motor ("Conductor/battery"). With regard to fuels for electricity generation, the gas-turbine stations will no doubt continue to use residual fuel oils (with sodium removed and vanadium inhibited), or natural and waste gases where available. In the steam-turbine stations, the trend from oil to coal is expected to continue together with the addition, where available, of processed solid wastes from municipal and industrial collection centres. Battery vehicles are quiet and non-polluting, but the extensive mass of battery restricts vehicle range, and the main alternative to necessarily lengthy recharging times (determined inversely by the crosssectional area and carrying capacity of the feeder cable) is an extensive network of battery-exchange facilities. Comparative studies of coal usage by liquefaction and by electricity generation/battery charging have shown the latter to be attractive, at present, only for specialized urban delivery vehicles operating over predictable cycles of limited duty, with liquefaction preferable for ranges above about 150kin (95mil. A

257

number of experimental electric passenger cars have appeared, but taxis, light vans and buses operated on a fleet basis appear to be best suited to electrical propulsion. The development work under way in the United Kingdom, United States, Japan, France, the Federal Republic of Germany, the Netherlands and elsewhere is applied to chargers and controllers as well as batteries and motors, a5 Advances in battery design, such as the replacement of lead-acid by sodium-sulphur technology, could raise the specific energy from about 0.13 to 0.30 or even 0.50 MJ kg- 1 and thus favour the electric vehicle for higher ranges in the future. However, the more attractive concept in the U.K. is the "engine/battery" hybrid vehicle, with the engine operating only at optimal conditions, particularly in the "series" version where the engine is confined to augmenting the external charge to the battery, and has no direct coupling to the driving wheels. Automotive liquid fuels will be required, as discussed earlier, but the rates of consumption will be less since the battery would also be charged from off-peak mains electricity overnight. Fuel consumptions of about 1.131/100 km (250mi/UKgal) are claimed for such hybrid vehicles. A vehicle designed on the "parallel" basis (i.e. with both engine and battery drive) is under construction at the University of Queensland. 46 Although still in the early stages of development, the fuel cell is an attractive alternative to conventional batteries since it operates indefinitely with continuous fuel supply rather than for periods limited by input charging or electrode consumption, and since its efficiency of chemical-electrical energy conversion is free from the constraints of the Carnot cycle. Potential fuels include hydrogen, methane, ammonia, methanol and light distillate liquids which are contaminant free. One further method of storing electrically-derived energy in a vehicle is by means of high-speed rotation of a heavy flywheel, recharged at suitably spaced stations en route. 4. Propulsion in rail transport is depending increasingly on schemes of electrification for main-line inter-city and highly-populated commuter services, using direct pick-up en route via an overhead wire or third rail conductor. An effective method of improving range and reducing battery mass of electricallypropelled road vehicles is the "conductor/battery" hybrid based on conductor pick-up over less than 20°~i of the route length, coupled with regenerative braking, as in the French 90 passenger "Biomode" system, and the German "Duo Bus". In the marine world, battery propulsion started in the 19th Century, and became popular for leisure boating before deferring to the less-expensive maintenance of the C-I engine, as Electrical transmission from heat engine prime movers (diesel or gas turbine enginesl has been employed in ships, as well as in rail locomotives, to give flexibility of power control. Later designs of some gas-turbine powered tanker ships capitalize on the ability to reduce the size of the all-important air and exhaust ducting by mounting

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the engines on deck aft, with power transmission effected electrically. Electrical propulsion energized by solar radiation has been demonstrated for cars, as in the Freeman model which generated up to 160W and achieved 24 k m h - l (15 mih- 1 ).49 Solar powered aircraft have also flown successfully, as exemplified by the crossChannel flight of the Solar Challenger in 1981.50 In general, electrical propulsion/transmission appears to offer some promise of higher utilization efficiency of liquid fuels in specific applications, but more development and field experience is required before this promise can be fulfilled.

barges at suitably remote off-shore points. There are also arguments in favour of nuclear propulsion for aircraft, with capability of virtually unlimited range and a variety of special missions. Nuclear aero engines have been built and tested, and the concern for crash safety met in part by the leakproof demonstration of a simulated reactor container projected at Mach 1 into a concrete wall. 52 However, the general conclusions to be drawn from all the above are that the liquid fuels, particularly those derived from petroleum, have proved themselves to be the most convenient and satisfactory form of packaged energy for in-vehicle storage. This suitability, together with the massive financial investments involved, lead to little incentive to change, and this very conservatism will hasten the decline of liquid fuel quantity. It will also lead inevitably to a gradual deterioration in quality as the use of less attractive crudes, and more severe processing, becomes unavoidable. Nevertheless, such changes will probably be of a slow evolutionary nature based on developments of existing engineering knowledge, rather than novel scientific breakthroughs. Temporary gluts in oil fuels (as existing at the time of writing) must never obscure the fact that a fuel based on a fossil source can have nothing but a finite life. New oil fields will undoubtedly be discovered, together with more extensive methods of recovery, but consumption will almost certainly reflect the increasing level of population, and the demands of the undeveloped countries for their rightful share of the energy wealth. An appreciation of the above situation has already led to a very creditable and effective awareness of the need for fuel economy, with progressively improved efficiency in the utilization of fuels and of their

12. C O N C L U S I O N S A N D F O R E C A S T

Although beyond the scope of this paper, it is relevant to take note of other sources of propulsive energy outside the sphere of liquid fuels which are also promoting interest. For example, the case has been made repeatedly for a return to coal as a marine fuel, and current buildings include coal-fired steam propelled bulk ore carriers for use in Australian waters. Following numerous developments in sail design and materials, interest is returning also to the much earlier based system of windships, and a twinmasted tanker, incorporating computer-controlled folding sails, has already appeared in Japan. Other windship projects include rotors, wind turbines, and also kites raised initially by in-built gas-filled lifting cells for capturing the higher velocity winds at altitude.51 Nuclear fuels are already seeing service in some Naval surface and submarine craft, and the safety requirements for merchant ships in port might well be met by the concept of the nuclear containership collecting and distributing its self-propelled container

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FIG. 14. Future transport engine and fuel types.

Liquid fuels for transport normally rejected exhaust heat. It is also proper to consider reducing fuel usage through shortened travelling distances between homes, amenities and jobs, and through replacing some types of travel for purposes of personal contact by advanced video systems of telecommunication. But as fuel shortages eventually arise, the logical next step appears to be one of supplementation with closely similar materials requiring minimal modification to the overall systems of fuel provisioning and utilization. The future fuel refinery may very well be accepting shale oil, tar and coal as well as supplies of crude oil, and may either augment the output of transport fuels on a cofeedstock basis, or produce the supplemental fuels direct from these alternative sources, possibly using part of the coal as a source of energy for the purpose. All this will provide an invaluable breathing period for new engine development to be effected, ready for the substitute type fuels when they become the only ones available. The possible options of the alternative fuels in the various branches of the transport sector are shown in Fig. 14. It is perhaps salutary to recognize from technological history that most of the alternative fuels discussed are not new and, in many cases, were c o m m o n p l a c e before being ousted by the more attractive petroleum fuels. However, the very availability of these alternatives at this time focuses on their re-adoption, and particular elegance centres on the renewable sources such as biomatter or, more directly, solar energy. F o r this overall complex of transition processes to be effected without serious perturbations, it must be remembered that a manufacturing scheme in economic quantities for any new material such as the supplemental fuels will occupy a lead time of the order of 15 years, and that conventional fuels may well have reached embarrassingly high price levels by that time. In essence, although there is no overall shortage of energy, there could well be shortages of its availability in convenient packaged form, i,e. as fuels, if considerations of supply, utilization and environmental protection are inadequate. In formulating energy policies, it is probably more effective to examine first the long-term future and then extrapolate backwards to the near term, in order to perceive an enduring solution, rather than to concentrate solely on the near term. Those entrusted to decide these policies must be provided with the best possible information on the potential availability and performance of alternative fuels and engines, so that the resulting policies force development by setting constructive targets, hopefully without dictating the routes to their achievement. Those who provide this information, in turn, must themselves possess an expertise that is comprehensive and continually updated. It is equally necessary to maintain communication, in both directions, with the public at large and so p r o m o t e awareness and acceptance on both sides of the needs and practices involved in providing the fuels for transport through the coming decades. All this highlights the invaluable nature of

259

discussion, debate and dissemination of knowledge in this vital area of technology.

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41. RUDEY, R. A. and GROBMAN, J. Characteristics and combustion of future hydrocarbon fuels, AGARD Lecture Series, 96, London (1978). 42. GOODGER, E. M. Fuels for discontinuous combustion, Symposium 'Liquid Fuels for the Future', Inst. Chem. Engrs. Birmingham, U.K., 25 March 1981. 43. ROBERTS. R., FIORENTINO, A. and GREENE, W. Experimental clean combustor program, Phase III, NASA CR-135253 (1977). 44. LARSEN, G. A. V.A.P. Turbine plant and its economy, Conference 'Steam Propulsion .for Ships in the Changing Economic Environment', Trans. Inst. Mar. Engrs, London (1978). 45. Electric Vehicle Development, Conference Publication 14, Peter Peregrinus Ltd, Hitchin, Herts, U.K. (1977). 46. GILMORE, D. B. and BULLOCK, K. J. Development of a hybrid petrol-electric vehicle, Queensland Division Tech. Papers, Institution of Engineers, Australia, 21, No. 27, pp. 19-24 (1980). 47. Flywheel Powers Road Vehicle, Autoworld, No. 77, p. 3 (1980). 48. DESMOND, K. The silent age of battery boating, Mot. Boat Yacht., 26, No. 252 (1979). 49. FREEMAN, A. Design and operation of a solar powered car, Automot. Engnr, April/May 1981, pp. 60, 61, 91. 50. SMITH, B. A. Use of composites key to solar aircraft, Aviat. Week, 11 May, 1981. 51. The Viability of Commercial Sailing Ships--Area for Research, The Naval Architect, London, September 1979, pp. 200-203. 52. GREY, J. Future engines and fuels, Esso Air Wld, 27, 9195 (1975).