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Exhaust gas recirculation for Nox control in a multicylinder hydrogen-supplemented S.I. engine
0360-3199/93 $6.00 + 0.00 Pergamon Press Ltd. International Association for Hydrogen Energy.
Int. J. Hydrogen Energy, Vol. 18, No. 12, pp. 1013-1018, 1993.
Printed in Great Britain.
EXHAUST GAS RECIRCULATION FOR NOx CONTROL IN A MULTICYLINDER HYDROGEN-SUPPLEMENTED S.I. ENGINE L. M. DAS and R. MATHUR Indian Institute of Technology, Delhi, New Delhi 110016, India (Received for publication 3 November 1992)
Abstract--This paper describes the results of an experimental investigation carried out on a hydrogen-supplemented multicylinder spark ignition (S.I.) engine to control the level of NO~ (oxides of nitrogen) emission by adopting exhaust gas recirculation (EGR). It was observed that the NO, level was substantially reduced over a wide range of engine operation conditions. Performance characteristics of the system were also evaluated corresponding to these operating conditions.
INTRODUCTION Our present-day lifestyle system is becoming increasingly dependent on the transportation sector with its network built on petroleum-fuelled automobiles. These automobiles, along with the growth of civilization have grown into the status of an indispensable prime mover. Unfortunately, in view of the present escalating energy (fuel) crisis, the very safe survival of the automobile has been put under question. Under such circumstances, it has now become all the more important that an acceptable alternative automobile fuel must be expeditiously developed so as to at least sustain the present growth rate of civilization. Recently, the eyes of the world were directed at the Earth Summit held in Rio de Janeiro, Brazil where it was unequivocally emphasized that our planet must be protected from further degradation. Logically, the simplest way of protecting our Earth is to stop hurting it. One of the most serious of such offences can be remedied by drastically reducing the level of pollutants which come out of the tail pipes of automobiles. These exhaust emissions from automobiles cause immense damage to human beings, as well as to plants and animals. Therefore, in essence, the search for an alternative automotive fuel must take into account environmental compatibility, in view of the cleanburning characteristics of the fuel. Hydrogen, undoubtedly is one such key to the energy-environment system. It possesses the potential to be generated from a host of non-fossil sources and it is almost non-polluting. Thus, hydrogen offers an eventual freedom from the twin crises of fuel depletion and environmental degradation. Coming to think of hydrogen generation from an infinite universal source such as water, this basic resource is recyclable within a short time period by completely natural means. It starts with the molecule of water being split using energy from nearly any non-fossil
source (such as nuclear, solar, wind or organic wastes, etc.), and, upon combustion, generates water as the principal constituent which goes back to the same biosphere from where it came. Tables 1 and 2 indicate the various properties of hydrogen fuel. The consequences of radiation of hazards from h y d r o g e n - a i r fires are far less damaging as compared with those from a corresponding hydrocarbon fire because of the low emissivity of the hydrogen flame. Moreover, radiation from a hydrogen fire is at a wavelength which is absorbed by the atmosphere. As hydrogen is light, leaking hydrogen rises rapidly through air, thus the risk of any possible explosion gets restricted to the space just above the leak. Spilled gasoline, on the other hand, spreads over the ground, thereby making a larger area susceptible to accidental explosion. Considering an accident in an actual vehicular system if it occurs due to some reason or other, the fire hazards would remain for a long time i n the gasoline-powered vehicle whereas hydrogen would leak and disperse within a short period. The merits of hydrogen as a fuel for internal combustion engines were identified a very long time ago but, unfortunately, no serious efforts were consistently put towards practical realization of a hydrogen engine vehicular prototype system, mainly because it was probably never thought that the apparently perennial petroleum reserves would also dry up. Such a false sense of self-sufficiency indirectly discouraged any sustained effort on nonpetroleum fuels and hydrogen was no exception. However, in the case of hydrogen engine research carried out over such a tong period, the merits (such as unthrottled operation, high efficiency) and drawbacks (such as backfire, pre-ignition, knocking and rapid rate of pressure rise) were glaringly brought out making a larger area susceptible to accidental explosion. A chronological evolution of hydrogen engine technology is described elsewhere [ 1].
1013
1014
L. M. DAS and R. MATHUR Table I. Thermodynamic properties of hydrogen, methane and gasoline (generally accepted values from the literature) Property Molecular weight Density of gas at NTP (g m -3) Heat of combustion (low) (kJ g- 1) Heat of combustion (high) (kJ g-l) Specific heat (cp) of NTP gas (J g- 1 K- 1) Viscosity of NTP gas (g cm -~ s ~) Specific heat ratio (v) of NTP gas Gas constant (R) (cm-' arm g -t K -i) Diffusion coefficient in NTP air (cm2 s- 1)
Hydrogen
Methane
Gasoline
2.016 83.764 119.93 141.86 14.89 0.0000875 1.383 40.7030 0.61
16.043 651.19 50.02 55.53 2.22 0.000110 1.308 5.11477 0.16
107.0 4400 44.5 48 1.62 0.000052 1.05 0.77 0.005
Table 2. Combustion properties of hydrogen, methane and gasoline (generally accepted values from the literature) Property Limits of flammability in air (vol ~ ) Stoichiometric composition in air (vol%) Minimum energy for ignition in air (MJ) Autoignition temperature (K) Flame temperature in air (K) Burning velocity in NTP air (cm s - ~) Quenching gap in NTP air (cm) Percentage of thermal energy radiated from flame to surrounding (%) Diffusivity in air (cm 2 s-~) Normalized flame emissivity (2000K, 1 atm) Limits of flammability (equivalence ratio)
HYDROGEN ENGINES - - E N V I R O N M E N T A L IMPACTS Carbon dioxide (CO2), nitrous oxide (N20), methane (CH4) and chloroflurocarbons (CFCs), along with other traces of gases absorb and reradiate outgoing infrared energy from the Earth. For over a century, anthropogenic emissions of these gases have substantially increased their concentration level in the atmosphere. Increase in the mean global temperature, thermal expansion of the oceans and, ultimately, melting of polar ice are some of the wellidentified disastrous consequences of these " g r e e n h o u s e " gases. Addition of a host of obnoxious pollutants (such as HCs, smoke, SO,, particulates, sulphuric acid deposition, ozone or any other polluting oxidants, benzene or any other carcinogenic aromatic compounds, lead or any other toxic metals) to the CO2 and other greenhouse gases worsen the environment-related problems. Unfortunately, most of the aforementioned pollutants come out of the tail pipes of conventional petroleum-fuelled engines. On the other hand, most of these polluting criminals are instrinsically absent in
Hydrogen
Methane
Gasoline
4.0 - 75.0 29.53 0.02 858 2318 265 - 325 0.064 17-25
5.3 - 15.0 9.48 0.29 813 2148 37 - 45 0.203 23 32
1.0 - 7.6 1.76 0.24 501 to 744 2470 37 - 43 0.2 30-42
0.63 1.00
0.2 1.7
0.08 1.7
0.1-7.1
0.53-1.7
0.7-3.8
a hydrogen engine system. Utilization of hydrogen fuel, obtainable from non-fossil sources, provides a ready answer to control effectively the very cause of the anthropogenic emissions. Hydrogen engines would produce no CO2, c a 4 nor any reactive hydrocarbon, a very necessary evil to overcome to prevent ozone formation in the troposphere. Of course, some investigators have reported traces of carbon monoxide and hydrocarbons in the hydrogen-fuelled engine system. In fact, in a welldesigned and properly maintained hydrogen engine, only a very small fraction of lubricant coating on the carbon walls is burnt. The amount of carbon monoxide and hydrocarbon thus formed can be neglected for all practical purposes. Some researches have also reported a very small amount of hydrogen peroxide [2, 3].
Oxides of nitrogen (NOx) The only pollutant of concern which is emitted from a hydrogen-operated engine system happens to be oxides of nitrogen.
EXHAUST GAS RECIRCULATIONFOR NOx CONTROL While looking at the kinetics of formation of oxides of nitrogen, it may be noted that nitric oxide (NO) and nitrogen dioxide are conventionally grouped together and referred to as NOx emissions. As far as the conditions inside an engine cylinder are concerned, nitric oxide happens to be the predominant oxide of nitrogen, which is formed by the oxidation of atmospheric nitrogen. Zeldovich suggested the importance of the following equation, well-known in literature as the Zeldovich mechanism: O + N2 -- NO + N, N + O2 -- NO + O. NO gets formed in both the flame-front and the postflame gases, but in the context of combustion in engines, the latter conribute substantially towards NO formation. Combustion-related studies in an engine cylinder reveal that combustion occurs at relatively larger pressures, so that the flame reaction zone is extremely thin, and thus residence time within this zone is also extremely small. Experimental studies, as well as chemical equilibrium analysis, agree that NO2/NO reactions are extremely small in the case of spark ignition engines -- of the order of about 2 % at an equivalent ratio of 0.85 Emission of NOx, if not restricted to a certain acceptable level in any combustion system, could prove extremely harmful for several reasons. NOx, in the first place can cause respiratory ailments which, in the long run, could be quite acute and dangerous for health. It has also been observed in some cases that NOx emission impairs visibility. It has already been discussed that NOx emissions, through a series of chemical reactions in association with reactive hydrocarbons, result in the formation of ozone. Sometimes, NO x has been found to be responsible for the formation of other toxic, mutagenic and carcinogenic compounds. It is in this context that the bad effects of NOx emissions cannot be underestimated, and thus the operating conditions of a hydrogen-fuelled engine system must be optimized for the least NOx emissions. As far as an internal combustion engine operation is concerned, it has been conclusively shown that the level of NO~ emission increases with the combustion temperature, the duration of the high-temperature combustion period and the availability of oxygen. Therefore, in general there could be several ways to control the NO x emission level, by monitoring one or more of the aforementioned operating parameters. If an engine runs very lean, the temperature is correspondingly lowered. On the other hand, a rich mixture operation results in decreased oxygen supply. Decreasing the burn-time or lowering the engine rpm and thus ensuring more effective dissipation of heat are also some of the very fruitful methods. Cooling the combustion environment by way of adding water or recirculating exhaust gases does also ultimately result in bringing down the NO~ level. However, overall smooth operation of a hydrogen-fuelled engine system depends on a number of other parameters, and NOx control techniques cannot be judged in isolation. The well-known obstacles often resulting in undesirable combustion phenomena (such as backfire) in a hydrogen engine cannot be ignored while deciding upon the practical
1015
operational mode. In addition, fuel induction techniques [4] and safety aspects [5] of a reliable engine system development play very significant roles in bringing down the exhaust emission level. Detailed accounts of performance, emission and combustion aspects of these studies involving both S.I. and C.I. engines are given elsewhere [6-9] EGR is one of the most well-known techniques for controlling the NOx level in an S.I. engine. In the present experimental investigation, EGR was adopted in which exhaust gas was mixed with the fuel-air mixture. Thus, EGR functionally plays the role of an additional diluent in the unburnt mixture, as a result of which the peak burned gas temperature gets lowered and hence the nitric oxide formation rate is also declerated. Thus, it is the total burned gas fraction in the unburnt mixture in the cylinder (which consists of both residual gas from the previous cycle and the exhaust gas recycled to the intake) which acts as a diluent. It is well-known that residual gas fraction is influenced by load and valve timing, particularly the degree of valve overlap. However, the compression ratio and the air-fuel ratio also affect this to some extent. It has also been found out that the absolute temperature reached after combustion varies inversely with the burned gas mass fraction. Therefore, an increase in the burned gas fraction results in an ultimate reduction in the rate of NO formation. The percentage of exhaust gas recirculated (EGR%) can be calculated from the following equation: EGR% = (MEcR~ X 100,
\Mi/ where MECRis the mass of the exhaust gas recirculated and M~ is the mass inducted per cycle. Therefore, the amount of burned gas fraction (BGF) in the fresh mixture is given by:
BGF
MEGR+ M~s M~h
where M~s is the residual mass and Mch is the mass of the charge in the cylinder. It has also been found out that the optimum amount of EGR necessary for efficient charge dilution depends upon the combustion characteristic and hence the design features of the combustion chamber. Important operating parameters such as load, speed and equivalence ratio have been observed to be playing extremely vital roles in the estimation of the required amount of EGR. An S.I. engine, operating under normal part-throttle conditions can accept a maximum quantity of EGR in the range of 15-30%. The faster engines can tolerate a reasonably large amount of EGR compared with slow-burning engine systems. PRACTICAL ENGINE OPERATIONAL MODE There exists a trade-off between the performance characteristics (power and efficiency) and NOx emission
1016
L. M. DAS and R. MATHUR
behaviour. Very lean operation of the engine increases efficiency and reduces NOx, but since the amount of fuel is less under lean operating conditions, the volumetric heating value of the air-fuel mixture gets reduced, thus resulting in decrease of power. This trade-off is obviously not specific to hydrogen operation. Maybe in the future when an engine is built specifically to operate on hydrogen fuel many of the problems arising in a converted system might hopefully be solved. At present, it is impossible to prescribe a set of universal optimum conditions of hydrogen engine operation, taking into account the performance as well as the emission characteristics. But, an acceptable balance could always be struck between such mutually conflicting requirements. The present study was carried out keeping a practical automobile engine in view. With the present level of practical experience acquired on running a hydrogen operated engine, it was thought that mixed hydrogen-gasoline operation in a practical engine system could be profitably used in existing vehicular engines. In the fuel supply and control system of an actual automobile, hydrogen could be utilized with an excess of air for starting and idling conditions. Subsequently, gasoline supply should be initiated to ensure engine operation, under loads. Thus, at part-load operation, which corresponds to the main in-city driving condition, both hydrogen and gasoline could be utilized. This driving condition has some specific advantages, in that high excess air, as well as city driving conditions at full throttle, results in high efficiency and subsequently, fuel consumption is reduced. If necessary, the vehicle could be run on pure gasoline under full-load conditions. Such a start-and-run system helps, in principle, to avoid the amount of power loss, which has often been pointed out as a strong demerit for a converted hydrogen engine system. Besides, the total quantity of hydrogen consumption can be reduced
Gasoline tank t
(~
significantly. Another advantageous feature which could be successfully adopted is that in the event of a failure in the hydrogen supply system, the engine could be switched over to total gasoline operation.
EXPERIMENTAL TECHNIQUE Since the primary objective of this investigation was to estimate the level of NOx emissions from an automobile, the engine selected for the present set of investigations was a typical automotive-type four-cylinder engine. To convert the system to a hydrogen-supplemented configuration, no substantial modification was necessary. Only an additional arrangement was incorporated to facilitate hydrogen induction. Figure 1 shows the schematic diagram of the experimental set-up. Since gaseous hydrogen was stored in bottles at a relatively high pressure, a pressure regulator and a needle valve were installed to reduce the line pressure to an almost atmospheric value at the intake to the engine. A specially designed two-pass shell and tube type of heat exchanger was fabricated and installed as part of the exhaust gas recirculation system. The location of the EGR tapping point was chosen at about 200 mm from the common junction of the exhaust. There was another condition which was observed to be playing an important role. Based on the low quenching distance of hydrogen, the spark-gap setting was slightly reduced, which ultimately led to stable and consistent ignition without any backfire. Exhaust was tapped from the exhaust manifold and cooled to ambient temperature before being admitted into the intake manifold. The present work discusses the variation of NO~ emissions at different operating conditions. A chemiluminescent gas analyser was used to measure the NOx level. The performance characteristics and emission of HC and CO generated in the system are described elsewhere [10].
I
~
metaasA~iign I
1 H2 gas tank ~ 1 0 ~ 6 i ~ 5 ~4 ! ~7--~11 2 Pressure regulator 3 Needle valve 4 Ha flow meter 5 Flame trap I ] 8 tl~ll I 7 ~ l l , W a t e r ° u t 9 U 6 Carburettor 7 Engine 8 Dynamometer Waterin 9 Heat exchanger 10 Gasoline flow meter 11 Exhaust gas flow meter 12 Chemiluminescent analyser
Fig. 1. Experimental set-up.
1017
EXHAUST GAS RECIRCULATION FOR NOx CONTROL The EGR system consists of the EGR tapping point, the heat exchanger to cool the exhaust sample, an EGR valve and a flow measuring device. The EGR pipe size was selected so as to permit a maximum amount of 30% of the exhaust gas to pass through it. The engine was initially run with neat gasoline and different quantities of exhaust gas were recycled and subsequently fed. Hydrogen fuel was introduced in phases to the system at different rates. RESULTS AND DISCUSSION Figure 2 clearly demonstrates the influence of burned gas fraction by adopting EGR to the intake system. It was possible to achieve a marked reduction in the NO~ concentration level with the aid of 15% EGR. It was also observed experimentally that under normal part-throttle operating conditions, the engine tolerated this amount of exhaust gas recirculation without exhibiting any undesirable combustion symptoms. In the course of the experiments, it was distinctly observed that EGR reduces the combustion rate, which makes stable combustion conditions difficult to achieve. This is probably because exhaust gas is relatively inert and thus slows down the combustion process by decreasing excess oxygen in the mixture. As is well-known, the unburnt mixture in the cylinder contains hydrogen, gasoline fuel vapour, air and burned gases. The burned gases in this case are composed of the residual gases from the previous cycle and the amount of exhaust gas recirculated. As far as the mechanisms of NO~ formation are concerned, it is obvious that the exhaust gas reduces the flame temperature by increasing the heat capacity of the cylinder charge. In the course of carrying out experiments for various practical engine system operating conditions, it was observed that both EGR and excess air have an almost similar effect as far as diluting the unburned mixture is concerned. However, EGR was found useful only at part-throttle operation and hence in such cases, the lean-mixture zone was the region of interest. Figure 3 indicates the variation of brake specific fuel
340
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330 320 310 300 0
I 5
t 10
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EGR % Fig. 3. Variation of BSFC with percentage of EGR.
consumption (BSFC) with increasing EGR rate. There is a distinct improvement in fuel consumption with increasing EGR, as is evident in the figure. One of the main reasons for this effect could be due to reduced pumping work. As the amount of E G R is increased (with the fuel and air flow rate remaining constant), the pump work get reduced. An additional important effect that influenced the above trend of fuel consumption is that the heat loss to the wall gets reduced, as the burned gas temperature is significantly reduced (because of recycling the exhaust). Figure 4 shows the variation of exhaust gas temperature with increase in EGR%. The trend of the curve in the figure clearly shows that exhaust temperature decreases with increasing EGR. The experiments showed that with the increase in level of hydrogen substitution, the NOn concentration does increase. The trend was pronounced over the entire range of engine operation. The peak emission level is reached at a certain load and diminishes on either side of this point (Figs 5-7). Increase in the level of nitric oxide emission may be
1400 1400
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9 1300
© z 700
1200 1100
0
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1
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0
5
10
15
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t 25
EGR %
1000
I
J
I
I
J
5
10
15
20
25
EGR % Fig. 2. Variation of NO emission level with percentage of exhaust gas recirculation.
Fig. 4. Exhaust temperature as a function of percentage of EGR.
1018
L. M. DAS and R. MATHUR 400 o • [] •
300 0,,
z
EGR 0 EGR 5% EGR 10% E G
R
~
lOO I
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8.40
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37.70
45.60
NOx formation was observed to be a very strong function of spark advance. Any small changes in spark timing resulted in a substantial change in NOx emissions. It seems that the advanced spark timing increased the temperature level in the burnt gases which, in turn, increased the NOx level. Variation in NOx concentration level was found to be very closely linked to equivalence ratio. It was increased with increasing equivalence ratio, reached a peak value and declined thereafter.
Power (kW) CONCLUSION 40o o EGR 0 • EGR 5% o EGR 10%
300
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~-~ 200 Z
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e~
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o EGR 0 300 -- • EGR 5% aEGR10% 200
--
100
-0
EGR is an effective method of controlling the NOx level in hydrogen-supplemented spark ignition engine. The method has a specific advantage in the sense that the existing system does not call for a substantial modification beyond incorporating an appropriate heat exchange unit. An engine system can be designed and operated so that its stable operating conditions can be optimized for least NOx emissions and to operate without any undesirable combustion phenomena. It reduces the possibility of backfire through the carburettor during valve overlap by slowing down the flame velocity. EGR also curtails the knocking tendency. REFERENCES
o / ~ ' ~ / _ -~
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I
I
1
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18.01
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Power (kW) Figs 5, 6 and 7. Effect of EGR on NOn emission with various degrees of hydrogen substitution. caused by the high rate of energy release by hydrogen. As a result of this process the overall temperature gets increased. In fact, the point at which the maximum NOx concentration point occurs is shifted towards the lower load side. It has been well-established by several investigators that lean operation with hydrogen redudes the NOx level drastically. A similar trend was experienced in the present investigation for a hydrogen-supplemented set-up (Figs 5 - 7 ) . In general, it was observed (over the entire range of operating conditions) that the effect of EGR on NO~ without any hydrogen flow remains the same. NOx emission first increases with power output, attains a maximum value at a particular load and then starts diminishing again. This effect of a certain quantity of EGR is clearly reflected with various levels of hydrogen substitution.
I. L. M. Das, Hydrogen engines -- a view of the past and a look into the future. Int. J. Hydrogen Energy 15, 425-443 (1990). 2. M. R. Swain, R. R. Adt, Jr and J. M. Pappas, Experimental hydrogen fueled automotive engine design data base project. Report prepared for the U.S. Department of Energy, University of Miami, FL (1983). 3. L. A. Sinclair and J. S. Wallace, Lean limit emissions of hydrogen-fuelled engine. Int. J. Hydrogen Energy 9, 123-128 (1984). 4. L. M. Das, Fuel induction techniques for hydrogen-operated engine. Int. J. Hydrogen Energy 15, 833-843 (1990). 5. L. M. Das, Safety aspects of a hydrogen fuelled engine system developement. Int. J. Hydrogen Energy 16, 619-624 6. H. B. Mathur and L. M. Das, Performance characteristics of a hydrogen-fuelled S.I. engine using timed manifold injection. Int. J. Hydrogen Energy 16, 115-127 (1991). 7. L. M. Das, Exhaust emission characterization of hydrogenoperated engine system -- nature of pollutants and their control techniques. Int. J. Hydrogen Energy 16, 765-775 (1991). 8. H. B. Mathur and L. M. Das, Automobile exhaust pollution control through hydrogen fuel substitution. Proc. 8th Worm Clean Air Congress, Holland (September 1989). 9. H. B. Mathur, L. M. Das and T. N. Patro, Effect of diluents on combustion and exhaust emission characteristics of a hydrogen operated diesel engine. Proc. 9th World Hydrogen Energy Conference, Paris (1992). 10. R. Mathur, Performance characteristic of an S.I. engine using hydrogen as fuel and with EGR. Master of Technology (M.Tech) thesis, Mechanical Engineering Department, Indian Institute of Technology, Delhi (1989).
Int. J. Hydrogen Energy, Vol. 18, No. 12, pp. 1013-1018, 1993.
Printed in Great Britain.
EXHAUST GAS RECIRCULATION FOR NOx CONTROL IN A MULTICYLINDER HYDROGEN-SUPPLEMENTED S.I. ENGINE L. M. DAS and R. MATHUR Indian Institute of Technology, Delhi, New Delhi 110016, India (Received for publication 3 November 1992)
Abstract--This paper describes the results of an experimental investigation carried out on a hydrogen-supplemented multicylinder spark ignition (S.I.) engine to control the level of NO~ (oxides of nitrogen) emission by adopting exhaust gas recirculation (EGR). It was observed that the NO, level was substantially reduced over a wide range of engine operation conditions. Performance characteristics of the system were also evaluated corresponding to these operating conditions.
INTRODUCTION Our present-day lifestyle system is becoming increasingly dependent on the transportation sector with its network built on petroleum-fuelled automobiles. These automobiles, along with the growth of civilization have grown into the status of an indispensable prime mover. Unfortunately, in view of the present escalating energy (fuel) crisis, the very safe survival of the automobile has been put under question. Under such circumstances, it has now become all the more important that an acceptable alternative automobile fuel must be expeditiously developed so as to at least sustain the present growth rate of civilization. Recently, the eyes of the world were directed at the Earth Summit held in Rio de Janeiro, Brazil where it was unequivocally emphasized that our planet must be protected from further degradation. Logically, the simplest way of protecting our Earth is to stop hurting it. One of the most serious of such offences can be remedied by drastically reducing the level of pollutants which come out of the tail pipes of automobiles. These exhaust emissions from automobiles cause immense damage to human beings, as well as to plants and animals. Therefore, in essence, the search for an alternative automotive fuel must take into account environmental compatibility, in view of the cleanburning characteristics of the fuel. Hydrogen, undoubtedly is one such key to the energy-environment system. It possesses the potential to be generated from a host of non-fossil sources and it is almost non-polluting. Thus, hydrogen offers an eventual freedom from the twin crises of fuel depletion and environmental degradation. Coming to think of hydrogen generation from an infinite universal source such as water, this basic resource is recyclable within a short time period by completely natural means. It starts with the molecule of water being split using energy from nearly any non-fossil
source (such as nuclear, solar, wind or organic wastes, etc.), and, upon combustion, generates water as the principal constituent which goes back to the same biosphere from where it came. Tables 1 and 2 indicate the various properties of hydrogen fuel. The consequences of radiation of hazards from h y d r o g e n - a i r fires are far less damaging as compared with those from a corresponding hydrocarbon fire because of the low emissivity of the hydrogen flame. Moreover, radiation from a hydrogen fire is at a wavelength which is absorbed by the atmosphere. As hydrogen is light, leaking hydrogen rises rapidly through air, thus the risk of any possible explosion gets restricted to the space just above the leak. Spilled gasoline, on the other hand, spreads over the ground, thereby making a larger area susceptible to accidental explosion. Considering an accident in an actual vehicular system if it occurs due to some reason or other, the fire hazards would remain for a long time i n the gasoline-powered vehicle whereas hydrogen would leak and disperse within a short period. The merits of hydrogen as a fuel for internal combustion engines were identified a very long time ago but, unfortunately, no serious efforts were consistently put towards practical realization of a hydrogen engine vehicular prototype system, mainly because it was probably never thought that the apparently perennial petroleum reserves would also dry up. Such a false sense of self-sufficiency indirectly discouraged any sustained effort on nonpetroleum fuels and hydrogen was no exception. However, in the case of hydrogen engine research carried out over such a tong period, the merits (such as unthrottled operation, high efficiency) and drawbacks (such as backfire, pre-ignition, knocking and rapid rate of pressure rise) were glaringly brought out making a larger area susceptible to accidental explosion. A chronological evolution of hydrogen engine technology is described elsewhere [ 1].
1013
1014
L. M. DAS and R. MATHUR Table I. Thermodynamic properties of hydrogen, methane and gasoline (generally accepted values from the literature) Property Molecular weight Density of gas at NTP (g m -3) Heat of combustion (low) (kJ g- 1) Heat of combustion (high) (kJ g-l) Specific heat (cp) of NTP gas (J g- 1 K- 1) Viscosity of NTP gas (g cm -~ s ~) Specific heat ratio (v) of NTP gas Gas constant (R) (cm-' arm g -t K -i) Diffusion coefficient in NTP air (cm2 s- 1)
Hydrogen
Methane
Gasoline
2.016 83.764 119.93 141.86 14.89 0.0000875 1.383 40.7030 0.61
16.043 651.19 50.02 55.53 2.22 0.000110 1.308 5.11477 0.16
107.0 4400 44.5 48 1.62 0.000052 1.05 0.77 0.005
Table 2. Combustion properties of hydrogen, methane and gasoline (generally accepted values from the literature) Property Limits of flammability in air (vol ~ ) Stoichiometric composition in air (vol%) Minimum energy for ignition in air (MJ) Autoignition temperature (K) Flame temperature in air (K) Burning velocity in NTP air (cm s - ~) Quenching gap in NTP air (cm) Percentage of thermal energy radiated from flame to surrounding (%) Diffusivity in air (cm 2 s-~) Normalized flame emissivity (2000K, 1 atm) Limits of flammability (equivalence ratio)
HYDROGEN ENGINES - - E N V I R O N M E N T A L IMPACTS Carbon dioxide (CO2), nitrous oxide (N20), methane (CH4) and chloroflurocarbons (CFCs), along with other traces of gases absorb and reradiate outgoing infrared energy from the Earth. For over a century, anthropogenic emissions of these gases have substantially increased their concentration level in the atmosphere. Increase in the mean global temperature, thermal expansion of the oceans and, ultimately, melting of polar ice are some of the wellidentified disastrous consequences of these " g r e e n h o u s e " gases. Addition of a host of obnoxious pollutants (such as HCs, smoke, SO,, particulates, sulphuric acid deposition, ozone or any other polluting oxidants, benzene or any other carcinogenic aromatic compounds, lead or any other toxic metals) to the CO2 and other greenhouse gases worsen the environment-related problems. Unfortunately, most of the aforementioned pollutants come out of the tail pipes of conventional petroleum-fuelled engines. On the other hand, most of these polluting criminals are instrinsically absent in
Hydrogen
Methane
Gasoline
4.0 - 75.0 29.53 0.02 858 2318 265 - 325 0.064 17-25
5.3 - 15.0 9.48 0.29 813 2148 37 - 45 0.203 23 32
1.0 - 7.6 1.76 0.24 501 to 744 2470 37 - 43 0.2 30-42
0.63 1.00
0.2 1.7
0.08 1.7
0.1-7.1
0.53-1.7
0.7-3.8
a hydrogen engine system. Utilization of hydrogen fuel, obtainable from non-fossil sources, provides a ready answer to control effectively the very cause of the anthropogenic emissions. Hydrogen engines would produce no CO2, c a 4 nor any reactive hydrocarbon, a very necessary evil to overcome to prevent ozone formation in the troposphere. Of course, some investigators have reported traces of carbon monoxide and hydrocarbons in the hydrogen-fuelled engine system. In fact, in a welldesigned and properly maintained hydrogen engine, only a very small fraction of lubricant coating on the carbon walls is burnt. The amount of carbon monoxide and hydrocarbon thus formed can be neglected for all practical purposes. Some researches have also reported a very small amount of hydrogen peroxide [2, 3].
Oxides of nitrogen (NOx) The only pollutant of concern which is emitted from a hydrogen-operated engine system happens to be oxides of nitrogen.
EXHAUST GAS RECIRCULATIONFOR NOx CONTROL While looking at the kinetics of formation of oxides of nitrogen, it may be noted that nitric oxide (NO) and nitrogen dioxide are conventionally grouped together and referred to as NOx emissions. As far as the conditions inside an engine cylinder are concerned, nitric oxide happens to be the predominant oxide of nitrogen, which is formed by the oxidation of atmospheric nitrogen. Zeldovich suggested the importance of the following equation, well-known in literature as the Zeldovich mechanism: O + N2 -- NO + N, N + O2 -- NO + O. NO gets formed in both the flame-front and the postflame gases, but in the context of combustion in engines, the latter conribute substantially towards NO formation. Combustion-related studies in an engine cylinder reveal that combustion occurs at relatively larger pressures, so that the flame reaction zone is extremely thin, and thus residence time within this zone is also extremely small. Experimental studies, as well as chemical equilibrium analysis, agree that NO2/NO reactions are extremely small in the case of spark ignition engines -- of the order of about 2 % at an equivalent ratio of 0.85 Emission of NOx, if not restricted to a certain acceptable level in any combustion system, could prove extremely harmful for several reasons. NOx, in the first place can cause respiratory ailments which, in the long run, could be quite acute and dangerous for health. It has also been observed in some cases that NOx emission impairs visibility. It has already been discussed that NOx emissions, through a series of chemical reactions in association with reactive hydrocarbons, result in the formation of ozone. Sometimes, NO x has been found to be responsible for the formation of other toxic, mutagenic and carcinogenic compounds. It is in this context that the bad effects of NOx emissions cannot be underestimated, and thus the operating conditions of a hydrogen-fuelled engine system must be optimized for the least NOx emissions. As far as an internal combustion engine operation is concerned, it has been conclusively shown that the level of NO~ emission increases with the combustion temperature, the duration of the high-temperature combustion period and the availability of oxygen. Therefore, in general there could be several ways to control the NO x emission level, by monitoring one or more of the aforementioned operating parameters. If an engine runs very lean, the temperature is correspondingly lowered. On the other hand, a rich mixture operation results in decreased oxygen supply. Decreasing the burn-time or lowering the engine rpm and thus ensuring more effective dissipation of heat are also some of the very fruitful methods. Cooling the combustion environment by way of adding water or recirculating exhaust gases does also ultimately result in bringing down the NO~ level. However, overall smooth operation of a hydrogen-fuelled engine system depends on a number of other parameters, and NOx control techniques cannot be judged in isolation. The well-known obstacles often resulting in undesirable combustion phenomena (such as backfire) in a hydrogen engine cannot be ignored while deciding upon the practical
1015
operational mode. In addition, fuel induction techniques [4] and safety aspects [5] of a reliable engine system development play very significant roles in bringing down the exhaust emission level. Detailed accounts of performance, emission and combustion aspects of these studies involving both S.I. and C.I. engines are given elsewhere [6-9] EGR is one of the most well-known techniques for controlling the NOx level in an S.I. engine. In the present experimental investigation, EGR was adopted in which exhaust gas was mixed with the fuel-air mixture. Thus, EGR functionally plays the role of an additional diluent in the unburnt mixture, as a result of which the peak burned gas temperature gets lowered and hence the nitric oxide formation rate is also declerated. Thus, it is the total burned gas fraction in the unburnt mixture in the cylinder (which consists of both residual gas from the previous cycle and the exhaust gas recycled to the intake) which acts as a diluent. It is well-known that residual gas fraction is influenced by load and valve timing, particularly the degree of valve overlap. However, the compression ratio and the air-fuel ratio also affect this to some extent. It has also been found out that the absolute temperature reached after combustion varies inversely with the burned gas mass fraction. Therefore, an increase in the burned gas fraction results in an ultimate reduction in the rate of NO formation. The percentage of exhaust gas recirculated (EGR%) can be calculated from the following equation: EGR% = (MEcR~ X 100,
\Mi/ where MECRis the mass of the exhaust gas recirculated and M~ is the mass inducted per cycle. Therefore, the amount of burned gas fraction (BGF) in the fresh mixture is given by:
BGF
MEGR+ M~s M~h
where M~s is the residual mass and Mch is the mass of the charge in the cylinder. It has also been found out that the optimum amount of EGR necessary for efficient charge dilution depends upon the combustion characteristic and hence the design features of the combustion chamber. Important operating parameters such as load, speed and equivalence ratio have been observed to be playing extremely vital roles in the estimation of the required amount of EGR. An S.I. engine, operating under normal part-throttle conditions can accept a maximum quantity of EGR in the range of 15-30%. The faster engines can tolerate a reasonably large amount of EGR compared with slow-burning engine systems. PRACTICAL ENGINE OPERATIONAL MODE There exists a trade-off between the performance characteristics (power and efficiency) and NOx emission
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L. M. DAS and R. MATHUR
behaviour. Very lean operation of the engine increases efficiency and reduces NOx, but since the amount of fuel is less under lean operating conditions, the volumetric heating value of the air-fuel mixture gets reduced, thus resulting in decrease of power. This trade-off is obviously not specific to hydrogen operation. Maybe in the future when an engine is built specifically to operate on hydrogen fuel many of the problems arising in a converted system might hopefully be solved. At present, it is impossible to prescribe a set of universal optimum conditions of hydrogen engine operation, taking into account the performance as well as the emission characteristics. But, an acceptable balance could always be struck between such mutually conflicting requirements. The present study was carried out keeping a practical automobile engine in view. With the present level of practical experience acquired on running a hydrogen operated engine, it was thought that mixed hydrogen-gasoline operation in a practical engine system could be profitably used in existing vehicular engines. In the fuel supply and control system of an actual automobile, hydrogen could be utilized with an excess of air for starting and idling conditions. Subsequently, gasoline supply should be initiated to ensure engine operation, under loads. Thus, at part-load operation, which corresponds to the main in-city driving condition, both hydrogen and gasoline could be utilized. This driving condition has some specific advantages, in that high excess air, as well as city driving conditions at full throttle, results in high efficiency and subsequently, fuel consumption is reduced. If necessary, the vehicle could be run on pure gasoline under full-load conditions. Such a start-and-run system helps, in principle, to avoid the amount of power loss, which has often been pointed out as a strong demerit for a converted hydrogen engine system. Besides, the total quantity of hydrogen consumption can be reduced
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significantly. Another advantageous feature which could be successfully adopted is that in the event of a failure in the hydrogen supply system, the engine could be switched over to total gasoline operation.
EXPERIMENTAL TECHNIQUE Since the primary objective of this investigation was to estimate the level of NOx emissions from an automobile, the engine selected for the present set of investigations was a typical automotive-type four-cylinder engine. To convert the system to a hydrogen-supplemented configuration, no substantial modification was necessary. Only an additional arrangement was incorporated to facilitate hydrogen induction. Figure 1 shows the schematic diagram of the experimental set-up. Since gaseous hydrogen was stored in bottles at a relatively high pressure, a pressure regulator and a needle valve were installed to reduce the line pressure to an almost atmospheric value at the intake to the engine. A specially designed two-pass shell and tube type of heat exchanger was fabricated and installed as part of the exhaust gas recirculation system. The location of the EGR tapping point was chosen at about 200 mm from the common junction of the exhaust. There was another condition which was observed to be playing an important role. Based on the low quenching distance of hydrogen, the spark-gap setting was slightly reduced, which ultimately led to stable and consistent ignition without any backfire. Exhaust was tapped from the exhaust manifold and cooled to ambient temperature before being admitted into the intake manifold. The present work discusses the variation of NO~ emissions at different operating conditions. A chemiluminescent gas analyser was used to measure the NOx level. The performance characteristics and emission of HC and CO generated in the system are described elsewhere [10].
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Fig. 1. Experimental set-up.
1017
EXHAUST GAS RECIRCULATION FOR NOx CONTROL The EGR system consists of the EGR tapping point, the heat exchanger to cool the exhaust sample, an EGR valve and a flow measuring device. The EGR pipe size was selected so as to permit a maximum amount of 30% of the exhaust gas to pass through it. The engine was initially run with neat gasoline and different quantities of exhaust gas were recycled and subsequently fed. Hydrogen fuel was introduced in phases to the system at different rates. RESULTS AND DISCUSSION Figure 2 clearly demonstrates the influence of burned gas fraction by adopting EGR to the intake system. It was possible to achieve a marked reduction in the NO~ concentration level with the aid of 15% EGR. It was also observed experimentally that under normal part-throttle operating conditions, the engine tolerated this amount of exhaust gas recirculation without exhibiting any undesirable combustion symptoms. In the course of the experiments, it was distinctly observed that EGR reduces the combustion rate, which makes stable combustion conditions difficult to achieve. This is probably because exhaust gas is relatively inert and thus slows down the combustion process by decreasing excess oxygen in the mixture. As is well-known, the unburnt mixture in the cylinder contains hydrogen, gasoline fuel vapour, air and burned gases. The burned gases in this case are composed of the residual gases from the previous cycle and the amount of exhaust gas recirculated. As far as the mechanisms of NO~ formation are concerned, it is obvious that the exhaust gas reduces the flame temperature by increasing the heat capacity of the cylinder charge. In the course of carrying out experiments for various practical engine system operating conditions, it was observed that both EGR and excess air have an almost similar effect as far as diluting the unburned mixture is concerned. However, EGR was found useful only at part-throttle operation and hence in such cases, the lean-mixture zone was the region of interest. Figure 3 indicates the variation of brake specific fuel
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consumption (BSFC) with increasing EGR rate. There is a distinct improvement in fuel consumption with increasing EGR, as is evident in the figure. One of the main reasons for this effect could be due to reduced pumping work. As the amount of E G R is increased (with the fuel and air flow rate remaining constant), the pump work get reduced. An additional important effect that influenced the above trend of fuel consumption is that the heat loss to the wall gets reduced, as the burned gas temperature is significantly reduced (because of recycling the exhaust). Figure 4 shows the variation of exhaust gas temperature with increase in EGR%. The trend of the curve in the figure clearly shows that exhaust temperature decreases with increasing EGR. The experiments showed that with the increase in level of hydrogen substitution, the NOn concentration does increase. The trend was pronounced over the entire range of engine operation. The peak emission level is reached at a certain load and diminishes on either side of this point (Figs 5-7). Increase in the level of nitric oxide emission may be
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1018
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EGR is an effective method of controlling the NOx level in hydrogen-supplemented spark ignition engine. The method has a specific advantage in the sense that the existing system does not call for a substantial modification beyond incorporating an appropriate heat exchange unit. An engine system can be designed and operated so that its stable operating conditions can be optimized for least NOx emissions and to operate without any undesirable combustion phenomena. It reduces the possibility of backfire through the carburettor during valve overlap by slowing down the flame velocity. EGR also curtails the knocking tendency. REFERENCES
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Power (kW) Figs 5, 6 and 7. Effect of EGR on NOn emission with various degrees of hydrogen substitution. caused by the high rate of energy release by hydrogen. As a result of this process the overall temperature gets increased. In fact, the point at which the maximum NOx concentration point occurs is shifted towards the lower load side. It has been well-established by several investigators that lean operation with hydrogen redudes the NOx level drastically. A similar trend was experienced in the present investigation for a hydrogen-supplemented set-up (Figs 5 - 7 ) . In general, it was observed (over the entire range of operating conditions) that the effect of EGR on NO~ without any hydrogen flow remains the same. NOx emission first increases with power output, attains a maximum value at a particular load and then starts diminishing again. This effect of a certain quantity of EGR is clearly reflected with various levels of hydrogen substitution.
I. L. M. Das, Hydrogen engines -- a view of the past and a look into the future. Int. J. Hydrogen Energy 15, 425-443 (1990). 2. M. R. Swain, R. R. Adt, Jr and J. M. Pappas, Experimental hydrogen fueled automotive engine design data base project. Report prepared for the U.S. Department of Energy, University of Miami, FL (1983). 3. L. A. Sinclair and J. S. Wallace, Lean limit emissions of hydrogen-fuelled engine. Int. J. Hydrogen Energy 9, 123-128 (1984). 4. L. M. Das, Fuel induction techniques for hydrogen-operated engine. Int. J. Hydrogen Energy 15, 833-843 (1990). 5. L. M. Das, Safety aspects of a hydrogen fuelled engine system developement. Int. J. Hydrogen Energy 16, 619-624 6. H. B. Mathur and L. M. Das, Performance characteristics of a hydrogen-fuelled S.I. engine using timed manifold injection. Int. J. Hydrogen Energy 16, 115-127 (1991). 7. L. M. Das, Exhaust emission characterization of hydrogenoperated engine system -- nature of pollutants and their control techniques. Int. J. Hydrogen Energy 16, 765-775 (1991). 8. H. B. Mathur and L. M. Das, Automobile exhaust pollution control through hydrogen fuel substitution. Proc. 8th Worm Clean Air Congress, Holland (September 1989). 9. H. B. Mathur, L. M. Das and T. N. Patro, Effect of diluents on combustion and exhaust emission characteristics of a hydrogen operated diesel engine. Proc. 9th World Hydrogen Energy Conference, Paris (1992). 10. R. Mathur, Performance characteristic of an S.I. engine using hydrogen as fuel and with EGR. Master of Technology (M.Tech) thesis, Mechanical Engineering Department, Indian Institute of Technology, Delhi (1989).