Internal combustion engines probable evolutions and trends

A. Frennet and J.-M. Bastin (Eds.) Catalysis and Automotive Pollution Control III Studies in Surface Science and Catalysis, Vol. 96 9 1995 Elsevier Science B.V. All rights reserved.

INTERNAL

33

COMBUSTION ENGINES PROBABLE EVOLUTIONS AND TRENDS

P. E y z a t

ENSPM- Director for Combustion Engines and Hydrocarbon Utilisations

1. I N T R O D U C T I O N

The first time I participate with a group of specialists, to forecast the type of converter to be used in the coming years was in 1964. The S.A.E. entitled the session "Should we have a new engine ?". By itself, the question mark indicates how confident the organiser was for the possible quality of the reply. Naturally, we have been more often wrong than right. The mistakes classicaly are made of over optimistics evaluations of new contenders and over pessimistic ones for the adjustment potential of the standard engines. Having this lesson in mind, I forecast that the present "4 stroke-fuel" ticket will maintain its leadership for at least a decade. This is to say that the market for the electric car use will be limited to certain over populated area, where a consensus will be agreed on the extra-cost sharing between citizens and vehicle users for the sake of local pollution abatement. Another big consequence of this hypothesis is that the thermal engine will still have to be adjusted and judged in city conditions : low speed, low torque, short trip. Secondly substitute fuels, biomass or others will have to adjust to basic standard engines or to limit their use to dedicated local fleets. As will be seen later on, the 4 stroke-fuel ticket might be very different from the present average : a lot of imaovations are in sight. In sight, is by no means, equivalent to predictable : who would have, 15 years ago, forecast the huge discrepancies between developped countries on the diesel share market ? 2

STROKE SPARK-IGNITION ENGINES

2.1. Standard systems The most common engines use a lot of fixed system which might well

34 become variable and adjusted more and more often. Let us particularly look at 9 Connection pipes : inlet and exhaust 9 Valves lift, number and profile 9 Compression and expension ratio 9 Supercharging All these characteristics, whenever constant, result from very accute balancing and compromise. Naturally low cost is the goal pursued by technicians. Sometimes the market allows some more degree of freedom in some niche : fixed items become variable ones. If users are satisfied, and if extracost becomes manageable for a bigger production line, then the solution is extended. Japanese firms are often in the front line for engine modifications, naturally not all of them are destined to spread and become a world standard. The other big problem created by an item which is modified from stable and fixed to variable is the real time control of this system. You need a CPU, pick-up sensors, actuators. You must define a strategy, create hysteresis, when not natural, in order to avoid instability. Furthermore you must never forget that the engine behavior is mainly transcient. The time constant of interest ranges from tenths of milliseconds, (noise and vibrations) to milliseconds, (cycle to cycle variation) and seconds or even minutes. The average trip is so short (1 or 3 km) that when you stop your car neither the water nor the oil are at their equilibrium temperature. When you remember that oil viscosity plays a major role in engine friction and is very sensitive to temperature (you need a log-log scale to draw a straight line for viscosity versus temperature law). You realise how difficult it must be to define a strategy and to check it.

2.2. Inlet Pipe adjustment Already performance car use inlet pipe of variable accoustic lengths. A well localised butterfly can be automatically actuated, offering two and sometimes three different design lengths. The origin of the variation of the maximum mass of air trapped inside a cylinder, a quantity directly colmected to the torque, is easy to understand. Imagine a pipe closed at 1 by a valve and colmected to the atmosphere at 0 (fig.l). If you create a brief suction at 1 you will observe your 2L pipe oscillate in quarter wave, e.g. after a time equal to + ~ , with C the sound C speed, you will have a pressure increase at your valve. Naturally, if you close 2L your valve around to + -C you will benefit from an increase of air filling. But

35 later at to + 4____LLit will be the reverse, you will empty your cylinder. So at high C nmning conditions, you need a short pipe and at low nmning conditions a long !

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Fig. 1 Wavepropagation pipe. Naturally the real phenomenon is for more comple. There is a volume effect, (Helmotz resonator) friction losses at the boundary layer, and losses due to curvature radius of pipes and to abrupt change of surface. Admission can also be thermally heated" in accordance to a thermal sensor you can heat up inlet gas during cold starting by ensuring a heat transfer from the exhaust gases. Or create an electrically heated plate in order to compensate for the latent heat of vaporisation of the fuel. The cooling potential of this effect can be very significant 9 about 30 K for gasoline but more than 200 K with methanol. Hysteresis effects are described in the (fig.2). S

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36 If you decide that you will change your configuration for a given value of a parameter, e.g. running condition No, and you try to stabilize at this point, any perturbation, however small, will make the system oscillate between state 1 and state 2. To avoid this phenomenon, you define two values : one, let us say No + is selected when N is going up and on the way back you change at No- which is lower than No + In the same circumstances, you can have two different equilibrium conditions, you create a bias in test reproducibility.

2.3. Exhaust pipe adjustment The waves behave in the exhaust as they do in the inlet. They are only travelling at higher speeds due to exhaust temperatures. So you can also imagine a variable exhaust pipe which creates a depression at exhaust valve closure, if you want to increase the torque, and an overpressure, if you want to increase the internal exhaust gas recirculation very efficient means for NOx abatement. However the cost of an exhaust butterfly is much higher than its inlet equivalent and they will probably be reserved for E.G.R. and may be for prompt E.G.R., an interesting idea which is described in the (fig.3). {r~mC}

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Fig3 Prompt E.G.R.:Recycling the last part of the exhaust If you analyse the concentration of unburned HC in an exhaust burst, you will find that the instantaneous value can be far higher than the average ones. Particularly at the end of the exhaust, at a time when the piston has scraped all the boundery layers, you see a pulse of high HC concentration. If you imagine this part of the exhaust being recycled then you will be more efficient : you will obtain the normal effect on NOx you are looking for and you will give HC a

37 second chance to burn. In some cases, you can recycle 3 5 % of the HC with only 10 % E.G.R. 2.4. Cam profiles and number It is obvious that at high nmning conditions the filling of the cylinder will be limited by the surface offered by the inlet valve when open. One of the biggest technological innovations for SI engines has been the 4 valve-per-cylinder concept. However at low running conditions, too high a surface, means lower gas speed and consequently smaller pressure wave amplitudes in the inlet duct. The 1 suction phase of (fig.2) is smaller in accordance with p+~pV2= cte , and the

2L later is smaller even for a well tuned inlet pipe. C It might be of interest to have a variable number of valves according to the running conditions. The possibility of modifying the cam lift profile can be fruitfully utilized for combustion speed adjustment. The combustion, expressed in percent bumed per crank-angle rotation of the engine diminishes with an increase in both gas dilution (air excess or E.G.R.) or running conditions. To improve your combustion speed, when it is needed, you must increase the turbulence in your combustion chamber. One way of doing this is to vary the tumble (an air rotation whose axis is parallel to the shaft) versus the swirl (an air motion of perpendicular axis). The creation of turbulence near top dead center (TDC) where you need it, comes from the tumble which is destroyed when the piston comes close to the head. So you can specialize your valves for tumble and swirl, and by adjusting their lift with the conditions you will correct the otherwise detrimental evolution of combustion speed. Finally, if you add the gas dynamic effect which, in COlmection with pipe length, explains why you must open your inlet valve earlier in the cycle and close it later at high running conditions than at low running conditions. You can imagine easily how fruitfull an adjustable cam mechanism can be. Some examples of cam mechanism adjustment and uses will be briefly described now. gain of pressure at At -

2.4.1 Mechanical systems The Mitsubishi system uses two cams, one for low the other for high speed range. Cams are driven, or not driven, according to the position of a hydraulic piston (fig.4). Here you Call have three situations. At low torque and low running conditions (fig.5) two cylinders are discolmected (their valves remain closed).

38 In such circumstances a gain in efficiency mainly from the reduction of pumping losses, is obtained. Apart from this case, one or other of the cams is driven. The Honda system is of the same type.

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39 2.4.2 Continuously adjusting system The potential of hydraulic and electronic control can be pushed a little bit ft~her. Siemens (fig.6) for example uses a solenoYd to control the oil pressure in an actuator located between cam and valve. CAM

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40 2.5. Lean burn concept Lean bum is a very old topic in engine research. The basics are well understood. Gains come mainly from pumping losses, which are those of the Diesel engine, and from thermodynamic advantages linked to the more favorable Cp/Cv ratio of air compared to a stoechiometric mixture.

2.5.1 Stratification A stoichiometric or rich mixture near the spark plug ; air in excess close to the botmdaries is one way of solving the problem (Baudry process, Texaco, Proco are well known examples). Besides fleet applications for Texaco, these solutions have not been able to fulfill industrial requirements. Problems arose from high pressure gasoline injection, reproducibility and wear, (a lot of progress has recently been reported in this area) and the quasi perfection of combustion needed. In fact for very lean mixtures the exhaust temperature is low enough to make oxidation catalysts inefficient. Then the unburned HC created by gas-gas coinching inside the cylinder may be far above legal treshold limits. 2.5.2 Homogeneous lean burn engines The goal is no longer to have an unthrotlled engine but to bum a nearly homogeneous mixture, at a point where NOx emissions are compatible with legislation, at least until NOx catalytic systems working in lean conditions and fulfilling automotive requirements are available. A famous example is the Honda VTEC-E engine (fig.7) Another type is the Mitsubishi (fig.8) which is a compromise between purely stratified charge engines and homogeneous lean bum. The Honda engines clearly demonstrate that the ambition of this manufacturer to create a"Diesel Killer" e.g. an engine emitting less CO2 than their best Diesel competitor is not out of reach (fig.9). However, with the breakthrough created for the sake of the two stroke engines in gasoline direct injection, notably by Siemens, or with air injection, (orbital or IAPAC) the 4 stroke lean bum engine is clearly a potential wi~mer for tomorrow's green house engines. Already existing results evidence this forecast. 2.5.3 Heavy_ EGR engines Instead of diluting by air you can think of diluting by burned gases. From a thermal stand point lowering burned gas temperatures can be equally attained by oxygen excess or CO2 excess. Drawbacks are linked to CO2 dissociation at high temperature, an obviously endothennic evolution, and kinetic effects. A comparison of both solutions is given in (fig.10). This clearly shows that an optimized piloted E.G.P. can offer a drastic reduction of NOx (this solution is

41

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43 compatible with 3-way catalytic requirements) even if C02 optimization remains in favor of air dilution.

2.6. Compressionratio 2.6.1 Mechanical adjustment Changing the shait to head distance is not easy the system must sustain heavy forces and adjusting time must be low to follow the ear transcient. In spite of these difficulties researchs is pursued" potential gains at part lead justify sustained effort in this direction (fig. 11). thermal engine efficiency

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Fig. 11 Variable compression ratio engine. Theoretical gains(VW) 2.6.2 Cam profile adjustmen.t The work done by gases occurs during the expansion stroke. Mechanical systems used today make compression equal to the expansion stroke. It is possible to avoid this combination by using a special cam profile adjustment along with the well known Miler principle. If, during the first part of the upward movement of the piston you maintain the admission valve open, you are in fact reducing the compression stroke. Mazda is said to be marketing such a solution soon (fig. 12).

2.7. Enginecontrol Closed loop control used to maintain the fuel/air ratio within narrow limits is well kaaown. A lambda sensor delivers a very sensitive signal to oxygen concentration around the stoechiometrie ratio. This signal is used to pilot the injection in order to correct observed deviations. Besides limitations coming from trine constants, (those of the lambda sensor and the injection system) the liquid gasoline film deposited in front of the inlet

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45 valve can seriously alter the accuracy of the control in transcient conditions. Part of the fuel injected is condensed on the pipe wall. This film evaporates and due to the high air turbulence intensity, secondary droplets are also created. Then a given cycle is fed with part of the injected qumatity and with part of the fuel film ~ a phenomena completely outside the simple closed loop control l o g i c s . A simulator of the car and engine system has been developed and placed on a work station at ENSPM. It will help us to understand the strange behavior of an engine with a simple control system. Imagine you force (fig. 13) the throttle to follow the law described in A. The instantaneous equivalence ratio will follow the curve drawn in B. The drop of 40 % which occurs in the first second is well known (air delay is smaller than injection delay). The very wide oscillation occuring between 6 and 9 s deserves some comments. At the begilming of the throttle opening rich mixture is measured by the lambda sensor. Naturally (C) the injection duration is adjusted. But even with a smaller injection duration the film continues to fill the cylinder (D). Then the C.P.U. interprets such behavior by continually reducing the injection duration. Just before 7 seconds, the injection has been halted and the equivalence ratio remain rich ! This lasts until the the liquid film disappears (D). As is shown in (B) the cycle by cycle equivalence ratio oscillates between .3 and 1.4. This is why the control system must be corrected for transcients. One way most often used today is to enter empirical laws in the C.P.U. Another way is to model the non linear fuel film system and to enter it in the C.P.U. of the engine : besides lower emissions, a more stable engine rumfing is obtained (fig.14). Obviously the direct injection S.I. engine definitely solves the fuel film trouble. As has been said earlier, on the lean bum engines, teclmological limitations of high injection pressure gasoline systems have now been mostly overcome. For those involved in the development of catalytic muflers the problem of engine control is of the upmost importance. Bearing in mind what happens with simple engines, one can easily imagine what problems can arise in the transcients of an engine in which duct length, cam profile and compression ratio are adjusted through non linear algorithms stored in the C.P.U. Lastly, optimisation will also have to include the exhaust pipe and its catalytic bed which add its own transcients : oxygen storage, thermal inertia, pressure drop...Comparing two catalytic systems at a ULEV level might not be possible without perfect knowledge of the C.P.U. program.

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Germany Italy Spain U.K. France Europe(17 countries)

1990 10,8 7,3 14,7 6,4 33,0 14,1

Fig. 15 European Diesel market share

% o f total car 1991 1992 11,8 14,8 5,7 7,6 12,7 16,6 8,7 12,5 38,4 39,0 14,6 17,0

1993 15 10 21 17 44 19-21

47 3. DIESEL ENGINES

The Diesel engine share of the automotive market has strongly increased in Europe during the last few years (fig.15). Peculiar drawbacks of this type of engine are well knowal : noise, particulates and NOx. Diesel engine specialists have made impressive progress in the last decade on the noise side. In front of catalyst specialists I will not comment further on particulate filter regeneration and NOx reduction in lean conditions ; these are two topics of research in your field. I will just remind you that diesel specialists will have a sigh of relief if you can offer them an industrial catalyst solution ! Contrary to their S.I. competitor, progression Diesel engines is not easily identified from outside by non specialists. Their teclmological breakthroughs appear mainly in clearance, reproducibility, machining precision. A bad engine is not very different from a good engine. Basic solutions for tomorrow are already on the market : prechamber of the Ricardo or Mercedes type, and direct injection engines (fig. 16). \

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Fig. 16 Diesel combustion chambers

Recent progress in prechmnber engines (I.D.I.) has come from the prechamber detailed optimization (localisation of the glow plug, maintained in action after the engine starts during an adjusted time, flow guidance at the outlet of the prechamber into the piston). Electronic control of the injection system can, and probably will be developped in future engines. Regardhlg the D.I. engines, noise reduction occurs through mechanical and combustion improvements. For mechanical noise the piston slap phenomenon (a vibration created by the piston to cylinder shock when the piston travels, near T.D.C., from left to right) decreases atier better adjustment of clearance, shape

48

and weight. Combustion noise is closely connected to fuel injected during the self-ignition delay. With a given fiiel, one way of reducing the delay period is to use a small pilot injection. A teclmological break-through enables such an injection with a two-spring nozzle (fig. 17). t

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Fig. 17 Bosch two spring nozzle A very small prelift (some htmdredths of mm) is limited by a second spring. The increase of force needed to complete the lift creates a delay. During this time the small amotmt of fuel injected during the prelift self-ignites and the main part of the fuel injected later in an already burning mixture bums less abruptly than with normal injection. If you add the fact that in order to obtain smaller droplets, injection pressure is increased (1200 bars today, 1500 to 1700 bars tomorrow) and at the same time nozzle hole diameters are reduced, you can understand where Diesel progress lies : in details, whose industrial management is the key point for competitivity. For newcomers, in the Diesel field, the question is "will it be an NOx regulation situated between what is possible for S.I. engines and what is not possible for Diesel engines ? Nothing of this kind is foreseeable in the coming years mad on the contrary the Diesel market share is increasing even in countries where there is no fiscal advantage for its use. However such a threat in an area where technological knowhow needs time to be created, acts as a brake.

4. STROKE ENGINES The more advanced contender of the standard 4-strokes for car powering deserves some COlmnents even if, as it has been assumed in the hatroduction, the

49 4-strokes has sufficient potential to remain the winner. And, furthermore, the two strokes has its own market for two and three wheeler vehicules. Reasons for two-stroke efficiency gains lie in reduction of friction losses and pumping losses at part load (fig.18). Friction losses are mainly related to cylinder capacity and running conditions : the two stroke which creates more power from the same capacity has lower friction losses. Improvement in pumping losses benefits from the fact that the S.I. four strokes functions like a vacuum pump during the suction stroke at part throttle. Gains in efficiency occur during city driving conditions : a 15 % reduction can be attained. Another potential big advantage of the two stroke is size reduction, a quality that designers appreciate even more than weight reduction for the freedom they gain in hood shaping (fig.19). To obtain these gains, engineers must have solved the fuel short circuiting which makes the standard carburetor 2 stroke a high pollutant and low efficiency enghle. In these engines, the burned gas is scavenged by the fuel air mixture which pushes it out. If we do this, part of the flesh mixture goes directly to the exhaust without having participated at all in the combustion phase. Injection offers the solution : scavenging will be done with pure air, and fuel injection occurs close to or after the exhaust port closure. Teclmological solutions are numerous and they can be examined through their injection system : Direct injection systems, mainly developed by Toyota, Chrysler, PSA and Renault to cite some of them. Key points are the high injection pressure system and the control of the fi~el air mixing which must be optimized in a very short time. 9 Air assisted injection systems. Two sub-classes must be defined : medium pressure, small amount of air, most often with an air compressor. The most famous name is ORBITAL, and a lot of licences has been taken by manufacturers. This is probably the closest to industrial car use of all solutions ; - low pressure, amount of air compressed below the piston in the crankase, typical of the IAPAC system. Air injection systems offer a finer atomization which make combustion and mixing phenomena a little easier to solve. However the lubrication of a dry crankase engine is not easy to solve with lost oil : low enough for pollution, high enough to prevent wear of moving parts. Toyota has avoided this by using a standard 4 stroke teclmology : 4 valves per

50

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51 cylinder, normal crankase, extemal blower for exhaust gas scavenging. NOx emissions are obviously the limiting factor.

5. CONCLUSION

Petroleum based fuels will remain the main sources of energy for transportation. In the car sector, the four stroke engine has a very good chance to maintain its predominance. Electronics offers it a great potential for real time optimisation of a lot of possible adjustable technologies : cams, ducts, compression ratio, firing cylinder with E.G.R. or lean combustion are some candidates for flexibility increases. The Diesel engine will maintain its impetus and the two stroke will enter into the game if efficiency and CO2 reduction become a greater priority (fig.20).

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52 While these trends define the backbone of the technological evolutions, as we see it, they leave some place for niche markets. However, and we learnt this with unleaded gasoline, the idea of having a new dedicaded source of energy for a new fuel seems hopeless. A new fuel or an extension of its use will benefit from the naturally more flexible engine of tomorrow, it may be the case for L.P.G. for example. Although Natural gas suffers from the constraint of its onboard storage, its availability and CO2 bonus explain the strong political pressure for its use elsewhere. Combustion characteristics of natural gas, low combustion speed and long self-ignition delay, create an incentive for a dedicated engine. It is forecasted that starting with fleet use, the number of filling stations will increase and this will help to spread tile use of gas engine.