Siat 2009 2 Stroke For Ulc

Symposium on International Automotive Technology 2009

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Pierre DURET

IFP School, France

Keynote Paper

Copyright © 2009 The Automotive Research Association of India, Pune, India

ABSTRACT

The main purpose of this paper is to discuss thepossibility of using a gasoline direct injected two-strokeengine to power innovative ultra low cost passengercars that are going to be introduced in India.

The principle advantages of the two-stroke cycle will bebriefly reviewed: lightweight and compactness, lowfriction losses, low pumping losses, double cyclefrequency and high specific power output. On the otherhand, the conventional carburetted two-stroke enginepresents also two major drawbacks, the fuel directlyshort-circuited in the exhaust during scavenging and theirregular combustion at part load, responsible of highfuel consumption and high unburned hydrocarbonemissions.

The use of gasoline direct fuel injection has beenextensively studied during the eighties and thebeginning of the nineties with the main purpose ofdeveloping a clean and efficient two-stroke engine forautomotive application. For different reasons, this newgeneration of DI two-stroke engines has not beenintroduced in passenger car applications but appearedas quite successful in some applications outside ofautomotive. Several OEMs worldwide have introducednew DI two-stroke engines in two-wheeler applicationsas well as in marine applications where lightweight,compactness, high efficiency and low emissions arerequired. The introduction of such technology in theIndian market for 3-wheeler auto-rickshaw has alsorecently started.

If we look now to the specifications of the first modelof ultra low cost passenger cars to be launched inIndia, we can see the following key requirements :

� low production cost

� minimum weight

� minimum size for easy rear engine placement

� compatibility with emission legislation

� acceptable performance (power output and low endtorque)

The characteristics of several examples of newgeneration two-stroke engines issued from the IFPextensive expertise and from current production DI two-stroke engines will be compared with thesespecifications. The main issues will be described anddiscussed. From this comparison, it will be possible toclearly demonstrate that the new generation of DI two-stroke engines could represent a very well adaptedchallenger to power the innovative ultra low costpassenger cars that are requested by the Indianautomotive market.

INTRODUCTION

In early 90’s, high fuel economy on a 500 kg conceptcar with a 2-cylinder 500 cc DI 2-stroke of 24 kW hasalready been demonstrated by the author [1,2,3] asshown in the Fig. 1. Nevertheless, this project was notfurther developed, in particular because gasoline DI(direct fuel injection) technology was not mature at thisperiod where in addition emissions regulations were lesssevere than today’s and future standards.

It is particularly interesting to see that the first ultra lowcost car (ULCC) introduced in the Indian market weights580 kg and is equipped with a 2-cylinder 623 cc 4-stroke of exactly the same power output of 24 kW. Itspublicly announced fuel economy achievements aresummarized in Fig. 2. Even if they are probably notobtained under the same conditions (same drivingcycle), they seem to be not as good as what wasobtained in the past with the DI 2-stroke.

The New Generation of Gasoline DI 2-Stroke Engines: a

Powertrain for Innovative Ultra Low Cost Passenger Cars ?


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In parallel, during the last decade the DI 2-stroketechnology has been further developed outsideautomotive and successfully applied in production formarine outboards and 2-3 wheelers engines.

Taking into account these two considerations (resultsachieved in the early 90s with a DI 2-stroke with a nonmature technology and recent availability of well provenDI 2-stroke technology outside automotive) it seemsnatural to wonder if for a small ULCC to replace asmall 4-stroke engine by an even smaller DI 2-strokeengine could perhaps provide the most adaptedpowertrain for several reasons that we will examine indetails in this paper.

Before that, the ULC car and its powertrain targetedspecifications that will be used in this paper have tobe defined. They are based on available characteristicsof the first Nano Indian ULCC and summarized in theTable- I here below.

THE PRINCIPLE ADVANTAGES OF THE 2-STROKE

CYCLE

The 2-stroke engine is well known for its main followingspecific advantages resulting from the principle of the2-stroke cycle [1]:

� Low Friction Losses : this is particularly true withpump crankcase configuration (roller bearings forcrankshaft, rod and piston pin; no oil ring retainer;no valves train to drive, one driving cycle everyrevolution), no oil pump to drive especially duringcold start;

� Low Pumping Losses : the pumping workdecreases in absolute value (almost constant inrelative value as shown by Fig. 3b) when the loaddecreases. It is the contrary in a SI 4-stroke

� Double combustion cycle frequency when comparedto a 4-stroke engine The advantages 1 and 2 resultin significantly higher effective power for the sameindicated power, especially at part load as shownby the Fig. 3a and 3b. This should give apotentially higher 2-stroke fuel economy than SI 4-stroke.

Figure 1 : Fuel Economy Achieved in the Early 90’swith a 24 kW DI 2-Stroke in a 500 kgConcept Car

Figure 2 : Announced Fuel Economy for the 580 kgIndian Ultra Low Cost Car Equipped witha 24 kW 4-stroke Gasoline Engine (Basedon Public Data)

Table- I : Specifications for an Innovative Ultra LowCost Passenger Car for the IndianMarket

Figure 3a : Distribution of Indicative Power in EffectivePower, Friction Losses and PumpingLosses Versus Engine Load in a 4-strokeEngine


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The third advantage results in higher specific torqueand power output but nevertheless lower than 2 timesthe power of an equivalent 4-stroke because all theexpansion stroke is not useful for producing power(exhaust port opens during the last part of theexpansion stroke). As a consequence, the size andweight of a 2-stroke can be much smaller. It also allowsto have drastically better 2-stroke NVH (noise, vibrationand harshness) characteristics as we will also see laterin this paper.

However in a classical carburetted 2-stroke engine, thefirst and second potential fuel economy advantages areunfortunately masked by the main 2-stroke drawbacks:

� The short-circuiting of fuel directly to theatmosphere (above 50 % of maximum engine load)solved by DI (Direct fuel Injection)

� The poor combustion or misfiring (below 50 % ofmaximum engine load) solved by combined CAI(Controlled Auto-Ignition) and DI These two differentsources of unburned fuel and therefore of poorefficiency are clearly illustrated in the Fig. 4 as afunction of engine load.

The technologies to solve these two drawbacks alreadyexist and have been successfully introduced inproduction several years ago outside of automotive :

� The gasoline direct fuel injection for HC emissionscontrol and best fuel economy with severalexamples of production available technologies: airassisted direct fuel injection on marine outboardengines, autorickshaw, 2-wheelers [5] IAPACcompressed air assisted fuel injection on marineoutboard engines [6,7] Direct liquid fuel injectionon marine outboard engines [8]

� Its combination with CAI combustion (ControlledAuto Ignition) for NOx emissions control andimproved combustion stability [2,9,10] with the AR(Activated Radicals) combustion as an example ofproduction available technology [4,11,12,13].

THE KEY SUCCESS FEATURES OF DI 2-STROKE

ENGINES FOR ULTRA LOW COST CARS

After this introduction, the content of this paper will beorganized in five main sections discussing the five mainissues that can be considered are key success featuresof the DI 2-stroke engine as a powertrain for ULC cars:

� Simple, lightweight and compact: DI + exhaustthrottling CAI

� NVH issues and low production cost: single-cylinder+ balancing shaft or 2-cylinder without balancingshaft.

� Easy maintenance and high fuel economy for lowoperating cost: 2-stroke principle advantages anddownsizing

� DeNOx free emissions control: oxidation catalystwith fast cold start lighting and CAI combustion foraftertreatment free NOx emissions control.

� Driveability with high low end torque

THE 2-STROKE ENGINE : A SIMPLE, COMPACT

AND LIGHTWEIGHT POWERTRAIN FOR ULCC

This is a well-known advantage of the conventional 2-stroke engine versus 4-stroke. The following Fig. 5 and6 clearly illustrate that this advantage is maintainedeven if two-stroke engines become direct fuel injected.

Figure 3b : Distribution of Indicative Power in EffectivePower, Friction Losses and PumpingLosses Versus Engine Load in a 2-strokeEngine

Figure 4 : Negative Effect of Losses from MixtureShortcircuiting and of Losses fromIrregular Combustion on Specific FuelConsumption of a Carbureted 2-strokeEngine [4]


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In the introduction, we already explained that the 2-stroke configuration adapted for an ULCC shouldinclude DI technology combined with CAI combustion.As shown by several previous studies, the most costeffective solution for implementing the CAI combustionis to use an exhaust throttling device [2, 14]. Thisdevice allows to control the exhaust back pressure andconsequently the upstream internal scavenging andstratification process between the fresh charge and theresidual gases. To use an AR exhaust valve [4, 11] or

a transfer throttling valve [15, 16 and 17] could beslightly more efficient solutions but at a rather significantincremental cost not justified for an ULC car.

The following Fig. 7 shows on the right a schematicexhaust throttling device, and on the left a view of the500cc 2-cylinder exhaust throttling arrangement.

Figure 6 : The Minimum Size of the 500 ccAutomotive DI 2-stroke Engine AllowingEasy Front or Rear In-Vehicle EnginePlacement [2]

Figure 5 : Compared Automotive Engine Size : DI 2-stroke Versus 4-stroke Engine of SamePower Output [15, 16]

THE DI 2-STROKE ENGINE: A LOW PRODUCTION

COST POWERTRAIN WITH SIGNIFICANT NVH

ADVANTAGES NVH ISSUES AND LOW

PRODUCTION COST

To improve the efficiency of automotive engines, oneof the most promising techniques so called downsizingis to reduce the engine displacement in order tooperate the engine in more efficient conditions at partload. The reduction of the overall engine displacementcan be achieved by two different ways: the reductionof the cylinder unit displacement and the reduction ofthe number of cylinders.

The reduction of the cylinder unit displacement isnevertheless limited towards low values by the increaseof losses and the decrease of efficiency. On the otherside, the reduction of the number of cylinders is limitedby turbo charging and Noise Vibration and Harshness(NVH) issues.

To use a downsized engine can also be an interestingstrategy in order to achieve better fuel economy forULC cars. Nevertheless, in the case of ULC cars therequested output power of the engine is limited (around25 kW) and then the engine used as shown by thespecifications of the Table- I is already a relatively small2-cylinder 4-stroke engine which cannot easily bedownsized. A 2-cylinder 4-stroke presents only onecombustion cycle every engine revolution. It needs abalancing shaft to improve its NVH characteristics. Forcost reduction, the best would be to use a single-cylinder engine, but if the 4-stroke cycle is still used, it

Figure 7 : The 500 cc Automotive 2-Stroke EngineCombining DI and Exhaust ThrottlingButterfly Valves (the Simplest Device toGet Part Load CAI Combustion)


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would mean only one combustion cycle every twoengine revolutions and therefore unacceptable NVHbehaviour for automotive application.

To apply engine downsizing in an ULC car by using thedouble 2-stroke cycle combustion frequency canprovide, an interesting solution to NVH issues atminimum production cost. A single-cylinder 2-strokeengine is even possible for minimum cost with NVHperformance equivalent at least to a 2-cylinder 4-strokeengine and even better as shown by the Fig. 8. ThisFigure compares the calculated instantaneous torquevariation of a 500cc 2-cylinder 4-stroke with those of asingle-cylinder 2-stroke and of a 2-cylinder 2-strokeengines of the same overall engine displacement. Thiscalculation undertaken at 3000 r/min with an averagetorque of 24 Nm shows that: the single-cylinder 2-stroketorque fluctuations have a slightly lower amplitudemainly due to the lower maximum in-cylinder pressureof the 2-stroke (about 30 bars) compared to the 4-stroke (about 90 bars), the 2-cylinder 2-stroke torquefluctuations are impressively lower than those of the 2-cylinder 4- stroke (in fact they are comparable and evenslightly lower than those of a 4-cylinder 4-stroke). Thisadvantage allows to achieve much more stable enginerevolutions while requiring less additional flywheelinertia.

There is also a way of significant further production costsaving (without sacrificing the NVH behaviour asexplained previously) if a single-cylinder 2-stroke is usedin place of a 2-cylinder 4-stroke :

� There is still no valve train (but a balancing shaftbecomes necessary as in 2-cylinder 4-stroke)

� The number of moving parts (pistons, rings, rods,...)is reduced (divided by two)

� The number of fixed parts (fuel supply andinjectors, ignition system,....) is also similarlyreduced

� Some parts become simpler: intake and exhaustmanifold, crankshaft,...

For all these reasons, the 2-stroke engine technologycan be considered as probably the cheapest to producewhile in parallel giving the best NVH characteristics.What can have a negative impact on the cost of a DI2- stroke are mainly the direct injection system and thepossible need for an expensive specific DeNOxaftertreatment. Concerning the cost of DI 2-stroketechnology, the progress done during the last few yearsand its various applications outside automotive showthat it can be probably considered as slightly higher butalmost similar to the cost of 4-stroke port fuel injectiontechnology. Concerning the NOx emissions control, wewill also see in a following section that there are somepossibilities to achieve it without specific aftertreatment.This is a key issue to keep the 2-stroke inherently lowproduction cost.

THE DI 2-STROKE ENGINE: EASY MAINTENANCE

AND HIGH FUEL ECONOMY FOR LOW OPERATING

COST EASY AND LOWER MAINTENANCE COST

FOR THE INDIAN CUSTOMER

The following 2-stroke engine specific features have tobe considered by the Indian customer as providingeasier maintenance at a lower cost :

� The 2-stroke mechanics is the simplest one andtherefore some limited maintenance operations canin some cases be directly done by the user himself,there is no requirement of oil change as in a4-stroke engine. The oil tank can be easily refilledby the user himself on a regular basis as it wouldbe recommended by the manufacturer

One of the largest marine outboard engine manufacturereven explains on its web site that a dealer visit intervalof three years is recommended for a 2-stroke enginewhile a visit interval of half a year is recommended fora 4-stroke engine (mainly due to the oil change). Thisis something which as to be positively considered foran automotive application in India.

LOW PRODUCTION COST ENGINE

CHARACTERISTICS

The 2-stroke engine is also particularly interesting interms of production cost. Its lighter weight means lessmaterials and therefore less raw materials cost. It is asimpler engine with much less components :

� The complete 4-stroke valve train system isdeleted, in addition if a 2-cylinder 2-stroke is used,there is no need of balancing shaft.

Figure 8 : Comparison of Instantaneous TorqueFluctuation between 2-Cylinder 4-Stroke,Single-Cylinder 2-Stroke and 2-Cylinder2-Stroke Engines


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HIGH FUEL ECONOMY FOR LOW OPERATING COST

Several examples of DI 2-stroke engines in productionoutside automotive show the 2-stroke versus 4-strokehigher fuel economy thanks to the principle advantagesof the 2-stroke cycle. To illustrate it, three differentexamples of applications will be described :

� 50 cc scooter application in Europe

� 3-wheeler application in India marine outboardapplication

The downsizing effect combined with CAI combustioncan help to further increase this fuel economyadvantage as shown by the two other followingexamples of advanced engine concepts :

� 1230 cc DI CAI 2-stroke automotive prototypecompared to 1360 cc 4-stroke

� 250 cc DI AR 2-stroke compared to 400 cc4-stroke for large scooter application

We can know examine these five examples of DI2-stroke engines and see how they perform in termsof fuel economy.

50cc Scooter Application in Europe : In a recentstudy done by the French Environment ProtectionAgency (so called ADEME), several types of 50ccscooters sold in Europe have been tested and theirperformance reported. Among the different types(carburetted 2-stroke, carburetted 2-stroke with catalyst,carburetted 4-stroke, DI 2-stroke), the best emissionsresults are achieved by the last two configurations. Butthis study also compares the fuel economy of thescooters tested under the European Driving Cycle formopeds. As shown by the next Fig. 9, the best fueleconomy is achieved by the DI 2-stroke 50cc scooterwith 35% advantage compared to the 4-stroke 50ccscooter.

3-Wheeler Application in India : A new 3-wheelerauto-rickshaw has been launched end of 2007 equippedwith a 2-stroke air assisted DI technology [19].Compared to the conventional carburetted 2-strokemodel, its performance improvements can besummarized as follows:

� 33 % better fuel efficiency

� 15% more engine torque

� 25 % more engine power

This again shows the potential of the DI 2-stroketechnology in terms of fuel economy improvement.

Marine Outboard Application : DI 2-stroke engineshave been introduced in the marine outboard market tosolve the already discussed main drawbacks of 2-strokeengines. Even if they are in competition with the4-stroke technology, we can observe a renewal ofinterest for this technology for several reasons: the lightweight, the highest engine torque, the high fueleconomy and the capability to meet the most severeemissions regulations without aftertreatment. On theother side the 4-stroke outboards are suffering of theirdifficulty to meet NOx emissions regulations withoutaftertreatment and without significantly increasing theiremissions of CO and decreasing their fuel economy(because of rich mixture operation for NOx control). Thisis probably the reason why one of the two big USmarine outboard manufacturers has decided two yearsago to stop the development of its 4-stroke models andto focus its future on DI 2-stroke only.

Considering that, it is interesting to see the comparisonof the fuel consumption along the boat load curve ofthree outboard engine models sold by the sameoutboard company and giving exactly the same poweroutput of 50hp. The Fig. 10 shows this comparison. Wecan see that to replace the conventional carburetted2-stroke engine by a 4-stroke one gives about half ofthe improvement got if a DI 2-stroke is used. It is evenmore impressive at the low speed (2000 r/min) low loadpoint where fuel consumption can be divided by almosta factor of two compared to the 4-stroke one.

1230 cc DI CAI 2-Stroke Automotive Prototype

Compared to 1360 cc 4-Stroke : The author has beeninvolved in the early 90s in the development of a CAI– IAPAC 1230cc direct injected two-stroke conceptengine for automotive applications. As shown by theFig. 11, this engine gave more than 20% improvementin fuel economy (and also in low end torque as we willsee later in this paper) compared to the equivalent1360cc four-stroke engine of same power output.

Figure 9 : Compared Fuel Economy Between DI 2-Stroke and 4-stroke 50 cc Europeanscooters [18]


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a carburettor, a technology with little future given thecoming more and more stringent emissions standards.

This explains why more recent Ishibashi work has beenfocused on high performance motorcycle two-strokeengines combining both the AR combustion and aninnovative air assisted direct fuel injection technologynamed PDI (which stands for Pneumatic DirectInjection) [13,20]. The choice to focus on highperformance engines is to target an ultra low emissionengine that can achieve power output that cannot bereached by equivalent four-stroke engine. With thisdownsizing strategy, a smaller displacement PDI-ARtwo-stroke engine can achieve significantly betterfuel economy than a four-stroke engine of similarperformance. This strategy is clearly illustrated inthe following Fig. 12 that compares the fuel economyof a 250 cc PDI-AR two-stroke (20kW @ 6500-7000 r/min), to a 385 cc four-stroke named ‘Original’ in thefigure with the same power output (20kW @ 7500-8000r/min) and a 250 cc four-stroke (about 13kW @ 7000r/min).

250cc DI AR 2-Stroke Compared to 400 cc 4-Stroke

for Large Scooter Application : At the beginning ofthe 90s, Ishibashi and his colleagues developed alsoa two-wheeler application of CAI combustion that theycalled the ‘AR combustion’ for Activated RadicalsCombustion [4,11,12]. The first AR combustion 250ccmotorcycle has been introduced in production in 1997in Japan. It was equipped with the so-called ‘ARCombustion’ Exhaust Control Valve allowing to have aneffect on both the exhaust port timing and crosssection. This was followed by the first AR combustiontwo-wheeler produced for the European market, a 125/150cc 2-stroke AR combustion scooter sold in Franceand in Italy from 1999. These less sophisticatedengines were equipped with a simpler exhaust throttlingbutterfly valve to get the AR combustion. Both the AR250 motorcycle and scooter demonstrated very goodfuel economy and emissions results meeting the currentlegislation in Japan and in Europe. Nevertheless theproduction stopped in 2001 for the motorcycle and in2003 for the scooter mainly because their engines used

Figure 11 : Comparison of Fuel Economy Between1230cc CAI-IAPAC 2-Stroke AutomotiveEngine Prototype and 1360cc 4-StrokeAutomotive Engine (910kg PassengerCar)

The Figure shows 68% improvement in fuel economyin favour of the PDI-AR two-stroke compared to thelarger displacement ‘Original’ four-stroke of same poweroutput. However when the same displacement is usedfor the four-stroke, the two-stroke fuel economyadvantage is reduced to only 23% but the two vehicleare not comparable in terms of power output (35% lessfor the four-stroke) and driveability.

As a conclusion of this section, the potential fueleconomy advantage (essential for the Indian customer)of a DI two-stroke engine has been clearly pointed outby the five examples presented in this paper.

Figure 12 : 250 cc PDI-AR 2-Stroke Fuel EconomyCompared to 385 cc 4-Stroke of samePower Output (Original) and to 250 cc4-Stroke for Large Scooter

Figure 10 : Compared Fuel Consumptions Along theBoat Curve Between 2-Stroke, 4-Strokeand DI 2-Stroke (LPDFI Technology) ofSame Power Output (50hp) and SameManufacturer [7]


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NOX AFTERTREATMENT FREE EMISSIONS

CONTROL: THE MAIN ISSUE OF DI 2-STROKE FOR

ULCC

This section deals with the emissions of a DI two-strokeengine applied for ULC car and about their control.

OXIDATION CATALYST WITH FAST COLD START

LIGHTING FOR HC AND CO EMISSIONS CONTROL:

DI 2-stroke engines present different emissions profilesthan 4-stroke engines. :

� HC emissions are generally higher (intake andexhaust open simultaneously in the 2-stroke cycle)but DI drastically reduces HC emissions to a levelalmost similar (or slightly higher) than 4-stroke,

� NOx emissions are significantly lower due again tothe principle of the 2-stroke cycle (one combustionevery cycle with half the 4-stroke IMEP) and of theinherent internal EGR dilution. There are somepossibilities for further reduction to ultra low levelat low load thanks to the CAI combustion,

� Raw emissions of CO are generally significantlylower (lean burn operation at part load)

A significant amount of scavenging air is directly short-circuited and lost in the exhaust which means that thereis always an excess of O

2 in the exhaust.

This has two main consequences:

� The exhaust conditions are highly favourable forproviding high efficient HC and CO conversion byan oxidation catalyst

� A conventional 3-way catalyst aftertreatment cannotbe the solution for NOx reduction and therefore theraw emissions of NOx have to be maintained verylow in order to avoid complex DeNOxaftertreatment in oxidizing conditions If we look nowagain to some examples of DI two-stroke engines,we can start first with the liquid direct fuel injected2-stroke outboard [8]. What is remarkable with thisengine is that it is the first (and only one) outboardengine that received the Clean Air ExcellenceAward of the US EPA ! Its raw emissionsperformances are compared in Fig. 13 with othertechnologies including fuel injected (EFI) 4-stroketechnology. Almost the same HC + NOx emissionsare obtained with significantly better CO emissions.

The emissions performances achieved with the CAI –IAPAC 1230 cc DI two-stroke concept engine [16] arealso presented in the Fig. 14 and 15.

Thanks to its rather wide CAI range, this engine wasable to really take all the benefits of the CAIcombustion process especially in terms of ultra low NOx

Figure 13 : Compared Total / HC+NOx / COEmissions Levels of Several MarineOutboard Technologies: Evinrude E-TECDI 2-Stroke Versus Other 4-Stroke and2-Stroke Technologies [8]

Figure 14 : CO Emissions for Euro- II Calibration(MVEG Cycle) of the 1230 cc CAI-IAPAC2-Stroke Automotive Prototype Comparedto 1360cc 4-Stroke Automotive Engine

Figure 15 : HC + NOx Emissions for Euro- IICalibration (MVEG Cycle) of the 1230ccCAI-IAPAC 2-Stroke AutomotivePrototype Compared to 1360cc 4-StrokeAutomotive Engine


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emissions combined with high fuel economy. It was ablein 1996 to meet the Euro- II emissions legislation witha safety margin of 32 % in total HC + NOx and withextremely low CO value (70 times lower than thelegislation). These results were obtained without exhaustafter-treatment device other than an open loop oxidationcatalyst for the control of the CO (0.03 g/km) and HC(0.07 g/km) emissions.

CAI COMBUSTION FOR AFTERTREATMENT FREE

NOx EMISSIONS CONTROL

The emissions specifications for a future ULC vehiclein India are nevertheless much more severe than whatwas achieved with a DI two-stroke Euro II passengercar. They will require to meet at least a level similarto Euro IV: on the NEDC driving cycle including theextra urban driving cycle for limited speed vehicles (Fig.16) with high efficient oxidation catalyst (close coupledmetallic substrate) and fast lighting control strategy forHC and CO emissions control and with aftertreatmentfree NOx emissions control.

engine speed the maximum power output curve, theroad load curves for the 5 vehicle gears, the limit ofthe auto-ignition (CAI range) and finally the powerrequired to drive the vehicle at 3 stabilized speeds (15,50 and 90 km/h). For each gear ratio, the vehicle wasfirst driven up to be inside the CAI range. The ignitionwas then cut off, and the vehicle speed wasprogressively decreased and similarly increased in orderto find both the lower and upper vehicle speedspossible without spark ignition. The Fig. 17 shows forexample that the vehicle was able to run without sparkfrom about 15km/h in first gear and in fourth gear fromabout 50km/h up to 95 km/h.

Figure 16 : The Extra Urban Part of the RegulatedDriving Cycle for Vehicles with LimitedMaximum Speed

From the HC and CO results achieved in 1996, it canbe seen that the control of the NOx emissions withoutthe incremental cost of a DeNOx aftertreatment wouldbe one of the key issue (may be the main one) to besolved in order to consider the 2-stroke engine for ULCpassenger cars. In the next sub section we are goingto examine which are the possibility of further NOxemissions reduction without DeNOx by looking to whatcan be learned from the previous Euro II experience.

NOx Emissions Control: what can be Learned from

the Euro- II IAPAC DI 2-Stroke Automotive

Concept ? : Thanks to the effectiveness of the transferthrottling system (especially when combined with theIAPAC air assisted direct fuel injection), this engineoffered a remarkably wide CAI operating range asshown by the Fig. 17. This CAI range has beenprecisely measured with the vehicle in the chassisdynamometer. The Figure shows as a function of the

The interest in having a sufficiently wide auto-ignitionrange is that CAI combustion operation provides ultralow NOx emissions as shown by the Fig. 18 and 19.

Figure 17 : CAI Operating Range (Spark Ignition CutOff) of the IAPAC 1230cc DI 2-StrokeAutomotive Concept Engine

Figure 18 : Specific NOx Emissions Versus EngineLoad for the IAPAC DI 1230 ccAutomotive Engine Operating in CAICombustion at Low Load


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At the light load operating point of Fig. 19, the NOxemissions are already remarkably low without CAIthanks to the two-stroke engine inherent advantages ofmainly the high dilution by internal EGR and the lowIMEP per cycle because of a combustion frequencyevery cycle [2] Nevertheless it is interesting to remarkthat operating the engine in CAI doesn’t increase theNOx. A tendency towards further reduction is evenobserved which would not be the case in stratifiedmode operation.

Thanks to its rather wide CAI range this engine wasable to really take all the benefits of the CAIcombustion process for the control of the emissions.Concerning NOx emissions, it is interesting to see onthe Fig. 20 that only 8 % of the total NOx (0.27 g/km)were emitted during the first 800 seconds correspondingto the urban section of the driving cycle. During thisextremely low NOx section (average NOx emissionsbelow 0.07 g/km without after-treatment !) the enginewas always running in CAI combustion mode (exceptedat idle). This really shows the potential of the CAIcombustion for ultra low NOx emissions.

The same Fig. 20 shows that among the total NOxemissions of 0.27 g/km, 30% and 50% of NOx areemitted during the two last accelerations (of the extraurban driving cycle) to respectively 100 and 120 km/h.This is important to point out the heavy weight of thesetwo accelerations on the overall level of NOx. It isparticularly interesting if we now consider the applicationof such type of two-stroke combustion in an ULCvehicle. Because of the limited speed of such kind ofvehicle, the driving cycle to follow for the measurementof the regulated pollutant emissions will be limited to amaximum speed of 90 km/h with only one lastacceleration (instead of two) to this maximum speed.These conditions become then much more favourablefor the control of the raw emissions of DI 2-stroke NOxemissions especially if this engine is designed to

operate in CAI combustion mode during the light loadurban driving part.

The following Fig. 21 shows a typical BMEP curve andcorresponding CAI auto-ignition range for a 500 cc two-stroke engine installed in an ULC car and delivering atleast the same power (and more torque) as the 623cc four-stroke engine of the Table- I. On the graph, theengine operating points to follow the extra urban partof the driving cycle are plotted in full line. It can beseen that some limited part of the cycle can beperformed within the CAI range and that theaccelerations from 50 to 70 km/h and from 70 to 90km/h correspond to the highest BMEP (between 2.5and 3 bars). The previous Fig. 18 shows that at suchBMEP the NOx emissions start to increase even if theyremain reasonably low.

One solution to minimize such increase of NOxemissions could be to operate the engine at higherengine speed / lower BMEP during these twoaccelerations. It would suppose to use a transmissionratio adapted for this purpose.

THE PROPOSED CONCEPT FOR NOX

AFTERTREATMENT FREE EMISSIONS CONTROL :

The previous discussion showed that Euro- IV can bemet at part load (urban driving cycle). For this purpose,DI has to be combined with CAI combustion (byexhaust throttling) for NOx emissions control. Stratifiedcharge DI operation has then to be avoided (except formisfire free idling control).

At medium load (extra urban driving cycle) the followingadditional measures will have to be considered to meetEuro IV :

� to chose a close coupled implementation of thecatalyst used mainly for the oxidation of CO andHC and to prefer a 3-way precious metalformulation (even if it is not necessary for HC and

Figure 20 : Cumulative NOx Emissions on the MVEGCycle – IAPAC/CAI Automotive 1230 ccDI 2-Stroke Concept Engine


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CO reduction) in order to obtain some (limited) NOxreduction (by the conversion of rich exhaustpulses),

� to use a calibration including an increase of theincylinder trapped fuel to air ratio during theaccelerations in order to help to further reduce theNOx emissions without sacrificing the aftertreatmentof the CO and HC; indeed due to the scavengingair losses, the exhaust fuel to air ratio will remainlean even if the trapped ratio becomes rich,

� to adapt the transmission in order to increase theengine speed and decrease the BMEP, if possibleduring the accelerations only in order to minimizethe negative impact on fuel economy; this couldfinally bring significant further reduction of NOx asdiscussed with the Fig. 21.

A low cost solution to apply the last strategy oftransmission adaptation would be to use anelectronically controlled CVT allowing to operate theengine at high revolutions during the strongaccelerations for NOx control and to operate atmedium zevolutions during steady state operation orlight accelerations for best fuel economy. This typeof CVT already exists in production in largeengine displacement scooters and could well fit in anULC car [21].

DI 2-STROKE FOR ULCC: IMPROVED DRIVEABILITY

THANKS TO HIGH TORQUE

One of the last specific feature in favour of the use ofa DI two-stroke engine for ULCC is its higher torque,

especially when compared to 4-stroke engine asillustrated by three examples already discussed in thispaper:

the marine outboard application for which the DI two-stroke gives more than 30 % higher torque (Fig. 22)for the same power output, the CAI IAPAC two-strokeautomotive engine concept with more than 20 % highertorque at lower engine speed (Fig. 23) again for thesame power output, and the PDI AR 250 cc two-strokefor two-wheeler application which gives impressivelyhigher specific torque.

This possibility of higher maximum engine torqueoffered by the DI 2-stroke technology without anyincremental cost can bring to ULC vehicles a betterdriveability that should be appreciated by the Indiancustomer.

Figure 21 : Estimated Load Points for a 580 kg IndianULC Car (of Table- I Specifications)Following the Extra Urban Driving Cyclefor Limited Speed Vehicles

Figure 22 : Torque Comparison for DI 2-stroke andFuel Injected 4-stroke Marine OutboardEngines of Same Power Output [22]

Figure 23 : Compared Low End Torque of the 1230cc CAIIAPAC 2-stroke AutomotivePrototype Versus 1360 cc 4-strokeAutomotive Engine [16]


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CONCLUSION

The main purpose of this paper is to review in detailsthe most recently available results from DI two-strokeengines recently produced outside automotive as wellas the performances achieved in the past of someadvanced DI two-stroke automotive concepts, and tocompare them with the required specifications for anultra low cost passenger car for India. From thistechnical constructive review presented here, it thenbecome clearly possible to point out the advantagesand limitations in considering the use of such enginetechnology in an ultra low cost passenger car. Thefollowing Table- II summarizes the main conclusions ofthis paper.

Compared to its two-cylinder four-stroke counterpart, itis expected that a DI two-stroke would have a smallerdisplacement, size and weight, a lower cost (significantlylower if a single-cylinder configuration is chosen), muchbetter NVH characteristics (if a twocylinder is chosen),easier and less expensive maintenance and significantlyhigher fuel economy. In addition the lower maximum in-cylinder pressure of the two-stroke would make it asparticularly adapted to be combined with a simple stopand start system for further fuel savings. The mainissue that would have to be carefully considered isprobably the control of NOx emissions to avoid the useof a costly DeNOx aftertreatment. We have shown thatseveral measures have to be taken to achieve thistarget.

Finally considering all these favourable conclusions andto answer to the question asked by the title of thispaper, we can conclude that it appears worthwhile toconsider this new generation of DI two-stroke enginesas a possible attractive powertrain for innovative ultralow cost passenger cars requested by the Indianmarket. A single-cylinder DI two-stroke gasoline engineof 450-500 cc able to operate in controlled auto-ignitionat part load installed in an ULC vehicle with a welladapted electronically controlled CVT could probably bethe best challenger along all the criteria of Table- IIwhen compared to a more conventional four-strokepowertrain.

ACKNOWLEDGMENTS

The author would like to particularly thank ThierryColliou of the IFP Powertrain Engineering for the mostrecent engines data he provided and for the very usefulcalculations he made and that were used in this studyand Yoichi Ishibashi of Honda R&D for his preciousadvices and support, and for his materials and resultsused in this paper .

REFERENCES

1. Duret P., Ecomard A and Audinet M., “A New Two-Stroke Engine with Compressed Air Assisted FuelInjection for High Efficiency Low EmissionsApplications”, SAE Paper N 880176, 1988

2. Duret P and Moreau J-F., “Reduction of PollutantEmissions of the IAPAC Two-Stroke Engine withCompressed Air Assisted Fuel Injection”, SAEPaper No. 900801, 1990

3. Duret P., “The Key Points for the Development ofan Automotive Spark Ignition Two-Stroke Engine”,IMEC 389/278, FISITA 925021, London 1992

4. Ishibashi Y and Tsushima Y, “A Trial for StabilizingCombustion in Two-Stroke Engines at Part ThrottleOperation, in Duret P, A New Generation of Two-Stroke Engines for the Future?, IFP InternationalSeminar, Rueil-Malmaison, Editions Technip, 1993

Figure 24 : 250cc PDI-AR 2-Stroke Specific Torque inNm per Liter of Engine DisplacementCompared to 385cc 4-Stroke of SamePower Output (Original) and to 250cc4-Stroke [20]

Table- II : Summary of the Selection Criteria of theMost Adapted Small Engine Configurationfor Ultra Low Cost Passenger Cars


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5. Bell, G., Brewster, S and Ahern, S., “Beyond 3 StarEmission Capability for Outboard Engines”, SAEPaper No. 2007-32-0052, 2007

6. Duret, P., Dabadie, J-C and Colliou, T., “Applicationof IAPAC Fuel Injection for Low Emissions SmallTwo-Stroke Engines”, SAE Paper No 951795, 4thSETC Conference, Milwaukee, 1995

7. Venturi S., et al., “From Development toIndustrialization of an IAPAC Marine Outboard DI2-Stroke Engine”, SETC Conference, Pisa Italy,2001

8. “The New Evinrude E-TEC Outboards”, IAME44-19. Onishi, S, et al., “Active Thermo-Atmosphere

Combustion (ATAC) – A New Combustion Processfor Internal Combustion Engines”, SAE Paper No.790501, 1979

10. Duret P., “Two-stroke CAI Engines” in Zhao H.,“HCCI and CAI Engines for the AutomotiveIndustry”, Woodhead Publishing Limited, 2007

11. Ishibashi, Y and Asai, M., “Improving the ExhaustEmissions of Two-Stroke Engine by Applying theActivated Radical Combustion”, SAE Paper No.960742, 1996

12. Ishibashi, Y., “Basic Understanding of ActivatedRadical Combustion and its Two-Stroke EngineApplication and Benefits”, SAE Paper No. 2000-01-1836

13. Ishibashi, Y., Nishida, K and Asai, M., “ActivatedRadical Combustion in High Speed High PowerPneumatic Direct Injection Two Stroke Engine”, inDuret P, A New Generation of Engine CombustionProcesses for the Future?, IFP InternationalSeminar, Rueil-Malmaison, France, EditionsTechnip, 2001

14. Tsuchiya, K., et al., “Emission Control of Two-Stroke Motorcycle Engines by the Butterfly ExhaustValve”, SAE Paper No. 800973

15. Duret, P., Venturi, S and Carey, C., “The IAPACFluid Dynamically Controlled Automotive Two-StrokeCombustion Process” in Duret P, A New Generationof Two-Stroke Engines for the Future ? – Rueil-Malmaison, France, Editions Technip 1993

16. Duret, P and Venturi, S., “Automotive Calibration ofthe IAPAC Fluid Dynamically Controlled Two-StrokeCombustion Process” SAE Paper No. 960363, 1996

17. Duret, P. et al. “The Air Assisted Direct InjectionELEVATE Automotive Engine Combustion System”,SAE Paper No. 2000-01-1899

18. Barbusse, S. “Motocycles, Cyclomoteurs; Energie etEnvironnement; Données et références – ADEME– June 2005

19. Bajaj Press Release, Pune 8th December 200720. Ishibashi, Y. and Sakuyama, H., “An Application

Study of the Pneumatic Direct Injection ActivatedRadical Combustion Two-Stroke Engine to Scooter”,SAE Paper No. 2004-01-1870

21. Asumi, M., et al., ‘Development of ElectronicallyControlled Belt-type CVT for Motorcycles’, SAEPaper No. 2005-32-0024, 2005

22. Mueller, E., Hull, D. and Reid, T., “PowerDevelopment of the New Boosted MercuryEngines”, 13th Aachen Kolloquium, 2004

CONTACT

Pierre DURETe-mail : [email protected]

DEFINITIONS, ACRONYMS, ABBREVIATIONS

AR : Activated Radicals (combustion)

DI : Direct Injection (of fuel)

CAI : Controlled Auto Ignition also namedaccording to the authors ATAC (ActiveThermo Atmosphere Combustion), HCCI(Homogeneous Charge CompressionIgnition), AR (Activated Radicals)Combustion,...

EPA : Environment Protection Agency

IAPAC : Injection Assistée Par Air Comprimé (whichstands for “Compressed Air Assisted FuelInjection Technology”), trade mark of the IFP-developed DI 2- stroke technology

LPDFI : Low Pressure Direct Fuel Injection (brandname used by the Selva Marine outboardcompany to market the IFP-developed IAPACDI 2-stroke technology)

PDI-AR : Pneumatic Direct Injection with ARcombustion

NVH : Noise Vibration and Harshness

ULCC : Ultra Low Cost (passenger) Cars

Health & Medicine

Siat 2009 2 Stroke For Ulc