2 MTZ 09|2006 Volume 67

Authors:Bas van den Heuvel, Werner Willems,Frank Krämer, Tim Morris and Evangelos Karvounis

Brennverfahrens-Entwicklung für die neuen Dieselmotoren in leichten

Nutzfahrzeugen von Ford und PSA

You will find the figures mentioned in this article in the German issue of MTZ 09|2006 beginning on page 606.

Combustion System Development for the New Diesel Enginesin Light and Medium Commercial Vehicles from Ford and PSA

This is the first engine family specifically designed for lightand medium commercial vehicles to be produced underthe joint Ford Motor Company/PSA Peugeot Citroën Dieselengine co-operation agreement. Although the new engineis based on an existing engine architecture, it introduces anumber of technical innovations, raising the bar evenhigher for engine technology in the commercial vehiclesector. Part of the engine development has focused on op-timising the combustion system and was led by the engi-neers of the Ford Research and Advanced EngineeringCentre FFA in Aachen, Germany. The new engines produceengine-out emissions that are well within the stringent lim-its given by the EU4 Light Duty Truck (LDT) emissions stan-dard for all of the vehicle applications, whilst maintaininggood fuel economy.

1 Introduction

The importance of having state-of-the-art com-mercial vehicle diesel engines is very high forboth Ford Motor Company and PSA PeugeotCitroën. Van sales play a crucial role in theEuropean market. Diesel engines have provedvery popular in Europe for many decades –indeed 95 % of commercial vehicles sold onthe continent are Diesel powered. For Ford

Motor Company and PSA Peugeot Citroën,Diesel popularity carries an even greater sig-nificance, as 98 % of all their light commer-cial vehicle sales in Europe in 2004 were ofdiesel models – thus making the need for aclass leading engine essential. Work on thenew commercial vehicle engine family beganin 2001. The engines will be built at Ford’sDagenham Diesel engine plant. Joint invest-ment on the new program of common rail

COVER STORYCombustion

3MTZ 09|2006 Volume 67

Diesel engines for light and medium commer-cial vehicles (LCVs) totals 120 million Euro.

All versions of this engine are either2,198 cm3 or 2,402 cm3 in displacement andhave dual overhead camshafts. The enginesfeature high-grade aluminium alloy cylinderheads with a cast-iron cylinder block. Thenew 2.2-l engine will be offered by Ford Mo-tor Company and PSA Group brands in fivedifferent power outputs ranging from 63 kWand 250 Nm to 96 kW and 310 Nm. These en-gines will be used by Ford of Europe in front-wheel-drive versions of its all-new Ford Tran-sit, and by PSA Peugeot Citroën in its upperrange LCVs (Boxer and Jumper). The new 2.4-lengine is a further development of the 2.2-lengine and features a choice of three poweroutputs ranging from 74 kW and 285 Nm to103 kW and 375 Nm. The 2.4-l engines willbe exclusively used by Ford in the rear-wheel-drive Transits. The entry and mid-level 2.2-land 2.4-l engines use a fixed geometry tur-bocharger (FGT), while the higher-poweredengines rely on a variable geometry turbo(VGT). The Table provides further technicaldetails.

The engine family has also been devel-oped with an objective to reduce overallweight. Compared to the unit that providedthe base architecture for the new engine, asubstantial reduction of weight has beenachieved. The block structure was optimised,which also allows for better noise perform-ance. The injector pump of the 2.2-l enginenow runs directly on the camshaft ratherthan off the engine block itself, which con-tributes to another significant weight loss.Also, a lighter front-end accessory drive isused. The camshaft is now driven by a sim-plex chain.

2 Combustion System Development

The main emphasis of the development hasbeen on optimising the combustion systemin all aspects. A clean and efficient combus-tion process requires an optimised combina-tion of various combustion parameters, suchas the combustion chamber definition, thein-cylinder charge motion and the fuel injec-tion strategy. State-of-the-art numerical andexperimental analysis tools form an integralpart of the combustion system developmentprocess in Ford. Thus, in-depth investigationshave successfully provided optimum settingsof the combustion parameters with regardto lowest emissions and best fuel efficiency,whilst satisfying the performance and dura-bility requirements for commercial applica-tions. The most important characteristics ofthe new EU4 combustion system and its de-velopment process are described below.

der head design. However, the validity ofsuch numbers for evaluating the real in-cylinder motion in a running engine is lim-ited. Computational Fluid Dynamics (CFD)can give valuable insights into the real flowstructures and the subsequent mixture for-mation. The left graph in Figure 3 shows thein-cylinder flow structure at BDC of the in-take stroke for the previous version of theseengines. Although the bulk air motion rep-resents a swirling flow around the cylinderaxis, there is clear evidence of substantialsecondary motion. This secondary motion islargely dissipated during the compressionstroke and does not add to the swirl momen-tum during injection and combustion. Con-sequently, the efficiency with which the in-take ports generate a required TDC swirllevel is limited. The right graph in Figure 3shows that the new design now generates asingle swirling motion that is well-centeredto the centre axis of the combustion cham-ber. No secondary motion is introduced.Much more of the trapped impulse momen-tum will now be preserved during the com-pression stroke in order to generate a morehomogeneous flow field with the requiredTDC swirl level. As a result, port efficiency ofthe new engines is best-in-class for parallelvalve arrangements. The improved quality ofthe mixture formation has substantiallycontributed to achieving the engine’s per-formance and emissions targets, as will beseen below.

2.2 Compression RatioThe compression ratio was reduced from19.0:1 to 17.5:1, which provides advantagesboth at part-load and full-load conditions.NOx emissions are reduced as the new com-pression ratio enables to run lower combus-tion temperatures at part-load conditions.Moreover, Soot oxidation is improved as a re-sult of a reduced rate of temperature declineduring the expansion stroke. At full-loadconditions, TDC pressure levels are lower, al-lowing to advance injection timing and toincrease injection duration without exceed-ing the maximum exhaust temperaturelimit of 760 °C. An optimised design of thecombustion chamber and the fuel injectionstrategy has eliminated any potential coldstarting issues that might occur as a resultof the reduced compression ratio.

2.3 Combustion Chamber DefinitionThe volume of the piston bowl has grownsubstantially as a result of the reduced com-pression ratio. The previous EU3 and thenew EU4 bowl designs are shown in Figure 4.Both designs feature a wide-bowl conceptwith reentrancy. The dome of the EU4 pis-

2.1 In-cylinder Air Motion and Flow EfficiencyThe engines apply four-valves-per-cylindertechnology and a centrally positioned injec-tor. The valve arrangement is close to paral-lel. In general, such a parallel valve arrange-ment is unfavorable for the definition ofhigh-efficiency, swirl-generating intake ports,contrary to twisted valve arrangements. Fur-thermore, the low base cylinder head heightand the existence of 6 head bolts per cylin-der leave little design space for the ports.The previous version of these engines hastwo helically shaped intake ports. An impor-tant objective with regard to combustionefficiency was the improvement of mixtureformation and flow efficiency withoutchanging the existing cylinder head conceptand manifolds.

A new intake ports concept was devel-oped within the boundaries given by theexisting cylinder head architecture whilstmaintaining the original valve sizes and po-sitions. Figure 1 shows the final design withone helical port and one directed port. Thenew ports fit into the original cylinder head,although small changes to the water jacketwere necessary. Both the new and the origi-nal cylinder heads run simultaneous on theexisting production facilities.

The new intake ports were equipped withseat swirl chamfers. These chamfers are theresult of machining operations at the flameface around the seat rings and are eccentricto the valve axis. They direct the flow intothe combustion chamber at low valve liftsuch that additional swirl momentum is in-duced. The valves were recessed into thehead to enable the swirl chamfer effects andto eliminate the need for valve pockets inthe piston surface. Eliminating piston pock-ets has a cost benefit, but also provides ad-vantages with regard to combustion unifor-mity.

Swirl ratio and flow coefficient versusvalve lift height are given in Figure 2 for boththe new and the previous design. A clearswirl chamfer effect exists at low valve liftheights for the new design. Port efficiency iscommonly expressed as the trade-off be-tween swirl number and mean flow coeffi-cient. The integral swirl number at IVC hasremained largely the same, but is generatedquite differently as will be seen below. Theflow coefficient has increased, indicatingthat the new intake ports concept providessubstantially improved flow efficiency.

The quality of the in-cylinder charge mo-tion is, however, even more important. Swirlnumbers are generally taken from steady-state flow bench testing and give an impres-sion of the swirl level generated by the cylin-

COVER STORY Combustion

4 MTZ 09|2006 Volume 67

ton bowl is significantly larger. The designof the bowl lip remained unchanged. Theshape of the piston bowl was optimised us-ing CFD methodology and through enginetesting.

Air/fuel mixing is one of the key parame-ters for the engine’s performance and emis-sions behavior. Mixture formation dependson the in-cylinder air motion, but also onthe injection characteristics and the designof the combustion bowl. CFD tools providevaluable insights into the mixing propertiesof a certain design. Figure 5 shows the globallevel of inhomogeneity of the in-cylindermixture for the EU3 and the new EU4 de-signs, at maximum torque conditions. Obvi-ously a low level of inhomogeneity is betterfor combustion efficiency. The new designshows faster homogenisation during injec-tion due to faster droplet break-up and va-porisation as a result of the reduced nozzlehole size and a reduced hydraulic flow rate.This condition is maintained during laterstages of combustion.

The mixing behavior is visualised in Fig-ure 6. During the early phases of combus-tion (e.g. 18° ATDC) it can be observed thatmixing takes place closer to the nozzle forthe new design. The EU3 design deflectsthe rich mixture back into the injectionspray, which results in soot production.This effect has been eliminated by the opti-mised EU4 design, which has reduced sootformation not only for full-load but also forhigh part-load conditions. Also during laterphases of combustion, a much more homo-geneous mixture exists compared to theEU3 design (e.g. 58° ATDC). The amountand extend of rich zones that occurred inthe previous design have now been largelyreduced. Consequently, a further reductionof soot emission is achieved.

2.4 Swirl Level and Number of Nozzle HolesThe swirl level at TDC is a decisive parame-ter for the mixture formation and the com-bustion process. The TDC swirl level de-pends on three engine properties: the cylin-der head-induced swirl level at IVC, thecompression ratio and the moment of iner-tia of the combustion bowl volume. The op-timum definition of the compression ratioand the combustion bowl are describedabove. The optimum swirl level may differbetween full-load and part-load operation.Moreover, it depends on the number of in-jector nozzle holes. At full-load conditions,the objective is to maximise engine per-formance and limit soot production, whileexhaust gas temperatures should not ex-ceed a given limit in order not to overheat

the turbocharger. The main emphasis atpart-load conditions is on emissions, whichmay require a different swirl level. Techni-cally it is possible to have a variable swirl lev-el by means of port deactivation techniques.However, such a system introduces addi-tional cost, and may introduce robustness is-sues.

The optimum combination of swirl leveland number of nozzle holes was investigat-ed at full-load as well as part-load conditionsby means of generic single-cylinder testingand CFD simulations. Figure 7 provides full-load CFD results at rated speed and at maxi-mum torque conditions. A general trend ofreduced power with increasing swirl level isobserved. Although this effect is further in-creased with increasing number of nozzleholes, the total effect on engine perform-ance remains limited. Therefore, the nozzledefinition and swirl level were mainly opti-mised for emissions.

Soot production results at maximumtorque conditions show that an optimumswirl level exists for each number of nozzleholes, Figure 7; a lower number of nozzleholes requires a higher swirl number. How-ever, for this specific application six- andseven-hole nozzles are clearly more robustagainst swirl variation where soot produc-tion is concerned than five-hole nozzles.Such robustness is important as swirl devia-tions exist in mass production. More impor-tantly, it provides the opportunity to choosethe swirl level for optimised part-load opera-tion, without major impact on full-load be-havior. Part-load investigations have clearlyshown improved emissions behavior withincreased number of nozzle holes combinedwith a moderate swirl level. Figure 7 illus-trates how a seven-hole nozzle improves theparticle/NOx trade-off in comparison to asix-hole nozzle, at part-load conditions.

Finally, based on the full-load and part-load investigations a seven-hole nozzle and amedium-high swirl level were found to com-bine optimum performance and emissionswith maximised robustness. This configura-tion satisfies both the full-load require-ments and the emission targets at part-loadconditions without the need for a variableswirl device.

2.5 Fuel Injection SystemThe new engine family applies the new Den-so second-generation common-rail fuel in-jection equipment (FIE), which is its firstEuropean commercial vehicle application.Solenoid-injectors and a maximum injectionpressure of 1600 bar are adopted. A seven-hole microsac type injector nozzle was foundto provide optimum performance and emis-

sions. Detailed matching of the interactionbetween injection sprays and combustionbowl geometry has resulted in an injectioncone angle of 153°. Although the fuel injec-tion system is capable of up to 5 injectionsper cycle, the current injection strategy islimited to a main injection event, combinedwith a single pilot injection event in most ofthe engine load map, as depicted in Figure 8.This injection strategy allows meeting theengine’s performance and emissions targetswhilst providing for the necessary robust-ness and durability for commercial vehicleapplication.

Specific power targets at rated speed haveincreased compared to the previous versionsof the engine. Nonetheless, nozzle flow ratescould be substantially reduced due to thevery good air utilisation of the new combus-tion system, and the favorable injectionrates of the new FIE. The new system satis-fies full-load power targets with a nozzleflow rate of 290 cm3/30s for the engineswith a FGT, down from 380 cm3/30s. The VGTengines apply a flow rate of 310 cm3/30s.

2.6 Exhaust Gas RecirculationThe engines apply water-cooled and electron-ically-controlled exhaust gas recirculation(e-EGR). In the lower speed and load ranges,and around idling speed, the new enginerealises EGR rates of more than 50 %.

3 Engine and Vehicle Results

3.1 Full-loadThe main thermo-dynamic properties of allfive models of the new 2.2-l engine and thethree models of the 2.4-l engine are listed inthe Table. The maximum power and torquecurves as well as the specific fuel consump-tion and soot levels at full-load are given inFigure 9 for the 63 kW, the 81 kW, and the96 kW variants of the 2.2-l. Torque levelsrange up to 375 Nm providing excellent dri-vability. The maximum torque levels arereached at engine speeds as low as 1500/min.The engine-out soot levels at full-load arevery low for the entire engine speed range.

3.2 Fuel ConsumptionEqually important for commercial applica-tion is the fact that these performance levelsare combined with state-of-the-art fuel econ-omy. The engine’s excellent fuel efficiencywas maintained despite the much-reducedemissions levels. Specific fuel consumptionreaches values as low as 205 g/kWh for the103 kW variant, Figure 10. At full-load and arated speed of 3500/min, fuel consumptionis still below 230 g/kWh. The fuel consump-tion numbers of several front-wheel-drive

5MTZ 09|2006 Volume 67

Ford Transit applications during the NEDC certi-fication tests are given in Figure 11.

3.3 EmissionsEngine-out emissions of the upgraded enginesare well within the stringent legal limits for EU4light duty truck targets for all of the output lev-els. The emissions of various Ford Transit LDT3applications with the new 2.2-l engine in theNEDC certification test are shown in Figure 11.Engine-out NOX, PM and HC emission levels aresubstantially below legal limits, CO emissionsare even limited to approximately half of the re-quired levels.

4 Summary and Outlook

The new common-rail Diesel engine family pro-vides a variety of clean and fuel-efficient powersources for light and medium commercial vehi-cles. State-of-the-art development tools were suc-cessfully applied to design the new combustionsystem. Figure 12 illustrates the technology walkfrom EU3 to EU4; the optimised mixture forma-tion by means of a new intake ports concept, thenew combustion bowl and reduced compres-sion ratio, and the optimised injection parame-ter settings were the main enablers for reducingemissions. Consequently, all of the eight enginevariants satisfy the 2007 EU4 light duty truckemissions standard. Despite these low emissionslevels, low fuel consumption was achieved.

The variation of power levels from the samefamily of engines ensures the commercial vehi-cle operators can choose the engine that is rightfor them: lower power for regular town or citydriving, or higher power for long distance cruis-ing. The new 2.2-l and 2.4-l diesel engines providea successful combination of lowest emissionsand low fuel consumption with good perform-ance, durability and reliability.

References[1] Lawrence, P.; Lake, P.; Turtle, D.; Taylor, T.; Carnochan, W.;

Finch, J.; Gallett, T.; Wölfle, M.: Die neuen Duratorq-Dieselmotoren mit Direkteinspritzung im Ford Transit. In:MTZ Motortechnische Zeitschrift 61 (2000), S. 8-17

[2] Lawrence, P.; Lake, P.; Turtle, D.; Taylor, T.; Carnochan, W.;Finch, J.; Gallett, T.; Wölfle, M.: The all new DuratorqDiesel Engines for the Ford Transit. In: proceedings to 8.Aachener Kolloquium Fahrzeug- und Motorentechnik1999, S. 65-91

[3] Willems, W.; Van den Heuvel, B.; Krämer, F.; Sommerhoff,A.; Karvounis, E.: Computational Methods for DieselCombustion System Development. In: proceedings to 10. Aachener Kolloquium Fahrzeug- und Motorentechnik2001, S. 279-298

[4] Van den Heuvel, B.; Willems, W.; Karvounis, E; Schulte,H.: In-Cylinder Flow Characterization of Modern HSDIDiesel Engines – Numerical and Experimental Evaluationof Current Practices. In: proceedings of THIESEL 2004conference, Valencia, S. 171-189

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