delphi

ABSTRACTToday turbo-Diesel powertrains offering low fuelconsumption and good low-end torque comprise a significantfraction of the light-duty vehicle market in Europe. GlobalCO2 regulation and customer fuel prices are expected tocontinue providing pressure for powertrain fuel efficiency.However, regulated emissions for NOx and particulate matterhave the potential to further expand the incremental cost ofdiesel powertrain applications. Vehicle segments with themost cost sensitivity like compacts under 1400 kg weightlook for alternatives to meet the CO2 challenge but maintainan attractive customer offering. In this paper the concepts ofdownsizing and downspeeding gasoline engines are exploredwhile meeting performance needs through increased BMEPto maintain good driveability and vehicle launch dynamics. Acritical enabler for the solution is adoption of gasoline directinjection (GDi) fuel systems. GDi provides the ability toutilize increased scavenging without sacrificing hydrocarbonemissions because fueling and air controls can be separated.In-cylinder injection with GDi also provides charge coolingbenefit yielding the knock reduction necessary forturbocharged applications. Several options within GDi areexplored including multi-hole and single pintle spraygenerators, as well as side-mount versus central-mountapplications. Both 3-cylinder and 4-cylinder base engineconfigurations are explored for turbocharged enginesdownsized to 1.2 L. Improvements in hydrocarbon emissions,heat losses and scavenging favor fewer cylinders. This is re-enforced by packaging and cost considerations. The nextsystem aspect considered is emissions aftertreatment.Stoichiometric turbocharged GDi provides the lowest costusing the established 3-way catalyst. Stratified GDi isexpected to require a lean NOx trap (LNT) and Diesel Euro 6systems require addition of a diesel particulate filter (DPF)

and possibly NOx aftertreatment. Although the aftertreatmentsystem to meet Euro 6 NOx requirement is not known today,addition of a DPF, DPF with LNT, or DPF with ureaselective catalytic reduction (SCR) represent significant costand packaging challenges. The analysis concludes bycomparing the 3-cylinder turbocharged GDi to other offeringsin the context of value tradeoff of CO2 reduction to systemon-cost. Stratified 3-cylinder turbocharged GDi systems offerup to 22% CO2 reduction compared to a baseline port fuelinjected 4-cylinder engine. CO2 reduction of 18% is possiblefor a stoichiometric GDi mechanization that employs variablevalve actuation (VVA) and a conventional 3-way catalyticconverter. At Euro 6 emission levels, stoichiometric 3-cylinder turbocharged GDi powertrains offer excellent value.The 3-cylinder turbo-Diesel offers similar value if it iscapable of meeting the NOx emissions standard without leanaftertreatment. Powertrains with higher engine-out NOxlevels that require lean aftertreatment are significantlydisadvantaged. Based on the price sensitivity of the compactcar segment, the value analysis predicts the 3-cylinderturbocharged GDi engine as the powertrain of choice.

INTRODUCTIONDramatic fuel consumption / CO2 reductions are necessary,both near-term and long-term, while tailpipe emissionstandards are becoming increasingly stringent. Figure 1indicates the rollout of US and European light-duty emissionstandards and fuel consumption targets over the next severalyears. Particularly interesting is the sharp reduction in NOxemissions and the trend toward elimination of relief in theEuro NOx emissions standard for vehicles with Dieselengines. The European overall light duty vehicle mix isroughly 50% powered by turbo-Diesels for vehicles meetingthe Euro 4 emissions standards. This is one reason that fleet-

3-Cylinder Turbocharged Gasoline Direct Injection:A High Value Solution for Low CO2 and NOxEmissions

2010-01-0590Published

04/12/2010

John E. Kirwan, Mark Shost, Gregory Roth and James ZizelmanDelphi Powertrain Systems

Copyright © 2010 SAE International

averaged CO2 emissions are lower in Europe compared to theUS, because turbo-Diesels at the same rated power emit lessCO2 compared to baseline naturally aspirated (NA) gasolinevehicles with multi-port fuel injection (MPFI).

However, the cost of Euro 4 turbo-Diesels is significantlygreater than a baseline Euro 4 gasoline engine due largely tothe common rail high pressure (1500 - 2000 bar) fuel system,the turbocharger, and differences in the powertrain structureto accommodate Diesel combustion that produces highermaximum cylinder pressures. As a result, the EuropeanDiesel market share is markedly lower than 50% for smallervehicles driven by vehicle purchase price sensitivity. A recentDRI survey [1] indicates that the Diesel market share isapproximately 30% for subcompact and compact vehicleslighter than approximately1400 kg. Euro 5 and Euro 6emission standards will increase the incremental Diesel costseven more due to more stringent NOx and particulatestandards. Lower cost alternatives for reduced CO2 emissionsare thus especially attractive for smaller vehicles.

Recent development efforts have been publisheddocumenting the benefits and challenges of 3-cylinderturbocharged gasoline direct injection (GDi) engines (see, forexample [2,3,4,5,6]). The present paper represents anevaluation of this technology as a high value solutionproviding low CO2 and NOx emissions for smaller vehicles.The discussion is divided into two sections. First is atechnology analysis of downsizing and turbocharging, GDifueling and its synergies with turbocharging, and an analysisof downsized 3-cylinder versus 4-cylinder engines. Thesecond major section is a value analysis comparing CO2benefits for 3-cylinder and 4- cylinder gasoline and Dieselpowertrains. Vehicle electrification is outside the scope of theanalysis. It is expected that hybrid vehicle growth continuesas well as widespread implementation of stop-starttechnology in Europe. These technologies complement Dieseland gasoline powertrains, and their implementation does notchange the stand-alone analysis or conclusions provided.

<figure 1 here>

TECHNOLOGY ANALYSISTURBOCHARGED GASOLINE DIRECTINJECTION OVERVIEWFigure 2 is a schematic diagram illustrating the concepts ofdownsizing, shown in the top half of the figure, anddownspeeding, depicted in the bottom half of the figure. Bothare effective methods to meet vehicle power needs withreduced fuel consumption. Downsizing refers to reducingtotal engine displacement. Downsizing the engine shiftsoperation from the solid line to the dashed line as shown inthe top-right graph of Figure 2. For a given vehicle power

requirement at constant speed, a downsized engine operatesat increased BMEP (specific load), which results in greateroverall efficiency and thus reduced fuel consumption.Downspeeding refers to reducing engine speed throughchanges to the transmission and/or final drive ratio.Maintaining a given vehicle power requirement at reducedengine speed also requires that the engine operate at higherspecific loads which again results in greater overall efficiencyand reduced fuel consumption. Viewed schematically on anengine speed-load map (see Figure 3), downsizing results inpurely vertical displacement to higher brake mean effectivepressures (BMEPs) at constant speed and downspeedingresults in a simultaneous upward and leftward movement tohigher load and lower engine speed.

Combining downsizing with downspeeding is particularlyeffective for reducing fuel consumption and CO2 emissions.However, sufficient torque is required across the enginespeed range, and particularly at lower speeds, to maintainvehicle gradeability and launch performance. Turbochargingis a well-known technology that uses exhaust energy to drivea compressor to increase charge air pressure. Withintercooling, turbocharging significantly increases themaximum mass of air delivered to each engine cylinder toincrease the maximum specific torque and specific power.Engine knock is a limiting factor for high load operation withturbocharging. Higher charge temperatures at increased loadsincrease the chemical reaction rates for auto-ignition. Atlower engine speeds, with increased cycle times, increasedreaction rates can lead to substantial knock and potentialengine damage.

GDi is an excellent complementary technology forturbocharged engines. Figure 4 shows a schematic for aturbocharged stoichiometric GDi engine. Major differencescompared to a baseline NA engine with multi-port fuelinjection (MPFI) are a higher pressure fuel system with 120 -200 bar typical maximum fuel pressure, and turbochargerwith intercooler. (We note here that the GDi fuel pump isdrawn to the side of the engine in Figure 4 for visual clarity.The pump is actually located near the top of the engine and isdriven by lobes on the camshaft.) Exhaust aftertreatment isaccomplished with a conventional 3-way catalytic converterfor stoichiometric engines. Due to the higher power densitiesprovided, turbocharged gasoline engines are being developedand implemented in production vehicles with increasedfrequency [7,8,9,10,11,12,13,14].

Figure 2. Effect of engine downsizing anddownspeeding.

Figure 3. Schematic representation of engine downsizingand downspeeding on an engine map.

<figure 4 here>

GDi offers a number of fundamental advantages that enableimproved engine performance compared to traditional portfuel injection. In-cylinder injection offers advantages duringengine warm-up. GDi improves fuel control compared to PFIin a cold engine when fuel vaporization characteristics arecompromised in the intake port. GDi also enables a splitinjection strategy during engine warm-up. Split injection canprovide a locally rich mixture near the spark plug. Thisimproves combustion robustness to enable greater sparkretard for catalyst heating while the globally leaner mixtureprovides reduced HC emissions compared to PFI [4, 5, 7, 9].

Additionally, GDi has key features that improve maximumtorque. First, injecting directly into the cylinder improves fuelcontrol and mixture motion to improve combustionefficiency. Direct injection also allows substantially betterscavenging in turbocharged engines at lower speeds and highload. Under these conditions, intake pressure is higher than

exhaust pressure, so appropriate valve overlap via camphasing allows intake air to blow through the cylinder with anopen exhaust valve to force additional residual gas into theexhaust. This process results in some of the intake charge toalso be exhausted. With MPFI, the intake charge comprises afuel-air mixture so that scavenging would lead to substantialamount of unburned fuel blown through to the exhaust.However with GDi, the fuel can be injected into the cylinderafter the exhaust valve has closed. The intake charge flowinginto the exhaust does not contain fresh fuel, and thusscavenging can be much more aggressive. In-cylinderinjection also results in charge cooling because the heat offuel vaporization is absorbed from the in-cylinder air mass.Scavenging and charge cooling provide increased volumetricefficiency and lower charge temperatures to reduce knock forimproved combustion phasing, efficiency and maximumtorque. This propensity to reduce knock can be augmented byuse of fuels containing increased alcohol content (such asE85) because these fuels have higher heat of vaporization andincreased octane number compared to gasoline [15,16,17].

GASOLINE DIRECT INJECTION FUELSYSTEMSGDi imposes substantially greater requirements on fueldelivery compared to MPFI. In an MPFI engine the fuel istraditionally injected onto the back of a closed intake valve.During standard, warmed-up MPFI engine operation, heatfrom the intake valve rapidly vaporizes the fuel in the portbefore the fuel and air are simultaneously inducted into thecylinder during the intake stroke. By contrast, fuelvaporization and mixing for GDi engines must occur rapidlyin the cylinder. This process is largely influenced by fuelspray characteristics such as droplet size and spraypenetration. These characteristics are achieved throughcareful injection system design with injectors operating atmoderate fuel pressures currently up to 200 bar.

Homogeneous GDi Fuel SystemsGDi operates in both homogeneous and lean stratified engineconfigurations. Figure 5 shows the major components forDelphi homogeneous GDi fuel systems comprising Multec 12inwardly-opening, multi-hole GDi injectors, a fuel rail and anengine-driven high pressure fuel pump. Key injectorrequirements for homogeneous GDi injection are thecapability to operate at fuel pressures up to 200 bar, goodlinearity over a wide flow range to ensure precise deliveryover the full engine map, and spray generation that providesgood vaporization and mixing without wetting in-cylindersurfaces. Injection is typically during the intake stroke toimprove vaporization and mixing. Stoichiometric operationwith homogeneous GDi allows the use of conventional 3-wayexhaust catalysts and thus worldwide application withoutconcerns for lean NOx production and aftertreatment.

Two different injector lengths are shown in the figure.Depending on the engine application, the injectors may be ineither side-mount (as shown in Figure 6) or central-mountconfigurations. The longer injector shown in the figure isrequired for some engines with central-mount injection. Side-mount injectors are frequently easier to package in an engine,but the off-axis mounting location makes uniform mixturepreparation more challenging and increases concerns forimpingement of the spray on the cylinder wall or piston topthat causes increased smoke emissions. Central-mountinjectors provide a more symmetric location that improvesmixing and generally reduces the potential for fuel dropletimpingement. However in-cylinder access through the headoften is prohibited due to packaging conflicts with thevalvetrain components and spark plug. Regardless of themounting location, multi-hole injectors produce distinct spraystreams from each hole as shown in Figure 7. Characteristicsof these streams are specific to a given engine to conform tospray targeting needs, and can differ substantially betweenapplications (see Figure 8). Designing the injector utilizesboth experimental and modeling tools to simultaneouslyoptimize the parameters required for spray formationappropriate to the specific engine [18].

Figure 5. Delphi homogeneous GDi fuel systemcomponents.

Figure 6. Side-mounted homogenous GDi.

<figures 7, 8 here>

Stratified GDi Fuel SystemsFor stratified GDi fuel systems, the characteristic fuelpressure is 200 bar, and the rail and pump characteristics aresubstantially similar between homogeneous and stratifiedconfigurations. However the injector required for stratifiedoperation is significantly different than the homogeneousmulti-hole injectors described above. Modern stratifiedsystems rely principally on spray characteristics forstratification. Because the fuel mixture burns with only aportion of the air mass in the cylinder, the fuel spray must becarefully controlled in both space and time so that a wellprepared, combustible charge is provided at the spark plugwhen it fires.

Figure 9 shows the Delphi Multec 20 stratified fuel injector.Side- and end-views of the spray are included in Figure 9 tohelp illustrate injector spray characteristics. Further detailsare available in [19]. As shown in Figure 10, the fuel injectoris centrally-mounted in close proximity to the spark plug, andfuel is injected during the compression stroke in a briefwindow around the timing of spark ignition. The injectoropens outwardly to produce a hollow cone, thin fuel sheet.When injected into elevated pressures characteristic of in-cylinder conditions during compression, the fuel sheet formsa recirculation zone to help place a combustible mixture atthe spark plug location. The circular white marker in Figure 9depicts the location of this spray recirculation zone. Modelingand simulation are used extensively in developing the injectorto ensure the spray characteristics are appropriately tailoredto the engine geometry [19, 20]. Multiple injection pulses aretypically used to reduce fuel penetration. Figure 9 shows thatthe fuel spray is better stratified in the area of the spark plug

by increasing the number of spray pulses from a single pulseto multiple, closely spaced fuel pulses. Thus, the injectordelivers multiple, closely spaced injection pulses with highprecision over a very short time window.

Stratified spray development can also have a significantimpact on ignition requirements. The fuel spray from theclosely-spaced injector imparts significant momentum andturbulence in the area of the spark gap around the time ofignition. This, combined with the substantial variations infuel air mixture associated with a stratified fuel charge, cancreate significant variability at the spark plug gap. A highenergy multi-charge ignition system can improve ignitionconsistency to reduce cycle-by-cycle combustion variability(see, for example [21, 22], for further discussion).

Figure 9. Delphi Multec 20 injector and spraycharacteristics.

Figure 10. Centrally-mounted stratified GDi.

3-CYLINDER VS 4-CYLINDERDOWNSIZED ENGINE EVALUATIONEngine downsizing can be accomplished via reduction in thenumber of cylinders, and/or a reduction in the displacedvolume of each cylinder. The value analysis in the nextsection begins with a 1.6L naturally-aspirated baselinevehicle and evaluates the benefit of 25% (modest)downsizing to 1.2L turbocharged engines withdownspeeding. Compared here are 4-cylinder naturally-aspirated configurations versus downsized turbocharged 3-cylinder applications.

Fuel consumption / CO2 and regulated emissions favorreduction in cylinder number. Weinowski et al [6] evaluatedthe brake specific HC emissions of 134 engines. While therewas a fairly wide scatter band, HC emissions plotted againstcylinder displacement showed a significant decrease in HCemissions with increasing cylinder volume (and thus fewercylinders at constant engine displacement). Removing acylinder results in smaller quench layers and crevice volume.This improves combustion efficiency for lower hydrocarbonemissions and reduced fuel consumption. Removing acylinder also reduces heat transfer surface area for a givendisplacement volume. Lower heat loss to the head andcylinder walls improves thermal efficiency for reduced fuelconsumption. Finally, fewer cylinders reduce friction for anadditional fuel consumption reduction. The overall effect ofreducing the number of cylinders on fuel consumption can besubstantial. Heil et al [23] evaluated downsized 2.2 Lturbocharged GDi engines configured either with 6-cylindersor 4-cylinders. Compared to a 6-cylinder 3.0 L naturally-aspirated baseline engine at a steady 10 KW operating point,downsizing to 2.2 L with 6-cylinders produced a 4%reduction in fuel consumption while downsizing to 2.2 L with4-cylinders reduced fuel consumption by 9% compared to thebaseline.

High load performance also favors a 3-cylinder enginebecause it offers better high load scavenging [2]. Scavengingefficiency in turbocharged engines is affected by thedifferential pressure between the intake and exhaust duringthe gas exchange process. With 4-cylinder engines, the firingfrequency is 180 deg. Consequently, while one exhaust valveis open near TDC during the gas exchange process, theexhaust valve for the next cylinder has already opened nearbottom-dead-center to begin the exhaust stroke. Initiation ofthe exhaust stroke creates a pressure wave that raises exhaustpressure and reduces scavenging for the cylinder undergoinggas exchange. The firing frequency for 3-cylinder engines is240 deg so that an additional exhaust valve is not open neartop-dead-center during gas exchange. Thus the three cylinderengine experiences a more favorable pressure differential andimproved scavenging to increase full load torque.

Finally, cost and packaging favor a 3-cylinder engine.Reducing cylinder count reduces the number of requiredcomponents leading to a direct reduction in cost.Additionally, the need for fewer components and the greatercylinder size offers packaging relief that can further reducecost and allow better optimization of the fuel injectorlocation.

One clear disadvantage to a 3-cylinder engine is degradedNVH. A 3-cylinder engine is inherently balanced for rotation.However, there are unbalanced 1st and 2nd order momentsalong the length of the engine. These unbalanced momentsinduce a front-back rocking motion, such that compensationshould be provided for a turbocharged 3-cylinder engine. Therequired measures include a counterbalancing shaft, andperhaps a modified engine mounting configuration.Counterbalancing results in sufficient compensation forcylinder displacements up to approximately 0.5 L, but resultsin increased cost, size and friction. Coltman et al [2]implemented roller bearings to the balance shaft in order tominimize friction effects.

Overall, for a downsized and turbocharged GDi engine a 3-cylinder configuration is favored for reduced fuelconsumption and regulated emissions. A 3-cylinder enginealso offers cost and packaging advantages and providesacceptable NVH with counterbalancing. We will thereforecarry forward a 3-cylinder mechanization as the 1.2 Lturbocharged engine considered in the value analysis below.

VALUE ANALYSISIn this section we evaluate CO2 emissions reduction potentialand OEM on-cost for 3-cylinder and 4-cylinder gasoline andDiesel powertrains in smaller vehicles (under 1400 kg). Welimit our scope only to powertrains and resultingaftertreatment needs. Additional cost measures related tonoise reduction, etc. are not included. For this analysis, wedefine the baseline to be an 1160 kg European gasolinevehicle with a 1.6L NA MPFI engine meeting Euro 4emissions standards. (Diesel powertrains are heavier, so CO2comparisons are made with 40 kg higher mass for Dieselvehicles.) Let us first consider attributes of this baselineengine compared to turbocharged Diesel engines in 2008Model Year. Figure 11 shows that the baseline gasolineengine and the turbocharged Diesel have comparable specificpower. The turbo-Diesels generate substantially greaterspecific torque. This enables a 25% longer gear ratio for theDiesel powertrains so that they generally operate at higherloads. Diesel vehicles from 2008 show approximately 24%lower CO2 emissions compared to baseline gasoline vehicle

To evaluate CO2 emissions reduction potential against thisbaseline, we look to the expected performance of futureturbocharged engines meeting substantially tighter emissions

standards (i.e. Euro 6). Figure 12 offers a look at theperformance attributes assumed in our analysis for thesefuture engines compared to series production vehicles oftoday. A survey of current production powertrains shows thatcurrent turbocharged vehicles deliver typical specific torquevalues of 150 N-m/L for both common rail Diesel and GDivehicles. The EU agreement defines first implementation ofEuro 6 standards in 2014. We expect moderate increases inspecific torque and power for turbocharged engines by thattime. Thus for our analysis we have assumed a 20% increasein specific torque for turbocharged vehicles to provide amaximum value of 180 N-m/L. Specific power for GDivehicles is assumed to be 80 kW/L and common rail Dieselsare assumed to deliver 65 kW/L. Clearly these values do notrepresent the “best in class” for future powertrains. In fact,engines exist today that exceed our assumed engineperformance for 2014, and we expect performance and fuelefficiency advancements to continue for GDi turbochargedengines by using cooled EGR [24 - 25] and other innovations.However, we have chosen performance estimates that weexpect to represent average vehicles 5 years from now usingsingle-stage turbocharging, no cooled EGR, andtransmissions designed to accommodate average torquelevels. These numbers are more appropriate for our analysisto better assess the value of the technologies for widespreadreduction of vehicle CO2 / fuel consumption and areappropriate for the smaller, lighter-weight, value-segment ofthe market.

Table 1 indicates the technologies considered in the analysis.The left-hand side of the table describes the enginetechnology. The columns in the center of the table offer arelative assessment of the major contributors to system cost.The right-hand section of the table estimates CO2 emissions(also depicted graphically in Figure 14). The first two rows inthe table comprise the 2008 gasoline and Diesel engines wehave discussed above meeting Euro 4 emissions. Theremaining rows consider turbocharged GDi and Dieselcommon rail technologies we project for smaller vehiclesmeeting Euro 6 emissions standards in 2014.

<figure 11 here>

<figure 12 here>

<table 1 here>

The major factors leading to cost differences between thepowertrain technologies fall into four categories: base engine,fuel system, air delivery system, and aftertreatment system.GDi fuel systems operate at maximum fuel pressures ofroughly 200 bar, resulting in a moderate cost increasecompared to MPFI. Fuel systems for the Diesel technologiesoperate at significantly elevated pressures (1500 - 2000 bar).This leads to a substantial increase in system cost for all

Diesel technologies compared to the baseline. Allturbocharged systems include a single stage turbochargerwith a cost premium compared to the baseline. The gasolinesystems also add a second cam phaser, and one homogeneousgasoline system includes 2-step variable valve actuation(VVA). Two-step is one VVA system that provides a cost-effective means to reduce pumping losses in both MPFI andGDi vehicles [26].

Considerable variability exists among aftertreatmentconfigurations for the technologies considered (see Figure13). The details of our aftertreatment analysis are proprietary.However, it should be noted that our efforts did includecatalyst volume and loadings needs for each configuration, aswell as the number and type of aftertreatment devicesrequired. The baseline gasoline and stoichiometric GDisystems rely on 3-way catalytic converters for aftertreatment.For the stratified lean gasoline system, a lean NOx trap(LNT) is added. Since turbo-Diesels normally operate underlean conditions, we considered a number of aftertreatmentsystem configurations to address NOx emissions. The Euro 4Diesel engine has only an oxidation catalyst. No NOx orparticulate aftertreatment devices are required to meet theseDiesel emissions standards. For Euro 6 Diesel engines, weconsidered three different aftertreatment combinations. Allthree combinations have kept the oxidation catalyst andadded a DPF. The simplest configuration includes no NOxaftertreatment device; a second has an added LNT, while athird configuration includes an SCR system.

<figure 13 here>

Diesel engine management systems are using increased fuelinjection pressures and injection strategies, higher EGR levelsand advancements in combustion control, so that engine-outNOx emissions continue to drop substantially (see, forexample, Schoeppe, et al [27]). Consequently, there is a goodchance that a number of smaller engines in compact vehicleswill be able to meet the upcoming Euro 6 emissions standardswithout NOx aftertreatment. However, US Tier 2 standardshave a more stringent NOx requirement with a much greaterlikelihood of needing NOx aftertreatment for all Dieselengines. And future NOx standards everywhere can beexpected to become continually more stringent. Especially fordownsized engines operating at higher loads that producegreater NOx emissions, meeting these tighter emissionsstandards without aftertreatment will be substantially moredifficult. Considering aftertreatment configurations with andwithout NOx aftertreatment in our study offers a valueassessment for a broader range of engines, and indicatessensitivity of the technologies to increasingly stringentemission standards.

Figure 14 shows the estimated effect of the various enginetechnologies on CO2 emissions over the NEDC drive cycle

for a compact (1160 kg baseline) vehicle. At the same enginedisplacement, downspeeding the turbocharged engine enablesa 9% reduction in CO2 compared to the baseline. Theremaining gasoline systems have been downsized to 3-cylinders. Stoichiometric 3-cylinder mechanizations offer upto 18% reduced CO2 with 2-step VVA. A turbocharged 3-cylinder stratified system offers up to 22% reduced CO2.

For the Diesel engines, we recall that a Euro 4 common railturbo-Diesel offers 24% reduction in CO2 compared to thegasoline baseline. Engine management system upgrades forEuro 6 (described previously) at the same enginedisplacement will offer some additional reduction in CO2emissions compared to a Euro 4 Diesel engine. Somevariation in CO2 occurs depending on NOx aftertreatmentchoice. Adding aftertreatment allows for more aggressivepursuit of CO2 reduction possible at higher engine-out NOxlevels. The SCR system offers the greatest overall reductionin CO2 because an LNT requires additional fuel to provideperiodic rich exhaust conditions to reduce NOx stored on theLNT. Downsizing the engine to 3 cylinders reduces CO2roughly 5% further for all NOx aftertreatment configurations.

An interesting result of across-the-board turbocharging anddownsizing is to reduce the CO2 emissions differencebetween gasoline and diesel powertrains. The 4-cylinderEuro-4 turbo-Diesel in our analysis emits 39 g/km less CO2than the baseline Euro-4 MPFI gasoline vehicle. Afterdownsizing both powertrains, and employing turbochargedGDi for gasoline, our analysis indicates the Diesel powertrainCO2 difference is roughly 25 g/km compared tohomogeneous GDi with 2-step VVA, and approximately 18g/km compared to stratified GDi.

<figure 14 here>

Figure 15 offers the final measure of value for thetechnologies. Plotted is CO2 reduction against vehiclemanufacturer (OEM) on-cost. By definition, the 4-cyl MPFIgasoline baseline point lies at the origin of this figure.Straight lines drawn through the origin represent lines ofconstant cost per unit CO2 reduction compared to thebaseline. Increasing slope indicates better value (greater CO2reduction per unit on-cost). The top graph considers onlyEuro 4 technologies - the gasoline MPFI baseline and atypical 4-cylinder Euro 4 turbo-Diesel. At the Euro 4emissions, level, a turbo-Diesel vehicle provides 24% CO2reduction at an on-cost rate of roughly 22 euros / percent. Thebottom graph shows a variety of Euro 6 mechanizations.Included are both gasoline and diesel powertrains with 3-cylinder and 4-cylinder engines. Overall, the Euro 6mechanizations that provide the greatest value for CO2reduction are the two stoichiometric 3-cylinder turbocharged

gasoline engines and the 3-cylinder turbo-Diesel applicationrequiring no NOx aftertreatment. These most favorablesolutions reduce CO2 at an on cost rate of 24 - 25 euros /percent. Downsized 3-cylinder engines offer improved valuecompared to 4-cylinder engines both through cost reductionand lower CO2 emissions. Whether gasoline or Diesel,systems implementing lean aftertreatment appear unfavorablebecause of the substantial added cost.

<figure 15 here>

SUMMARY AND CONCLUSIONSTurbocharged GDi engines are a key enabler for downsizingthat can substantially reduce CO2 emissions from gasolineengines. Good low end torque is critical to maintain gooddriveability with downsizing and downspeeding inturbocharged engines. Low end torque increases with GDibecause it allows improved scavenging efficiency and coolsthe intake charge to reduce knock. GDi also improves fuelcontrol and mixture motion to improve combustionefficiency.

For downsized engines, reducing the number of cylindersoffers advantages over simply reducing cylinder displacementvolume. In smaller vehicles, for a given engine displacement,3-cylinder engines provide less heat transfer surface area anda reduction in the quench layer and crevices for improvedcombustion efficiency and lower engine-out emissionscompared to 4-cylinders. Lower firing frequency per enginecycle with 3-cylinders reduces exhaust pressure pulsationsduring the gas exchange process providing better scavengingat high loads. Reduction in number of cylinders also results inreduced friction and lower cost, but introduces unbalancedtorque pulsations inducing a front-back rocking action of theengine. For higher specific outputs in turbocharged engines,counterbalancing compensations can provide acceptableNVH for cylinder displacements up to approximately 0.5liters. Consequently, 3-cylinder turbocharged engines areattractive for engines up to 1.5 L displacement.

Our value analysis estimates of OEM on-cost and CO2reduction considered 3-cylinder and 4-cylinder turbo-Dieseland turbocharged GDi mechanizations. For a compact vehiclemeeting Euro 4 standards, a 4-cylinder turbo-Diesel reducesCO2 emissions at an on cost rate of 22 euros / percent CO2reduction. Our analysis of compact turbocharged vehiclesconfigured to meet Euro 6 emissions in 2014 shows twogasoline configurations and one Diesel powertrainmechanization to have the best value. They reduce CO2 at anon cost rate of 24-25 euros / percent. The turbo-Dieselconfiguration is a downsized 3-cylinder engine with engine-out NOx that must be capable of meeting the Euro 6 NOxstandard without lean aftertreatment. Both preferred

turbocharged GDi applications are 3-cylinder stoichiometricengines with conventional three-way catalytic converters.

The turbocharged 3-cylinder stoichiometric GDi powertrainswe considered noticeably reduce the CO2 emissionsdifference between gasoline and Diesel powertrains. And thestoichiometric GDi solutions have substantially lower cost, afactor that can be particularly important for lower-pricedvehicles. The turbocharged 3-cylinder stoichiometric GDisolutions also employ a traditional 3-way catalytic converterto meet the most stringent NOx emissions regulations such asthe US Tier 2 standards and future standards being postulatedfor Euro 6+. Thus turbocharged 3-cylinder stoichiometricGDi powertrains offer a robust method to reduce CO2 withlow NOx for worldwide application in the compact vehiclesegment.

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CONTACT INFORMATIONJohn E. KirwanChief ScientistGas EMS and Powertrain ProductsDelphi [email protected]+1 248.836.1879

ACKNOWLEDGMENTSThe authors thank our following colleagues from Delphi forvaluable discussions and information: Walter Piock, KeithConfer, Dominique Mormont, Joseph Bonadies, and GalenFisher. We are also grateful to Timothy Johnson (Corning)and Ken Price (Umicore) for their feedback on aftertreatmentsystems:

NOMENCLATUREBMEP

Brake mean effective pressure

CCClose-coupled

DOXCDiesel oxidation catalyst

DPFDiesel particulate filter

EGRExhaust Gas Recirculation

GDiGasoline direct injection

LLiter

LNTLean NOx trap

MPFIMulti-port fuel injection

NANaturally aspirated

NVHNoise, vibration and harshness

SCRSelective catalytic reduction

TDCTop dead center

TWCThree-way catalyst

VVAVariable valve actuation

Figure 1. Rollout of light duty regulated emissions and fuel consumption / CO2 targets.

Figure 4. Turbocharged stoichiometric GDi engine schematic.

Figure 7. Schematic of spray streams for homogeneous GDi injectors.

Figure 8. Plan view of three different injector targeting patterns.

Figure 11. Performance attributes for 4 cylinder naturally aspirated gasoline MPFI and turbo diesel vehicles meeting Euro 4emission standards.

Figure 12. Specific load and specific power attributes for average engines assumed in the value analysis.

Figure 13. Aftertreatment system configurations.

Figure 14. CO2 reduction potential for turbocharged GDi and turbo-Diesel technologies.

Figure 15. CO2 reduction potential vs. OEM On-cost for turbocharged GDi and turbo-Diesel technologies.

Table 1. Technology effects on cost and CO2 emissions.

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ISSN 0148-7191

doi:10.4271/2010-01-0590

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