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At the end of 2002 BMW will be launching a Partial Zero Emission Vehicle (PZEV) onto theAmerican market. The vehicle's emissions levels are below the Super Ultra Low EmissionVehicle (SULEV) limits and thereby meet the most stringent emissions regulations in the

world. At the same time, however, typicalBMW characteristics, such as dynamic,comfort and, especially, driving pleasure,have been retained. The vehicle's power-train is based on the current 2.5 liter inlinesix-cylinder engine. Through targeted revi-sion and optimization of this power unit, ithas been possible to achieve a powertrainthat produces minimum emissions withoutcompromising on performance, torque andfuel consumption.

1 Motivation

To improve the air quality in California, therelevant environmental authorities (CARB)passed a Zero Emission Vehicle (ZEV) man-date. This stipulated that, from model year2003 on, 10 % of all new cars sold must beemission-free, in other words, they must beelectric vehicles. This proportion will be in-creased in stages to 16 % by 2018.

Medium-volume manufacturers (andBMW still falls into this category) can com-ply with the ZEV mandate in full using Par-tial Zero Emission Vehicles (PZEV). A vehicleis regarded as a PZEV if its emission levelsare below the Super Ultra Low Emission lim-its and if it produces no fuel evaporative

emissions at all, complies with the OBD IIregulations for on-board diagnostics, andguarantees all emissions-related compo-nents for a useful life of 15 years and a dis-tance of 150,000 miles (241,000 km). Themanufacturer can choose between variouspowertrain concepts, e.g. gasoline, naturalgas, hydrogen or hybrid power units. A vehi-cle with a gasoline engine scores a credit of0.2, i.e. five PZEVs must be sold instead of oneelectric vehicle. Thus the required proportionof PZEVs is 50 %, instead of 10 % for ZEVs.However, a phase-in system will be in forceup to 2006.

Each manufacturer has to decide whichalternative suits it best. BMW has opted foran inline 6-cylinder gasoline engine and

By Norbert Klauer,

Wolf Kiefer and

Bernd Ofner

Der Antrieb für den BMW 325i

SULEV – Ein Meilenstein in der

Emissionsreduzierung

You will find the figures mentioned in this article in the German issue of MTZ 9/2002 beginning on page 676.

The Powertrain forthe BMW 325i SULEVA Milestone in Emissions Reduction

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DEVELOPMENT Gasoline Engines

thereby for the typical BMW characteristicsof handling, comfort and, above all, sheerdriving pleasure. The current ULEV inline 6-cylinder engine was used as the basis for theSULEV engine.

2 The ULEV Inline 6-cylinderEngine

Since 2000, the 330i and 530i models for Cal-ifornia have been powered by an inline six-cylinder engine with emissions that are low-er than the ULEV (Ultra Low Emission Vehi-cle) limits, currently the most stringent. Thisengine required the incorporation of somespecial components and functions and somespecial operating modes for them. For themost part, these components and functionshave been retained for the SULEV engineand will be referred to briefly in the follow-ing remarks, Figure 1.

The ULEV engine's turbulence system hasexcellent mixture preparation in the idle andpart-load ranges and thereby facilitates astable, reproducible combustion processwith a high lean-burn capability and per-mits extremely retarded ignition during thecatalyst warm-up phase.

The infinitely adjustable inlet and ex-haust camshafts (double VANOS) facilitatean emission-optimized charge cycle with in-ternal exhaust gas recirculation. Raw emis-sions are already reduced to a very low levelthanks to a combustion chamber designvery suitable for the engine's bore/stroke ra-tio, and a piston with a top land of just 4 mmfor a low HC pollutant volume.

The two shell-type manifolds with mini-mum gas flow lengths and low surface areasreduce heat loss from the exhaust gas flowthrough the walls, thereby enabling the con-verter monoliths located next to the engineto heat up quickly. A cascade design favoringgood thermodynamic and fluid-dynamicperformance was used for the monolith con-verters.

A secondary air system is used to blow airinto the manifolds during the catalystwarm-up phase. This causes exothermal ox-idation of the unburned hydrocarbons,which serves both to reduce HC mass emis-sions and to achieve faster light-off andwarm-up of the two converter monoliths.When compared with engines without sec-ondary air system, the catalyst temperatureis already 100°C higher after just 20 s.

The lambda control probes for the ULEVengine are located close to the engine. Thisensures that gas travel time is reduced and,as a result, the mixture control system's re-sponse time is quicker. The central positionof the control sensors at a point where theducts converge offers the possibilty of deter-mination of the individual cylinder lambda.

3 Package of Measures to Achieve Emission Levelsbelow the SULEV Limit

To develop an engine with emission levelslower than the SULEV limits, it was neces-sary to optimize the entire chain of effectsencompassing both the spark-ignition com-bustion and exhaust gas treatment process-es. A distinction can be made here betweendesign measures and functional measures.The following explains in greater detail themain optimization steps in the sequence inwhich they take effect, Figure 2.

3.1 Design Measures to Reduce Raw EmissionsTo achieve low levels of raw emissions it isessential for the fuel to be burned complete-ly. In cold start conditions, however, the coldintake pipe and cylinder walls and the coldpiston surface result in poor evaporation ofthe liquid fuel. To improve mixture prepara-tion during this phase on a SULEV enginewith multipoint injection, the injection noz-zle hole diameter was reduced and the injec-tion pressure increased from 3.5 bar to 5 bar.This caused the average droplet velocity toincrease by 50 %, Figure 3. As a result of thehigher velocity, spray disintegration is im-proved and the droplet size (Sauter diame-ter) is reduced by 19 %. The result is fasterfuel evaporation and a better mixture prepa-ration. Doubling the number of nozzle holesfrom two to four and improving the nozzlegeometry helped to optimize spray targetingon the intake pipe and inlet valve positions.

To improve the starting process, the inletcamshaft is stopped by a hydraulic systemwhen the engine is switched off. At maxi-mum spread, the inlet valve would not closeuntil 54 °CA after BDC and would conse-quently expel a large proportion of thecharge again. The stop action (10 °CA beforethe end position) increases cylinder chargeat engine start by approx. 6 %, Figure 4. Thisresults both in better engine run-up, becauseof the higher torque, and also in faster evac-uation of the intake manifold. The latter isparticularly desirable for better mixturepreparation. If the intake pipe pressuredrops below the vapor pressure of the fuel,the volatile components of the fuel will be-gin to evaporate, even at low ambient tem-peratures. In addition, because the vacuumis available more quickly, the secondary airvalve is opened sooner, which is particularlyadvantageous where HC emissions are con-cerned.

One other major step towards a reductionin HC raw emissions has been developmentof the piston. Despite a very high power out-put of 55 kW/l (the same as the previous en-gine) and the high thermal and mechanical

loads that this entails, it has been possible toreduce the top land of the piston to just 3mm. As a result, the HC volume in this areafell by approx. 25 %. An optimized pistonring set keeps oil consumption at a very lowlevel. This is particularly necessary for reduc-ing HC emissions to a minimum, especiallyfrom the point of view of long-term emis-sion stability.

3.2 Functional Measures toReduce Raw EmissionsThe design measures shown were the basicessential requirements for a SULEV concept.In addition, raw emissions had to be reducedusing a host of functional measures. Here,the focus was on optimizing engine startand the early post-start phase. It is in thesephases that most emissions are released,since at this point, the catalytic convertersare still cold and raw emissions are quitehigh. Without suitable exhaust gas treat-ment measures, a BMW ULEV vehicle wouldexceed the LEV limits approx. 65 s after en-gine start. Within only 18 s the SULEV limit(aged type approval limit) could be exceed-ed, even though, at this point, the vehicle isstill stationary or idling in the FTP-75 test cy-cle.

For this reason, and from a functionalpoint of view, it was necessary to pursuenew paths. Unlike earlier approaches, con-sideration of emissions on a global or inte-gral level in the start and post-start phasesno longer sufficed for the SULEV engine. In-stead, each individual combustion processhad to be investigated in detail and opti-mized. This made the use of fast HC measur-ing techniques essential. Such techniquesmade it possible to carry out cycle-resolvedmeasurements of HC emissions in the ex-haust gas of the individual cylinders.

Figure 5 shows an example of cylinderpressure, engine speed and HC raw emis-sions curves for an engine start process thathas not yet been optimized. The first com-bustion process takes place on the third com-pression stroke (here cylinder 5) and demon-strates moderate combustion pressure be-cause of the poor fuel conversion. The upperhalf of the diagram shows the associated, cy-cle-resolved raw HC emissions (before thecatalytic converter). We can see that the poorcombustion process in cylinder 5 has causedan increase in HC emissions that remains inevidence for several working strokes and de-cays exponentially. Such detailed informa-tion enables conclusions to be drawn aboutthe causes of emissions and thereby formsthe basis for targeted optimization of thestart and post-start phases.

Another example of functional measurestaken to improve emissions is start synchro-nization. It has been possible to shorten the

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MATERIALSTitanium

synchronization time during the starterphase by revising the synchronizing func-tions in the engine management system.This facilitates emission-optimized injectiontiming for the first combustion processes. Asa result, injection into the open inlet valveand the high HC emission levels that thiscauses, and which are then evident over sev-eral cycles, is systematically avoided.

With the aid of high-resolution enginespeed recording, it has been possible to im-prove the engine speed gradient during theengine run-up phase. This facilitated target-ed optimization of the injection timing, theignition timing and the injection quantityfor each individual working stroke.

Through intelligent selection of engineparameters, namely engine speed, ignitionangle, Lambda curve, VANOS spread and airroute (main manifold, turbulence manifold),particularly during the first idle phase in theFTP-75 test cycle, it has been possible both toreduce engine load and also to shorten thecatalyst warm-up phase by faster catalyticconverter light-off. All these measures to-gether have resulted in a considerable reduc-tion in HC raw emissions, as can be seen inFigure 6.

3.3 Exhaust Gas TreatmentDespite the fact that a low level of raw emis-sions has been achieved, exhaust gas treat-ment continues to play a decisive role.

3.3.1 Secondary Air SystemTo reduce HC emissions and accelerate cat-alytic converter light-off behavior, the sec-ondary air system was optimized. The de-sired aim was to achieve a mass air flow ofas high a volume as possible and commenc-ing at an early stage. At the same time, how-ever, the power consumption of the pumpwas to be kept low.

In comparison with the ULEV engine, itwas possible to shorten the delay beforemeasurable amounts of secondary air flowinto the exhaust manifold by approx. 50 %.This was achieved by de-throttling the pipesystem and using a vacuum-opened sec-ondary air valve. At the same time, the ab-solute mass of the air flow was increased by15 %, although pump power output re-mained the same, Figure 7.

3.3.2 Linear Lambda Closed-loop ControlThe lambda control system, a central compo-nent of an exhaust gas treatment system,was thoroughly revised. Instead of the jump-action-jump sensors used in the ULEV en-gine, the SULEV engine has linear lambdasensors. The main advantage of the linearlambda sensor is its faster operational readi-ness, which means that the post-start and

warm-up phases in particular can operatewith a regulated mixture much earlier. Inaddition, the linearity of the sensor signal fa-cilitates shorter regulating times in the caseof temporary mixture mismatches, becausethe absolute deviation is always known. It isalso possible to achieve a more accurate oxy-gen balance in the lambda control systemand thereby better catalyst conversion per-formance. In addition, post-oxidation oxy-gen expulsion from the converter, for exam-ple, can be carried out more accurately andmore quickly by overrun fuel cut-off, whichreduces oxides of nitrogen.

3.3.3 Catalytic Converter ConceptAs the converters need to warm up veryquickly and the conversion process to startearly, the first monoliths of the cascade-structured converters, mounted near to theengine, featured high-cell technology andreduced wall thickness. The high cell densi-ty and thinner walls increase the effectivesurface area of the catalyst and at the sametime reduce thermal loads, Figure 8.

In addition to the cell structure, preciousmetal application and the coating composi-tion of the monoliths were also optimized.In the first monoliths, the emphasis was onfaster converter light-off; in contrast, thesecond monoliths were designed for ashigh a level of NOx conversion as possible.In view of the fact that the emission limitsmust be complied with for a period of 15years and 150,000 miles, the SULEV enginewas fitted with an additional underfloorcatalytic converter, the structure and coat-ing of which have been adapted to the spe-cific area of use.

3.3.4 Emissions Results andCompliance with LimitsAs can be seen from Figure 9, it has beenpossible to reduce the post-converter emis-sions significantly in the first phase of theFTP-75 test cycle. The hydrocarbon emissionsof the SULEV engine are so low that after just30 s, the concentrations are comparable withthose in ambient air. A similar result is ob-tained for NOx emissions, which are negligi-bly small.

The effectiveness of the measures andtheir contribution to the achievement of theaim are demonstrated in Figure 10, using hy-drocarbon emissions as an example. Thisanalysis is based on the current ULEV typeapproval limit. The aim is to reduce emissionlevels to below the SULEV limit, with a suffi-cient safety margin to ensure that durabilityrequirements are met.

The PremAir cooler of the 325i SULEV hasa special coating that is capable of convert-ing ozone into oxygen. The US legislator re-

wards this by increasing the limit for ozone-forming exhaust gas components.

3.3.5 DiagnosticsThe low SULEV limits also impose severe re-quirements on the OBD II functions. In par-ticular, the limit-related diagnostic proce-dures for lambda sensor and catalytic con-verter monitoring are technically consider-ably more complex than those required tocomply with ULEV legislation. With the ex-ception of secondary air diagnosis - whichrequires a hot-film air mass meter for directmeasurement - it has been possible to satisfyall the legal requirements without using ad-ditional sensors.

4 Engine Data

When compared with the previous engine,the design and functional measures thathave been implemented do not have any ef-fect on the engine's full load figures. Thiswas essentially achieved by optimizing thehigh-performance air intake system and de-throttling the secondary muffler. As a result,the customer identifies no change in on-roadperformance. Fuel consumption accordingto the US driving cycle also remains thesame as for the previous model. The engine'stechnical data are shown in the Table.

5 Summary

Next to Europe, the USA is BMW's most im-portant market. To secure the company'ssuccess here and expand its business in thelong-term, it was necessary to develop aSULEV (PZEV) model. The BMW inline 6-cylinder ULEV engine proved an excellentstarting point for this development. By usingvarious design and functional measures, ithas been possible to make the 325iA a SULEV(PZEV) model. In the process, no compromis-es had to be made in terms of on-road per-formance, fuel consumption or refinement.However, the necessary measures have ledto substantial increases in manufacturingcosts. The 325iA SULEV (PZEV) will be avail-able on the US market from autumn 2002,showing that BMW can still uphold its tradi-tional “sheer driving pleasure”, even with itsminimum emissions concept.

References

[1] Liebl, J.; Hofmann, R.; Melcher, T.: The BMWSULEV (PZEV) concept – emissions reductionwithout compromises, Vienna Engine Sympo-sium, 25th-26th April 2002

[2] Albrecht, F.; Fischer, H.-D.; Kiefer, W.; Mertl,R.; Otto, E.; Griebel, C.-O.: The technology ofthe new BMW six-cylinder engines, MTZ (61)2000, Nr. 9, pp. 536-548

[3] Liebl, J.; Otto, E.; Albrecht, F.; Zinecker, R.:The development of exhaust gas after-treat-ment concepts for future legislation, MTZ (58)Nr. 12, pp. 728-737