The Exhaust System of theBMW 325i SULEV / PZEV The US version of the BMW 325i is the first BMW to meet the world’s strictest emissionslimit, the SULEV (Super Ultra Low Emission Vehicle) standard. A central feature of the exhaustsystem, which was developed by Boysen as a full system partner of BMW, is the use of ultra-thinwall substrates from NGK.

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The requirements of the AmericanSULEV standard go beyond thelimitation of tailpipe emissions.Compliance with this strict stan-dard requires far-reaching func-tional and constructive measuresin the vehicle as a whole.

The constructive and function-al measures implemented by thedevelopers of the BMW 325iSULEV in order to comply withthe emissions standard are de-scribed by BMW in [1]. Within theframework of the BMW 325iSULEV project, Boysen was thedevelopment partner responsiblefor the exhaust system. The mono-liths were developed by NGK. Thisarticle focuses on the special fea-tures of the exhaust system.Above all, these include the cat-alytic converter system, which isfitted with ultra-thinwall ceramicmonoliths in order to optimise theexhaust treatment performance.

The SULEV StandardSULEV is the world’s most strin-gent emissions standard for pas-senger cars with conventional in-ternal combustion engines. Thestandard was developed by CARB,the California Air ResourcesBoard. It stipulates that part of thefleet of every vehicle manufactur-er must comply with the standardfrom 2008, at the latest. In themeantime, a number of other USstates have also decided to intro-duce the strict emissions limits [2].

The SULEV standard is thefifth stage of a process that goesback decades, during which emis-sions limits have been increasing-

ly tightened, Figure 1. Californiahas been a forerunner in the in-troduction of new exhaust emis-sion standards since the early1990s. In model year 1995, BMWfirst marketed cars that compliedwith the TLEV (Transitional LowEmission Vehicle) standard. In1998, the LEV (Low EmissionVehicle) standard came into force,stipulating a 40 % reduction in thelimit values for hydrocarbons to0.075 g/mile. Compliance with thesubsequent ULEV (Ultra LowEmission Vehicle) standard de-manded a further halving of thelimit values for hydrocarbons.Now, SULEV sets the upper limitfor hydrocarbons at 0.01 g/mile. Inorder to reduce pollutants by theamount required, the catalyticconverter must be able to reach itsoperating temperature (‘light-off’)within just a few seconds. TheBMW 325i SULEV not only com-plies with the SULEV limit valuesbut also fulfils the ZEV (ZeroEmission Vehicle) requirements.Compliance must be guaranteedfor the "full useful life” of the ve-hicle, i.e. for a period of 15 yearsor for 150,000 miles [2].

Like its predecessor model,which complied with the ULEVstandard, the BMW 325i SULEV isfitted with a 2.5-litre six-cylinderinline engine. The further devel-oped engine still has an output of135 kW (184 hp), and maximumtorque is also unchanged at 237Nm (175 ftlb) at 3500 rpm.

The constructive and function-al measures implemented by theengine developers in order to re-duce emissions to the SULEV lev-

Figure 1: SFTP emission standards over the ‘useful life’ ofthe vehicle, USA/California, for passenger cars and light-duty trucks (up to 3855 kg)

Figure 3: Relationship between pipe cross-section and gastemperature at the manifold exit

Figure 2: CAD representation of exhaust treatment system –BMW 325i SULEV

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el were primarily aimed at achiev-ing optimum mixture formation asa prerequisite for complete com-bustion and the ensuing low en-gine-out emissions [1]. The designof an internal combustion enginemeans that emissions are particu-larly high in the first few secondsafter a cold start. In order to com-ply with the emission standard, itis therefore essential that the cat-alytic converter reaches its light-off temperature as quickly as pos-sible, in order to ensure effectiveexhaust treatment.

Constructive andFunctional MeasuresOne functional measure, for ex-ample, was to improve the startsynchronisation with a view toachieving emissions-optimised in-jection timing. The mixture, injec-tion timing and ignition timingare regulated individually by elec-tronics for each working stroke. Aprecise choice of these engine pa-rameters achieved particularly lowengine-out emissions, especiallyin the first idle phase of the FTP-75 test cycle with an optimumlight-off of the catalytic converterat the same time.

The six-cylinder inline engineof the BMW 325i SULEV is fittedwith a dual-line exhaust system.

lower heat losses in the earlywarm-up phase – and thereforehigher gas temperatures at themanifold exit. The results of com-putation were supported by testson the real engine.

In order to increase the con-version performance to the re-quired level, both close-coupledcatalytic converters have a cas-cade design with ultra-thinwallceramic substrates, the first mono-lith in each case being consistent-ly designed with a high cell densi-ty and a minimal wall thickness.

The volume of the first sub-strate is 0.3 litres, and that of thesecond 0.77 litres. The frontmonolith has a cell density of1200 cpsi (cells per square inch)and a minimal wall thickness of2.0 mil (1 mil = 1/1000 inch). Therear monolith has 900 cpsi and awall thickness of 2.5 mil.

Compared with a thinwall sub-strate with 400 cpsi and 4.4 mil,the geometrical surface area of anultra-thinwall substrate with 1200cpsi and 2.0 mil is up to 77 % larg-er with the same volume, Figure 4.Furthermore, the ultra-thinwallsubstrate features a much lowerthermal mass. The corresponding-ly lower specific thermal capacitypromotes the fast light-off of thecatalyst [3, 4].

Using extensive CFD(Computational Flow Dynamics)

The front section consists of twosingle-wall steel manifolds, twoclose-coupled catalytic convertersin a cascade design and two un-derfloor catalytic converters,Figure 2. The rear section of thesystem comprises a middle andrear silencer for each line.

The Catalytic ConverterConcept In order to achieve the SULEVemissions limits, the passive cat-alytic converter concept for theULEV six-cylinder inline enginewas improved. The aim of furtherdevelopment was not only to min-imise the time until catalyst light-off, but also to increase the over-all efficiency of the catalyticconverter system.

The flow cross-sections of themanifold were optimised with re-gard to charge exchange. At thesame time, the large pipe diame-ters result in a reduction in flowvelocity and therefore to a lowerheat input into the manifold wall.Furthermore, the short flow dis-tances serve to minimise the heatloss from the exhaust gas. Figure3 shows some results of compara-tive calculations of the gas exittemperature as a function of theflow cross-section. According tothis, large cross-sections ensure

A dual-lineexhaust systemwhich uses ultra-thinwall sub-strates for meet-ing the strictestUS SULEV emis-sion standard.

Conversion performance

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calculations, the geometry of themanifold pipes was optimised tosuch an extent as to guarantee themost favourable impact of the ex-haust gas on the inlet of the firstcatalyst substrate in all load con-ditions.

In addition to the close-coupledcatalytic converters, the BMW 325iSULEV is fitted with two underfloorcatalytic converters. They eachhave two ceramic substrates with acell density of 400 cpsi and a wallthickness of 4.3 mil. The underfloorcatalytic converters guarantee thatthe strict HC and NOx limit valuesare met for a period of 15 years orfor 150,000 miles. The pressurelosses occurring in the catalyticconverter system are compensatedfor by an exhaust valve in the rearsilencer to such an extent that thereis no loss of power and acoustictargets are achieved.

CanningThe embedding of the monolith inthe catalytic converter shell is of

crucial importance for the long-term durability of a ceramic cat-alytic converter. In order to cancelout both the tolerances of the ce-ramic substrate and the differentcoefficients of thermal expansionof the monolith and the metalshell, the catalyst substrate is en-closed in a support mat. For thispurpose, Boysen uses a mat madeof polycrystalline fibres, which,unlike commonly used intumes-cent support mats, does not causean increase in mounting pressuredue to heat. From its pre-stress ina cold condition, the mat mustgenerate sufficiently high support-ing forces in every conceivable op-erating state and even with an un-favourable combination ofgeometrical tolerances. If the sup-porting forces were insufficient,the substrate would be free tomove in the shell and would quick-ly be destroyed. The maximumholding pressure is determined bythe load-bearing capacity of theceramic substrate. Ultra-thinwallceramic substrates react much

more sensitively to stress thanstandard monoliths. Compared to aceramic structure with 400 cpsiand 6.5 mil, the peak pressure foran ultra-thinwall ceramic substratecan be a maximum of 20 % [4].

A special stuffing technique isused for ‘canning’ the roundmonolith and the mat in the cat-alytic converter shell. A hydraulicpress forces the substrate togetherwith the mat through a funnel in-to the shell. Before this takes place,the weight and the actual pressurecapability of the mat are deter-mined and the maximum diameterof the monolith is optically meas-ured. The catalytic converter shellis individually expanded to pre-cisely match the dimensions of themonolith and the mat. If one of themeasured values is outside the tol-erance range, the part is rejected.The resulting tolerance of the gapinto which the mat is compressedis so tight that the set limits – min-imum holding force and peak pres-sure – can be achieved with maxi-mum reliability.

Figure 4: Relationship between geometrical surface area,isostatic compressive strength and specific thermal capacityfor four differently structured substrates [1,5]

Figure 5:Thermal imagingon the enginetest stand: themanifold temper-atures increaseto as much asH800° C underfull load

Underfloor cat-alytic converters

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TestingThe requirement of the SULEVstandard to meet the emission lim-it values for 15 years or 150,000miles goes far beyond that of theULEV standard, which prescribes aperiod of 10 years or 100,000miles. The problem of representingsuch long guarantee periods with-in a relatively short testing phase isexacerbated by the fact that engi-neers have hardly any experienceregarding the durability of exhaustsystems over such long periods oftime or mileages.

Most of the tests in the vehicleand on the engine test stand aredesigned to reproduce, in an inten-sified manner, the entire range ofloads that are to be expected dur-ing the vehicle’s lifetime. In orderto take account of the stricter re-quirements in the tests, the testprogramme had to be extendedand the test conditions had to bemade more severe, in some casesdrastically. In addition, specialcomponent tests with speciallymanufactured "bad samples” were

carried out. These tests took up tothree times the amount of timenormally required.

Summary andConclusions

The world’s strictest emissions lim-its, as prescribed in the SULEVstandard, require innovative ex-haust treatment solutions and closecooperation between vehicle man-ufacturers, system partners andspecialised suppliers. Compared tothe predecessor model, the meas-ures to comply with the standard inthe case of the BMW 325i SULEVresulted neither in a loss of powernor a reduction in maximumtorque. However, the additional ex-penditure both in research & devel-opment and in production leads toa significant increase in costs.

by Rainer Diez and Nils Rippert,Friedrich Boysen GmbH & Co. KG,Dr. Jürgen Wiehl, NGK EuropeGmbH.

[1] Liebl, J.; Hofmann, R.; Melcher, T.:Das BMW SULEV (PZEV) Konzept –Emissionsreduzierung ohneKompromisse - 23. WienerMotorensymposium 2002, S. 139-155

[2] CARB, California Exhaust EmissionStandards and Test Procedures for2001 and Subsequent ModelPassenger Cars Light-Duty Trucksand Medium-Duty Vehicles, 1999

[3] Schmidt, J.; Franz, J.; Merdes, N.;Brady, M.J.; Müller, W.; Lindner, D.;Bog, T.; Clark, D.; Buckel, T. Stöpler,W.; Henninger, R.; Ermer, H., Abe,F.; Makino, M.; Kunz, A.; Vogt, C.D.:Utilization of Advanced Three-Way-Catalyst Formulations on CeramicUltra Thinwall Substrates for FutureLegislation. SAE-Paper 2002-01-0349, 2002.

[4] Wiehl, J.; Vogt, C.D.; Abe, F.;Makino, M.: KeramischeUltradünnwandsubstrate:Technologien für zukünftigeEmissionskonzepte. Symposium“Entwicklungstendenzen beiOttomotoren 2002", Ostfildern, 5. -6. 12. 2002, S. 139ff.