29MTZ worldwide 7-8/2003 Volume 64

MATERIALSTitanium

Part 1 dealt with the design and mechanical features of the V6 TDIengine [1]. This second part describes the thermodynamic develop-ment work that was necessary to comply with the extremely strin-gent Euro 4 exhaust emission limits in a large luxury car. The ambi-tious target of a power output of 132 kW from a displacement of 2.5

litres while remaining within the Euro 4 limits was achieved byadopting new or further optimised exhaust emission reductiontechniques.

1 Introduction and Task

The familiar 2.5-litre V6 TDI engine with apower output of 132 kW, which complieswith the Euro 3 exhaust emission limits [2],was used as a starting point and developedfurther with the aid of internal design mea-sures until it was capable of operating be-low the stringent Euro 4 exhaust emissionlimits. All pollutant constituents in the ex-haust gas were reduced, the reduction be-ing particularly great in the case of NOx andparticulates. The fuel injection system withits VP44 radial-piston distributor-typepump exhibited particularly promisingemission improvement potential while re-taining the engine’s high specific poweroutput. It also proved possible to achieve afurther significant improvement in the en-gine’s refinement.

2 Development Priorities

Until now, the only diesel engine passengercars to satisfy the stringent Euro 4 exhaustemission limits were in midsize cars orsmaller. Audi’s target was to comply withthis standard in the large-car category (fly-wheel mass class ≤ 4250 lbs) as well. Thehigh potential possessed by the 2.5-l V6 TDIengine enabled this challenging target tobe reached. This engine is used in the AudiA4 and Audi A6, and also in the VW Passatand Skoda Superb, in conjunction with amanual-shift gearbox with either front-wheel drive or quattro all-wheel drive, andalso with a conventional automatic trans-mission and with Multitronic.

Customer benefits in the form of perfor-mance on the road, response to acceleratorpedal movement and fuel consumptionwere further enhanced despite compliance

By Richard Bauder,

Sven Bechle,

Wolfgang Dorsch,

Hans-Werner Pölzl

Ralph Riegger and

Hans-Josef Schiffgens

Der neue V6-TDI-Motor

von Audi

Teil 2: Thermodynamik

You will find the figures mentioned in this article in the German issue of MTZ 7-8/2003 beginning on page 606.

The New AudiV6 TDI Engine

Part 2: Thermodynamics

30 MTZ worldwide 7-8/2003 Volume 64

3.2 Fuel Injection HydraulicsAs mentioned above, the fuel injection sys-tem with its VP 44 radial-piston distributor-type injection pump offered promising po-tential for the achievement of very lowemission values. Its high delivery rate andprecise volumetric measuring action areamong the relevant factors. The injectionsystem proves to be extremely robust andto exhibit excellent long-term stability.Compared with the Euro 3 version, injec-tion pressure at the nozzle was raisedagain, Figure 4. With injection pressures ofup to 2000 bar, the hydraulics are leading-edge in character. By increasing the deliv-ery rate it was possible to lower the flowthrough the nozzle and thus achieve a fur-ther improvement in mixture formation,which had a positive effect on particulateemissions.

The following detail aspects of the fuelinjection system were optimised:■ dynamism of the solenoid valve ■ minimized leakage at the distributorshaft ■ lower dual-spring injector holder ope-ning pressures ■ detail optimisation of the injector nozzle(rounded-off contour, reduced flowthrough nozzle).

An important factor in complying withthe low Euro 4 emission limits is ensuringvolumetric stability in the emissions area.To achieve this, functional improvementswere developed in the pump control unitarea. Special note should be taken of thestart-of-injection correction facility. Thispermits corrections to be made to the actu-ating time during the actual injectionprocess, in order to allow for scatter in thesolenoid valve closing times, and this con-siderably reduces scatter between onestroke and another.

In addition to the total injected volume,the pilot injected volume is of major impor-tance, particularly with regard to particu-late emissions and combustion noise. Thereduction in flow through the nozzle andmodification of the injector opening pres-sures enabled the pilot injection volume tobe reduced. Figure 5 shows the advantagepossessed by the Euro 4 version comparedwith the Euro 3 version in terms ofNOx/particulate trade-off at a selected op-erating point. The first major step forwardwas obtained with the introduction of theEuro 4 package of measures (combustionchamber, lower compression ratio, higherinjection pressures, EGR cooler), but the de-cisive change contributing towards Euro 4compliance was to reduce the pilot injec-tion volume.

A further development priority was toreduce scatter between individual exam-

DEVELOPMENT Diesel Engines

with the more severe exhaust emissionlimits. The engine’s high power output wasmaintained and its moderate fuel con-sumption either improved slightly or main-tained according to the version installed. Acontribution to this success, thanks to theresulting reduction in friction losses, wasmade by the adoption of cylinder headswith roller cam followers. The mechanicalnoise level and combustion noise were alsosignificantly reduced.

In addition, EOBD (European On-BoardDiagnostics) were installed for the firsttime on the Euro 4 engine in order to moni-tor the reliably functioning of all compo-nents with exhaust-emission relevance.

Figure 1 shows the areas in which ther-modynamic development work took place.

3 Thermodynamic OptimisationMeasures

In order to reach the Euro 4 from the Euro 3standard, untreated NOx and particulateemissions had to be approximately halved– an extremely challenging task in view ofthe already low Euro 3 emission levels. Theprincipal measures that were adopted aredescribed below.

3.1 Combustion ChamberThe combustion chamber shape and thecompression ratio have a decisive effect onemissions. For low-emission combustion,airflow in the combustion chamber and theinjection of fuel must be carefully matchedtogether. Optimised fuel injection, which isdiscussed in detail in Chapter 3.2, called forthe piston bowl geometry to be modified.The previous and new bowl patterns areshown in Figure 2.

A significant reduction in exhaust emis-sions compared with the Euro 3 combus-tion chamber could only be achieved bysystematic optimisation of the fuel spraycontact point, modification of the combus-tion chamber to suit the free fuel spraylength and a simultaneous reduction in thecompression ratio of 0.7 of a unit to 17.8:1.Figure 3 shows the improvements at select-ed stationary points. Improved mixture for-mation and the more efficient combustionobtained as a result have significantly low-ered NOx and in particular particulateemissions.

Subsequently, a further emission reduc-tion was obtained by optimising the appli-cation (start of injection, boost pressure,EGR rates, etc.) to suit the modified com-bustion chamber. At the same time, thechange in peripheral piston-bowl geometryfurther improved the piston’s mechanicalstrength.

ples of all fuel injection system compo-nents. Further development of the VP44 in-jection system could only be carried outsuccessfully because of close and dedicatedcooperation between Bosch and Audi.

3.3 Fresh Air and EGR SystemExternal exhaust gas recirculation (EGR) isknown to be an effective means of lower-ing NOx emissions. In order to achieve a fur-ther reduction in combustion temperature,a highly effective water-cooled EGR heatexchanger was therefore integrated intothe recirculation path. The cooled and recir-culated exhaust gas reduced emissions con-siderably by comparison with the non-cooled EGR [3]. At the same time, introduc-tion of the EGR cooler further improved thecompatibility of the combustion processwith exhaust gas recirculation. TheNOx/particulate trade-off in the dy-namometer test with the EGR cooler in usewas displaced as a whole towards a muchlower emissions level compared with thesystem not equipped with a cooler, Figure6.

In addition to the EGR cooler, the Euro 4engine has a position-controlled throttlebutterfly. This permits reduced cylinder fill-ing at low-load points, while retaining highlevels of exhaust gas recirculation. Figure 7shows the positive effect this has on NOxand particulate emissions. Based on theprevious Euro 3 ratings, the boost pressurewas initially lowered, which proved to re-duce particulate emissions distinctly but tocause a slight rise in NOx emissions. By ad-vancing the throttle butterfly, the deterio-ration in NOx emissions was more thancompensated for and the particulate emis-sions further improved. The EGR rate waskept almost constant. A further advantageis that EGR control stability is improvedand thus contributes to reliable compliancewith the exhaust emission and EOBD lim-its.

3.4 Exhaust Gas AftertreatmentIn addition to internal measures on the en-gine to reduce emissions, the oxidating cat-alytic converter was given an improvedcoating in order to reduce its light-off tem-perature still further and increase its long-term stability.

4 European On-Board Diagnostics (EOBD)

Engine management malfunctions and de-fective components can lead to a signifi-cant rise in pollutant emissions from pas-senger cars. The Euro 4 exhaust emissionlegislation not only lays down the type ap-proval values but also calls for permanent

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MATERIALSTitanium

monitoring of components relevant to ex-haust emissions by the engine manage-ment control unit. The driver is informed ofmalfunctions by a warning lamp and calledupon to take the car into the workshop,where computer-aided diagnosis permits arapid, localized repair to be carried out.

The following emission limits apply forEOBD:NOx ≤ 1,2 g/kmParticulates ≤ 0,18 g/kmCO ≤ 3,2 g/kmHC ≤ 0,4 g/km

Figure 8 is a schematic view of all thesensors and actuators needed for EOBD.

Monitoring of the air mass meter is ex-plained below as an example.

Precise diesel-engine combustion de-pends, among many other factors, on pre-cise metering of the recirculated exhaustgas. The mass of fresh air needed for this ismeasured by the air mass meter and regu-lated by the EGR valve.

In parallel with the air mass meter mea-surement, the air mass is calculated bymeans of the values available to the enginemanagement control unit: boost pressure,charge air temperature, engine speed andengine stall tendency. The calculated airmass is compared with the measured val-ue. Before the first deviations are of suffi-cient magnitude to exceed the EOBD ex-haust emission limits, the exhaust emis-sion warning lamp comes on to indicatethat this situation is about to arise. The reli-ability of this check has been confirmed bytests on a large number of vehicles in prac-tical operating conditions.

5 Results from the Engine andVehicle

Table 1 contains the technical data of theengine described here.

5.1 Full LoadSystematic development work in the areasof action described above enabled the Euro4 version of the engine to maintain the al-ready good full-load behaviour of the Euro 3version in terms of torque characteristic,power output and black smoke emissions,Figure 9. It thus has a specific power outputof 52.8 kW/l and a specific torque of 148Nm/l.

5.2 Fuel Consumption and PerformanceDespite optimisation to comply with theEuro 4 emissions levels, the minimum spe-cific fuel consumption across the operatingmap was held down to the existing value of204 g/kWh. Fuel consumption on the MVEGtest cycle was further improved. Table 2

shows by way of an example the values foran Audi A4 with manual-shift gearbox andquattro driveline. The improved responsefrom low engine speeds is demonstrated bya reduction in the time needed to acceleratefrom 60 to 120 km/h.5.3 Exhaust EmissionsFigure 10 shows the NOx and particulateemission values obtained for various vehi-cle and transmission versions with the Euro4 engine, compared with the Euro 3 enginepreviously available. It will be seen that theemissions have been reduced by half.

It should also be noted that the stablelong-term emission values typical of adiesel engine were again confirmed for theEuro 4 version by a series of endurancetests.

6 Summary

Audi’s competence in TDI engine design isclearly demonstrated by the emission lev-els achieved by the V6 TDI engine com-pared with competitors’ vehicles, Figure 11.This engine is so far the only one that alsosatisfies the severe Euro 4 limits in higherflywheel mass categories. Its low exhaustemissions have been achieved solely by in-ternal measures.

As a result, this V6 TDI Euro 4 engine canbe regarded as a successful combination ofextremely low emissions, excellent perfor-mance and good refinement with low fuelconsumption and therefore makes a fur-ther genuine contribution towards the pro-tection of our environment and our re-sources. ■