Titanium MATERIALS You will find the figures mentioned in this article in the German issue of MTZ 7-8/2003 beginning on page 606.
Der neue V6-TDI-Motor von Audi Teil 2: Thermodynamik
The New Audi V6 TDI Engine Part 2: Thermodynamics Part 1 dealt with the design and mechanical features of the V6 TDI engine [1]. This second part describes the thermodynamic development work that was necessary to comply with the extremely stringent Euro 4 exhaust emission limits in a large luxury car. The ambitious target of a power output of 132 kW from a displacement of 2.5 litres while remaining within the Euro 4 limits was achieved by adopting new or further optimised exhaust emission reduction techniques.
By Richard Bauder, Sven Bechle, Wolfgang Dorsch, Hans-Werner Pölzl Ralph Riegger and Hans-Josef Schiffgens
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1 Introduction and Task
2 Development Priorities
The familiar 2.5-litre V6 TDI engine with a power output of 132 kW, which complies with the Euro 3 exhaust emission limits [2], was used as a starting point and developed further with the aid of internal design measures until it was capable of operating below the stringent Euro 4 exhaust emission limits. All pollutant constituents in the exhaust gas were reduced, the reduction being particularly great in the case of NOx and particulates. The fuel injection system with its VP44 radial-piston distributor-type pump exhibited particularly promising emission improvement potential while retaining the engine’s high specific power output. It also proved possible to achieve a further significant improvement in the engine’s refinement.
Until now, the only diesel engine passenger cars to satisfy the stringent Euro 4 exhaust emission limits were in midsize cars or smaller. Audi’s target was to comply with this standard in the large-car category (flywheel mass class ≤ 4250 lbs) as well. The high potential possessed by the 2.5-l V6 TDI engine enabled this challenging target to be reached. This engine is used in the Audi A4 and Audi A6, and also in the VW Passat and Skoda Superb, in conjunction with a manual-shift gearbox with either frontwheel drive or quattro all-wheel drive, and also with a conventional automatic transmission and with Multitronic. Customer benefits in the form of performance on the road, response to accelerator pedal movement and fuel consumption were further enhanced despite compliance
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DEVELOPMENT
Diesel Engines
with the more severe exhaust emission limits. The engine’s high power output was maintained and its moderate fuel consumption either improved slightly or maintained according to the version installed. A contribution to this success, thanks to the resulting reduction in friction losses, was made by the adoption of cylinder heads with roller cam followers. The mechanical noise level and combustion noise were also significantly reduced. In addition, EOBD (European On-Board Diagnostics) were installed for the first time on the Euro 4 engine in order to monitor the reliably functioning of all components with exhaust-emission relevance. Figure 1 shows the areas in which thermodynamic development work took place. 3 Thermodynamic Optimisation Measures
In order to reach the Euro 4 from the Euro 3 standard, untreated NOx and particulate emissions had to be approximately halved – an extremely challenging task in view of the already low Euro 3 emission levels. The principal measures that were adopted are described below. 3.1 Combustion Chamber
The combustion chamber shape and the compression ratio have a decisive effect on emissions. For low-emission combustion, airflow in the combustion chamber and the injection of fuel must be carefully matched together. Optimised fuel injection, which is discussed in detail in Chapter 3.2, called for the piston bowl geometry to be modified. The previous and new bowl patterns are shown in Figure 2. A significant reduction in exhaust emissions compared with the Euro 3 combustion chamber could only be achieved by systematic optimisation of the fuel spray contact point, modification of the combustion chamber to suit the free fuel spray length and a simultaneous reduction in the compression ratio of 0.7 of a unit to 17.8:1. Figure 3 shows the improvements at selected stationary points. Improved mixture formation and the more efficient combustion obtained as a result have significantly lowered NOx and in particular particulate emissions. Subsequently, a further emission reduction was obtained by optimising the application (start of injection, boost pressure, EGR rates, etc.) to suit the modified combustion chamber. At the same time, the change in peripheral piston-bowl geometry further improved the piston’s mechanical strength.
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3.2 Fuel Injection Hydraulics
As mentioned above, the fuel injection system with its VP 44 radial-piston distributortype injection pump offered promising potential for the achievement of very low emission values. Its high delivery rate and precise volumetric measuring action are among the relevant factors. The injection system proves to be extremely robust and to exhibit excellent long-term stability. Compared with the Euro 3 version, injection pressure at the nozzle was raised again, Figure 4. With injection pressures of up to 2000 bar, the hydraulics are leadingedge in character. By increasing the delivery rate it was possible to lower the flow through the nozzle and thus achieve a further improvement in mixture formation, which had a positive effect on particulate emissions. The following detail aspects of the fuel injection system were optimised: ■ dynamism of the solenoid valve ■ minimized leakage at the distributor shaft ■ lower dual-spring injector holder opening pressures ■ detail optimisation of the injector nozzle (rounded-off contour, reduced flow through nozzle). An important factor in complying with the low Euro 4 emission limits is ensuring volumetric stability in the emissions area. To achieve this, functional improvements were developed in the pump control unit area. Special note should be taken of the start-of-injection correction facility. This permits corrections to be made to the actuating time during the actual injection process, in order to allow for scatter in the solenoid valve closing times, and this considerably reduces scatter between one stroke and another. In addition to the total injected volume, the pilot injected volume is of major importance, particularly with regard to particulate emissions and combustion noise. The reduction in flow through the nozzle and modification of the injector opening pressures enabled the pilot injection volume to be reduced. Figure 5 shows the advantage possessed by the Euro 4 version compared with the Euro 3 version in terms of NOx/particulate trade-off at a selected operating point. The first major step forward was obtained with the introduction of the Euro 4 package of measures (combustion chamber, lower compression ratio, higher injection pressures, EGR cooler), but the decisive change contributing towards Euro 4 compliance was to reduce the pilot injection volume. A further development priority was to reduce scatter between individual exam-
ples of all fuel injection system components. Further development of the VP44 injection system could only be carried out successfully because of close and dedicated cooperation between Bosch and Audi. 3.3 Fresh Air and EGR System
External exhaust gas recirculation (EGR) is known to be an effective means of lowering NOx emissions. In order to achieve a further reduction in combustion temperature, a highly effective water-cooled EGR heat exchanger was therefore integrated into the recirculation path. The cooled and recirculated exhaust gas reduced emissions considerably by comparison with the noncooled EGR [3]. At the same time, introduction of the EGR cooler further improved the compatibility of the combustion process with exhaust gas recirculation. The NOx/particulate trade-off in the dynamometer test with the EGR cooler in use was displaced as a whole towards a much lower emissions level compared with the system not equipped with a cooler, Figure 6. In addition to the EGR cooler, the Euro 4 engine has a position-controlled throttle butterfly. This permits reduced cylinder filling at low-load points, while retaining high levels of exhaust gas recirculation. Figure 7 shows the positive effect this has on NOx and particulate emissions. Based on the previous Euro 3 ratings, the boost pressure was initially lowered, which proved to reduce particulate emissions distinctly but to cause a slight rise in NOx emissions. By advancing the throttle butterfly, the deterioration in NOx emissions was more than compensated for and the particulate emissions further improved. The EGR rate was kept almost constant. A further advantage is that EGR control stability is improved and thus contributes to reliable compliance with the exhaust emission and EOBD limits. 3.4 Exhaust Gas Aftertreatment
In addition to internal measures on the engine to reduce emissions, the oxidating catalytic converter was given an improved coating in order to reduce its light-off temperature still further and increase its longterm stability. 4 European On-Board Diagnostics (EOBD)
Engine management malfunctions and defective components can lead to a significant rise in pollutant emissions from passenger cars. The Euro 4 exhaust emission legislation not only lays down the type approval values but also calls for permanent
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Titanium
monitoring of components relevant to exhaust emissions by the engine management control unit. The driver is informed of malfunctions by a warning lamp and called upon to take the car into the workshop, where computer-aided diagnosis permits a rapid, localized repair to be carried out. The following emission limits apply for EOBD: NOx ≤ 1,2 g/km Particulates ≤ 0,18 g/km CO ≤ 3,2 g/km HC ≤ 0,4 g/km Figure 8 is a schematic view of all the sensors and actuators needed for EOBD. Monitoring of the air mass meter is explained below as an example. Precise diesel-engine combustion depends, among many other factors, on precise metering of the recirculated exhaust gas. The mass of fresh air needed for this is measured by the air mass meter and regulated by the EGR valve. In parallel with the air mass meter measurement, the air mass is calculated by means of the values available to the engine management control unit: boost pressure, charge air temperature, engine speed and engine stall tendency. The calculated air mass is compared with the measured value. Before the first deviations are of sufficient magnitude to exceed the EOBD exhaust emission limits, the exhaust emission warning lamp comes on to indicate that this situation is about to arise. The reliability of this check has been confirmed by tests on a large number of vehicles in practical operating conditions.
MATERIALS
shows by way of an example the values for an Audi A4 with manual-shift gearbox and quattro driveline. The improved response from low engine speeds is demonstrated by a reduction in the time needed to accelerate from 60 to 120 km/h. 5.3 Exhaust Emissions
Figure 10 shows the NOx and particulate emission values obtained for various vehicle and transmission versions with the Euro 4 engine, compared with the Euro 3 engine previously available. It will be seen that the emissions have been reduced by half. It should also be noted that the stable long-term emission values typical of a diesel engine were again confirmed for the Euro 4 version by a series of endurance tests. 6 Summary
Audi’s competence in TDI engine design is clearly demonstrated by the emission levels achieved by the V6 TDI engine compared with competitors’ vehicles, Figure 11. This engine is so far the only one that also satisfies the severe Euro 4 limits in higher flywheel mass categories. Its low exhaust emissions have been achieved solely by internal measures. As a result, this V6 TDI Euro 4 engine can be regarded as a successful combination of extremely low emissions, excellent performance and good refinement with low fuel consumption and therefore makes a further genuine contribution towards the protection of our environment and our resources. ■
5 Results from the Engine and Vehicle
Table 1 contains the technical data of the engine described here. 5.1 Full Load
Systematic development work in the areas of action described above enabled the Euro 4 version of the engine to maintain the already good full-load behaviour of the Euro 3 version in terms of torque characteristic, power output and black smoke emissions, Figure 9. It thus has a specific power output of 52.8 kW/l and a specific torque of 148 Nm/l. 5.2 Fuel Consumption and Performance
Despite optimisation to comply with the Euro 4 emissions levels, the minimum specific fuel consumption across the operating map was held down to the existing value of 204 g/kWh. Fuel consumption on the MVEG test cycle was further improved. Table 2
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