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ENGINE DESCRIPTION: MODULAR DESIGN

The key development objective was to implement a modular design of the engine family, utilising many identical components across the various power and emission variants. The basic variant in the new engine generation is the 200 kW Euro 6. ❶ shows the relevant full-load curve. The combustion process, turbocharging and exhaust gas recirculation have been improved over the previous generation. In order to achieve the CO2 emission targets, the innovative thermo-management system and the engine’s friction loss were optimised. A major element of this was the integration of a fully variable oil pump into the oil

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circuit. This makes it possible to adapt the power consumption of the pump and the flow losses optimally to the specific engine load. An additional benefit is the demand-oriented application of oil circuit control for the engine warm-up phase. Increasing the swirl level and adjusting the timing maximised the efficiency of the 160 kW power variant. The lesser cooling requirement of the “small” V6 TDI permits downsizing of the water pump and oil cooler, to reduce the flow and heat losses further. A low-viscosity engine oil is used, by which further potential for reducing friction loss is realised. The ULEV125 variant for North America features optimised piezo-injectors

with reduced flow so as to improve the NOx/soot trade-off. The cooling power of the EGR system has been enhanced by the installation of a pre-cooler in addition to the main cooler. To safeguard combustion stability with the (lower cetane number) diesel fuels used in North America, the familiar cylinder pressure sensor based control of the centre of combustion is employed. The Euro 5 derivative is designed for use in countries where fuels have a sulphur content of up to 500 ppm. Highly sulphur-resistant catalytic coatings of the oxidation catalyst and the diesel particulate filter are used for these applications. In order to retain good starting capability, the compression ratio is increased to 1:16.8. This configuration

NEW GENERATION OF THE AUDI V6 TDI ENGINE PART 2: THERMODYNAMICS, APPLICATION AND EXHAUST CLEANING The new Audi V6 TDI range marks a major advance in V6-engines. The object of its development, in compliance with the future Euro 6 emissions standard, was to cut consumption while at the same time improving power and torque. The key factor in achieving the objectives was the integration of a modular, close-coupled exhaust gas cleaning system. A redesign of the chain drive freed up space which was fully utilised to deliver optimum exhaust gas cleaning with minimal CO2 emissions. The design and mechanical construction of the new V6 TDI were described in the first part of the article in MTZ 9 [1]. This second part details the thermodynamics, application and exhaust gas cleaning system of the new V6 TDI.

AUTHORS

DR.-ING. STEFAN KNIRSCH is Head of Development Powertrain at the Audi AG in Ingolstadt (Germany).

DIPL.-ING. ULRICH WEISS is Head of Development Diesel Engines at the Audi AG in Neckarsulm (Germany).

DIPL.-ING. STEFAN MÖHN is Head of Derived Application V6 Diesel Engines at the Audi AG in Neckarsulm (Germany).

DIPL.-ING. GIOVANNI PAMIO is Head of Thermodynamic V6 TDI Exhaust Gas Aftertreatment and OBD V-Diesel Engines at the Audi AG in Neckarsulm (Germany).

enables all known international market requirements to be met. COMBUSTION PROCESS AND FUEL INJECTION SYSTEM

To cut consumption while at the same time increasing power output to 200 kW for the Euro 6 basic version, Audi’s current four-valve combustion method had to be optimised. As focus points of the upgrade, the swirl and flow rate of the intake and exhaust ducts were redesigned and flow-optimised. This resulted in a significant improvement in filling while at the same time reducing charge cycle losses. The geometry of the piston bowl was additionally optimised, with a reduction in compression ratio to 1:16. In 10I2014

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❶ Full load curve 200 kW Euro 6 engine

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DE VELO PMENT DIESEL ENGINES

❷ Fuel consumption map of the new 3.0-l V6-TDI engine, consumption reduction in comparison to the predecessor engine generation [%]

conjunction with an adjusted injection jet orientation, the NOx/soot trade-off and fuel consumption were improved throughout the engine map, ❷. Optimum responsiveness from low revs was implemented by means of an adjusted inlet valve stroke curve and a redesign of the turbocharger. The predecessor engine’s 2000 bar fuel injection system was likewise upgraded. To increase the hydraulic pressure level on the injector nozzle, the throttle bore in the injector throttle plate was made 20 % larger. This enabled the engine power to be increased while the nozzle flow rate remained

unchanged. For the ULEV125 variant, a nozzle with a reduced hydraulic flow was specified, in order to additionally cut raw emission levels. The piston stroke has also been increased in order to increase power output further. AIR SYSTEM: TURBOCHARGER, AIR INTAKE, EXHAUST GAS RECIRCULATION

The new V6 TDI engine family marks the launch of a new turbocharger generation, ❸. Its key features are reduced play of the variable turbine geometry and a re duction in flow losses. This

❸ Turbocharger and comparison of the new VNT system to the predecessor generation

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enabled a power spread from 160 to 200 kW without having to employ differing turbocharger designs. Improved responsiveness of the engine and a marked improvement in driving dynamics can thus be assured for all application cases. The intake manifold in the inside V is of twin-pipe design, thereby ensuring load- and engine speed-dependent swirl control by the central swirl control flap. To optimise uniform distribution and charge cycle efficiency, much attention was paid to the cross-section, positioning and geometry of the EGR intake in the upstream charge air pipe. The EGR system was executed in a modular design to cover the various applications, ❹. The Euro 6 basic version comprises a higher-powered main cooler with integrated bypass. The ULEV125 variant additionally features a pre-cooler, likewise with a bypass. The pre-cooler is switched by a pneumatic valve. The EGR valve and main cooler bypass valve are electric, and are used in all variants across the engine family. There are thus four different cool power options for the application. The modular design concept means that an optimum solution to the conflict of aims between cooling power and pressure loss can be found for any application case. EXHAUST GAS CLEANING

Against the background of more stringent emissions legislation worldwide, the design of the exhaust system for the new V6 TDI was integral to the development of the new engine family. A key feature in achieving compliance with the latest emissions standards is the selective catalytic reduction of nitrogen oxide emissions. Audi first deployed this technology back in 2008 in the A4 and Q7 3.0-l V6 TDI [2] models. The SCR systems located away from the engine in the vehicle’s underbody have been continually upgraded since [3]. The continual reduction in fuel consumption over recent years has resulted in a significant loss of exhaust gas energy, so that underbody systems entail disadvantages in terms of conversion performance. ❺ shows the trend in exhaust gas temperature in the Audi A6 with the V6 TDI from engine generation 1 (Euro 4) through to the new generation (Euro 6). Consequently, the

file of the SCR catalytic converter. ❽ shows the distribution of the NOx conversion rate at the outlet of the SCR catalytic converter at 200 °C exhaust gas temperature. The launch of the 160 kW engine and front-wheel drive entails need for further action, as the efficiency of the drive results in additional temperature losses. Here, in place of the oxidation catalyst a NOx storage-type catalytic converter of the same volume supports the reduction in nitrogen oxides. The abbreviation NOC stands for NOx Oxidation Catalyst [6]. This combination enables the lowest pollutant emissions accompanied by

substantial CO2 savings. Application of an intelligent operating strategy delivers the following benefits: : optimum low-temperature activity of NOx storage : avoidance of heat-up measures (e.g. e-cat) which for purely SCR systems are necessary below a certain CO2 threshold : high NOx conversion rates at medium and high engine loads thanks to the SCR system. A specially developed NOC/SCR coordinator optimises the interaction of the NOC and SCR to minimise emissions with optimised CO2 emissions.

❹ Modular design of the EGR system

underbody layout has been gradually replaced by close-coupled SCR systems [4] or assisted by electric heating [5]. ❻ shows the development steps in recent years. The specific areas of development focus in designing the exhaust system, ❼, for the new generation of V6 TDI engines were: : minimal distance to the turbocharger outlet in order to reduce temperature loss : oxidation catalyst enlarged to 1.6-l : optional replacement of the oxidation catalyst by a NOx oxidation catalyst (NOx storage and oxidation of CO/HC) : enhancement of the SCR coating integrated on the highly porous DPF with the aim of creating a modular, unified exhaust system for future vehicle models : extended AdBlue mixing distance and application of a full-flow mixer to provide optimum preparation of the dosed AdBlue for high NOx conversion rates with minimal NH3 slip. A close-coupled AdBlue dosage system generally poses a major challenge owing to the restricted space. To that end, the spray was increased to six jets, with a spray angle of 25°. In conjunction with an oval full-flow mixer, outstanding uniformity of NH3 distribution was achieved. This ensures highly homogeneous conversion of the nitrogen oxides over the cross-sectional pro10I2014

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❺ Trend in exhaust gas temperature in the Audi A6 3.0-l V6 TDI

❻ Technological development steps about engines and exhaust gas aftertreatment systems (schematic view)

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❼ Modular exhaust gas aftertreatment system with “SCR at DPF”

ENGINE MANAGEMENT AND APPLICATION

The key objective of the application was to fully utilise the achieved potential for improvement in the various components

and modules in terms of consumption, performance and comfort. To cut CO2 emissions, it was necessary to translate the lower pressure losses of the air system into less charge cycle work. This was supported by the optimised

combustion process and sustained implementation of the downspeeding strategy. The seven-speed S-tronic gearbox fitted in the Audi A6/A7 has been configured longer in its overall transmission and equipped with a centrifugal pendulum absorber. This enables driving at engine speeds of less than 1000 rpm without any loss of comfort. Another major challenge for the application was control of the modular exhaust gas cleaning system. The combined exhaust gas cleaning comprising NOC and “SCR at DPF” in particular demanded new functional approaches. To cope with the wide variety of different operating states and switching operations, model-based air regulation was developed for the new V6 engine family. For this, the parameters relevant for running of the engine, such as the EGR rate or cylinder charge, are calculated based on the recorded sensor inputs (pressures, temperatures). In addition, the respective characteristics of the various actuators (e.g. turbocharger or EGR valve) are mapped in the engine control unit. This provides the basis for further development of model-based control structures. The advantage, in addition to improved control quality, is that no complex balancing of control parameters dependent on the environmental conditions and engine

❽ Development steps about improvements of the NOx conversion rate at SCR outlet by 200 °C exhaust gas temperature

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THANKS The authors thank Dipl.-Ing. (FH) Ralph Riegger, Head of Application V6 TDI B-/C series, Dr. rer. nat. Henning Lörch, Head of Exhaust Gas Aftertreatment, and Dipl.-Ing. Danilo Rossi, Head of Thermodynamic and Application V6 TDI, all Audi AG, for their support.

➒ CO2- and acceleration values of the Audi A6 200 kW quattro in comparison to competitors

operating states is required. Moreover, a finished application can be transferred to other vehicle models with comparatively little effort.

tion was necessary. A functionally optimised engine-gearbox interface was essential to achieving the combination of low-rev driving, improved engine responsiveness and fast gear-shifting. The performance and CO2 emission figures subsequently attained set a new benchmark within the direct competitive environment, ❾. ❿ shows the improvement in the characteristic data of the new Audi A6 compared to the predecessor model.

PERFORMANCE AND FUEL CONSUMPTION

The new V6 TDI is first deployed in the new A6/A7 model range. The version developing 200 kW and featuring the S-tronic transmission and quattro drive will be available initially. It will be followed later by the efficiency version developing 160 kW in combination with the new S-tronic front transmission [7]. To meet the demands of improved driving dynamics accompanied by reduced fuel consumption, intensive tuning of the engine and gearbox applica-

SUMMARY

With the new V6 TDI, Audi has updated its successful diesel engine family in line with future requirements. It has succeeded in implementing the lowest emissions standards such as ULEV125 and Euro 6 while at the same time cutting

AUDI A6 V6 TDI

AUDI A6 V6 TDI

PREDECESSOR MODEL

NEW GENERATION

S-tronic quattro

S-tronic quattro

Euro 5

Euro 6

Torque [Nm]

580

580

Power [kW]

180

200

Acceleration 0–100 km/h [s]

6.1

5.5

250 (limited)

250 (limited)

CO 2 emission (MVEG) [g/km]

156

133–138*

Consumption (MVEG) [I/100 km]

5.9

5.1–5.2*

Gear box Emission category

Max. speed [km/h]

* Depending on tyres

❿ Characteristic data of the new Audi A6 compared to the predecessor model 10I2014

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CO2 emissions, and also further improving performance. The modular design of the basic engine and exhaust system permits optimal integration into different vehicle concepts with differing gearboxes. As a result, the V6 TDI continues to guarantee outstanding levels of comfort, driving enjoyment and efficiency. REFERENCES [1] Knirsch, S.; Weiß, U.; Fröhlich, A.; Helbig, J.: New Generation of the Audi V6 TDI Engine, Part 1: Design and Mechanics. In: MTZ 75 (2014), No. 9 [2] Gruber, M.; Hatz, W.; Bauder, R.; Pamio, Z.-A.; Reuss, T.; Lörch, H.; Burkardt, A.: Der neue Audi 3,0 l V6-TDI mit Ultra Low Emission System [The new Audi 3.0l V6 TDI with Ultra Low Emission System]. 17 th Aachen Colloquium Automobile and Engine Technology, 2008 [3] Reuss, T.; Bauder, R.; Weiß, U.; Macher, A.; Lörch, H.; Pamio, G.: The new, second-generation Audi 3.0 V6 TDI EU6 – powerful and economical. 21st Aachen Colloquium Automobile and Engine Technology, 2012 [4] Weiß, U.; Lörch, H.; Pamio, G.; Bauer, R.; Kahrstedt, J.; Düsterdiek, T.; Schütte, T.; Kösters, M.: The new EU6 R4 and V6 TDI Engines from Volkswagen and Audi Integration of SCR Functionality in a Close Coupled Particulate Trap. 22 nd Aachen Colloquium Automobile and Engine Technology, 2013 [5] Lörch, H.; Möhn, S.; Weiß, U.; Haas, J.: Combination of electrically heated catalytic converter and SCR@DPF for challenging V-TDI projects. 14 th International Stuttgart Symposium, 2014 [6] Knirsch, S.; Weiß, U.; Fröhlich, A.; Pamio, G.; Helbig, J.; Ritter, H.: The new generation of the Audi V6-TDI engine 25 years of Technology – Dynamics – Innovation. 35 th Vienna Engine Symposium, 2014 [7] Knirsch, S.; Schöffmann, M.; Fleischmann, H.; Deimel, A.; Schumm, S.; Aldrian, W.: Die neue S tronic-Getriebegeneration von Audi. 35 th Vienna Engine Symposium, 2014

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