You will find the figures mentioned in this article in the German issue of MTZ 10/2005 beginning on page 744.
Der neue 4,2-l-TDI-V8-Motor von Audi Teil 1: Konstruktion und Mechanik
The New 4,2-l-TDI-V8 Engine from Audi Part 1: Design and Mechanics Audi’s 4,2-l-V8-TDI engine reveals the consistent onward development of the 4,0-l-TDI engine that hitherto had been offered in the Audi A8. The new V8 TDIs are characterised by very high engine power – 240 kW – and torque of 650 Nm. By using a CSF diesel particle filter the EU 4 exhaust gas norm is satisfied. In the new engine, the cylinder gap is increased from 88 to 90 mm and the injection system converted to piezo technology. The exhaust recirculation system, combustion and turbo-charging have been further optimised. The development of the new engine is documented in two reports. The first part describes the design and mechanics, the second part the thermodynamics, application and exhaust-gas treatment.
1 Introduction
Authors: Manfred Bach, Richard Bauder, Andreas Fröhlich, Henning Hoffmann, Steffen Muth and Gerd Seifried
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The story of the Audi 8-cylinder diesel engines started in 1993 with an initial concept study that was presented at the IAF. In 1999 the first V8 TDI went into production with the 3.3-l engine. In the run-up to the current Audi A8, in 2003 the 4,0-l engine with 202 kW and 650 Nm of torque was deployed [1]. The new 4,2-l engine represents consistent development in the direction of the Audi-V family of engines and of enhanced performance and is initially offered in the Audi A8. Its 240 kW of power and 650 Nm of torque will meet the sportiest of demands in the premium sector. In conjunction with a double-flow CSF particle filter system the EU 4 exhaust gas norm is satisfied. With this new engine all Audi V diesel engines have now been converted to chain
drive and a cylinder gap of 90 mm. The V8 TDI operates according to the Audi-4 valve process already familiar from the V6-3,0-lTDI [2]. It possesses a third-generation Bosch common-rail system and twin exhaust turbochargers with third-generation variable turbine geometry by Honeywell-Garrett. The recirculated exhaust gases are cooled by an EGR-cooler switched according to the engine’s operating point. The Audi-V family of engines feature many equivalent parts and multiple synergies as well as development on a par with petrol and diesel engines with various numbers of cylinders [3]. They are characterised by their very short and compact construction and their maintenance-free chain drive on the transmission side. Whereas the 4,0-l engine with a cylinder gap of 88 was an intermediate step, the 4,2-l engine with a
cylinder gap of 90 mm is a fully-fledged member of the engine family. Hence it was possible to extend component and manufacturing synergies.
2 Main Points of Development The further development of the V8 TDI was purposefully directed at meeting the following principal requirements: – lowest emissions in accordance with the EU 4 exhaust norm with CSF diesel particle filter – increase in the cylinder gap from 88 to 90 mm and hence full integration into the Audi-V family of engines – further increase in the 'sporty' feel by raising performance and improved driving dynamics in the vehicle while still achieving excellent consumption figures – further optimisation of acoustics and driving comfort – compact, short construction in combination with very low engine weight. Very high requirements were set for the 4,2l engine in terms of technology and economy. Additionally, the very short development time of only 24 months from project launch to SOP presented a major challenge. This could only be overcome with the efficient use of existing synergies within the Audi-V family of engines and with rigorous project management.
3 Engine Description By increasing the cylinder gap it was possible to increase the cylinder bore by 2 mm while retaining the same web-width. The piston stroke was not altered so that the cubic capacity rose from 3936 cm3 to 4134 cm3. The maximum power output is 240 kW. A maximum torque of 650 Nm is available in the revolution range 1600 to 3500/min. Conditioned by the engine package already familiar in the predecessor with transmission chain-drive and lateral twin-turbo arrangement, it was possible to achieve an extremely short distance of only 520 mm between the transmission flange and the front belt-pulley damper. This dimension plus various optimisations, mainly in the engineblock components, enabled the weight of a V8 TDI to be reduced to a very low 255 kg. The preceding engine was 15 kg heavier. In order to attain the high power and torque values, the Audi 4-valve combustion process already familiar from the V6-3,0-lTDI was optimised with the third-generation common-rail system. Particular attention was given to the EGR components with an eye to the EU 4 emis-
4,2-l-TDI-V8 Engine from Audi
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sion category. The two EGR valves are operated electrically and the EGR cooler is switchable according to the engine’s operating point. The good consumption values were achieved inter alia by reducing friction in various components. Further technical data can be derived from the Table .
diameter of 60 is also deployed on the new V8 engine. The strength of the journal radii is increased by means of a rolling process. The crankshaft is spatially crimped so that free forces and 1st and 2nd order moments are avoided. Owing to limited flank widths and counterweight radii in the compact form of construction, it is not possible to effect balancing of the crank drive solely with counterweights on the crankshaft. By using additional masses in the oscillation damper and tappets, optimal balancing was attained. The belt-pulley damper with decoupled poly-V belt-track of the predecessor engine was replaced in the 4,2-l-TDI with a plain torsion oscillation damper, Figure 3. Owing to the simplified geometry of the plain torsion damper, greater degrees of freedom are obtained in dimensioning the rubber track and the balancing mass in the available space. Hence optimal damping of torsion effects was able to be achieved. The measured extent of torsion moments of the torsion oscillation damper in comparison with the belt-pulley damper used previously is displayed in Figure 4. In this way a reduction of 13 % was attained with a maximum torsion moment. In addition to significantly lower crankshaft loading, this result also has a positive effect on engine acoustics. The oscillatory loading of the poly-V beltdrive increased by the omission of the beltpulley damper is compensated by using pacifier rollers and generator freewheeling.
3.1 Crankcase When designing the crankcase and the main bearings the tried and tested principle of the preceding engine was adopted, Figure 1. The crankcase is divided along the centre of the crankshaft. The five main bearing covers are combined with a very rigid bearing frame of spheroidal cast iron (GJS-600). The material of the housing uses a vermicular graphite casting (GJV-450). A web-width of 7 mm between the cylinders with a simultaneous potential for low component weight is only possible owing to the high basic strength of the vermicular graphite casting. In very close collaboration with the rawmaterial supplier it was possible to lower the weight of the crankcase incl. bearing frame by 10 kg in comparison with the predecessor to 62 kg. In essence, this could be achieved by rigorously minimising wall strengths and the skilful integration of cast channels. The honing process by exposure to UV laser light, already familiar from the V6-3,0l-TDI, was also used for this engine. The advantages are reduced oil consumption and a clear improvement in the frictional qualities of the operating surfaces. The upper oil-sump of pressure-cast aluminium is free from direct effects of combustion owing to the free-standing bearing frame. The overall design selected also offers significant acoustic advantages.
3.2 Crank Drive and Belt Drive Particular attention was given to piston design during development. The high thermal loading was borne by optimal calculations for piston-cooling, Figure 2. Hence the geometry of the 'salt-core' cooling channel was arranged variably with respect to loadings generated and for the most efficient cooling possible both at the bowl edge and at the first ring-groove. Optimisation of the burn process resulted in a piston bowl diameter of 52 mm and compression of 16.5. With the 5-bearing crankshaft of 42CrMoS4, the tried and tested main bearing of the predecessor engine with a diameter of 65 mm was adopted. The big-end bearing, familiar from the V6-3,0-l-TDI, with a journal
3.3 Chain Drive, Auxiliary Unit Drive The layout of the 4-stage chain drive on the transmission side for driving the camshaft and the oil-, water and power-steering pump is the main characteristic of the Audi-V family of engines and was essentially adopted from the 4,0-l-TDI engine, Figure 5. The encapsulation of the 4 simplex chain drives in only 2 chain tracks contributes decisively to the short dimension of the engine. The translation of drive B was modified. Via an intermediate gear it drives the inlet camshaft on the left side and, via an extra maintenance-free toothed belt, it also drives the HP pump and hence is under maximum load. This step was necessary as the suction control of the high-pressure pump was deployed to optimise consumption across the complete rpm range. This led to enhanced rotary oscillation effects. Figure 6 shows the rotary oscillation amplitudes against rpm in comparison with the predecessor engine. It is clear that, despite the omission of a visco-damper and activation of suction control, the amplitudes are at almost the same level as for the predeMTZ 10/2005 Volume 66
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4,2-l-TDI-V8 Engine from Audi
cessor engine. The chain forces also measured via telemetric data transmission show that the design of the simplex chains lies within the fatigue-strength safety margin.
3.6 Addition of Injection Components
3.4 Cylinder Head and Valve Drive The synergies between the V6 and V8 TDIs are most clearly seen in the cylinder head unit. Hence, both cylinder heads of the new V8 are based on the tried and tested design principle of the V6 as regards channel arrangement and valve drive, Figure 7. The full valve drive incl. the roller cam follower and the tensed gearwheels were adopted from the V6 to compensate for gear side play in the camshaft drive. The camshafts are designed as built shafts with pre-formed barrels as body material. The individual cams are connected to the barrel by a combined form-fit and interference-fit connector. The head is cooled by the transverse cooling design established in the meantime for Audi-V TDI engines for optimal cooling of all cylinders. On the transmission side an EGR channel is integrated into the cylinder head. This space-saving routing of exhaust gases from the exhaust manifolds to the inner-V also entails pre-cooling of the exhaust gases by means of the coolant water present in the cylinder head. For the meantime, the tried and tested principle of the camshaft bearing frame with a flat, decoupled valve cover of synthetic materials is the standard on Audi's V-TDI engines. This design principle has sealing and acoustic advantages.
3.5 Friction During the further development of the V8 TDI engine, steps were also taken to improve mechanical efficiency. Despite an increase in cubic capacity there has been a reduction in the absolute resistance in the engine. The following measures contribute to this improvement: – Use of UV-light honing of the cylinders bores, familiar from the V6-TDI-3,0-l engine – Optimised piston and ring packet – Adoption of the valve arrangement from the V6-TDI-3,0-l engine – Optimisation of the chain drive. By adopting auxiliary units such as oil, water and servo pump from the 4,0-l-TDI and increasing the cubic capacity, there has been a further improvement in friction mean effective pressure. Figure 8 shows friction mean effective pressure in the new engine in comparison with the predecessor over the rpm range with an oil temperature of 90 °C. An improvement of up to 15 % has been achieved. 4
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The third-generation common-rail system by Bosch with up to 1600 bar injection pressure with piezo-inline injectors is used as the injection system, Figure 9. The 3-plunger high-pressure pump sits behind the inner-V. Fuel is channelled from the pump to the piezo-injectors via both rails. It was possible to omit the distributor block used in the predecessor engine. The rail pressure sensor and the pressure regulation valve are bolted directly onto the rails. The rails have been welded instead of forged, as in the predecessor engine. Significant cost savings motivated this decision. The system is based on a seamless rifled steel tube. The open ends of the tube are closed with threaded stoppers. The connector fittings for HP-lines and RDS are welded onto the tube by means of the capacitor-discharge process. By means of a patented nipple arrangement the high-pressure lines are sealed directly to the tube so that the welding seam does not have to be pressure-tight. The required pressure resistance is ensured by appropriate production processes. The fuel filter with an integrated preheat valve is located on the side facing the vehicle in the suspension strut area.
3.7 Oil Circuit and Crankcase Ventilation The oil circuit and crankcase ventilation of the 4,2-l engine were adopted almost in their entirety from the predecessor engine. The engine has an external-gear oil-pump that in the V family of engines has been designed identically to that of the V8 petrol engine. An oil-water heat-exchanger is located in the inner V as oil-cooler, and this has been designed so that the maximum oil temperature remains below the 150 °C limit even in extreme conditions. Owing to its easy access from above the vertical arrangement of the oil-filter housing in the inner V simplifies the replacement of the fully incinerable filter unit. The engine has an oil-change volume of 9,5 litres and can be operated with flexible intervals between services. Oil can be used for a maximum of 30,000 km or for two years. It was possible to integrate the Zyklon oil separator into the oil-filter module in the inner V with a maximum saving of space. The blow-by gases are routed in the chain housing or behind the front seal flange into a temporary reservoir in the inner V of the engine. From here the blow-by gas flows via a three-fold Zyklon, in which the best part of the available oil has been separated, to the suction part of both turbochargers. The oil separated in the Zyklon is routed back via a
channel integrated into the crank housing and an engine-internal line under the oil mirror into the sump.
3.8 Exhaust Gas Recirculation The EGR system, Figure 10, was redesigned so as to meet the EU 4 emission requirements. The aim was to make the EGR distribution as homogenous and even as possible across all cylinders and to make EGR cooling switchable for exhaust gases. The exhaust gases flow via integrally-cast channels at the back end of the cylinder heads of the exhaust manifolds in the direction of the inner V to the EGR valves. Initial pre-cooling of the exhaust gases occurs in the EGR channels by means of water cooling of the cylinder head. The valves are designed as electrically operated lifting valves with integrated motionfree status reporting. This arrangement has advantages in terms of control and diagnosis when compared to the pneumatic operation of the predecessor engine. The EGR valve housings are water-cooled to protect against high temperatures. The exhaust gas stream is cooled in a central, U-shaped EGR cooler. The input flow of exhaust gases is regulated by a pneumatically operated flap, so that cooling of the exhaust gas stream can be adjusted according to operation. After the exhaust gases have passed through the cooler or bypass, the EGR stream is divided and passed to the two cylinder banks. This is achieved by means of separate channels via which the exhaust gases are mixed with the fresh air stream shortly after the control flaps. When creating the channels and input points, optimal mixing of both gas streams was given particular attention. Comprehensive CFD calculations were made when designing the components and in order to optimise mixing and even distribution. The results are displayed in Figure 11 and Figure 12. The exceptional computational results were confirmed during engine testing. By means of good, even distribution of the recirculated exhaust gases both to the two cylinder banks and then to the individual cylinders an important step was taken towards satisfying the emission level limit despite the double-flow of the intake section.
3.9 Exhaust Manifold and Turbocharger In the V8-4,2-l-TDI cast manifolds were able to replace the air-gap insulated sheet-metal manifolds with internal high-pressure shaped tubes used in the predecessor engine. This is possible owing to the short gas pathways between cylinder head and ex-
haust turbocharger which have a positive impact on exhaust heat losses before the chargers and pre-catalyser, Figure 13. GJV SiMo was used as a casting material. This adaptation was motivated by significant cost savings as opposed to the expensive IHU technology. On account of the lower oscillation effects in the manifold/turbocharger-connection because of the more rigid cast manifold, it was also possible to simplify the charger supports. Two water-cooled exhaust-gas turbochargers with variable turbine geometry from Honeywell-Garrett, size GT 17, are used for charging. Chargers of the latest generation, so-called Step 3, are used. Electric servomotors are used to move the control vanes, Figure 14. To increase specific performance various changes had to be made to the turbocharger unit. The aim was to raise the maximum exhaust gas temperature as well as the charge pressure and the rpm. The following technical innovations were deployed to this effect: – Efficiency improvement by optimising compressor, turbine wheel and adjustor vanes – turbine wheel closed on one side – thermal and mechanical decoupling of the turbine control equipment of the turbine housing. As in the predecessor engine, exhaust gas temperature sensors are also deployed in the turbine housing in front of the turbine in the 4,2-l engine. By means of temperature monitoring and associated regulation, the VTG mechanism is protected from exceeding the maximum permitted temperature, and the critical temperature value for maximum engine performance can be determined and reached. For internal sealing, the single sealing ring used hitherto for the turbine was replaced by a double one. This ensures a good gas seal even during short-lived increases in exhaust-gas counter pressure that can occur as a result of overloading the particle filter.
3.10 Air Inlet, Inlet Module, Intercooler The air intake was designed as a 2-groove system with 2 air filters and intercoolers positioned on the side facing the vehicle, Figure 15. The filter inserts are circular cartridges; the volume of the filter housing measures 2 x 11.5 l. Both intercoolers are located at the front of the vehicle under the headlamps so as to maximise flow. In an effort to make maximum use of the available space, the intercoolers were designed in the shape of a parallelogram.
The oversupply air-intake also contains EGR conduits and inputs, and in view of the resultant higher thermal load is made of cast aluminium. The unremarkable input of exhaust gases without any cross-sectional narrowing of the fresh-air stream is to the advantage of the power potential under full load. On the grounds of cost and weight heatstabilised PA6 was selected as the material for both input pipes. The rifled valves familiar from the V6-3,0-l engine are integrated into the component by means of a welded valve frame. The valves control the flow mixture of the spiral channel and adjust the rifling according thermodynamic requirements. There is a bi-directionally operating electro-motor that operates the valves by means of a push/pull-rod. According to operational status, adjustments can be open, closed or in an intermediate position.
4 Summary With the new 4,2-l-V8-TDI engine, the last engine in the Audi TDI range has been converted to the EU 4 emissions directive that comes into effect in 2006. The most up-todate diesel technologies and many innovative solutions are implemented. The result is an extremely compact and powerful engine. At 240 kW and 650 Nm this is currently the most powerful car diesel engine in the world. In combination with the light aluminium bodywork it enables the Audi A8 to achieve excellent driving performance with low fuel consumption. In addition, its exemplary quality and balance and the engine noise-levels are especially persuasive.
References [1] Bach, M.; Bauder, R.; Fröhlich, A.; Hatz, W.; Hoffmann, H.; Marckwardt, H.; Pölzl, H.-W.: Audi 4,0-lTDI-Motor – Teil 1: Konstruktion und Mechanik. In: MTZ 64 (2003) [2] Anton, C.; Bach, M.; Bauder, R.; Franzke, G.; Hatz, W.; Hoffmann, H.; Ribes-Navarro, S.: Der neue 3-lV6-TDI-Motor von Audi – Teil 1: Konstruktion und Mechanik. In: MTZ 65 (2004) [3] Bauder, A.; Clos, R.; Hatz, W.; Hoffmann, H.; Pölzl, H.-W.; Reichert, H.J.: Die neuen V-Motoren von Audi. Wiener Motorensymposium 2002
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