COVER You STORY will find theW12 figures Engine mentioned in this article in the German issue of MTZ 4/2004 beginning on page 254.

Der neue 6-l-W12-Motor im Audi A8

The New 6-l-W12 Engine in the Audi A8 The new W12 engine installed in the Audi A8 is the result of continuous development of the W12 engine concept. By optimising the intake and exhaust systems and by reducing friction within the moving parts, power and torque have been increased whilst at the same time reducing fuel consumption. Installed in the Audi A8, as in the new Bentley Continental GT [1], is the new 6 speed AL600 transmission, in which the front differential is located forward of the torque converter. In conjunction with the extremely short W12 engine, this allows permanent four-wheel drive to be realised whilst allowing a very short front overhang of the vehicle [2].

1 Introduction

By Frank Thomas Metzner, Norbert Becker, Wolfgang Demmelbauer-Ebner, Robert Müller and Matthias W. Bach

2

Developed for the top luxury models of the Volkswagen Group, this W-engine series is derived from the V-type range of engines [3]. With a cylinder angle of 15°, the V engines have a good track proven history in many vehicle configurations with 5 and 6cylinders [4]. They combine the advantages of a very good in-line engine with those of a conventional V engine. The 15° V engine is 25% shorter than similar sized in-line engines and about 80% narrower than comparable V engines. The result is a very compact engine, which is very similar to an ideal in-line six cylinder unit in terms of

smooth running properties. If two of these compact V6 engines are joined to form a V engine with 72° cylinder angle and common crankshaft, the result is a V-V-12 engine or, simply, a W12 engine, see Figure 1 [3]. This engine was first installed in the A8 6.0 ltr [5] and as an 8-cylinder version in the Volkswagen Passat W8 [6], and later in the Volkswagen Phaeton W12 [7]. A version of the W12 with 2 turbochargers is now installed in the Bentley Continental GT [1], which has just started series production. The W16 engine in the top luxury Bugatti Veyron 16.4 [8] is also based on the W engine principle. The new Audi A8 is now installed with

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the revised W12 engine with increased performance and higher torque. 2 Further Development of W12 Engine

The following chapter concentrates on the most important changes to the main components, compared with the W12, installed in the VW Phaeton. Figure 2 shows the lateral and longitudinal cross-section. The Table shows the technical data for the W12 engine in the new Audi A8. 2.1 Crankcase

The crankcase has been further developed from the previous model. The main external changes are the oil filter/cooler module, which is now directly mounted to the crankcase, and the modified gearbox flange for connection of a new 6-speed transmission. One of the main goals in the further development of the W12 was the reduction of the friction losses of the moving parts. Part of these friction losses is attributable to the pump action losses of the air forced down by the pistons. These losses are relatively high on the W12 because the pulsating air mass beneath the pistons hits small crank chambers, which result from the engine’s compact design. Until now, a means to compensate this pulsation was provided by 5 holes, of 24 mm in diameter, drilled into the cylinder block bearing pedestals. However, since with this solution the pillar of air must first pass into its own crank chamber and then via the pedestal bores into the adjacent crank chambers which are also covered by the crank webs, this results in pulsation losses. In order to minimise these losses, pulsation cross-sections were engineered that are formed by two cores cast into the structure, Figure 3. The geometry, developed from a series of FEM calculations, is distinguished by the fact that mechanical stress is focused in areas with high cross-sections and areas with low cross-sections have been placed away from the stress peaks, Figure 4. This way, it was possible to reduce the maximum mechanical stress on the crankcase by about 5% in calculation, despite an increase in the pedestal hole sizes. In order to implement this calculated advantage in real terms on the cast part, a high level of quality has to be achieved in the surfaces of the pulsation cross-sections. At Kolbenschmidt, a combination of core sand and fine finishing was developed in a series of casting trials, which brings the de-

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sired high surface quality. Since the cores are located in the supply cross-section of both cylinder banks, their influence on the form filling and hardening were checked and optimised using a computerised process. Machining of the crankcase was simplified as the former pulsation drillings are no longer needed. Particularly complicated in the manufacturing process were the vent drillings, which were necessary due to the collision of the lower cylinder bore edges. The core mounting is placed at the position where the vent drillings were previously made. The leak testing systems can therefore be taken over without change. 2.2 Crankshaft Drive

One of the most striking design features of the W engines are the narrow dimensions of the conrod bearings and main bearings. By virtue of the specified cylinder spacing of 65 mm in the VR engines, the main bearing widths were 15.8 mm and the conrod widths were just 11.6 mm at a bank offset of 13 mm. The demand for a higher nominal output led to an increase in the nominal engine speed from 6000 rpm to 6200 rpm. In order to prevent a further rise in mechanical stress on the bearings, the crankshaft drive was completely reconfigured. 2.2.1 Pistons

The cast piston was given a fundamentally new design with drawn-in piston pin boss area beneath the piston ring pack (ECOFORM from the company Mahle), Figure 5. The compression height was reduced from 33 to 30 mm. The piston pin diameter was reduced by 1 mm to 19 mm. Due to the special geometry of the W-engines, the following parameters have to be considered: ■ asymmetrical force and weight distribution by angled piston crown ■ installation positions with positive and negative cylinder angle offset ■ axially low piston skirt. A common machine grind pattern for all pistons was achieved through fine tuning the process. The achieved machine finish provides an even wear pattern, despite the tough requirements, and eliminates the use of pistons of different types. 2.2.2 Piston Ring Pack

In order to reduce oil consumption and friction losses of the moving parts in the crankshaft drive, a new piston ring pack was developed. The following ring sizes are installed: 1st ring: Square ring with 1.2 mm axial

W12 Engine

height (instead of 1.5 mm) 2nd ring: Taper faced compression ring with 1.0 mm axial height (instead of 1.75 mm) 3rd ring: Double-bevelled oil control ring with spiral-type expander with 2.0 mm axial height (instead of 3 mm). This combination of measures results in a reduction in piston weight, inc. rings and piston pin, by 56 g to 391 g. In particular, the tribology of the oil control ring was improved. By diagonally brushing the face of the rings, an optimal wear pattern was achieved, which, at the same time, brought with it outstanding oil scraping properties. Oil consumption was reduced even further, compared with the previous engine model, and at the same time an improvement in the blow-by characteristic was made. 2.2.3 Conrod

During optimisation of the forged conrod, there was to be no compromise in rigidity and safety against buckling to facilitate a reduction in weight. This was particularly important as the conrod length increased by 3 mm, compared with the series production conrod with 171.5 mm, as a measure to reduce the piston mass. The safety against buckling of the conrod was maintained through a series of optimisation cycles with FEM calculations. The bearing cap and rod are now fixed by means of two dowel pins. This design is characterised by a higher fitting accuracy compared to the previously used fitted bolt joint. In addition, it prevents the bearing cap from being fitted upside down, thereby improving process chain safety at the assembly facility, Figure 6. The combination of measures resulted in a weight reduction of the conrod by 113 g. The conrod, inc. bearing shells and bushes, now weighs just 529 g. 2.2.4 Crankshaft

The hardening process for the forged crankshaft of the W12 engine had already been changed in series production from short time gas nitriding to plasma coating. With plasma coating, a greater hardening depth is achieved and the hardened surface is also more ductile. The fatigue limit was improved by about 10% thanks to this measure. The crankshaft balancing was adapted to the lighter crankshaft drive. 2.2.5 Main and Conrod Bearings

Particular attention was given to the conrod and main bearings. Due to the extremely compact design, the specific mechanical loads are considerably higher than those of

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W12 Engine

standard production vehicles. In addition to high demands in production accuracy combined with maximum rigidity in the crankcase and crankshaft, high demands are placed on the bearing material. For this reason, the main and conrod bearings were changed to bi-material bearings from the Japanese manufacturer Taiho. These bearings, made from an aluminium and tin alloy, have a 6 μm thick molybdenum/sulphate overlay, so that an improved run-in characteristic can be achieved. This design, combined with the light crankshaft drive, allowed a reduction in bearing wear compared with previously used bearing shells, despite a rise in mechanical load. Figure 7 is an example of the optimised run-in wear characteristic of the

conrods and the influence of the light crankshaft drive on run-in wear. As can be seen in the diagram, the W engine is in the lower range of variation relative to comparable series production engines. Since the aluminium/tin alloy has absolutely no lead additive, the bearings already fulfil the new environmental legislation with regards to the future heavy metal ban. These bearings are also being installed in all other W engines as of now. 2.3 Oil Lubrication Circuit and Oil Pump

The oil lubrication circuit of the W12 engine, fitted in the previous Audi A8 model, was of the dry sump type due to restric-

2 Further Development of W12 Engine Table 1: Technical data of W12 engine in the new Audi A8

Engine type

[ -- ]

W

Displacement

[dm3]

5.998

Power

[kW]

331

at engine speed

[rpm]

6200

Torque

[Nm]

580

at engine speed

[rpm]

4000

Mean effective pressure

[bar]

12.15

Specific power

[kW/dm3]

55.19

Bore

[mm]

84

Stroke

[mm]

90.168

Cylinder spacing

[mm]

65

Conrod length

[mm]

171.5

Conrod ratio

[ -- ]

0.262

Bank angle

[°]

72

V-angle of bank

[°]

15

Bank offset

[mm]

13

Cyl. offset

[mm]

12.5

Piston pin offset

[mm]

0

Main bearing ∅

[mm]

65

Main bearing width

[mm]

15.8

Conrod journal bearing ∅

[mm]

54

Conrod journal bearing width

[mm]

11.6

Split pin

[°]

12

Inlet camshaft timing range

[°]

52 (continual)

Exhaust camshaft timing range

[°]

22 (continual)

Compression ratio

[ -- ]

11

Firing order

[ -- ]

1-12-5-8-3-10-6-7-2-11-4-9

4

tions in space [5]. By the time the W12 engine was introduced in the VW Phaeton, the oil lubrication circuit had already been revised considerably and converted to the less complex wet sump lubrication system [7]. This oil lubrication circuit has now been further optimised, within the scope of additional engine development measures, for installation in the new Audi A8. The simplified areas are essentially the shortened internal oil passages and journals and the minimisation of internal oil pressure loss and leakage. For one, the oil filter/cooler module, previously located on the vehicle, was bolted to the crankcase. In addition to the filter and cooling function, this highly modular component features a bracket for the watercooled alternator. This further underlines the compact overall concept of the engine. Secondly, the split main oil channel has been discontinued and replaced by a single channel, located in the V chamber. Supply to the main bearings is now directly from above, Figure 8. Further simplifications can be seen in the way of the cam drive. Here, the intermediate shaft with sleeve bearings has been replaced by a sprocket wheel with a needle roller bearing, and spray lubrication of the primary chain sprocket has been discontinued. Lubrication of both bearings and bushes is now assured by means of the sufficient return oil flow in the sprocket chamber from the cylinder heads and the chain lubrication jets of both upper secondary chains. Furthermore, the primary chain drive has been changed from the previous duplex double roller chain to a single roller chain. These measures made it possible to reduce the dimensions of the oil pump by a quarter. This was achieved by reducing the gear width in the external spur gear pump from 60 mm to 45 mm and thereby reducing friction losses in the moving parts of the engine. 2.4 Oil Foaming

As part of the measures for friction loss optimisation, strong emphasis was placed on minimisation of loss through ventilation and churning. Churning can occur in the crank chamber if the oil and blow-by gases cannot be channelled passed the baffle plate gap quickly enough. Once the oil and gases have escaped from the crank chamber, the gases must be passed through the vent system and the appropriately calmed oil has to be fed back into the oil pump. After comprehensive trials, it was possible to channel the blow-by gases to the extraction points in the cylinder heads by optimisation of the openings in the oil sump

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upper part without affecting the back flowing oil. In addition to the desired reduction in friction losses, these optimisations also resulted in an improvement in oil foaming at 6000 rpm and 140°C oil temperature, and thus equate at this operating level to 16.2 %, based on the FEV testing model. 2.5 Variable Valve Timing

The W12 engine features continually variable inlet and exhaust valve timing [9]. The four vane-type adjusters are fed with oil under pressure by the engine oil lubrication circuit. On the latest W12 engine, installed in the Phaeton, variable valve timing is guaranteed at all operating conditions. Due to the optimisation of the oil circuit, the reduced pump volume guarantees lubrication of all friction bearings, but at hot idle there is an insufficient oil supply for the variable valve timing. To facilitate comfortable idling, the inlet camshafts are adjusted to the retarded position, which is the optimum for this operating condition due to frictional momentum. When this occurs, the oil pressure of 0.45 bar at 140° C is insufficient to advance the exhaust camshafts against the drag momentum. An increase in the hydraulically effective rotor vane surfaces is not possible due to restrictions of space. Furthermore, the balanced motion of the engine, and a valve train practically free of counter-torque reaction, prevent automatic "pumping up” of the camshaft adjuster. By the use of a coil spring, this oil temperature and oil pressure sensitive problem was solved, Figure 9. The wire coil spring engages on the face side of the exhaust camshaft adjuster in the sender wheel, fixed on the camshaft, and counterbalances itself on the other side of the adjuster housing. At this operating condition, the drag momentum in the retarded position is practically neutralised. The available oil pressure is sufficient to guarantee adjustment of the exhaust camshafts in the advanced position. Therefore, at hot idle, smooth running and thereby comfort orientated exhaust timing can be set spontaneously and efficiently. 2.6 Oil Separator Modules

Further emphasis was placed on improving the oil separation system. The goal here was to reduce the external pipework to a minimum and further improve efficiency in oil separation. To do this, forces were joined with the suppliers Mann and Hummel to develop a separator module, which is identical for both cylinder banks in its basic form and differs only in that the symmetrical inlets are opposed, Figure 10.

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The blow-by gases are extracted at each cylinder bank from the cylinder head covers. The extraction points in the covers have already been designed respectively with rib features to act as primary coarse oil separators. From this point on, the gases follow a path along very short and gradually rising lines to the separator modules, which are installed left and right on the intake manifold. The gases which enter here are first channelled to a baffle plate separator where the majority of the oil droplets carried along is separated. From here the gases reach the fine cyclone separators. These parallel centrifugal filters separate the last fine droplets of oil from the gases so the gases that escape are of a very pure composition. From here, the blow-by gases pass through pressure control valves, also located in the module, to the intake manifold of the respective cylinder bank. The oil trapped in the baffle plate separator and in the cyclones is collected in the lower part of the module and is siphoned back directly into the cylinder heads via very short lines. The complete module has a twin wall, shell-like structure and all lines are insulated with silicon hoses to ensure the system works even at extremely low ambient temperatures up to -40°C without the need for electric auxiliary heaters. The modules are supplied to the engine installation facility of the factory as preinstalled and checked units, together with the intake manifolds. 2.7 Friction

To summarise, the above mentioned optimisation measures meant that friction in the moving parts of the W12 engine was significantly reduced across the whole engine speed range. For the customer relevant speed range below 4000 rpm, the friction coefficient is at the lower limit of the FEV variation range, Figure 11. 3 Intake and Exhaust System 3.1 Air Intake

The intake system in the new Audi A8 is of the twin path type, like the one of the previous model. Both lower parts of the air cleaner are based on parts from the V8 4.0 ltr. TDI. The upper parts of the air cleaners have been redesigned due to the vertical position of the HFM (hot film air mass meter) and the W12-specific position of the throttle valve parts. The cold air intake before the air cleaner is common on all engines with twin path intake. The gross pressure loss from the intake system is < 35 mbar.

W12 Engine

3.2 Intake Manifold

The intake manifold comprises of 4 parts for reasons of maintenance and assembly. The intake manifold upper part is split into one centre piece and two side mounted manifolds to allow accessibility of the spark plugs. By the use of elastomer seals with aluminium supports, each manifold requires only two bolted connections, despite a spacing of 458 mm. Therefore, the manifolds can be dismantled for service in just a few work steps. The intake manifold lower part provides the connection of the intake pipes to the inlet ports at the cylinder heads and allows good accessibility to the bolted connections. The layout was suggested by the Promo simulation program. During the design phase, several variations were created in an attempt to meet the target specifications. By means of fully parametric construction with the CAD Pro Engineer system, several variations were derived from the basic model within a short space of time. For verification on the fired test bed, the intake manifold variants were manufactured in the prototype foundry in Hanover from aluminium sand cast parts. The core and cast moulds for this procedure were manufactured by means of Rapid Protoyping within one week in VW’s own Technology Centre with the aid of laser sintering. A variant was chosen with short, conical intake pipes with integrated partial longitudinal compensation. This was the optimal solution. In this way, the excellent figures of the previous 12 cylinder engine, with regards to power and torque output, were again exceeded. By including the engine compartment designers in the project at an early stage, the appearance of the engine was adapted very well to the geometrical boundary conditions, Figure 12. The goal in this instance was to cover the engine as little as possible. 3.3 Exhaust Manifold/Catalyst Module and Exhaust System

The exhaust system was completely revised in line with the new power and torque requirements. The four 3-in-1 manifolds, both front pipes and four main converters, in close vicinity of the engine, were combined into four exhaust manifold/catalyst modules, Figure 13. The inner shells of the air gap insulated exhaust manifolds within the same outer shells were optimised, in terms of gas flow, by the introduction of larger port openings and a revised pipe junction. Due to the removal of the flange connection between front pipe and manifold, the following advantages were gained:

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W12 Engine

■ improved flow to converter, located in close vicinity of engine ■ removal of starter catalyst heat reducer (measure brought about by flange) ■ more freedom in pipe design with regards to engine bay package ■ weight reduction. The improvement in gas flow, in conjunction with larger starter catalysts, meant that the underbody catalysts could be removed. With the optimised package, spacing between manifold flange and starter catalyst is now equal, which helps to simplify the application. To increase the torque, an extension was introduced in the twin pipe version of the exhaust system down into the underbody area. The double D front pipe on the previous engine model has been replaced by single pipes with greater cross-section. To further reduce the exhaust back pressure, pneumatically controlled exhaust flaps have been installed in the left and right rear silencers. With this revised exhaust system, the exhaust back pressure was reduced by approx.100 mbar, whereby a major contribution towards greater power and torque delivery of the engine was achieved. 4 Thermodynamics

The very demanding development goal, from a thermodynamic viewpoint, was the maximum torque of up to 580 Nm and a maximum power output of 331 kW, which is an increase of 30 Nm and 22 kW compared to the previous model, Figure 14. At the same time, the goal was to further reduce fuel consumption under customer driving conditions. Excellent idle characteristics and compliance with EU4 emission limits supplemented the specifications catalogue. 4.1 Torque and Power

The described measures to reduce friction and open up the intake and exhaust paths led to the desired target figures. The torque in particular was increased, even at low engine speeds, by 30 Nm by means of a carefully balanced extended twin down pipe in the exhaust system. Therefore, more than 95% of the maximum figure is available between 2300 rpm and 5300 rpm, even without the use of a variable intake manifold. The mentioned measures relating to the engine parts and the highly effective crankcase ventilation, especially at high engine speeds, allowed a moderately chosen increase in nominal output engine speed to 6200 rpm. In the four-wheel driven Audi D3, performance is outstanding with very favourable fuel consumption fig-

6

ures compared to the conventionally driven vehicles of the competition, Figure 15. 4.2 Vehicle Application

The development targets for application of the engine and gearbox control systems were to set new standards with respect to driving properties and spontaneity. Furthermore, a reduction in fuel consumption and fulfilment of the EU IV emissions standard were at the forefront of development. The emissions figures were achieved by the implementation of 4 catalysts, located in close vicinity of the engine, with ceramic carriers and a cell density of 600 cpsi. The catalyst heating strategy is based on a classic secondary air reaction and was derived and refined from the previous project. Unlike on the previous engine, the secondary air valves open automatically. The complicated vacuum hose structure could therefore be eliminated. The continually variable inlet and exhaust valve timing was mostly carried over from the previous engine. The maximum adjustment angle of the inlet camshafts of 52° and exhaust camshafts of 22° remain unchanged, as do the event periods. The timing was adapted to the new boundary conditions of the charge exchange process (intake and exhaust system). The positive effects of mapped cooling were also carried over. Together with the especially effective adaptation of the new 6-speed Tiptronic, the detail measures in engine optimisation resulted in a drop in super unleaded fuel consumption by 5.5% to 13.8 litre / 100 km during the test cycle. At the characteristic operating point of 2000 rpm with 2 bar mean effective pressure, the engine measures alone resulted in a drop in the specific fuel consumption by 2% to 358 g/kWh. If, with the previous project, reference is to be made to a points system with regards to the basic application (i.e. it was only possible to adapt the intake manifold pressure model and the actual data at a certain operating condition to the cam timing and the respective optimal ignition timing angle), it has now been possible to adapt the intake manifold pressure model and the actual data to each useful cam timing figure and the respective optimal ignition timing angle. The application complexity, which is theoretically far greater as a result, was limited considerably by means of a DOE process [9]. The advantage of this process is that, with a variation in cam timing (e.g. for exhaust gas application), it is no longer necessary to adapt the filling data. To minimise the use of test vehicles and emissions test bench periods, the pilot ap-

plication for emissions data and the application for overlap compensation were carried out predominantly on a dynamic test bed. The fine application was then carried out in the vehicle during various test runs and on a roller test bench. 5 Summary

The new W12 engine from Volkswagen’s W engine family is the highest powered engine, with a nominal output of 331 kW and a maximum torque of 580 Nm, in the new Audi A8. The consistent efforts made to reduce friction losses and to open up the intake and exhaust paths led to a further increase in the brake specific power and torque to secure the vehicle’s place among the very top of the competition. A fuel consumption of 13.8 ltrs. per 100 km in MVEG mix represents a very low figure for a vehicle in this class.

References [1]

Szengel, R.; Metzner, F. T.; Dehmke, M.; Gush, B.: Der neue 6,0-l-W12-Biturbomotor für das Bentley Continental Coupe GT. Beitrag zum 12. Aachener Kolloquium “Fahrzeug und Motorentechnik”, Oktober 2003 [7] [2] Bauder, A.; Dirschnabel, T.; Pischke, A.; Schöffmann, M.: Der Antriebsstrang des Audi A8. In: ATZ/MTZ-Sonderausgabe Audi A8, August 2002, S. 46-58 [3] Metzner, F. T.; Becker, N.; Bohnstedt, K.; Herzog, R.: Die neuen W-Motoren von Volkswagen mit 8 und 12 Zylindern. In: MTZ 62, April 2001, Nr. 4, S. 280-290 [4] Metzner, F. T.; Kirsch, U.; Demmelbauer-Ebner, W.; Ebbinghaus, W.; Ebel, B.: Der 3,2l-V6Motor von Volkswagen – Ein Motor für sehr unterschiedliche Anwendungsfälle. Beitrag zum 24. Int. Wiener Motorensymposium, Mai 2003 [5] Endres, H.; Bauder, A.; Müller, R.; Möndel, A.; Gorenflo, E.: Der neue 6.0-l-Zwölfzylindermotor für den Audi A8. MTZ 62, Juli/August 2001, Nr. 7/8, S. 534-543 [6] Aschoff, G.; Busch, H.; Hofmann, R.; Metzner, F. T.: Der neue W8-Motor von Volkswagen. Beitrag zum 10. Aachener Kolloquium “Fahrzeug- und Motorentechnik”, Oktober 2001 [7] Endres, H.; Szengel, R.; Metzner, F. T.; Becker, N.; Uphoff, K.: Der neue W12-6.0-l-Motor für den VW Phaeton. In: ATZ/MTZ-Sonderausgabe Phaeton, Juni 2002, S. 30-40 [8] Neumann, K.-H.; Kurowski, A.: Der Hochleistungsmotor des Bugatti Veyron 16.4. Beitrag zum 24. Int. Wiener Motorensymposium, Mai 2003 [9] Metzner, F. T.; Flebbe, H.: Doppelnockenwellenverstellung an V-Motoren. Beitrag zum 8. Aachener Kolloquium “Fahrzeug und Motorentechnik”, Oktober 1999

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