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The New Audi V8 TFSi eNgiNe Part 1: Design anD Mechanics
Audi has developed a new generation of turbocharged 4.0-l V8 gasoline engines to replace the naturally aspirated 5.2-l V10 units in the A6 and A8 series. A key feature of the new V8 TFSI is the arrangement of the turbochargers and the exhaust manifold in the inner V of the engine. The V8 is available in two power versions, with outputs of 309 and 382 kW. The first part of the report describes the design and mechanical components of the new engine. Thermodynamics and application aspects will be presented in the second part in MTZ 3.
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AuThorS
Dipl.-ing. (FH) MiCHael SCHäFer is head of Mechanics Cylinder head/ Charge Cycle for V-gasoline Engines at the Audi Ag in neckarsulm (germany).
Dipl.-ing. guiDo SCHieDt is Design Engineer with responsibility for the V8 Turbocharged gasoline Engine at the Audi Ag in neckarsulm (germany).
Motivation
In order to meet present and future consumption and emissions requirements, the issue of downsizing will play an increasingly key role in engine development. It is for that reason that Audi’s successful 5.2-l V10 FSI engines fitted in the A6 and A8 models are to be replaced by the new 4.0-l V8 TFSI generation of engines [1]. The V8, featuring two turbochargers, is fitted in the new S6, S7, S8 and A8 models. A key feature of the newly developed 4.0-l V8 TFSI gasoline engines is the location of the turbochargers, including the exhaust manifold, in the inner V. The main development goal, alongside delivering sporty and emotionally linked attributes such as spontaneity, power, fast-revving responsiveness and sound, was to cut fuel consumption substantially while significantly improving performance. To that end, the new
engines for the first time feature Audi’s new COD (Cylinder on Demand) efficiency technology. DeSign
The design of the new V8 TFSI engine is similar to that of the 4.2-l FSI induction engine, though capacity has been reduced to 4.0-l. A key feature of the new V8 TFSI engines is the location of the turbochargers, including the exhaust manifold, in the inner V. The inlet and exhaust sides in the cylinder heads have been swapped in comparison to conventional V-configuration engines. The fresh air supply to the inlet ducts comes from the turbocharger, through the throttle valves, by way of the indirect charge air cooler (likewise located in the inner V), two distributor pipes on the front face and intake manifolds on the engine’s outside.
unit
4.0-l v8-tFSi
Dipl.-ing. robert Müller is head of Design for V-gasoline Engines at the Audi Ag in neckarsulm (germany).
CylinDer CapaCity
cm 3
3993
–
8
Stroke
mm
89
bore
mm
84.5
CylinDer gap
mm
90
nuMber oF CylinDerS
CrankSHaFt bearingS
Dipl.-ing. Jürgen JablonSki is head of Mechanics for V-gasoline Engines at the Audi Ag in neckarsulm (germany).
4.0-l v8-tFSi pluS
V8-90°
DeSign
–
5
Main bearing DiaMeter
mm
65
ConroD bearing DiaMeter
mm
54
67
ConroD lengtH
mm
153
valveS per CylinDer
–
4
inlet valve DiaMeter
mm
33.85
exHauSt valve DiaMeter
mm
28
°CA
42
°CA
42
CaMSHaFt aDJuStMent range – inlet CaMSHaFt aDJuStMent range – exHauSt event lengtH – inlet
°CA
180
190
event lengtH – exHauSt
°CA
200
210
–
10.1
9.3
309 at 5500
382 at 5800
CoMpreSSion ratio power
kW at rpm
SpeC. power
kW/l
77.25
95.5
Max. torque
nm at rpm
550 at 1400–5200
650 at 1700–5500
iDling SpeeD
rpm
550
–
1-5-4-8-6-3-7-2
DeSeleCtable CylinDerS
Cyl.
2, 3, 5, 8
engine lengtH
mm
497
CylinDer bloCk HeigHt
mm
228
ignition SequenCe
engine weigHt (to Din gz) Fuel
kg ron
eMiSSion StanDarD
219
224 98/95 Euro 5/uLEV 2
❶ Main dimensions and characteristic data of the V8 TFSI engine 02I2013
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To achieve the smallest possible vehicle overhang, the engine was made extremely short – just 497 mm in length, ❶. Another factor in the engine’s compact design is the highly integrated vent module, likewise located in the inner V. The COD function is realised by means of the AVS (Audi valve lift system). The tried and proven layout of the timing drive has been adopted from the predecessor engine. The broad spread of power output, from 309 to 382 kW, and the integration into various Group models was achieved with a minimal number of different components.
❷ Engine block
engine bloCk
As in the 4.2-l V8 FSI induction engine, the two banks of the 4.0-l V8 TFSI are arranged at an angle of 90° to each other. The 90 mm cylinder gap, the 84.5 mm bore diameter and the 18.5 mm bank offset are also adopted from the predecessor engine. The engine block is made as a homogeneous low-pressure gravity diecast block from the hypereutectic aluminium-silicon alloy ALSi17Cu4Mg. Owing to the increased specific power output and the associated increase in thermal and mechanical loading, the 382 kW variant is additionally heattreated by the T6 process. The cylinder bores are spiral slide honed using bolted hone plates. In order to reduce cylinder deformation under combustion chamber pressure, the cylinders are connected to the surrounding engine block structure by longitudinal
fins, ❷. To strengthen the bearing block, five inserts made of cast-iron with nodular graphite are cast into the aluminium bed and additionally cross-bolted. This measure enhances the strength and circularity of the bearing points and improves the acoustics. oil CirCuit
The oil circuit is essentially based on that of the 4.2-l V8 FSI. The vane pump is executed as a two-stage control oil pump. Up to 4000 rpm the oil pump operates in the low-pressure range at 2 bar oil pressure, then it switches to the high-pressure range. Except for the turbocharger oil supply lines, all pressure oil and return channels are inte-
❸ Oil circuit and piston spray nozzle map
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grated into the cast of the cylinder heads, the engine block and the oil pan top section, ❸ (left). The pressure losses of the oil circuit have been greatly reduced further compared to the predecessor engine. In combination with the two-stage control oil pump, this significantly reduces friction and therefore also fuel consumption. Use of map-controlled piston spray nozzles enabled churning losses to be minimised. At engine speeds below 2500 rpm the piston spray nozzles are activated only under high loads of above 400 Nm, ③ (right), thereby reducing the oil throughput in this operating range by as much as 25 %. The higher piston temperature in the warm-up phase and under partial load also brought some emission
benefits. Based on the totality of measures, an oil throughput rate of < 60 l/min at 120 °C oil temperature was achieved – a very good rate for V8 engines. The high power variant additionally features an air-oil cooler in the front end. Crank Drive
In all power variants of the 4.0-l V8 TFSI the connecting rods are executed as cracked rods with 17 mm wide threematerial bearings. The upper connecting rod end has a trapezoidal angle of 13° with a piston bolt diameter of 22 mm. The connecting rod bushing is made of a copper alloy. The five-bearing mounted crankshaft was adopted in its basic design from the 4.2-l V8 FSI; the stroke has been reduced to 89 mm. To increase strength, all fillet radii are induction-hardened. The material used for the V8 TFSI is C38 mod. with 65 mm main bearing diameter; for the higher-powered variant the material is 42CrMoS4 with 67 mm main bearing diameter. Cast aluminium pistons with ring carriers are used. Owing to the differing compression, the piston head is adapted to the respective power variant. The first piston ring and the piston bolt are DLCcoated (diamond-like carbon) for friction-related reasons. Cooling CirCuit
Different cooling peripherals are used depending on the vehicle model and power variant; the engine-side cooling circuit is always identical, ❹. For homogeneous temperature distribution, the flow through the engine is transverse, and the engine also features an innovative thermal management (ITM) system. The heated annular valve thermostat on the intake side of the coolant pump exhibits much lower pressure losses in the control range than commonly used plate thermostats. In order to provide more benefits in terms of friction, the coolant temperature is controlled to 105 °C in the partial load range when the engine is warmed up. A ball valve downstream of the coolant pump additionally realises the principle of standing water throughout the engine in the warm-up phase. This happens on every engine start at a coolant temperature below 80 °C. When the operating temperature is reached, the 02I2013
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coolant flows into the inner V of the crankcase; from there on the two cylinder heads are filled with coolant via manifolds. A further special feature of the ITM is the autonomous heating function. This additional heating circuit is connected directly to the cylinder head, and is supplied via an electric pump. The passenger compartment can thus be heated despite standing water in the engine block. This results in significant consumption savings in customer driving, particularly in the warm-up phase when heating is required.
: frictionless axial fixing of the AVS cam units : reduced piston ring pre-tension of the third ring and DLC coating on the first ring and piston bolt : less cylinder deformation thanks to combination of plate honing and longitudinal stiffeners on the cylinder pipes. The totality of these measures significantly reduced friction compared to the 4.2-l V8 FSI induction engine, especially in the customer-relevant lower engine speed range, ❺.
FriCtion loSS
CylinDer HeaD anD variable valve liFt SySteM
The friction loss of the 4.0-l V8 TFSI was effectively reduced by a large number of single measures. They include: : enlarged ventilation cross-sections in the engine block and oil sump : increased vacuum in the engine block : derestriction of the oil and coolant circuits : oil pressure lowering to 2 bar in the low-pressure stage : map-controlled piston spray nozzles : enhanced crank drive rigidity thanks to reduced piston stroke : frictionally optimal main bearing diameters for both power variants : reduced inlet valve spring forces : three-chamber camshaft adjuster with reduced oil throughput
The high power output and resultant thermal loading on the cylinder heads of the V8 TFSI demands optimally balanced temperature distribution and effective cooling of the combustion chamber top in the area of the exhaust valves. To ensure this, the flow through the cylinder head is based on the transverse flow principle. The increased coolant demand of the cylinder heads was met by CFD simulation of the water chambers allied to low throttling losses. The material chosen was the high heatresistant aluminium alloy AlSi7. The AVS uses components from the Group modular platform, with enhancements having been made in particular
❹ Cooling circuit
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: : : : :
AVS actuators hydraulic compensating elements rocker arms of the fixed cams proportional valves camshaft adjuster.
CrankCaSe breatHer
❺ Friction loss diagram
with regard to the package and acoustics. The built camshafts consist of an externally splined basic shaft with bearing rings, two fixed cam units, two moveable AVS cam units for cylinder shut-off, a pump cam and an incremental encoder wheel. A special assembly concept had to be devised for this whereby the bearing rings and incremental encoder wheel are joined to the tooth tip, valve and pump cams on the tooth flank. In order to implement a sound in keeping with a luxuryclass model, the AVS cam units are for the first time held in position solely by the standard-fit detent balls, in order to prevent contacting on the ladder frame. A special detent groove was developed for this. The AVS valve gear in the V8 TFSI is subject to the highest loading of any Group engine with AVS. As a result, the rocker arms – likewise from the Group modular platform – had to be adapted for the higher load. The exhaust valve seats are reinforced and the valves are sodiumcooled. The modification of the camshaft adjuster, reducing it from four to three chambers, enabled the oil throughput to be significantly reduced while at the same time increasing the adjustment speed. Reducing the AVS rocker arm spray nozzles in size from 0.4 to 0.3 mm reduced their oil throughput by a further 30 %. The oil demand of a cylinder head – 2.5 l/min at 120 °C oil temperature – is thus very low for a V8. In addition to reducing the oil throughput, the spring force of the inlet valve springs was minimised in order to further reduce the friction loss of the cylinder heads. The integration of the camshaft bearing into the valve hood eliminated the
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need for an additional cylinder head cover. Moreover, the hood also holds the high-pressure injection pump, the AVS actuators and a number of other engine components, ❻. In line with the V8-specific ignition sequence, the AVS actuators are mounted on the cylinder head cover of cylinders 2/3 (right bank) and 5/8 (left bank). For cost and synergy reasons, special attention was paid in the development of the V8 TFSI to a high rate of standard part use. Consequently, the cylinder heads incorporate the following parts adopted from the Audi valve gear kit used by the R4, R5, V6, V8 and V10 gasoline engines: : inlet and exhaust valves, springs, guides and shaft seals : valve spring plate : exhaust seat rings
The development goal with regard to the crankcase breather was to implement a highly integrated and compact complete system. The core system component is a plastic crankcase breather module flanged onto the plenum chamber housing which performs the following functions: : coarse oil separation : fine oil separation : pressure control by way of the pressure regulating valve : positive crankcase ventilation (PCV) including restrictor bores for fresh air dosing : demand-oriented operation of full and part load venting : return of the separated oil into the oil sump. The blow-by gases are discharged from the engine block by way of integrated ducts in the bearing traverse, crankcase, cylinder heads, CVTS (Continuous Variable Tumble System) flanges and the plenum chamber housing and are then routed via a steel pipe with a short length of hose into the breather module, ❼ (left). This compact, pressure loss-optimised blow-by guidance out of the crankcase minimised the required number of external lines.
❻ AVS valve gear
❼ Crankcase breather
The integration of the camshaft bearing into the cylinder head covers eliminated the need for the large volume in and beneath the conventional cylinder head covers previously used as a coarse oil separator. So as to nevertheless meet the high demands in terms of oil separation, a large number of detail developments of the breather system were required. By optimising the position, shape, sequence and cross-section of the blowby terminal units and the line routing to the breather module, the raw oil discharge from the engine block was reduced by more than 95 % in the course of development. The subsequent coarse oil separation is implemented by means of centrifugal force separation based on multiple flow reversal in combination with gravity separation in the first chamber of the new separator module. Then the pre-cleaned blow-by gas is routed into a second chamber, ⑦ (right). There the fine oil is separated by way of the impactor, an impact separator with fleece. The separated oil from the coarse and fine oil separator is fed back below the oil level via two separate channels cast into the engine block. To safeguard the full functionality of the separator system even under the high driving dynamics demands imposed on the engine, a spring-loaded non-return valve was additionally developed for each return channel, providing a very high degree of leak-tightness allied to low opening pressure. The blow-by module is supplied with the plenum chamber housing, and is pushed into the inner V during assembly with no need of additional procedures, and then bolted on to the engine block 02I2013
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and CVTS housing. The positioning of the crankcase breather module in the inner V and the integration of all mediacarrying lines into heat-protected areas of the engine ensures freeze protection down to below -40 °C with no additional heating measures. In the V8 TFSI, as previously in the V8 4.2-l FSI so-called HDZ engine from the RS5, a modified control spring realised an increased crankcase vacuum of 150 mbar, as a result of which the friction loss and thus the fuel consumption was further reduced and the oil separation improved. The PCV system was likewise adapted to the high demands of a turbocharged direct-injection gasoline engine. By designing a long mixing section of the blow-by gases with fresh air, the potential of the PCV system was fully utilised in terms of water and fuel discharge from the crankcase and the engine oil. air Supply anD turboCHarging
The air supply was also adapted to the new space conditions due to the posi-
tioning of the turbochargers in the engine’s inner V. The fresh air is drawn in at the front end, cleaned by the air cleaners and routed to the two compressors. Depending on power variant and vehicle model, the intake air passes through an air cleaner housing on the right (S6, S7, A8) or through two air cleaner housings on the left and right (S8). The compressed intake air flows through the throttle valve module into the air collector, where the indirect charge air cooler is located, ❽ (right). Then the charge air is routed via two distributor arms on the front face to the intake manifolds on the outside of the cylinder heads. The tumble flaps are located upstream of the inlet ducts in the CVTS flanges. In conjunction with the ducting geometry, the duct partition plates and the piston form, the tumble flaps generate a rollerform charge motion and so improve the mixture formation in the combustion chambers. As a result, fuel consumption and exhaust emissions are significantly reduced at low revs and under partial load. Thanks to the highly compact design of the charge air control, very small air volumes were achieved in the pressure system. This is reflected in the spontaneous responsiveness of the engine. Two electric divert-air valves prevent excessive braking of the turbochargers when the throttle valve is closed, so enhancing the responsiveness of the engine on renewed acceleration, ⑧ (left). The indirect air-water charge air cooler is of two-row design. The flow through it is implemented according to the counter-flow principle, in order to achieve greater efficiency. The energy extracted from the charge air is routed via the low-temperature cooling circuit
❽ Air intake and guidance
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to the radiator in the front end, where it is discharged to the outside. The electric coolant pump of the charge air circuit is actuated by the engine control unit dependent on load, engine speed and ambient temperature.
❾ Turbocharger and manifold (manifold of two cylinders of one bank are directed together respectively; bank 1: cylinders 1 and 3 as well as cylinders 2 and 4; bank 2: cylinders 5 and 6 as well as cylinders 7 and 8)
exHauSt ManiFolD anD turboCHarger
The V8 TFSI is the first Volkswagen Group engine to feature twin-scroll technology. By separating the scrolls through to the turbine inlet, exhaust-side cross-talk of the cylinders which fire in direct sequence on one bank is greatly reduced, thereby significantly improving torque build-up, especially at low engine speeds. Owing to the positioning in the inner V, very high demands had to be met in terms of heat shielding to prevent the surrounding components from being thermally overloaded. To minimise the surface temperature on the outside of the exhaust manifolds, and also to retain the exhaust energy through to the turbine for as long as possible, the turbocharger turbine housing is welded directly to the double air gap insulated exhaust manifold and executed as an integrated module. The exhaust manifold itself consists of gas-carrying inliners, supporting shells and heat-insulating outer shells, ❾. Eliminating the connecting flange between the turbine housing and exhaust manifold avoided the critical thermal radiation which is otherwise present on flanges and produced a flow-optimised contour on the way to the turbine. A built-in silicate fibre insulation with a stainless steel cladding insulates the turbine housing from the outside. By these measures, and based on targeted flow through the engine compartment, the
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need for additional costly insulation on surrounding components in the area of the inner V was largely avoided. In view of the high thermal loading by exhaust gas temperatures of up to 980 °C and a very high mass throughput of up to 1400 kg/h, a key area of development focus was on the durability of the exhaust manifold and the twin-scroll turbine housing. Extensive thermo-mechanical calculations were applied to optimise the geometries of the exhaust manifold and the turbocharger right from an early stage of development. This reduced the plastic elongation by as much as 70 % in places. The turbine housing is made of high heat-resistant cast steel 1.4849. The inliners and the supporting shell of the exhaust manifold are made of Inconel. The vacuum-controlled waste gate actuators are positioned on the cylinder heads, delivering key benefits in terms of the package and thermal loading. This made it possible to make the vacuum
units, including their holders, out of plastic. The vacuum control enables quick actuation even on cold-starting, meaning the catalytic converter heats up more rapidly and so emissions are reduced. The only difference in the turbocharger module between the 309 and 382 kW engines is that the variant with higher power output has a larger compressor wheel diameter, and so the machining in the compressor housing is adapted accordingly. outlook
The second part of this article will appear in the MTZ 3 issue. It will cover the subject of thermodynamic development and application of the new engine in detail. reFerenCe [1] Königstedt, J.; Aßmann, M.; Brinkmann, C.; Eiser, A.; grob, A.; Jablonski, J.; Müller, r.: The new 4.0l V8 TFSI engines from Audi. 33 rd Vienna International Engine Symposium, 2012
