Audi D2 S8: AQH/AVP/AYS/BCS 40 valve V8 engine

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Introduction

Audi’s AQH, AVP, AYS and BCS were 4.2-litre, 40 valve V8 engines that powered the Audi D2 S8 from 1999 to 2003. Effectively replacing the 4.2-litre 32 valve AHC/AKH engines, key features of the 40 valve AQH/AVP/AYS/BCS engines included:

• A cast aluminium alloy block with 90-degree ‘V’ angle;
• Die-forged steel crankshaft with five main bearings;
• Cast aluminium alloy cylinder head;
• Double overhead camshafts driven by a toothed belt and simplex chain;
• Five valves per cylinder actuated by roller rocker cam followers;
• Variable intake camshaft timing;
• A variable intake manifold;
• Electronic throttle control;
• A compression ratio of 11.0:1;
• Maximum engine speed of 7200 rpm; and,
• Mass of 200 kg.

For simplicity, this article will refer to the AQH, AVP, AYS and BCS engines as the 4.2-litre 40 valve engine.
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Cylinder block

TTe 4.2-litre 40 valve engine had a cast aluminium alloy block with 84.5 mm bores and a 93.0 mm stroke for a capacity of 4172 cc. The die-forged steel crankshaft was supported by five main bearings and, attached to the crankshaft, the 40 valve V8 engine had forged connecting rods.

It is understood that the 4.2-litre 40 valve engines had cast aluminium alloy pistons with specially-designed valve recesses.

Cylinder head

The 4.2-litre engine had cast aluminium alloy cylinder head that was mounted on a triple-layer metal gasket. The double overhead camshafts were driven by a toothed belt and simplex chain and used roller cam followers – with hydraulic adjustment – to actuate the five valves per cylinder (three intake and two exhaust).

The 4.2-litre 40 valve engine had variable intake camshaft timing.

Roller rockers

To minimise inertia forces and facilitate the 7200 rpm maximum engine speed, the roller rockers were made of die-cast aluminium. Compared to the mechanical bucket tappets of the 32 valve V8 engine, the roller rockers reduced frictional losses in the valvegear and halved the oil delivery rate in the cylinder heads. The rockers were supported by a spindle shaft which was also used to supply oil to the bearings and the hydraulic valve lifters. Valve actuation, however, differed between intake and exhaust valves:

• The two exhaust valves per cylinder were actuated by a twin roller rocker that, in turn, was actuated by a single cam by means of a roller located between the rocker arms; and,
• The three intake valves per cylinder were actuated by a triple roller rocker that was actuated by a double cam by means of two rollers between the rocker arms. For uniform compression between the two cams and rollers of the roller rockers, the spindle shaft of the inlet roller rocker had a convex shape to prevent the roller rockers from tilting.

For the 40 valve V8 engines, coolant flow direction was changed as per Audi’s concurrent V6 engines. As such, coolant flowed from the cylinder heads, merged in the rear coolant pipe and then flowed to the cooler.

Intaeke manifold

The 4.2-litre 40 valve engine had a fresh air intake system with two branches to reduce pressure losses, with the branches joined upstream of the throttle valve. While the 3.7- and 4.2-litre 40 valve V8 engines had a three-stage variable intake manifold (VIM) made from die-cast magnesium alloy, the AQH/AVP/AYS/BCS engines for the Audi S8 had a two-stage variable intake manifold. The VIM was controlled by the engine management system and, at low engine speeds, a long intake runner was employed to increase pulsation effects and torque output. At higher engine speeds, however, the short intake runner was used to reduce intake resistance and increase power output.

Injection and ignition

The 4.2-litre 40 valve engine had electronically-controlled, common-rail multi-point sequential fuel injection via eight injectors that were positioned in the intake manifold. The injection and ignition process was controlled by the Bosch Motronic ME 7.1 engine management system which used a hot film air mass meter to measure intake air volume, two sensors for cylinder selective knock control and two heated lambda probes for adaptive lambda control.

The 4.2-litre 40 valve engine had a compression ratio of 11.0:1 and 1-5-4-8-6-3-7-2 firing order.

Exhaust

The 4.2-litre 40 valve engine had an air-gap insulated exhaust manifold, with the exhaust pipes of the individual cylinders were assembled in a cloverleaf configuration for each cylinder bank (i.e. a 4-into-1 arrangement).

Secondary air injection system

The 4.2-litre 40 valve engine had a secondary air injection system so that the catalytic converters reached their operating temperature faster. In this process, the fuel mixture was enriched to produce a higher percentage of unburnt hydrocarbons in the exhaust gas. The secondary air injection – located downstream of the exhaust valves – then enriched the exhaust gases with oxygen, causing oxidation (i.e. after-burning) of the hydrocarbons and carbon monoxide.