Audi 8X S1: CWZ engine

Introduction

Audi’s CWZ engine was a 2.0-litre turbocharged petrol engine that powered the Audi 8X S1 and S1 Sportback. A member of Volkswagen’s EA888 Gen 3 engine family, key features of the CWZ engine included:

• A grey cast iron block;
• Forged steel crankshaft;
• Alusil cylinder head;
• Double overhead camshafts with four valves per cylinder;
• Variable intake and exhaust valve timing;
• Two-stage exhaust valve lift;
• An IHI turbocharger which provided peak boost pressure around 1.4 bar;
• Direct injection and multi-point port injection;
• A ‘Start/Stop’ function which enabled the engine to shut down when the vehicle was stationary in traffic; and,
• A compression ratio of 9.3:1.

Maximum engine speed for the CWZ engine was 6800 rpm.

Block

The CWZ engine had a closed-deck, grey cast iron (GJL 250) block with 82.5 mm bores and a 92.8 mm stroke for a capacity of 1984 cc. For the CWZ engine, upright pouring was used for the casting process and the cylinder walls had a nominal thickness of to 3.0 mm +/- 0.5 mm.

Within the cylinder block, the CWZ engine had two balance shafts to counteract second order inertial forces. The horizontally-staggered balance shafts rotated at twice the speed of the crankshaft in opposite directions from one another, with the direction of the second shaft reversed by an idler gear. The balance shafts were made from spheroidal graphite cast iron and, for the CWZ engine, lower-friction roller bearings were introduced.

Crankshaft, connecting rods and pistons

The CWZ engine had a forged steel crankshaft that was induction hardened and operated on five main bearings. The crankshaft for the CWZ engine had four counterweights and 48 mm diameter main bearings. It is understood that the connecting rods for the CWZ engine were made from 36MnVS4 and were cracked for precision fitting and to reduce movement of the bearing cap under load.

For the CWZ engine, a new ‘strength-enhanced alloy’ piston coating was applied. Furthermore, an electronically-controlled system was introduced to control the oil-jet cooling for the pistons.

Cylinder head

The CWZ engine had a cross-flow cylinder head that was made from AlSi10Mg and mounted on a three-layer metal head gasket; the exhaust manifold was integrated into the cylinder head. Furthermore, the cylinder head had an integrated water-cooled exhaust gas circulation loop so that full load enrichment (i.e. enriching the fuel/air mix at higher loads) was no longer required for cooling, thereby improving fuel consumption at high loads.

Camshafts and valves

The CWZ engine had hydro-formed, double overhead camshafts that were driven by gear chains (as opposed to roller chains). For the CWZ engine, the intake and exhaust valves were actuated by roller cam followers with needle bearings and hydraulic valve clearance adjusters. It is understood that both the intake and exhaust valves were chrome-plated and had reinforced seats. While the intake valves had solid stems, the exhaust valves were also tempered and had sodium-filled stems for heat dissipation.

Variable valve timing and exhaust valve lift

The CWZ engine had variable intake and exhaust valve timing as well as variable exhaust valve for better control of the charge exchange process. The variable exhaust valve lift system is understood to be based on the ‘Audi Valve Lift System’ (AVS) whereby the camshaft had two valve lift contours for each exhaust valve (small and large). In this system, change-over between the cam lobe contours was achieved by the longitudinal displacement of the cam elements via electromagnetic solenoid-type actuators. One actuator moved the cam element on the camshaft for large valve lift, while the other actuator reset the cam element for small valve lift.

At low engine speeds (up to around 3100 rpm), the small profile cam lobe contour was used to:

• Provide late exhaust valve opening;
• Prevent the back-flow of exhaust gas during the valve overlap phase; and,
• Enable advanced intake valve timing.

Specifically, the positive cylinder pressure gradient allowed the combustion chamber to be effectively purged – this enhanced fuel/air mixture formation by reducing the residual gas content in the cylinder and enabled advanced intake valve timing since less intake air was expelled after BDC (bottom dead centre). As a result, greater torque was produced at low engine speeds and charge pressure could be accumulated faster.

At high engine speeds, the large profile cam lobe contour was used.

For the CWZ engine, the intake camshaft was continuously adjustable over a range of 60 degrees relative to the crankshaft, while the exhaust camshaft was adjustable over a 30 degree range.

IHI turbocharger

The CWZ engine had an IHI turbocharger that was integrated with the exhaust manifold and provided peak boost pressure of 1.4 bar (20.3 psi). Features of the IHI turbocharger included:

• An alloy for the turbine wheel that could withstand exhaust gas temperatures up to 980 degrees Celsius;
• An electric wastegate actuator that reduced boost pressure when power was not needed, thereby reducing fuel consumption;
• A pulsation damper; and,
• An oxygen sensor mounted directly upstream of the turbine wheel.

Cooling

The CWZ engine introduced a fully-electronic coolant control system for more efficient thermal management. The system included a new type of rotary vane module that could block coolant entry into the engine or adjust to low volumetric flows in the engine’s warm-up phase. As a result, shorter warm-up times were achieved, thereby reducing frictional losses and improving fuel economy. At higher engine temperatures, coolant temperature could be adjusted quickly and variably as a function of engine load and external constraints.

Port and direct injection

The CWZ engine had both direct and port fuel injection via separate sets of injectors. The CWZ engine adopted a dual injection system to reduce particulate emissions and comply with Euro 6 emissions standards. The direct injection system used six-hole injectors that provided fuel pressure up to 200 bar.

Based on inputs from sensors, the engine management system controlled the injection volume and timing of each type of fuel injector, according to engine load and engine speed, to optimise the fuel:air mixture for engine conditions. The injection system is understood to have the following operating conditions:

• Cold start: the port injectors provided a homogeneous air:fuel mixture in the combustion chamber, though the mixture around the spark plugs was stratified by compression stroke injection from the direct injectors. Furthermore, ignition timing was retarded to raise exhaust gas temperatures so that the catalytic converter could reach operating temperature more quickly;
• Low engine speeds: port injection and direct injection for a homogenous air:fuel mixture to stabilise combustion, improve fuel efficiency and reduce emissions; and,
• Medium to high engine speeds and loads: direct injection only to utilise the cooling effect of the fuel evaporating as it entered the combustion chamber to increase intake air volume and charging efficiency.

Ignition

The CWZ engine had four single spark ignition coils and cylinder-selective anti-knock control. Furthermore, the CWZ engine had a 1-3-4-2 firing order and compression ratio of 9.3:1.