Audi C7 S6 and S7: CEUC 4.0 TFSI engine

Introduction

Audi’s CEUC engine was a 4.0-litre V8 petrol engine with two twin-scroll turbochargers that powered the Audi C7 S6 and C7 S7 Sportback. Marketed as Audi’s ‘4.0 TFSI’ engine, key features of the CEUC engine included its:

• Hypereutectic aluminium-silicon alloy block with 90-degree ‘V’ angle;
• Forged steel crankshaft;
• Aluminium alloy cylinder head with ‘hot side in’ design;
• Twin-turbochargers mounted in the inner ‘V’ which generated relative charging pressure (or boost pressure) of 1.0 bar;
• Chain-driven double overhead camshafts with variable intake and exhaust camshaft timing;
  Cylinder deactivation system (or cylinder on demand);
• Start-stop system;
• Direct fuel injection (Audi’s ‘Fuel Stratified Injection’ or FSI);
• Bosch MED 17.1.1 engine management;
• Double air-gap insulated exhaust manifolds; and,
• Active engine mounts.

Furthermore, the CEUC engine had a mass of 219 kg.

Compared to the CGTA engine in the Audi D4 S8, differences for the CEUC engine included:

• Single air induction for the turbochargers (rather than for both cylinder banks);
• For the turbine, a smaller compressor;
• Different camshaft profiling;
• Omission of the power steering pump (due to its electromechanical steering); and,
• Different location of the secondary air pump motor.

While the CEUC engine produced peak outputs of 309 kW at 5800 rpm and 550 Nm from 1400 to 5300 rpm, 400 Nm was available from 1000 rpm.

Block

The CEUC engine had a hypereutectic aluminium-silicon alloy crankcase that was produced through low-pressure chill casting. With its 84.5 mm bores (spaced at 90 mm intervals) and 89.0 mm stroke, the CEUC engine had a capacity of 3993. An aluminium alloy bedplate – produced in a pressure die-casting process – was sealed to the crankcase (and oil pan) by liquid sealant and formed elastomer seals. The CEUC engine was 228 mm tall and had a length of 460 mm.

Rather than having separate liners, the cylinder bores of the BSM engine were mechanically stripped to separate the aluminium in a three-stage honing and exposure process. By stripping the aluminium from the cylinder surfaces, silicon was exposed in the form of very small and hard particles. Prior to honing the cylinder walls, a plate was bolted onto the cylinder block to simulate the stresses of the installed cylinder head; a textured finish was applied to the cylinder walls during the final honing process.

Crankshaft

The CEUC engine had a forged steel crankshaft that was supported by five bearings; for the CEUC engine, the main bearings were a three-material composite design and had a 65 mm diameter, while the crank pins had a 90 mm diameter. To strengthen the bearing support, the five cast-in-place crankshaft bearing caps were made of GGG 60 spheroidal cast iron and cross-bolted at a 45 degree angle to the bedplate.

A plastic windage tray separated the crankshaft assembly from the oil pan. As a result, the crankshaft webs were not immersed directly in the engine oil and this prevented oil foaming at high engine speeds.

Connecting rods

The lower ends of the connecting rods for the CEUC engine were cracked after forging so that the breaking surface could be used for high joining precision of the two mating parts. Furthermore, the small ends of the connecting rods had a trapezoidal angle of 13 degrees. While the lower connecting rod bearing shell was a three-material composite, the upper connecting rod bearing shell was a two-material composite. Furthermore, the connecting rod bushings were made from bronze.

Pistons

The cast aluminium pistons for the CEUC engine had a cast-in ring carrier for the compression ring. Compared to the related CGTA engine, however, the piston crowns for the CEUC engine had a much smaller dished area.

For the CEUC engine, the piston pins had a 22 mm diameter and a fine diamond-like carbon (DLC) coating to reduce friction and wear. Furthermore, oil jets within the cylinder block switched on at engine speeds above 2500 rpm to cool the pistons.

Cylinder head

The CEUC engine had an aluminium alloy cylinder head that was mounted on a triple-layer gasket and had cross-flow cooling. Unusually, the CEUC engine had a ‘hot side in’ cylinder head which positioned the exhaust valves on the inside of the engine’s ‘V’ and the intake valves on the outside. According to Audi, the ‘hot side in’ design provided compact construction, improved thermodynamics and shorter gas flow paths for lower flow losses.

Camshafts and chain drive

The CEUC engine had chain-driven, double overhead camshafts per cylinder bank. Both the intake and exhaust camshafts were continuously adjustable over a range of 42 degrees relative to the crankshaft. Furthermore, internal exhaust gas recirculation was achieved by adjusting camshaft timing to create valve overlap – this occurred in both four- and eight-cylinder operation.

Compared to the related CGTA engine, the CEUC engine had different valve timing.

Located on the rear side of the engine, the chain drive for the CEUC engine consisted of four roller-type chains that were arranged in two planes and hydraulically tensioned. In this arrangement,

• Chain drive A acted as the distributor drive from the crankshaft to the intermediate gears;
• Chain drives B and C were driven by intermediate gears and, in turn, drove the camshafts for each cylinder head; and,
• Chain drive D drove a gear module from which all accessories, except the generator, were driven.

Valvetrain

The camshafts actuated the four valves per cylinder via roller cam followers with hydraulic valve lash compensation. However, roller cam followers with wide rollers were used for the cylinders that remained operational at all times, while cam followers with narrow rollers were used for cylinders with on-demand operation.

The CEUC engine had:

• Solid stem intake valves with hardened seats;
• Bronze intake valve guides;
• Exhaust valves with sodium-filled stems for heat dissipation, also with hardened seats;
• Sintered lead steel exhaust valve guides; and,
• Valve lift of 11 mm.

Air supply: pre-turbochargers

Whereas the CGTA engine had a dual branch air intake, i.e. each cylinder bank had its own intake path and air filter, the CEUC engine had a single branch air intake that was located on the right side of the vehicle. The intake air was then routed from the air filter module through two separate passages to the two turbochargers.

All intake air was drawn from the front of the engine and cleaned by the air filter before flowing to the turbochargers

Twin-scroll turbocharger

The CEUC engine had two twin-scroll turbochargers whereby separate exhaust passages from paired cylinders (1-3, 2-4, 5-6, 7-8) were joined just before the turbine to prevent reciprocal interference of exhaust flows. The turbochargers had two exhaust gas inlets and two nozzles: a smaller sharper angled one for quick response and a larger one for peak performance. In addition to improving turbine efficiency and reducing turbo lag, the twin-scroll design enabled exhaust scavenging. Audi opted to use twin-scroll turbochargers rather than variable turbine geometry because of the latter’s higher exhaust temperatures.

The two turbochargers for the CEUC engine generated maximum relative charge pressure of 1.0 bar (14.5 psi).

For the CEUC engine, the wastegate had a vacuum actuator and was opened by mechanical spring force. Relative to regulating charge pressure by a pressure actuator, the advantages of this design included:

• Less heat loss during the warm-up phase of the catalytic converters because the heat flow was in a direct path to the catalytic converters (not over the turbines);
• At part throttle, there was reduced exhaust backpressure because of the wastegate valves were open; and,
• During the transition to over-run phase, the wastegate was opened briefly to maintain high turbine speeds.

Charge air cooling

After the intake air passed the turbochargers, it flowed through the throttle valves to the air-to-water charge air cooler (or intercooler) that was located in the air duct in the inner ‘V’ of the engine. The charge air coolant circuit was separate from the primary cooling system, though they shared the coolant expansion tank.

Intake manifold and flaps

After the intercooler, the intake air passed through the air collector box to the intake manifolds on the outer side of the engine. Intake manifold flaps were integrated into the intake manifold and, when actuated, divided the lower air passage in the cylinder head to create a tumbling motion in the intake air. For each cylinder bank, the intake manifold flaps were mounted on a common shaft which was driven by a spring-loaded vacuum actuator.

Beyond the intake manifold flaps, however, tumbling air movement in the combustion chamber was enhanced by the shape of the intake runners, divider plates in the cylinder heads and the shape of the pistons.

Cylinder Deactivation (Cylinder on Demand)

The CEUC engine had a cylinder deactivation system since it greater fuel efficiency could be achieved by four cylinders operating at a higher load than by eight cylinders operating at a lesser load.

The cylinder deactivation system used Audi valvelift system (AVS) technology. To deactivate cylinders 2, 3, 5 and 8, the camshafts switched to a round lobe that did not create any valve lift for those cylinders (i.e. a “zero-lift cam”), so the valves for those cylinders remained closed. When cylinder deactivation commenced, the exhaust valves remained closed after ignition and combustion so that exhaust gas is trapped in the cylinder. As such, the deactivated cylinders operate as air springs, with temperatures remaining at a high level. In four cylinder mode, the firing mode was 1-4-6-7.

The conditions for cylinder deactivation were:

• Engine speed in the range of 960-3500 rpm (such that it did not operate at idle);
• Oil temperature greater than 50 degrees Celsius;
• Coolant temperature greater than 30 degrees Celsius; and,
• The transmission was in third gear or higher.

For the CEUC engine, the upper limit for torque output was 25 to 35 per cent of the engine’s maximum (i.e. 160 Nm to 250 Nm). Furthermore, the engine could operate continuously in four-cylinder mode for up to seven minutes; at this point, all cylinders were engaged to prevent excessive heat loss and efficiency. In four cylinder mode, it was possible to maintain a constant speed of 140 km/h.

When the engine operated in four-cylinder mode, more severe vibrations were produced because the ignition pulses are halved. To reduce the oscillations generated in V4 operation, the CEUC engine had active engine mounts which used the crankshaft position sensors and built-in accelerometers to detect V4 vibrations between 25 and 250 Hz.

The engine mount’s counter-oscillation was generated by the up and down movement of a membrane ring that was attached to an electromagnetic coil; this movement was then transferred to hydraulic fluid (glycol) in chamber and, from there, to the engine mount. The mounts also worked during V8 idle and when driving over rough roads (low frequency vibrations between 7 and 15 Hz).

To reduce engine noise during V4 operation, the Active Noise Control system used four microphones embedded in the headliner to detect acoustic changes and then emitted reverse phase audio signals to diminish unwanted engine noise.

Oil supply and lubrication

The CEUC engine had wet sump lubrication and a flow-rate controlled, vane-type oil pump that was bolted to the bed plate and driven by a shaft from the spur gear drive. The oil pump had an eccentrically mounted adjusting ring which could change the size of the pump interior and its output – or the pressure in the system after switching from low to high delivery – by rotating. Oil supply to the engine was constantly adjusted in two pressure stages:

• For lower engine speeds, the pump operated in a low pressure stage at a relative pressure of approximately 2 bar (29 psi); and,
• The high pressure level was used at engine speeds above 4000 rpm and controlled to a value of approximately 4.5 bar (75.3 psi). The switch back to the lower pressure level occurred when engine speed fell below 3500 rpm.

At around 11 bar (159.5 psi), the pressure relief valve in the pump opened.

From the main oil gallery, engine oil was supplied to:

• The crankshaft;
• The electronically-controlled piston cooling jets;
• Chain drives and tensioners;
• Valve train;
• amshaft adjusters;
• The oil pump;
• Exhaust turbochargers; and
• Vacuum pump.

Both the CEUC and CGTA engines had an oil-to-water oil cooler that was bolted below the vibration damper on the upper part of the oil pan.

Fuel system

The fuel system for the CEUC engine consisted of low- and high-pressure circuits:

• For the low-pressure fuel circuit, fuel pressure between 5.0 and 6.5 bar (72.5-94.2 psi) was generated by an electric transfer fuel pump; and,
• For the high-pressure fuel circuit, fuel pressure of 20 to 120 bar (290-1740 psi) was generated by a Hitachi high-pressure fuel pump that was driven by a three-lobed cam on the exhaust camshaft of each cylinder bank. A mechanical pressure relief valve opened at a pressure of 145 bar (2103 psi).

Both cylinder banks had their own high pressure circuit such that there was no connection between the high pressure lines of the individual cylinder banks.

Injection and ignition

Once coolant temperature exceeded 70 degrees Celsius, the CEUC engine used homogeneous, direct injection. For cold starts, however, there were three injection modes:

• Injection during compression stroke: the engine is started in ‘high pressure stratified start mode’ using one injection event per stroke;
• Catalytic converter heating: once the engine had started, the heating phase for the catalytic converters commenced immediately and would operate for a maximum of one minute. In this mode, there were three injection events per stroke; and,
• Warm up: after the catalytic converter phase, the warm-up phase would begin. In this phase, there were two injection events and warm-up continued until coolant temperature reached 70 degrees Celsius.

The CEUC engine had mapped direct ignition with centrally-mounted spark plugs and eight direct-acting single spark coils. The ignition process was controlled by the Bosch Motronic MED 17.1.1 engine management system. Furthermore, the CEUC engine had a compression ratio of 10.1:1 and 1-5-4-8-6-3-7-2 firing order.

Exhaust system

The CEUC engine had a dual exhaust system with exhaust flaps that were operated by an electric motor:

• In four-cylinder mode, the exhaust flaps were closed to reduce exhaust noise;
• In eight-cylinder mode, the exhaust flaps are open to the maximum to reduce exhaust back pressure and provide a sporty exhaust sound; and,
• For the CEUC engine in the Audi C7 S6 and S7 Sportback, the exhaust flaps were also open at idle.

Emissions control and secondary air system

The CGTA engine had two close-coupled ceramic main catalytic converters with:

• Pre-catalytic converter oxygen sensors (Bosch LSU 4.9 wide-band planar sensors); and,
• Post-catalytic converter oxygen sensors (Bosch LF 4.2 jump sensors).

To make the catalytic converters reach operating temperature faster, the CEUC engine had a secondary air injection system. 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.

Crankcase breather and ventilation

The CEUC engine had a ventilation system for both cylinder heads whereby blow-by gases were taken into the crankcase breather module through separate ducts that were connected to the intake manifolds and charge air module. Positioned in the inner ‘V’ of the engine, the crankcase breather module performed the following:

• Coarse oil separation: the larger oil drops in blow-by gases would collect on the chamber wall, drip into the collector and pass through a return passage to the oil pan;
• Fine oil separation: blow-by gases were cleaned in a fine oil separator that worked on the principle of an impactor and operated in conjunction with a pressure restriction valve. The oil was returned to the oil pan over a separate connection; and,
• Positive crankcase ventilation: fresh air was admitted into the crankcase at idle and partial load.