Audi 8J TT RS: CEPA/CEPB 2.5 TFSI engine

Introduction

Audi’s CEPA and CEPB were 2.5-litre inline five-cylinder turbocharged engines that powered the Audi 8J TT RS and TT RS Plus, respectively. Hand-built in Gyor, Hungary, the CEPA engine was developed by Audi’s high-performance subsidiary quatttro GmbH and based on the naturally aspirated 2.5-litre engine which powered the Volkswagen Bora and Jetta for North American markets. Key features of the CEPA engine included:

• Vermicular graphite cast iron cylinder block;
• Die-forged steel crankshaft supported by six main bearings;
• Forged and cracked connecting rods;
• Cast aluminium alloy pistons;
• Cast aluminium-silicon alloy cylinder head;
• Chain-driven double overhead camshafts;
• Variable intake and exhaust camshaft timing;
• Four valves per cylinder actuated by roller finger cam followers;
• BorgWarner K16 turbocharger which provided peak boost pressure of 1.2 bar (17.4 psi);
• Direct fuel injection (Audi’s ‘Fuel Stratified Injection’ or FSI) at a pressure of up to 120 bar;
• Compression ratio of 10.0:1;
• Bosch MED 9.1.2 engine management system; and,
• Euro V emissions compliance.

The CEPA and CEPB engines were 494 mm long, 220 mm tall (block height) and weighed 183 kg.

The higher-output CEPB engine differed from the CEPA engine in that its turbocharger provided peak boost pressure of 1.25 bar (18.1 psi) and injection pressure increased to 130 bar.

Block

The cylinder block for the CEPA/CEPB engine was based on the naturally aspirated 2.5-litre engine that was used in the North American Volkswagen Bora and Jetta. With its 82.5 mm bores – spaced at 88 mm intervals – and 92.8 mm stroke, the CEPA/CEPB engine had a capacity of 2480 cc.

For the CEPA/CEPB engine, the crankcase was made from compacted vermicular graphite cast iron which had a tensile strength of 450 N/mm²– while this substance had previously been used in naturally aspirated racing engines in the 1990s and Audi’s V6 and V8 TDI engines, the CEPA was the first petrol engine to be made from this substance. According to Audi, vermicular graphite cast iron was used because of the relatively narrow connecting rod bearings and main bearings. The cylinder block also had targeted reinforcements on the main bearing seat and the main bearing cover to increase its load-bearing capacity.

As an inline five-cylinder engine, the CEPA/CEPB had a firing interval of 144 degrees and did not require balance shafts.

Crankshaft

The CEPA/CEPB engine had a die-forged steel crankshaft (C38modBY) that was supported by six main bearings, each of which had a 58 mm diameter. At the front end of the crankshaft, the CEPA/CEPB had a torsional vibration damper to reduce radial vibration caused by the transmission of force from the piston to the crankshaft via the gudgeon pin and the connecting rod.

Connecting rods

The CEPA/CEPB engine had forged and cracked steel connecting rods that were 144 mm long; the connecting rod bearings had a diameter of 47.8 mm. Furthermore, the pin on the small-end side of the connecting rod had a 22 mm diameter.

Pistons

The pistons for the CEPA/CEPB engine were developed by Mahle and cast from a heat-resistant aluminium alloy. The pistons were recessed in the area of the skirt towards the gudgeon pin (known as a box or window piston) – this design increased rigidity for the piston skirt greater rigidity and enabled a shorter gudgeon pin to be used.

The top piston ring groove had an asymmetric spherical nitride steel ring with a PVD coating and inside bevel. The second and third grooves had a taper face ring and ventilated oil ring with bevelled outer edges and chromated conical ring lands. Furthermore, the pistons had asymmetrical skirts and bevelled box walls on the thrust and counter-thrust sides. Each piston, ring and pin weighed a combined 492 grams.

For cooling, oil jets in the crankcase sprayed the underside of the pistons.

Cylinder head

The CEPA/CEPB engine had a cast aluminium-silicon (‘Alusil’) alloy cylinder head that was adapted from the naturally aspirated 2.5-litre engine. Due to the higher stresses of a turbocharged engine, the following modifications were made:

• A more heat-resistant aluminium casting alloy (AlSi7MgCu) was used;
• The water jacket was positioned lower around the spark plug;
• Tempered exhaust valve seat rings were used;
• The high-pressure pump was attached to the ladder frame;
• The exhaust cam contour was revised;
• An additional exhaust cam phaser was fitted; and,
• Sodium-filled exhaust valves were used for heat dissipation.

The CEPA/CEPB engine had chain-driven double overhead camshafts. Specifically, the CEPA/CEPB engine had two chain drives:

• A primary chain drive which consisted of a 3/8″ toothed chain that extended from the crankshaft to an intermediate gear. The primary chain drive powered the oil pump; and,
• A secondary chain drive which consisted of a 3/8″ roller chain that extended from the intermediate gear to the camshafts.

For variable valve timing, the intake and exhaust camshafts had an adjustment range of 42 degrees.

The CEPA/CEPB engine had four valves per cylinder that were actuated by roller finger cam followers which featured automatic hydraulic valve clearance compensation. Specifications were as follows:

• Intake valve diameter: 33.85 mm;
• Exhaust valve diameter: 28.0 mm;
• Intake valve lift: 10.7 mm; and,
• Exhaust valve lift: 10.0 mm.

BorgWarner K16 turbocharger

The CEPA engine had a cast steel (1.48.49 grade) exhaust manifold with integrated BorgWarner K16 turbocharger. The BorgWarner K16 turbocharger had a 64 mm diameter compressor wheel at the outlet end and provided maximum relative charge (or boost) pressure of 1.2 bar (17.4 psi). At full throttle, the turbocharger could compress 290 litres of air per second.

For the CEPB engine, however, maximum relative charge pressure was 1.25 bar (18.1 psi).

The turbocharger casing had a separate oil supply and a cooling system serviced by a separate water pump. After the engine stopped, the coolant run-on pump operated to dissipate accumulated heat.

Intercooler

Since the compressing effect of the turbocharger increased the temperature of the intake air mass, the CEPA/CEPB engine had a charge air cooler (or intercooler) to increase its density.

According to Audi, total pressure loss along the entire compressed air flow path was only 135 millibars at maximum flow. Furthermore, the intercooler achieved an efficiency of more than 80 per cent at full load.

Intake

For the CTSA and CZGA engines, the intake manifold was a two-piece sand casting which comprised the intake plenum and the intake arm gallery. In the intake gallery, pneumatically actuated flaps – controlled by the ECU – generated a controlled tumbling motion. The intake ports were also designed to generate a tumbling air motion to produce the turbulence for better air/fuel mixing, with the objective of achieving a homogeneous mix.

Fuel supply and injection

The CEPA/CEPB engine had a demand-controlled fuel supply with low- and high-pressure circuits:

• For the low-pressure circuit, the ECU regulated the fuel pump control unit and, hence, the delivery rate of the fuel pump that was located in the fuel tank;
• For the high pressure circuit, the ECU regulated the fuel metering valve directly at the high-pressure pump. The demand-controlled, single-piston high-pressure pump was produced by Hitachi and driven by a three-lobe cam that was seated on the exhaust camshaft.

The CEPA engine had a common fuel rail from which injected petrol directly into the combustion chamber at a pressure of up to 120 bar; for the CEPB engine, however, injection pressure is understood to be 130 bar. The fuel swirled within the combustion chamber and its evaporation cooled the cylinder walls to suppress the knocking tendencies.

The injection system had three operating modes:

• High-pressure stratified starting;
• Catalyst heating and engine warm-up with twin injection; and,
• Homogeneous injection.

The CEPA/CEPB engine had a compression ratio of 10.0:1.1.

Ignition

The CEPA/CEPB engine had direct ignition with five individual direct-acting single spark coils and long-life Beru spark plugs that were centrally positioned within the cylinder chamber. The CEPA/CEPB engine had a firing order of 1-2-4-5-3; alternating between adjacent and distant cylinders provided the distinctive rhythm characteristic of five-cylinder engines.

The Bosch MED 9.1.2 engine management system controlled the injection and ignition process; engine load was measured via an intake manifold pressure sensor and the engine speed sensor. Two knock sensors were used for cylinder-selective knock control.

Exhaust

To minimise exhaust gas back-pressure, the CEPA/CEPB engine had a twin-flow exhaust system with large diameter pipes. The exhaust system included a flap in the left tail-pipe which, when closed, routed exhaust gases from the rear muffler to exit at the right tail-pipe. When accelerating at higher loads and engine speeds, however, the flap would open so that exhaust gases took the direct route, producing a ‘fuller and more intense sound.’ Furthermore, the driver could open and close the exhaust flap by pressing the standard ‘Sport’ button on the centre tunnel.

Crankcase ventilation

The CEPA/CEPB engine had a crankcase ventilation system which extracted blow-by gases from the cylinder block via ‘rise channels’ in the bearing saddle of main bearings two, three and four. The blow-by gases were ‘pre-cleaned’ as they passed through the riser channels and entered the rocker cover through the cylinder head. Initially, the blow-by gases admitted into the rocker cover flowed into a relatively large hollow chamber so that oil droplets were deposited onto the walls; the gases then passed through a fine oil separator which operated on the centrifugal force principle.

The fine oil separator consisted of four permanently open swirls and six packs of up to nine swirls, which could be activated – by locking springs – according to flow rate. The separated oil from the rocker cover and the fine oil separator were recirculated into the oil pan below the engine oil level via the oil return line.

The cleaned blow-by gases were then directed to the engine for combustion via a single-stage pressure-regulating valve. Depending upon the compression ratio in the intake manifold, the gas then flowed:

• Through the non-return valves into the intake manifold downstream of the throttle valve; or,
• Upstream of the exhaust turbocharger turbine.

Positive crankcase ventilation system

The CEPA/CEPB engine had a positive crankcase ventilation (PCV) system which sought to separate the fuel and water which could mix with the engine oil by purging the crankcase with fresh air at part throttle applications. If fuel and water remained in the oil sump, they could freeze at low ambient temperatures and cause engine damage (e.g. loss of oil pressure or oil leaks due to excessive pressure in the engine).

By admitting fresh air that was extracted downstream of the air filter, fuel and water were channelled through the rocker cover and into the cylinder head. The air then flowed through the chain shaft and into the crankcase, absorbing moisture and fuel along the way, thereby ‘flushing’ the engine dry. This process also extended the service life of the engine oil.