Audi 8U RS Q3: CTSA/CZGA 2.5 TFSI engine

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Introduction

Audi’s CTSA and CZGA were 2.5-litre inline five-cylinder turbocharged engines that powered the Audi 8U RS Q3. Hand-built in Gyor, Hungary, the CTSA engine was closely-related to the CEPA engine which powered the Audi 8J TT RS Coupe, but differed in that the CTSA engine had:

• Relative boost pressure of 1.0 bar (compared to 1.2 bar for the CEPA);
• A ‘start-stop’ system which enabled it to shut down when the RS Q3 was stationary to reduce fuel consumption; and,
• A Bosch MED 17.1.1 engine management system.

The CZGA engine replaced the CTSA in late 2014 and differed in that:

• It was compliant with Euro 6 emissions standards;
• Its turbocharger provided relative boost pressure of 1.2 bar; and,
• Peak power and torque increased to 250 kW and 450 Nm, respectively.

Key features of the CTSA and CZGA engines included:

• A 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;
• A BorgWarner K16 turbocharger;
• Direct fuel injection (Audi’s ‘Fuel Stratified Injection’ or FSI); and,
• A compression ratio of 10.0:1.

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Block

The cylinder block for the CTSA and CZGA engines had 82.5 mm bores that were spaced at 88 mm intervals and a 92.8 mm stroke for a capacity of 2480 cc. The crankcase was made from compacted vermicular graphite cast iron which had a tensile strength of 450 N/mm²– this substance had previously been used in naturally aspirated racing engines in the 1990s and Audi’s V6 and V8 TDI engines. 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 inline five-cylinder engines, the CTSA and CZGA engines had firing intervals of 144 degrees and did not require balance shafts.

Crankshaft

The CTSA and CZGA engines 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, there was 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 CTSA and CZGA engines 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 CTSA and CZGA engines 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.

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

Cylinder head

The CTSA and CZGA engines had a cast aluminium-silicon (AlSi7MgCu) alloy cylinder head and 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 double overhead camshafts.

For variable valve timing, both the intake camshafts had an adjustment range of 42 degrees relative to the crankshaft.

The CTSA and CZGA engines 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.

For heat dissipation, the stems of the exhaust valves were filled with sodium.

BorgWarner K16 turbocharger

The CTSA and CZGA engines 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. However,

• For the CTSA engine, maximum relative charge pressure was 1.0 bar (14.5 psi); and,
• For the CZGA engine, maximum relative charge pressure was 1.2 bar (17.4 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.

Since the compressing effect of the turbocharger increased the temperature of the intake air mass, the CTSA and CZGB engines had a charge air cooler (or intercooler) to increase its density.

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 CTSA and CZGB engines 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 CTSA and CZGB engines had a common fuel rail from which injected petrol directly into the combustion chamber at a pressure of up to 125 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 CTSA and CZGB engines had a compression ratio of 10.0:1.

Ignition

The CTSA and CZGB engines 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 firing order was 1-2-4-5-3.

The Bosch MED 17.1.1 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 CTSA and CZGB engines 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 for a louder exhaust note.

Crankcase ventilation

The CTSA and CZGB engines 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 CTSA and CZGB engines 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.