With the newly developed 2.9 TFSI, Audi has drawn upon the legendary 2.7 liter V6 from the first RS 4 Avant. Produced between 1999 and 2001, it delivered 280 kW (380 hp). Just like the 2.7 liter engine, the new high-performance V6 is also designed as a bi-turbo engine. In comparison with its direct predecessor, a freely-aspirated, high-revving V8 unit, its pulling power and efficiency attain a whole other dimension.
From 1,900 to 5,000 rpm, the 2.9 TFSI supplies the crankshaft with an impressive 600 Nm (442.5 lb-ft) of torque, which is 170 Nm (125.4 lb-ft) more than on the previous model. With its 331 kW (450 hp) Рwhich equates to a power ratio of 155.5 hp per liter displacement Рthe unit accelerates the new RS 5 Coup̩ (combined fuel consumption in l/100 km: 8.7; combined CO2 emissions in g/km: 197) from zero to 100 km/h (62.1 mph) in 3.9 seconds and thus six tenths of a second faster than before. If the customer orders the RS dynamic package, Audi increases the electronically restricted top speed from 250 km/h (155.3 mph) to 280 km/h (174.0 mph).
Modular concept: the new V6 generation
The 2.9 TFSI was derived from the 3.0 TFSI used in the Audi S5. Because of the greater forces inside the engine, the valve lift has been shortened by 3 millimeters (0.1 in) to 86 millimeters (3.4 in). With its unchanged bore of 84 millimeters (3.3 in), the displacement is 2,894 cm3. The crankcase integrates thin-walled cylinder liners made of gray cast iron and the crankshaft main bearing has a 2-millimeter (0.08 in) larger diameter compared with the 3.0 TFSI.
The crankcase is created from an aluminum-silicon alloy by means of sand casting and weighs just 34 kilograms (75.0 lb). The so-called “deep-skirt” design extends the walls of the crankcase far downwards, which also saves weight. The entire engine weighs just 182 kilograms (401.2 lb) which makes it 31 kilograms (68.3 lb) lighter than its V8 predecessor. Gross weight and axle load distribution are the predominant beneficiaries of this in the new RS 5 Coupé.
Typically Audi is the positioning of the exhaust side within the V
Each of the two turbochargers on the 2.9 TFSI is assigned to a single cylinder bank and can build up a maximum of 0.95 bar charge pressure. Like with all new V6 and V8 engines from Audi, the turbochargers are installed within the 90-degree angle of the cylinder banks, and thus the exhaust side of the cylinder heads is on the inside, while the intake side is on the outside of the engine. This layout enables compact construction and short gas flow paths with minimal flow losses – the 2.9 TFSI responds extremely spontaneously to movement of the accelerator pedal.
Fuel savings: the new B-cycle combustion process
The high-output V6 doesn’t just impress with its strong performance, but also with its high level of efficiency. In the NEDC, it consumes just 8.7 liters of fuel per 100 kilometers (27.0 US mpg), which corresponds to 197 grams CO2 per kilometer (317.0 g/mi). This means a 17 percent consumption saving compared with the previous model. A decisive factor in this is the new TFSI combustion process from Audi, which is known as the B-cycle. It has been specifically designed for the partial-load range, which is the predominant operating mode during normal use.
In the induction tract, the intake valves close well before the piston reaches bottom dead center. This very short opening time keeps the fresh gas flow comparatively small. When the piston moves back up again after reaching bottom dead center, the compression phase starts later than usual. This allows a high geometric compression ratio of 10.0:1 – the combustion then takes place in a relatively small volume. In comparison to the short compression phase, the expansion phase is extended, which results in a high level of efficiency.
In the classic Miller cycle, the reduced cylinder charge is detrimental to torque and power output. These aspects have been neutralized by Audi thanks to the implementation of turbocharging and the two-stage Audi valvelift system (AVS). The four camshafts of the 2.9 TFSI can each be adjusted by 50 degrees crank angle. At higher load and engine speed ranges, AVS closes the inlet valve later. The opening time increases from 130 to 200 degrees crankshaft angle, while at the same time the valve lift increases from 6.0 to 10.0 millimeters (0.2 to 0.4 in). Cylinder charging also increases considerably – the 2.9 TFSI revs up powerfully and delivers an impressive output.
An important component of the B-cycle combustion process is the central position of the injectors in the combustion chamber. In the area around the inlet valve, this makes possible a geometry which, in interaction with the inlet ports, allows for targeted mixing up of the gas charge. The common-rail system injects the fuel into the combustion chamber with 250 bar of pressure – the high injection pressure ensures a homogeneous spray pattern and consequently a uniform propagation of the flame front.
Separate circuits: thermal management
A further efficiency component of the 2.9 TFSI is the thermal management system. The crankcase and the cylinder head have separate coolant circuits. After a cold start, the switchable water pump controls the flow of coolant through the engine so that the oil is warmed up as quickly as possible. The exhaust manifolds are integrated in the cylinder head, and coolant circulates around them which helps ensure rapid warming up. When the engine is warm, this technology lowers the exhaust temperatures – consumption drops drastically, especially when driving in a sporty way. The great efficiency of the V6 bi-turbo is also partly to do with low friction. For example, the fully adjustable oil pump only builds up as much pressure as necessary.
The chain drive uses a new concept to reduce the drive power required: the crankshaft drives the balancer shaft via gears, with the shaft lying far down in the V of the cylinder banks. From here chains run to the four camshaft sprockets. These are trioval in shape, i.e. they have a slightly triangular shape, in order to compensate for peak forces. The balancer shaft rotates on roller bearings with particularly low levels of friction.
Powerful: the sound of the V6 bi-turbo
Even just starting the vehicle, it has a distinctive sound. At high loads, the flaps in the exhaust system open in order to provide an even fuller sound. Using the standard dynamic handling system Audi drive select, the driver can control the opening and closing of the flaps themselves – both on the standard RS exhaust system as well as on the optional RS Sport exhaust system with black tailpipe trims. The intake system of the V6 bi-turbo has also been extensively optimized: pipes in polished stainless steel allow the unhindered flow of the air inwards.