The V8-Biturbo 4.0 TFSI
The new Audi RS 6 Avant and the RS 7 Sportback use the most powerful series-production engine in the Audi lineup. From a displacement of 3,993 cm3, the 4.0 TFSI generates 700 Nm of torque between 1,750 and 5,500 revs. Its rated power stands at 412 kW (560 hp).
The new RS 6 Avant and the RS 7 Sportback accelerate from 0 to 100 km/h in 3.9 seconds – a top figure in this class. Depending on customer preference, the top speed is 250, 280 or even 305 km/h. In the NEDC, both high-performance models consume just 9.8 liters of fuel per 100 km – around 30 percent less than for the previous RS 6 Avant model with the V10 engine.
The V8 biturbo is a high-tech engine. Its aluminum-silicon crankcase is made using a low-pressure gravity die-casting process that delivers an exceptional degree of homogeneity. A robust frame for the crankshaft’s lower bearing bridges further increases the overall stiffness of the block. The chain drive is located at the back of the engine where it saves on packaging space. This compact configuration reduces the length of the engine to 497 millimeters; bore and stroke stand at 84.5 and 89.0 millimeters respectively. With all its key ancillaries, the unit weighs a total of 224 kilograms.
Fuel supply is handled by Audi’s FSI direct injection. Switchable flaps in the intake channels send the inflowing air into a cylindrical rotation; depending on their setting, this improves the chamber fill and thus the combustion. The direct injection of intensely swirled fuel cools the combustion chambers, while the two turbochargers generate up to 1.2 bar of relative charge pressure. Twin-scroll technology, by which the exhaust gas from two cylinders streams into the turbine through separate channels, avoids undesirable interaction between the gas columns – an effect that results in an early and steep build-up of torque.
The large turbos and their air/water intercooler are located within the V of the cylinder banks, instead of in the usual position on the outside next to the crankcase. In the cylinder heads, the exhaust side is on the inside and the intake side on the outside. This layout ensures short gas paths with minimal flow losses and highly responsive characteristics. Sophisticated insulation of the hot components maintains thermal stability inside the V.
The four-liter V8 has a dedicated engine control system and is supplied with air by a dethrottled intake system. There are sound flaps in the two-channel exhaust system that bring added volume to the engine’s tone. They are controlled on the basis of load and torque by the mode set in the Audi drive select system and by the operating level of the eight-speed tiptronic. quattro GmbH also offers an optional sports exhaust system.
The 4.0 TFSI is equipped with technologies from Audi’s modular efficiency platform. These include a bundle of friction-reducing measures, the start/stop system and the innovative thermal management; it regulates the flow through the engine of cooling water during the warm-up phase to ensure that the oil heats up as quickly as possible. The regulated oil pump varies the oil pressure in two stages, while the piston oil injectors are map controlled.
A further groundbreaking innovation is the cylinder on demand (COD) technology based on the Audi valvelift system (AVS). At low to medium load and revs – up to around 250 Nm and 3,500 rpm – it deactivates cylinders 2, 3, 5 and 8 by closing the valves and stopping injection and ignition. The efficiency of the active cylinders increases because the operating points shift to higher loads.
The switchover occurs in a matter of hundredths of a second via sleeves pushed axially along the camshafts. To achieve this, electromagnetically actuated pins are fired into spiral grooves on the outside of the sleeves. This process takes place so smoothly that the driver is virtually unaware of the switch to four-cylinder mode. The cylinders are reactivated as soon as he steps firmly on the gas pedal. COD technology reduces fuel consumption in the NEDC by around five percent and, at a constant speed of 100 km/h, by around as much as ten percent.
When the V8 is running temporarily as a V4, it fires only every 180 degrees of crank angle. As a consequence, its crank gear produces higher torsional vibrations, which are counteracted by active engine mounts. These use electromagnetic actuators to generate phase-offset counter-vibrations that largely cancel out the vibrations from the engine. The active engine mounts also function during idling.