The powerful 4.0 TFSI is very efficient and represents a new and trailblazing type of sportiness. Its power rating of 309 kW (420 hp) is available as of 5,500 rpm and – between 1,500 and 5,200 rpm – the twin-turbo V8 continuously transmits 550 Nm (405.66 lb-ft) of torque. On average, though, it consumes less than ten liters per 100 km (less than 23.52 US mpg). Compared to its predecessor model, the new Audi S6 consumes 25 percent less fuel while offering much-improved driving performance.
The highly cultivated 4.0 TFSI, covered by an elegant designer fascia, offers a displacement of 3,993 cc; its bore measures 84.5 (3.33 in) and its stroke 89.0 (3.50 in) millimeters. It has all the typical features of an Audi V engine: The cylinder angle is 90 degrees, the chain drive for the four camshafts and the engine’s ancillaries are located on the back.
The cylinder body made of an aluminum/silicon alloy is manufactured by means of low-pressure chill casting, which ensures excellent homogeneity. A bedplate, which is a sturdy frame for the lower bearing bridges of the crankshaft, further boosts the block’s rigidity. The V8 is very light: even with all key add-on parts, it weighs just 220 kilograms (485.02 lb).
Like nearly every Audi engine, the 4.0 TFSI embodies Audi’s downsizing philosophy: substituting engine displacement with supercharging. One twin-scroll turbocharger per cylinder bank compresses the fresh air. Exhaust-gas ducts from two cylinders each are kept separate in the exhaust manifold and the turbocharger housing; these ducts only converge right at the turbine. This approach eliminates unwanted interactions between the gas columns. As a result, considerable torque is generated faster than otherwise.
The turbocharger and its intercooler are in the inner V of the cylinder banks, instead of their usual external position next to the crankcase. As regards the cylinder heads, the exhaust-gas side is inside and the inlet side outside. This layout ensures compact construction and short gas travel paths, with minimal friction losses. The 4.0 TFSI responds instantaneously when the driver depresses the gas pedal. Sophisticated insulation of the hot components, in particular the manifold, stabilizes the thermal conditions in the inside V.
The intake system is external, on the cylinder banks. Switchable flaps in the intake ports mix the incoming air in a cylindrical motion. The directly injected fuel is very turbulent, thus cooling the chambers. The reduced tendency to knock enables a high compression ratio of 10.1 : 1.
Impressive package: Technologies of the Modular Efficiency Platform
The 4.0 TFSI uses technologies from Audi’s modular efficiency platform, from the Start-Stop system to friction-reducing measures. At the Audi plant in Györ, Hungary, high-end technologies such as panel honing are used to assemble the engine. The piston pins have a fine, diamond-hard carbon (DLC) coating.
Innovative thermal management uses a ball valve to disconnect the water pump while the engine is warming up. The coolant stays in place throughout the engine and the oil heats up quickly, considerably shortening the phase of elevated friction losses. The demand-controlled oil pump varies the oil pressure in two stages.
A groundbreaking technology in the new 4.0 TFSI is the cylinder on demand system. It activates during low to mid-range engine loads. Depending on engine speed, its upper limit ranges from about 25 to 35 percent of maximum torque, about 160 to 250 Nm (118.01 to 184.39 lb-ft). Engine speed must be between 960 and 3,500 rpm, the coolant temperature must be above 30 degrees Celsius (86.00° F) and the transmission’s third gear or higher must be engaged.
If these conditions are met, then the system will close the intake valves and exhaust valves of cylinders 2, 3, 5 and 8 in both banks. At the same time, the engine management system shuts off fuel injection and the ignition system. The V8 is now operating as a V4: it fires just twice instead of four times per revolution of the crankshaft (firing order 1 – 4 – 6 – 7). Efficiency in the active cylinders is increased because the operating points are displaced toward higher loads.
In the deactivated cylinders, the pistons continue to move because they are being driven by the crankshaft. Before the valves close, the combustion chambers are again filled with fresh air. This fresh air-fuel mixture results in minimal pressure in the cylinder and, accordingly, a small amount of energy used.
The closing of valves is handled by an enhanced Audi valvelift system (AVS). Its sleeves, which can be adjusted electromagnetically on the four camshafts, have so-called zero cams as additional sections. Since these do not move the cam followers as they rotate over them, the valve springs keep the valves closed. The driver is alerted to four-cylinder status by means of a text notification as well as a green bar in the fuel-consumption indicator on the screen of the driver information system (DIS) in the instrument cluster.
Mere hundredths of a second: the gear shifts
As soon as the driver presses firmly down on the gas pedal, the deactivated cylinders cut in again. Switching to and from eight-cylinder mode is so smooth and speedy that it is barely noticeable. Depending on the operating point, shifting lasts between one hundredth and nearly four hundredths of a second.
Audi therefore developed a control-logic system which monitors a driver’s steering movements and use of the gas pedal. If this system detects an irregular pattern – while the vehicle is driving around a traffic circle, for example, or during sporty driving on a rural route – then no cylinders are deactivated in order to achieve the best possible efficiency.
The cylinder on demand system is operational at all times, even in the automatic transmission’s S mode and Audi drive select’s dynamic mode of operation. It offers the very best performance whenever the driver steadily maintains a moderate speed, which is common on many expressways. At 100 km/h (62.14 mph), for instance, it reduces fuel consumption of the new S models by more than ten percent. As per the NEDC (New European Driving Cycle), savings amount to approximately five percent. Including the Start-Stop system, which deactivates an idling engine, savings rise to some twelve percent.
Four-cylinder engines generally do not run quite as quietly as V8 engines; this is true of the new 4.0 TFSI, as well. When the cylinder on demand system is active, the crankshaft drive – depending on the load and engine speed – produces stronger torsional vibrations, which enter the vehicle’s interior as structure-borne sound waves and, consequently, as airborne sound. The exhaust system also emits certain humming noises, which cannot be prevented entirely despite an intelligent flap-control system.
Targeted interference: Active noise control
The new Audi active noise control (ANC) technology counteracts unwanted noise by generating a targeted cancellation sound. It puts to use the principle of destructive interference. When two waves of the same frequency overlap, their amplitudes cancel each other out – provided that they are equal and 180 degrees out of phase.
Four small microphones are integrated in the headlining of the new S models. Each of them registers the complete noise spectrum in its immediate area. Based on these signals, the ANC control unit generates a 3D ultrasonic pattern. It also utilizes engine-speed data supplied by the crankshaft sensor.
Whenever the system recognizes unwanted noises in a zone covered by pre-programming, it emits a precisely modulated cancellation sound via the sound system’s loudspeakers. It counteracts the noise so effectively that passengers will not hear it. Active noise control works with every sound system supplied by Audi. It works even when the system’s volume is low, muted or switched off.
Irrespective of the ANC, the S models come equipped with a second system: active engine mounts. They offset second-order engine vibrations which occur in four-cylinder mode by generating phase-shifted counter-vibrations.
The centerpiece of the active engine mount is an electromagnetic oscillating-coil actuator, which functions like a loudspeaker. Its fast movements – up to one millimeter of stroke between 25 and 250 hertz – are transmitted by means of a rubber diaphragm to the mount’s hydraulic fluid, which also bears the engine’s vibrations. The vibrations overlap in the fluid and cancel each other out; the point of connection between the mount and the body is quiet.
The control units for the active engine mounts receive their information from two sources. The crankshaft sensor supplies information about engine speed and its signals are used to calculate the phase and frequency of the actuator signal. Acceleration sensors on the engine mounts supply the data which describes the required amplitude. The active engine mounts are also active at idle speed with all eight cylinders in operation. In this case they largely eliminate engine excitation of the fourth order.
In addition, a centrifugal governor in a dual-mass flywheel also helps counterbalance the vibrations caused by the V8 operating in four-cylinder mode. It thus contributes to convenient and efficient driving in these modes of operation.
The equipment, data, and prices specified in this document refer to the model range offered in Germany. Subject to change without notice; errors and omissions excepted.