The new 4.0 TFSI from Audi combines supreme power with groundbreaking efficiency. It produces its rated output of 382 kW (520 hp) at 6,000 rpm. The twin-turbo V8 delivers a constant 650 Nm (479.42 lb-ft) of torque to the crankshaft between 1,700 and 5,500 rpm. Its average fuel consumption of just 10.2 liters per 100 km (23.06 US mpg) is much lower than that of its competitors. And compared to its predecessor, the new S8 consumes on average 23 percent less fuel.
The sonorous 4.0 TFSI has a displacement of 3,993 cc, a bore of 84.5 millimeters (3.33 in) and a stroke of 89 millimeters (3.50 in). As the youngest member of the large Audi family of V engines, the 4.0 TFSI has all of their typical features. The included angle between the cylinder banks is a classic 90 degrees. As a space-saving measure, the chain drive to the four camshafts and the auxiliaries is located at the rear of the engine.
The cylinder case is made from an aluminum-silicon alloy using a low-pressure chill casting process that achieves a high standard of homogeneity. The high proportion of silicon content makes the cylinder walls extremely resistant to wear. The bedplate – a stable frame for the crankshaft bearing bridges – further enhances the block’s rigidity. Weighing a good 220 kilograms (485 lb) including all key add-on parts, the V8 is very light.
Like almost every Audi engine, the new 4.0 TFSI also follows the principle of downsizing: Forced induction replaces displacement. This also harmonizes perfectly with FSI direct fuel injection. One twin-scroll turbocharger per cylinder bank compresses the fresh air. From each pair of cylinders, the exhaust ports lead separately to a manifold and the turbocharger housing; they are not combined until immediately ahead of the turbine.
The twin-scroll technology prevents the exhaust gas flows from influencing each other. The result is powerful torque build-up at an early stage when accelerating from idle speed. Even at 1,000 rpm, the 4.0 TFSI reaches a torque of roughly 400 Nm (295.02 lb-ft).
Special feature: turbocharger in the V, “cold side” on the outside
The turbochargers and the common intercooler – an air-to-water heat exchanger – are located in the inside V of the cylinder banks rather than in the usual location outside next to the crankcase. The cylinder heads have a new layout, with the exhaust side on the inside and the intake side on the outside. This layout provides for compact dimensions and short gas paths with minimal flow losses, and the 4.0 TFSI reacts immediately to the gas pedal as a result. Sophisticated insulation of the hot components, in particular the manifold, stabilizes the thermal conditions in the inside V.
The fresh air system is mounted on the outside of the cylinder banks. Switchable flaps in the intake ports induce a rolling type of movement in the incoming air. The directly injected and intensively swirled fuel cools the combustion chambers.
The 4.0 TFSI employs all the technologies from Audi’s modular efficiency platform. They range from the start-stop system to a wide variety of measures aimed at reducing friction.
High-end technologies such as plate honing are used during assembly of the V8 at the Audi plant in Györ, Hungary. The piston pins have a fine, diamond-like carbon (DLC) coating.
The innovative thermal management system uses a ball valve to disconnect the water pump during the engine warm-up phase. Because there is no coolant flow at all through the engine, the oil reaches its operating temperature more quickly, shortening the phase in which high frictional losses occur. The demand-controlled oil pump varies oil pressure in two stages.
Attractive innovation: the cylinder on demand system
The most interesting technology in the new 4.0 TFSI, however, is the cylinder on demand system. It is activated at low to moderate engine loads, with the upper limit between 25 and 35 percent of peak torque, or 160 and 250 Nm (118.01 and 184.39 lb-ft), depending on engine speed. Coolant temperature must be at least 30 degrees Celsius (86 degrees Fahrenheit), third gear or higher must have been selected and the engine must be running at more than idle speed, namely between 960 and 3,500 rpm.
If these preconditions are satisfied, the system closes the intake and exhaust valves of cylinders 2, 3, 5 and 8 on both banks. The V8 continues to run as a V4 with a regular firing order (1 – 4 – 6 –7), but with only two cylinders instead of four being ignited on each revolution of the crankshaft. Efficiency in the active cylinders is increased because the operating points are displaced toward higher loads.
The valves on the four camshafts are closed by an enhanced version of the Audi valvelift system (AVS). Its sleeves can be slid sideways electromagnetically, and have “zero-lift cams” as additional profiles.
Since these do not move the cam followers as they rotate over them, the valve springs keep the valves closed. At the same time, the engine management system shuts down fuel injection and ignition. Four-cylinder status is indicated to the driver on the instrument cluster’s driver information system display. The fuel consumption indicator bars turn green and a text message is also displayed.
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 intake of fresh air minimizes pressure in the cylinder and reduces the energy needed to move the pistons accordingly – an important factor for increased efficiency.
As soon as the driver presses firmly down on the gas pedal, the deactivated cylinders cut in again. The return to eight-cylinder operation, like the cylinder deactivation process, takes place so smoothly and quickly that it is essentially imperceptible. The changeover takes between just one hundredth and nearly four hundredths of a second, depending on the operating point. The changeover is accompanied by parallel interventions in the ignition and the throttle valves, resulting in a temporary drop in efficiency during the transitional phase.
Audi has therefore developed a control logic that monitors the movement of the accelerator and the steering wheel by the driver. Cylinder deactivation may be inhibited if an irregular pattern is detected, for instance at a roundabout or when the car is driven hard on interurban roads. Cylinder deactivation lasting only a few seconds would tend to increase fuel consumption rather than reducing it.
Savings: more than ten percent at 100 km/h
The cylinder on demand system is ready to operate at all times, even in the S mode of the automatic transmission or the dynamic setting of Audi drive select. It saves the most fuel when the car is driven steadily at a moderate speed, as is typically the case on many main roads. At around 100 km/h (62.14 mph), the new technology reduces the fuel consumption of the 4.0 TFSI by more than 10 percent. In the standard NEDC test cycle, the increase in efficiency is around five percent. If the effect of the start-stop function that switches off the engine when idling is added to this, the total savings effect is around 12 percent.
Four-cylinder engines generally do not run as smoothly as classic V8 engines, and this is also true of the new 4.0 TFSI when the cylinder on demand system is active.
During these phases, the crankshaft drive produces higher torsional vibrations as a function of load and engine speed. These vibrations penetrate into the cabin in the form of solid-borne sound and the resultant airborne sound. The large exhaust system also emits certain thrumming noises that cannot be suppressed completely despite intelligent flap control.
The new Audi active noise control (ANC) technology counteracts unwanted noise by generating a targeted cancellation sound. This principle is known as destructive interference: If two waves of the same frequency are superimposed, their amplitudes – the peaks and troughs that determine the sound pressure – cancel each other out. The amplitudes must be of the same strength and the phases opposed by 180 degrees.
In the Audi S8, four small microphones are integrated inconspicuously into the headlining. Each of them registers the complete noise spectrum in its immediate area. From these signals the ANC control unit computes a highly differentiated spatial sound image; information on actual engine speed is also obtained from the crankshaft sensor. In all of the previously calibrated zones in which the system identifies disturbing noise, it emits a precisely modulated cancellation sound. The ANC must react particularly quickly and accurately during the moments in which cylinder deactivation or reactivation takes place.
The cancellation sound is played through the speakers of the sound system. Because each microphone and each speaker must cover a certain area, there is a slight fuzziness. The cancellation sound therefore does not completely eliminate the disturbing noise, but reduces it until the vehicle occupants can no longer hear it. ANC is always active, regardless of whether the sound system is on or off, or volume is set to loud, soft or mute. It works with both of the available sound systems.
Vibration killers: active engine mounts
Audi has developed a second system in parallel to ANC: The active engine mounts generate out-of-phase counter-oscillations to compensate for the second-order engine oscillations that occur during four-cylinder operation. The key component is an electromagnetic oscillating-coil actuator. A flexible diaphragm transmits the rapid strokes of the actuator to the hydraulic fluid in the mounting, which also absorbs vibrations from the engine. In the fluid, the counter-oscillations overlay the engine vibrations and cancel them out.
The control units for the active engine mounts receive their information from two sources. Engine speed is detected by the crankshaft sensor; these signals are used to compute the precise phase and frequency of the actuator signal. Acceleration sensors on the two engine mounts supply the data that determine the amplitude necessary to cancel out the vibration. 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.
Audi normally uses firm, sporty settings for its engine mounts. For many years now, Audi has therefore used switchable electromagnetic mounts in certain models and with certain engines. These have two operating settings: During idling, their soft characteristics keep disturbing noises and vibrations away from the interior; during driving the damping is increased, in order to suppress engine vibrations.
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.