Continuation of the series for the brand's drive units
The eight - cylinder 4.0 TFSI of Audi and Bentley is the epitome of downsizing in the upper classes. It replaced the naturally aspirated 4,2-liter engine and 5,2-liter V10 engine of the S6, S7 and S8 and is available with power levels of 420 through 520 hp. up to 605 hp depending on the model. In these respects, Audi's engine is a direct competitor to the 4,4-liter N63 biturbo engine. BMW and its S63 version for the M-models. As with BMW, the two turbochargers are placed on the inside of the cylinder rows, which are located at 90 degrees as in the previous 4,2-liter unit. This arrangement achieves more compactness and shortens the exhaust path. The twin-scroll configuration (in BMW it is used only in the S-version) allows to reduce the mutual negative influence of the pulsations from the different cylinders and to extract most of their kinetic energy, and is realized by a complex combination of channels of cylinders of different rows. This operating principle provides a solid reserve of torque when accelerating even in modes slightly above idle speed. Even at 1000 rpm 4.0 TFSI already has 400 Nm. The more powerful version is ready to provide its maximum torque of 650 Nm (700 in the versions with 560 and 605 hp) in the entire range from 1750 to 5000 rpm, and the 550 newton meters of the standard are available even earlier - from 1400 to 5250 rpm. The engine block is made of aluminum alloys with homogeneous casting of aluminum at low pressure, and in the powerful versions it is additionally heat treated. To strengthen the block, five inserts of ductile iron are integrated in its lower part. As with the smaller EA888, the oil pump has a variable capacity, and at low speeds and loads the piston bottom cooling nozzles are switched off. The logic of engine cooling is similar, in which the control module regulates the temperature in real time and the circulation is maintained until the operating temperature is reached. When it is present, the fluid begins to move from the inside of the cylinders in the direction of the cylinder head, and if heating is required, an electric pump directs water from the head to the cabin. Here, too, in order to eliminate the piston almost completely, several fine fuel injections per cycle are performed with a cold engine.
Switch off part of the cylinders
The partial load cylinder shut-off system is not a new approach to reducing fuel consumption, but with Audi's turbocharged engine, this solution has been perfected. The idea of such technologies is to increase the so-called. operating point - when the engine requires a power level that would handle four of the eight cylinders, the latter operate in a much more efficient mode with a wider throttle. The upper limit of the cylinder deactivation operation is between 25 and 40 percent of the maximum torque (between 120 and 250 Nm), and in this mode the average effective pressure in the cylinders increases significantly. The coolant temperature must have reached at least 30 degrees, the transmission must be in third or higher gear, and the engine must be running between 960 and 3500 rpm. If these conditions are met, the system closes the intake and exhaust valves of two cylinders of each cylinder row, whereby the V8 unit continues to operate as a V4.
The closing of the necessary valves on the four camshafts is carried out with the help of a new version for controlling the phases and stroke of the valves Audi valvelift system. The bushings with cams for opening two valves and channels are moved to the side with the help of electromagnetic devices with pins, and in the new version they also have cams for "zero stroke". The latter do not affect the valve lifters and the springs keep them closed. At the same time, the engine management system stops fuel injection and ignition. However, before the valves close, the combustion chambers are filled with fresh air - the replacement of exhaust gases with air reduces the pressure in the cylinders and the energy required to drive the pistons.
The moment the driver presses the accelerator pedal harder, the deactivated cylinders start working again. The return to eight-cylinder operation, as well as the reverse process, is extremely precise and fast, and virtually imperceptible. The entire transition takes place in just 300 milliseconds, and the mode change leads to a short-term reduction in efficiency, so that the actual reduction in fuel consumption begins about three seconds after deactivation of the cylinders.
According to Audi, people from Bentley, who use the advanced 4.0 TFSI for the new Continental GT (2012 debut), have also been involved in the development process of this technology. Such a system is not new to the company and works in the 6,75-liter V8 unit.
V8 engines are known not only for their traction and harmonious throttle response, but also for their smooth operation - and this applies in full force to the 4.0 TFSI. However, when a V8 engine functions as a V4, depending on the load and speed, its crankshaft and reciprocating components begin to generate high levels of torsional vibration. This in turn leads to the appearance of specific noises that penetrate into the interior of the car. With its large size, the exhaust system also generates specific bass sounds that are difficult to suppress, despite the intelligent gas flow control system with valves. In search of ways to reduce vibration and noise, Audi's designers have taken an unusual technological approach, creating two unique systems - anti-sound generation and vibration damping.
Thanks to the intensive vortex process during filling and the increased burning rate, the degree of compression can be increased regardless of the presence of turbocharging without the risk of causing detonations in the combustion process. There are also some technological differences between the different power versions of the 4.0 TFSI, such as the use of a single- or double-circuit intake system, different operating settings of the turbochargers and the presence of an additional oil cooler in the more powerful units. There are also structural differences in the crankshafts and their main bearings, the degree of compression, the phases of gas distribution and the injectors are different.
Active noise control and vibration damping
Active Noise Control (ANC) counteracts unwanted noise by generating "anti-sound". This principle is known as destructive interference: if two sound waves of the same frequency overlap, their amplitudes can be "arranged" so that they are mutually attenuated. For this purpose, their amplitudes must be the same, but they must be out of phase at 180 degrees to each other, ie they must be in antiphase. Experts also call this process "reverse noise elimination". Audi's models, which will offer the new 4.0 TFSI unit, are equipped with four small microphones integrated in the roof lining. Each of them registers the full noise spectrum in the adjacent area. Based on these signals, the ANC control module creates a differentiated spatial noise picture, while at the same time the crankshaft speed sensor provides information about this parameter. In all pre-calibrated areas where the system identifies disturbing noise, it purposefully generates precisely modulated elimination sound. Active noise control is ready for operation at any time - whether the audio system is on or off and whether the sound is amplified, reduced, etc. The system also works regardless of the system with which the car is equipped.
The way to dampen the vibrations is very similar as an idea. In principle, Audi uses rigid, sporty settings for the engine mounts. For 4.0 TFSI, engineers have developed active mounting brackets or pads that aim to eliminate motor vibrations with phase-shifted reverse oscillations. A key component in the system is an electromagnetic device that creates vibrations. It has a permanent magnet and a high-speed coil, the movement of which is transmitted by means of a flexible membrane to a chamber with liquid. This fluid absorbs both the vibrations caused by the motor and those that counteract them. In addition, these elements limit vibrations not only in atypical operating modes such as V4, but also in normal V8 mode, with special attention paid to idling.
Text: Georgy Kolev