Continuation of the series for the brand's drive units
Nowadays, designers of modern gasoline engines are looking for more and more diverse methods to increase their efficiency. It is true that diesels have also experienced downsizing in recent years by reducing the displacement, increasing the boost pressure and the injection system, and sometimes using a cascade turbocharging system. However, they have long used forced filling and, unlike their gasoline counterparts, have already skipped the evolutionary stage of the transition from atmospheric to forced filling. The principle of operation of high-pressure diesels in the cylinders and the lack of a throttle valve makes them initially effective. Therefore, downsizing in gasoline engines becomes far more extreme with a reduction in the volume and number of cylinders and the transition to forced filling. However, the high exhaust gas temperature compared to diesels still makes the use of variable geometry turbochargers inaccessible (with the exception of BorgWarner units for Porsche 911 Turbo), the throttle valve continues to create resistance to air and designers are looking for all sorts of alternative methods to improve their efficiency. Ten years ago from Audi first introduced the combination of turbocharging and direct gasoline injection with their TFSI, and now with their new 2.0 TFSI engine, the company's engineers have returned to Miller's well-known cycle - only in a rather modified form. The company's marketing calls the philosophy of creating the new engine with 190 hp. and a maximum torque of 320 Nm "rightsizing", in the sense of "precisely selected working volume". However, the term is very different from the message of their colleagues from Mazda, who refer in this case to the avoidance of forced charging.
On the contrary, in Audi, turbocharging is an essential element in the workflow strategy of the new engine, just as the compressor is an invariable attribute of Miller cycle machines, the most typical of which is the Mazda Millenia of the 90s. This principle of operation involves keeping the intake valve open long after the piston has begun to move from a lower to a dead center. As the air thus begins to return to the intake manifolds, the mechanical compressor, which creates a back pressure, takes care of its retention. At first glance, this seems pointless, but in practice the dynamics of the flow is such that in this case it experiences less resistance than if it is compressed in the cylinder itself. On the other hand, the degree of expansion stroke becomes higher at a normal degree of compression without the danger of detonation. That is, Miller's principle allows a different degree of compression and expansion to be achieved, rather than the same as with the standard Otto engine. A positive effect is also the ability to work with a wider open throttle valve.
Audi's interpretation of the Miller cycle
Audi designers interpret this theme in their own way. However, unlike the main process, instead of keeping the intake valve open to reduce the degree of compression, they simply close it much earlier - before the piston has reached a bottom dead center. Instead of the opening time being 190-200 degrees of crankshaft rotation as usual, the valve remains open only for 140 degrees. However, in practice, the same effect of reducing the degree of compression is achieved. Compensation for the reduced opening time is done by increasing the filling pressure with the help of the turbocharger. Thus, the engine achieves the cost of a downsizing engine, and at full load has the dynamic performance of a large machine. When operating at partial load, additional fuel injection is performed as the piston moves upwards by means of the direct injection system, which complements another injection system in the intake manifolds. In addition, the Audi Valvelift System (AVS) for variable gas distribution allows the suction valve opening phase to be increased to 170 degrees at full load. To this are added intelligent cooling control, integrated in the head exhaust manifold and further friction reduction by using low viscosity oil (0W-20). Thanks to the many high-tech solutions, the new 2.0 TFSI has a maximum torque in the range of 1450 to 4400 rpm and consumes less fuel.
3.0 TFSI: Mechanical instead of turbocharger
Porsche colleagues preferred biturbo filling for its three-liter V6 engine with 420 hp. For the 3.0 TFSI, Audi uses mechanical compressor filling (Eaton sixth generation, R1320) with water / air intercooling. The process of creating the engine was extremely short, which is perhaps one of the explanations for this decision, although Audi claims that this concept is preferred because of other advantages - such as the popularity of this type of forced filling in the United States. The specifics of Audi's solution include a compressor located behind the throttle valve, which significantly increases the filling efficiency. At partial load, a special valve in the compressor housing returns some of the compressed air to its inlet, thus reducing losses and the power required to rotate it. In practice, up to certain modes the unit works almost like an atmospheric motor and only at high load the compressor starts operating at full capacity.
2.5 TFSI: Five-cylinder for sporty compact versions
This unit follows many of the postulates of the company's other engines, taking into account the specifics of the five-cylinder engines. 2.5 However, the TFSI has a more limited scope and only drives models such as the Audi RS 3, TT RS and RS Q3. In the Audi TT RS plus version, the engine with a displacement of 2,48 liters has an output of 360 hp. - the same as AMG's new four-cylinder engine for the A-Class and its derivatives. However, the five-cylinder engine provides its maximum torque of 465 Nm significantly earlier (in the range from 1650 to 5400 rpm) than the machine of colleagues from Stuttgart.
Text: Georgy Kolev
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