Audi Drive Test Range - Part 3: 2.0 TFSI, 2.5 TFSI, 3.0 TFSI

Continuation of the series for the drive units of the brand

Nowadays, designers of modern gasoline engines are looking for more and more diverse methods to increase their efficiency. It is true that diesels in recent years have also experienced downsizing with a reduction in displacement, an increase in boost pressure and the injection system, and sometimes with the use of a cascade turbocharging system. However, they have long used forced filling and, unlike their gasoline counterparts, have already skipped the evolutionary stage of switching from atmospheric to forced filling. The principle of operation of diesels with high pressure in the cylinders and the absence of a throttle valve makes them initially efficient. Therefore, the downsizing of gasoline engines takes on a far more extreme character with a reduction in volume and number of cylinders and a switch to forced filling. However, the high temperature of the exhaust gases compared to diesels still makes the use of variable geometry turbochargers unaffordable (with the exception of BorgWarner units for the Porsche 911 Turbo), the throttle valve continues to create air resistance, and designers are looking for all possible alternative methods to improve their efficiency. Ten years ago, Audi first introduced the combination of turbocharging and direct gasoline injection with its TFSI, and now with its new 2.0 TFSI unit, the company's engineers have returned to the well-known Miller cycle - only in a rather modified form. The company's marketing calls the creation philosophy of the new motor with a power of 190 hp. and a maximum torque of 320 Nm "rightsizing", in the sense of "exactly 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 filling.

On the contrary, at 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. Unlike the basic process, however, instead of holding the intake valve open to reduce the compression ratio, they simply close it much earlier - before the piston has even reached bottom dead center. Instead of the opening time being 190-200 degrees of crankshaft rotation as usual, the valve only stays open for 140 degrees. However, in practice, this achieves the same effect of reducing the compression ratio. Compensating for the reduced opening time is done by increasing the boost pressure using the turbocharger. Thus, the engine achieves the consumption of a downsizing engine, and at full load it has the dynamic performance of a large machine. In part-load operation, additional fuel injection is performed on the upward stroke of the piston using the direct injection system, which complements another injection system in the intake manifolds. In addition, the Audi Valvelift System (AVS) for variable valve timing allows the opening phase of the intake valves to be increased to 170 degrees under full load. Added to this are intelligent cooling management, a head-integrated exhaust manifold and further friction reduction through the use of low-viscosity oil (0W-20). Thanks to numerous high-tech solutions, the new 2.0 TFSI has 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 their three-liter V6 engine with 420 hp. For the 3.0 TFSI, Audi uses a mechanical compressor charge (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 does the compressor start operating at its 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 five-cylinder engines. The 2.5 TFSI, however, has a more limited field of application and only powers 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 a power 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 of 1650 to 5400 rpm) than the machine of colleagues from Stuttgart.

(to follow)

Text: Georgy Kolev

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2020-08-30