Audi pushes efficiency boundaries
Audi has presented what it terms the most efficient two-litre gasoline engine in its class, the new 2.0 TFSI. Audi will be using the engine for the first time in the next generation of the A4.
The four-cylinder engine produces 140kW) of power and 320Nm of torque from a displacement of 1,984 cc. Because the high torque is applied in a wide speed range of 1,450 to 4,400 rpm, and boasts ground-breaking low fuel consumption levels at less than 5L/100 km.
Technology in detail
The breakthrough of the 2.0 TFSI lies in a new combustion method. At its core its principle is comparable to the Miller cycle. Audi engineers have further developed that method in crucial ways, however. The increase in efficiency is based on the following factors:
The intake time has been significantly shortened (140 deg. crank angle (CA) rather than 190 to 200 deg.CA).
Owing to a higher boost pressure on the inlet side, the engine attains optimal cylinder charges despite the shorter intake time.
The intake valve also closes earlier – well before the bottom dead centre is reached. This lowers the medium pressure, allowing a high, efficiency-- boosting compression ratio.
In the partial load range, an additional injection upstream from the intake valve yields an efficient mixture formation that is already complemented by the direct injection in the intake manifold and in the combustion chamber.
The Audi Valvelift System (AVS) on the inlet side allows a short intake timee at partial load and a longer time at higher loads (full load: 170 deg. CA).
In the Miller cycle, the intake valve is left open longer than it would be in an Otto cycle engine. In effect, the compression stroke is two discrete cycles: the initial portion when the intake valve is open and final portion when the intake valve is closed. This two-stage intake stroke creates the so-called “fifth” stroke that the Miller cycle introduces.
As the piston initially moves upwards in what is traditionally the compression stroke, the charge is partially expelled back out through the still-open intake valve. Typically this loss of charge air would result in a loss of power, but in the Miller cycle, this is usually compensated for by thee use of a; however Audi has managed to make it work with turbocharging.
In the Miller cycle engine, the piston begins to compress the fuel-air mixture only after the intake valve closes; and the intake valve closes after the piston has travelled a certain distance above its bottom-most position: at around 20 percent to 30 percent of the total piston travel of this upward stroke. So in the Miller cycle engine, the piston actually compresses the fuel-air mixture only during the latter 70 percent to 80 percent of the compression stroke.
During the initial part of the compression stroke, the piston pushes part of the fuel-air mixture through the still-open intake valve, and back into the intake manifold.