More than simply fine tuning: the engine technology
What can the production FSI engine, in which petrol is injected directly into the cylinders, do better than a conventional engine with fuel injection into the intake manifold?
The answers are:
This progress is made possible above all by the use of both types of operation: stratified charge with part throttle and homogeneous charge with full throttle.
How does the FSI technology differ operationally from a conventional engine? The term "petrol direct injection" makes the central feature clear: in contrast to indirect injection, the fuel is injected directly into the combustion chamber.
More than just fine tuning was required by the Audi technicians in order to achieve this. Many new components and modules had to be specifically developed for this. They include the following:
Two operating states
The injector, which is located under the input channels, is supplied by a high-pressure pump driven by the camshaft and a single distribution pipe for all cylinders, the common rail system. The injector controls the injection time to within thousandths of a second at injection pressures of up to 110 bar.
A two-stage intake manifold is used with two length settings for use at higher and lower engine speeds. A continuous adjuster on the inlet camshaft varies the inlet valve opening times as required.
Another special feature can be found in the intake port - the so-called "tumble flap". This is infinitely variable on the 2.0 FSI engine, thus controlling the movement of the incoming air.
With the flap in the open position, the air passes by unobstructed. If the flap is tilted, it channels the intake air via a controlled path into the combustion chamber and to the piston crown.
This makes two different operating modes possible - these are the fundamental requirement for the versatility of an FSI engine: homogeneous and stratified-charge operation. Depending on load status and accelerator position, the engine electronics always switch to the optimum mode - without the driver noticing.
Homogeneous operation - more power, less fuel
A conventional indirect injection engine produces a petrol-air mixture in the entire combustion chamber with an ignitable ratio of 14.7 : 1 (lambda = 1). An FSI engine also operates in this mode, known as "homogeneous operation": namely whenever the engine is required to produce a high power output.
At full load, the fuel is injected synchronously with the air intake phase. This fills the combustion chamber uniformly. However, as the fuel - in contrast to the indirect injection engine - is completely vaporised in the combustion chamber, the cold from the vaporisation process cools the mixture there. For this reason, the direct injection spark-ignition engines are less susceptible to engine knock and the compression ratio can be increased accordingly (2.0 FSI: 11.5:1). Much greater efficiency is therefore possible.
In other words: even in homogeneous mode, considerably lower fuel consumption can be achieved together with higher output and torque values - a fact that has been proven by the winning Le Mans engine which operates permanently in the homogeneous mode.