"So how does an FSI engine differ from a conventional one? On this engine, fuel is not injected into the intake port, but directly into the combustion chamber. The injector, which is supplied by a high-pressure pump and common rail fuel line, is in the side of the cylinder head, and controls the injection time to within milliseconds, at injection pressures of up to 110 bar.
In the stratified-charge operating mode, fuel is injected during the engine's compression stroke and is picked up by the movement of the air that has been drawn into the combustion chamber. A movable flap in the intake manifold, the intake port and the special shape of the piston crown impart a type of movement in the air that is known as "tumble".
The main factor contributing to the engine's efficiency is the stratified-charge principle at part load. In other words, in this operating mode the engine only needs a fuel-air mixture capable of immediate ignition in the area around the spark plug. The remainder of the combustion chamber is filled with a leaner mixture, that is to say one with a considerable degree of excess air. As a result of this, the engine can be run with the throttle valve open. The direct injection engine also benefits from reduced heat losses,
This means that the strata of air around the 'cloud' of ignitable mixture isolate the latter from the cylinder and cylinder head; less thermal energy is lost via the cooling system.
The desired "stratified-charge effect" is obtained in this way: the cloud of fuel which has become swirled with sufficient air to form an ignitable mixture surrounds the spark plug at the moment of ignition.
After combustion, a layer of air insulates the ignited mixture from the cylinder wall. This cuts the amount of heat lost to the engine block and increases the engine's operating efficiency.
In stratified-charge operation, incidentally, significantly higher lambda values related to the combustion chamber as a whole are achieved. This is essential if fuel consumption is to be reduced at low and medium engine speeds.
At full load, the fuel in injected synchronously with the air intake phase. This fills the combustion chamber homogeneously. Here again, this produces a definite reduction in fuel consumption together with higher power-output and torque figures than would be possible with indirect fuel injection. This was demonstrated on the race-winning Le Mans engine, which runs permanently in the homogeneous mixture mode"
