Variable valve timing

The intake valves and the exhaust valves define when air enters the cylinders and when exhaust gas is expelled. By means of variable valve timing, it is possible to optimize the air/fuel mixture charge in the cylinder in line with the actual operating conditions. The opening and closing points for the intake and exhaust valves are defined by the engine’s cam-shafts. Bosch incorporates variable valve-timing functions as
a software package in the engine management system.

One example is “scavenging” with direct gasoline injection.
At the heart of this function is the DI-Motronic engine manage-ment, which controls the start-of-injection point, as well as the intake and exhaust-valve timing, and the turbocharger. With scavenging, at low engine revs, the intake and exhaust valves are controlled so that they open simultaneously for a very brief period. The pressure gradient between intake and exhaust tract leads to the combustion chamber being filled with fresh air, while at the same time the residual gas from the last combustion expelled.

This improves cylinder cooling and leads to the cylinder being better filled with fresh intake air. This higher cylinder charge increases the spontaneous torque and due to the stronger exhaust-gas flow also increases turbocharger pressure.