Strategies to Improve SI-Engine Performance by Means of Variable Intake Lift, Timing and Duration

This paper reports the results of theoretical and experimental investigations in the field of variable intake - valve control of spark-ignition engines. Different degrees of freedom for a variable intake profile such as variable intake opening and closing events, variable valve lift, as well as the deactivation of one of the intake valves per cylinder of a multi-valve engine are considered and evaluated concerning their potential to reduce pumping losses, to support mixture formation, and to improve combustion. The investigations show that additional efforts are necessary to convert the potential of minimized pumping losses due to unthrottled SI-engine load control into reduced fuel consumption and good driveability. Increased gas velocities during intake for low engine speed and load and adjusted residual-gas fractions according to the different operating conditions prove to be very efficient parameters to improve engine performance under unthrottled conditions. Another important aspect is the optimized intake-lift profile for maximum volumetric efficiency over the whole engine speed range, taking into account the dimensions of the intake manifold and the possibility to deactivate one of the intake valves of a 4-valve-per-cylinder engine.