The Application of Delta-Sigma Modulation to A Continuously Variable Valve Lift Engine

A continuously variable valve timing and lift system (CVTL) can improve the fuel consumption of gasoline engines by reducing of pumping losses, and provide high torque and power by optimizing intake valve lift, valve opening and valve closing according to engine load and speed conditions. In general gasoline engines, engine load is controlled by throttling. On the other hand, CVTL can control engine load by altering intake valve lift and valve opening period under all engine load and speed conditions. Hence, CVTL requires high-precision control of intake valve lift in order to realize small engine load change at low engine speed condition such as idle condition. However, the dynamics of CVTL indicates non-linear characteristics, for example mechanical friction, backlash and hysteresis, because CVTL is generally composed of some gears and multi-link mechanisms. Therefore, it is difficult to realize smooth and small micro step of valve lift. This difficulty worsens the stability of idle speed control and smoothness of drivability. Consequently, a sliding-mode controller using delta-sigma modulation instead of previous non-linear switching input was developed and introduced to the valve lift control of CVTL. The sliding-mode controller could significantly improve the precision and smoothness of valve lift control.