Abstract
Continuously variable valve lift gasoline engines can improve the fuel economy of vehicles by reducing pumping losses. In addition, valve timing and cam phase settings optimized for the engine speed can increase torque. However, there are the following issues coexistence of low fuel consumption, high output and low emission.
The exhaust system aiming for higher output features larger heat mass and area of heat-transfer surface to maintain low exhaust back pressure. Therefore it makes longer delay time until the catalyst activity. The relationship between the suction air and the valve lift is highly sensitive under low engine speed condition. The valve lift is low at 2 mm or less, and slight variations in the lift of each cylinder cause disparities in the amount of each cylinder suction air. This is turn causes disparities of each cylinder air-fuel ratio, which decreases the catalytic conversion efficiency and increases emissions. To reduce emissions during cold start condition, an ignition timing control was developed that combine I-P control with a sliding mode control algorithm, which enables the engine speed to reach the target value smoothly and promotes early activation of the catalyst.
To reduce influence of disparities of each cylinder air-fuel ratio, the new optimal control was designed using a sliding mode control mechanism with an expanded delta-sigma modulation algorithm. By applying this to secondary O2 feedback, the post-catalyst O2 sensor output can be controlled precisely and the catalytic conversion efficiency can be improved. Using the above technologies, this research was able to achieve ultra low emission levels of vehicles equipped with continuously variable valve lift gasoline engine.
Keywords:engine, variable valve, emissions, adaptive control, sliding mode control