The gasoline engine with continuously variable valve actuation is recently focused on as a method to improve fuel consumption and performance of a vehicle. The suction air amount is controlled without using throttling, and therefore can dramatically reduce pumping losses.
On the other hand, the suction air amount is significantly affected when the valve lift is slightly changed under low lift condition. In other words, the response of the suction air amount against the change of the valve lift is very sensitive. Furthermore, a conventional gasoline engine with a throttle has a self-stabilizing effect on engine speed, but an engine controlled by a valve lift is not. Therefore, accurate idle speed control is required in order to reduce engine idle speed and improve fuel consumption.
The conventional control is composed of a two-degree-of-freedom sliding mode controller and an adaptive disturbance observer. This method can provide better tracking performance and disturbance suppression characteristic than PID controls when it controls engine speed. However, the conventional method can not provide sufficient control performance for the engine with the variable valve actuation because this engine controls engine speed by only adjusting the continuously valve lift.
In this research, a new concept for idle speed control is designed for the engine. This control is based on a two-degree-of-freedom sliding mode algorithm. It controls engine speed by altering both the valve lift and the ignition timing of the engine. Altering the valve lift performs to compensate for large error of engine speed caused by the change of engine load, and altering the ignition timing to correct small error.
As a result, this control can reduce the idle engine speed and dramatically improve the stability of idle speed control.