A Hydraulic Tappet with Variable Timing Properties 930823
A new hydraulic cam follower with variable valve timing (VVT) properties is described. Experimental results show that the point of closure of the valve may be delayed as a linear function of engine speed without external control. No other parameter of the valve event is modified by the device. An obvious application is the control of intake valve timing for engines with a wide speed range, where the point of valve closure could be scheduled with engine speed in order to maximise the trapped mass, hence improving the torque curve at low and high speeds. The device is considered for application to the Ford 2.5 litre DI diesel engine, where it may be used to retard inlet valve closure from close to bottom dead centre (BDC) at cranking speed to 50-60 deg after BDC at rated speed. This reduces the compromise in inlet valve timing across the speed range, thereby improving starting or allowing a small reduction of compression ratio, and improving the volumetric efficiency at low and high speeds. An experimental program is reported which consists of rig and engine work based on the Ford 2.5 litre DI diesel engine. The engine testing shows that the device may be operated quietly, albeit on a diesel engine, and reliably over a wide speed range. The device is seen to be sensitive to oil inlet temperature such that when cold, measures are required to prevent late inlet valve closure.