Development of Continuous Variable Valve Event and Lift Control System for SI Engine (VEL) 2008-01-1348
This paper describes a newly developed variable valve control system called VEL (Variable Valve Event and Lift Control System), which enables continuous control of both valve events (opening duration) and valve lifts, from the lowest lift state below 1mm to the highest lift state over 12mm. VEL has actually been adopted to the new V6 production engine of Nissan since last summer, and contributes not only to fuel economy and low emissions, but also to driving pleasure.
VEL is composed of three subsystems. First is a mechanical valve train system, which converts crankshaft rotation into output cam oscillation via a transmission mechanism. The valves are moved by the output cam oscillation. Second is an electric powered actuator system, which varies valve events and lifts according to driving conditions by controlling the angular positions of a control shaft. Third is the drive control system, consists of the Engine Control Module and the Electric Control Unit to drive the VEL, which controls the actuator system to the optimum valve lifts and events of each engine running conditions.
The basic design of the VEL was decided in consideration of mechanical superiority, that is, high rotational speed limit and low driving friction. In order to realize this, a forced drive mechanism was introduced as the VEL transmission structure, because no additional spring is needed for driving the output cam back to the original position. As a result, both the allowable maximum speed of 7500rpm and the lower driving friction under ordinary driving conditions can be realized.
The electric powered actuator uses a low friction ballscrew element for quick response and lower energy consumption. This simple ball-screw type actuator needs only a 100W motor for its adequate operation with the assistance of a suitable control method.