Development of a Valve Timing Control System 890680
There has been a general trend recently toward applying double overhead camshafts (DOHC) and four valves per cylinder to automotive engines to obtain better performance. The use of such valve train designs can produce not only higher maximum power but also better idle stability, and higher torque at low speed as well as improved fuel economy, if the valve timings are optimized over the full range of engine speed and load.
A newly developed valve timing control system is presented which makes it possible to obtain the expected improved engine performance. This system employs an annular piston with helical splines which is positioned by engine oil pressure and is controlled according to the engine speed and load. The mechanism initially produced excessive noise problem caused by camshaft torque fluctuation but this was solved by eliminating system backlash through the use of split piston with phase offset splines and another spring.
This valve timing control system achieves a 7 to 10% improvement in wide open throttle torque in the low engine speed range by advancing the inlet cam phasing by 20 crankshaft degrees, while normal phasing delivers the same maximum power and maintains idle stability. The system has been adopted in Nissan's new V6 DOHC engine.