The Dynamics of Valvetrains with Hydraulic Lash Adjusters and the Interaction with the Gas Exchange Process

Models are presented for:

• (i)
  the dynamics of valvetrains with hydraulic lash adjusters. The effects of the adjuster can be split, the elasticity being included in the dynamics model, and the leak-down solved explicitly. This eliminates the numerical difficulties encountered with straightforward formulations.
• (ii)
  the gas force on exhaust valves. The exhaust flow and the valve dynamics are solved simultaneously to obtain the full effect of cam modifications. Advantage is taken of the positive displacement flow in four-stroke engines to avoid numerical difficulties associated with throttling flow.
• (iii)
  the efficient generation of accurate lift curves for gas exchange predictions.

In this paper we describe developments of a model for the dynamics of valvetrains /13/* presented at this Congress two years ago. This model was very successful for mechanical valvetrains under the action of spring and inertia forces. Numerical difficulties were, however, encountered when simulating valvetrains with hydraulic lash adjusters. Moreover, no model was available to simulate the gas force on exhaust valves, a third important source of valve vibrations. This caused us to study the properties of valvetrains with hydraulic lash adjusters, and to examine the effect of the exhaust flow on valve dynamics. Finally, we show that the small deviations of the valve lift from the kinematic lift have a substantial effect on the gas exchange, both at idle and full load. We therefore outline a simple model, based on what we learned for the detailed dynamics model, for generating accurate valve lift curves.