Effect of cylinder pressure on engine valve-train friction under motored and fired conditions

In the last three decades, there has been a resurgence of interest in reducing friction in the engine valve-train for better fuel economy and improved emissions. Research studies and scientists have developed complex mathematical models and advanced test rigs to understand the interaction between cam and tappet. Most of the experimental valve-train work has been carried out on motored test rig assuming there is less difference in valve-train friction under motored and fired conditions. For complete understanding of the difference of camshaft friction under motored and fired conditions, experiments have been carried out on a real engine under both conditions allowing for the first time the detailed study of the effect of cylinder pressure on exhaust camshaft friction. A new method of directly measuring camshaft friction that offers excellent accuracy is described in this article. The technique uses strain gauges to measure the torque exerted upon the camshaft drive pulley sprocket. The advantage of this method over previous techniques is the instantaneous valve-train friction as a function of crank angle can be measured, simultaneously for both inlet and exhaust camshafts, under motored and fired conditions. Experiments have been carried out for both motored and fired conditions on a single-cylinder gasoline engine. Results are presented for instantaneous and mean valve-train friction as a function of engine speed and oil temperature, indicating a decrease in friction with increase in engine speed and an increase with increase in temperature. A significant difference in valve-train friction between motored and fired conditions was found, especially for the exhaust camshaft.