Experimental evaluation of engine valve train friction under motored and fired conditions

Abstract

One way of improving fuel economy whilst maintaining acceptable exhaust emissions is to reduce engine friction. In the last three decades there has been a resurgence of interest in reducing friction in the engine valve train. However, further reduction in friction requires better understanding and measurement in the valve train friction on a real firing engine. A new method of directly measuring camshaft friction that offers exceptional accuracy is described in this paper. 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 from both inlet and exhaust camshafts, under both 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 increasing temperature. The fuel economy benefit of friction modified lubricants was also observed. A significant difference in valve train friction between motored and fired condition was found, especially for the exhaust camshaft.