In reciprocating internal combustion engines, understanding the friction characteristics between piston ring and cylinder liner is very important for the engine performance and its reliability. Measurement of the friction force of the piston-ring assembly provides very useful information for clarifying complex tribological phenomena.
The present study deals with the improvement of measuring technique in the instantaneous IMEP method, the development of the direct measuring system employing a moveable bore system, and the investigation of the friction force characteristics of the piston-ring assembly under various modes of engine operations.
First, in order to improve the confidence of measured friction force data by the instantaneous IMEP method, following steps were taken. For minimization of the error in the final result, the frequency response function was determined for each measuring system, and the relative time delays among the measured signals were obtained by the application of the cross-correlation function.
Secondly, a new system was developed for the piston-ring assembly friction force measurement. This system had a relatively high fundamental frequency at 884 Hz and a fine resolution of 0.5 N in friction force measurement. Comparing with existing floating liner systems this system required small installation space and, at the same time alleviated the system noise problem induced by piston thrust and slap impulse forces.
Finally, this new system was applied to the friction force measurement to find its functional relationship with variables such as engine speed, oil viscosity and engine load. The friction mean effective pressure (fmep) was found to have a linear relationship with $(\upsilon{U})^{0.42}$ under motoring and with $(\upsilon{U})^{0.45}$ under firing operations, where $\upsilon$ is the kinematic oil viscosity and U is mean piston speed.
