Break-in liner wear and piston ring assembly friction in a spark-ignited engine

Cylinder liner wear and surface roughness were measured at the top-ring reversal point of a single-cylinder, air-cooled, spark-ignition (gasoline) engine during break-in. In addition, the instantaneous frictional torque of the engine was determined and correlated with the wear rate and surface roughness during this period. The engine friction, liner surface roughness and wear had their highest values at the beginning of the break-in period. The wear rate dropped sharply during the first hour, after which it maintained a steady lower rate. The surface roughness and friction took longer than the wear rate to stabilize. Friction was found to change in three modes. The first mode took one hour and was characterized by a sharp drop in wear rate and a small drop in friction. The second mode was a transition to the third mode, and took about 2.5 hours. The third mode lasted about 20 hours and was characterized by a drop in both surface roughness and friction. The wear rate did not change much during this mode. After the third mode, the friction and surface roughness changed at a slow steady rate. The total test took 28 hours. Correlations developed for wear rate, surface roughness and friction indicated that friction was a linear function of the surface roughness over the whole period. However, friction was not an indicator of the wear rate.