Lubricating Oil Droplets in Cylinder on Abnormal Combustion in Supercharged SI Engine

The supercharged spark ignition engine has a problem of abnormal combustion at low speed and high load operating condition. This paper focuses on the sauce and mechanism of the abnormal combustion, namely, the behavior of lubricating oil droplets in cylinder, ring crevice, piston crown and ring gap. The experimental approach and the numerical analysis have been carried out. The two experimental approaches namely direct photography by high speed camera and measurement of scattering oil quantity at low speed condition have been tried. The photographs which is in engine operation show, 1^st The oil droplets from ring crevice scatter every reciprocating motion and the diameter of oil droplets is between 0.10mm and 0.30mm. 2^nd The oil droplets from piston crown has three steps as follows, firstly, the lubricating oil which reaches piston crown continues to accumulate, secondly, the accumulated lubricating oil scatters by the reciprocating motion. It is needed the time of several thousand crank shaft revolution from engine start. After, the accumulated lubricating oil scatter for a few cycles. Finally, almost lubricating oil which is accumulated on the piston crown has scattered away and it stops a series of the scattering process suddenly. The experimental data are able to explain the behavior of the abnormal combustion occurrence, namely it appears suddenly at low speed operation and continues several cycle and suddenly return to normal combustion again. The estimated frequency of oil scattering from piston crown is 2 to 15 times per 1000 cycles approximately. 3^rd The behavior of oil droplets from ring gap is not able to explain the occurrence of the abnormal combustion. The calculated results show the lubricating oil droplet during the compression stroke has the potential of abnormal combustion source and if the droplet size is under 0.10mm, the temperature of oil droplet rise up sufficiently for spontaneous ignition.