Hiroshi Moritani, Yu Nozawa

The relationship between oil degradation and environmental conditions (temperature and gaseous components) was clarified by characterizing the oil and blow-by gas collected from the second land region of a piston by using a link mechanism during the actual operation of the engine. The oil extracted from the land region exhibited a lower total base number and higher total acid number than that in the crankcase. Also, more degradation products were found to form, and additive consumption was greater. This could be explained by combustion heat leading to higher temperatures in the land region than in the crankcase, and by the effect of the blow-by gases (hydrocarbons, NOx, and oxygen) which lead to oil degradation. The degree of oil degradation in the land region was found to be higher at lower rotational speeds combined with higher loads. NO2 bubbling tests proved that the total base number of the oil falls as the its temperature and the NO₂ concentration increases. Reducing the amount of oil accelerated this tendency. Therefore, in addition to temperature and the blow-by gases, the amount of oil was also identified as being a major factor affecting the degradation of oil in the land region.

Minimizing the oil pump capacity of an engine while maintaining reliability is one of the most effective ways of reducing mechanical loss of an engine. Care is required to assure reliability, because an excessively low oil flow may result in poor lubrication that can cause engine components to seize.

The oil flow was observed by visualization and the pressure was measured in two types of oil passage that link main bearings and con-rod bearings, namely, "V type" which has an oil passage in the main journal, and "I type" which does not have this passage. The oil flow in the passage was observed using a CCD camera and a crankshaft made of acrylic resin. The oil flow rate was measured at the same time as the flow was observed. The pressure at which the oil supply failed due to the occurrence of aeration differed with the oil passage types.

Then, the oil flow rate from the oil pump through the main bearing to the con-rod bearing was predicted by using a model that combined a mass-conserved elastohydrodynamic lubrication calculation with an oil flow equation to calculate the effect of the oil supply conditions on aeration. Consequently, the validity of the model was confirmed by attaining a good agreement with the measured oil flow rates, as well as the limit pressure without aeration in the two oil passage types.