Effects of Post Injection on Piston Lubrication in a Common Rail Small Bore Diesel Engine

The authors et al. developed a unique single cylinder diesel engine equipped with a floating liner device for the measurement of piston friction force, aimed at the clarification of the relationship between the multiple fuel injections and the state of piston lubrication. Using this engine, the effects of the post fuel injections on the state of piston lubrication have been checked under this study, according to the variation in the piston friction force. As a result, it is verified that the oil film on the cylinder near the top dead center is diluted immediately with the injected fuel, and that the piston lubrication deteriorates more than anticipated even for the case using the SAE 5W/30 oil, compared with the case where the SAE 0W/20 oil with a lower viscosity is used. It is confirmed, on the other hand, that the deterioration in piston lubrication can be suppressed by increasing the post injection pressure, and conducting the injection at a proper timing prior to the opening of exhaust valve.