Simplicity in diesel combustion structure suggested from agreement of experimental results with similarity theory and simple spray flame model

Abstract

The author proposed a combustion similarity theory among different sized diesel engines about thirty years ago. This theory predicts combustion becomes similar among different sized diesel engines, when similarity conditions are satisfied. The theory suggests that it may be possible to have better thermal efficiency in small engines designed similarly to the large size engines. The results of the numerical simulation show that the engines designed to have a shallow dish type combustion chamber with double side injection nozzles, which is typical for large size engines, have improved thermal efficiency than the typical small sized engines with deep dish with a center injection nozzle. The difference is mainly due to the cooling loss effect and the main combustion characteristics are similar between the two types of engine systems. On the other hand, the author and his colleagues have investigated the development of high-speed diesel combustion simulator based on the HTB-DECS model developed by Hiroyasu, et al. The results show that it simulates combustion well for the injection timings, rates and engine speeds with the same model constants for an individual engine. Additionally, there are simple tendency in the model parameters among different engines. The results suggest that there is a simple structure in diesel combustion, although it appears to be very complex in the phenomena.