A Study on Heat Loss in DI Diesel Engines(Diesel Engines, Performance and Emissions, Heat Recovery)

Abstract

The objective of this study is to investigate the relationship among heat loss through the chamber wall of diesel engines and the spray flame impingement on the wall, the flame motion and the combustion characteristics such as flame temperature and rate of heat release. Local heat flux at 6 locations on the piston head of a rapid compression and expansion machine, by which experimental conditions such as injection conditions, ambient conditions, can be independently controlled, were measured by thin film thermocouples. The spray flame in an optically accessible combustion chamber with a quartz piston head was imaged by a digital high speed camera to observe the flame motion. From the high speed direct photographs, flame temperature distribution was obtained via two-color method. These measurements were conducted with changing the nozzle orifice diameter of fuel injector and the swirl ratio to investigate the effects of relative flame motion to the chamber wall on the heat loss. The ambient oxygen concentration was also changed as parameter to investigate the effects of combustion characteristics including flame temperature, ignition delay, combustion duration, and rate of heat release, which are affected significantly by oxygen concentration, on the heat flux on the chamber wall. Experimental results showed that the magnitude of heat flux is determined mainly by flame temperature and the relative distribution of heat flux is determined by the distribution of high temperature region in the flame. However, at flame impinging region on the wall, flow motion induced by the spray contributes to increase in heat flux. As nozzle orifice diameter is decreased, average flame temperature is increased, however, local heat flux at the bottom of the piston cavity decreases due to decrease in spray flame penetration. As ambient oxygen concentration is decreased, average flame temperature decreases, and this results in the decrease in local heat flux in the piston cavity. On the other hand, the relative distribution of heat flux is not affected by ambient oxygen concentration. The distribution of heat flux on the piston cavity wall is strongly affected by the swirl motion because the relative flame location to the chamber wall is changed significantly by the change in swirl ratio.