Comparisons of predicted and measured results on performance and emission of engine effected by intake air dilution and supercharging

Abstract

An experimental study was conducted on a single cylinder direct injection diesel engine to investigate the effects of diluting intake air, with different gases and increasing intake pressure on combustion process and exhaust emissions. The intake O2 concentration is changed from 15% to 21% by diluting intake air with different gases (CO2, Ar, N2), and the intake pressure is changed from one to two bar by a screw compressor. A modified program for calculating heat release rate, is used to study the characteristics of combustion and exhaust emissions in detail. The main results show that the addition of either CO2 or Ar to the intake air increases the ignition delay. The variations of ignition delay with CO2 are much larger than those of ignition delay with Ar for the same O2 concentration. The emission of NOx decreases with the decrease of O2 concentration and the smoke level is lower with the addition of the CO2 than with that of Ar. As the intake pressure is increased, the ignition delay is shortened. Furthermore the high intake air pressure enhances the air-fuel mixing and diffusion combustion, and reduces the premixed combustion, so that NOx emission is decreased without increasing smoke emissions. The addition of CO2 at high intake pressure, drastically reduces NOx emissions and smoke emission simultaneously at a high load condition, and the addition of CO2 reduces NOx emissions without affecting the smoke emissions substantially at a low load condition. A zero-dimensional combustion simulation program incorporated with the present heat release correlation and ignition delay correlation is used to predict ignition delay, cylinder pressure and engine power. The results show that the correlations are likely to be adequate for the engine operating under diluted intake air and various intake pressure.