A Study on the Effects of a Higher Compression Ratio in the Combustion Chamber on Diesel Engine Performance

摘要

In order to improve the brake thermal efficiency of the engine, such as cooling and friction losses from the theoretical thermal efficiency, it is necessary to minimize various losses. However, it is also essential to consider improvements in theoretical thermal efficiency along with the reduction of the various losses. In an effort to improve the brake thermal efficiency of heavy-duty diesel engines used in commercial vehicles, this research focused on two important factors leading to the engine's theoretical thermal efficiency: the compression ratio and the specific heat ratio. Based on the results of theoretical thermodynamic cycle analyses for the effects of the above two factors, it was predicted that raising the compression ratio from a base engine specification of 17 to 26, and increasing the specific heat ratio would lead to a significant increase in theoretical thermal efficiency. In the next step, by using a single-cylinder research engine, effects of the above factor on the indicated and brake thermal efficiencies were investigated. The geometric compression ratio was changed by variation of the combustion chamber volume, and the specific heat ratio in cylinder gas was controlled by adjustment of excess air and EGR rate through an external air supply system. Relative to 8 points improvement for theoretical thermal efficiency derived from initial theoretical analyses (by the combination of higher compression ratio and higher specific heat ratio), 6 points and 4 points improvements were observed in the indicated thermal efficiency and the brake thermal efficiency, respectively.