Effects of Compression Ratio on Combustion Characteristics of a Direct-Injection Diesel Engine

The effects of compression ratio on the history of the fuel-burning process -was investigated in a single-cylinder direct-injection diesel engine. Two compression ratios of 21.4 and 18.5 were studied by using two wide and shallow bowls. The engine was operated at different speeds and overall air-fuel ratios, but with constant start-of-combustion timing at top dead center.

The measured cylinder pressure was used to derive fuel-burning rate. At all conditions, the low-compression-ratio bowl had a larger mass of premixed-burned fuel and a higher peak specific fuel-burning rate. The fuel-burning rate in the diffusion-controlled phase of combustion decreased with decreasing compression ratio. The resulting combustion duration became longer at reduced compression ratio, and the difference could be as large as 14 crank-angle degrees.

For the engine conditions examined in this study, the mass of fuel present in the combustion chamber at ignition appears to be the dominant parameter determining the mass of premixed-burned fuel. The mass of premixed-burned fuel is also expected to be influenced by the spray development during the ignition-delay period. However, the effects cannot be determined in this study.