HEAT TRANSFER EFFECTS ON THE PERFORMANCE OF AN AIR STANDARD OTTO CYCLE

Öz

There are heat losses during the cycle of a real engine that are neglected in ideal air standard analysis. In this paper the performance of an air-standard Otto cycle with heat transfer loss and variable specific heats of working fluid is analyzed by using finite-time thermodynamics. Heat transfer from the unburned mixture to the cylinder walls has a negligible effect on the performance for the compression process. Additionally, the heat transfer rates to the cylinder walls during combustion are the highest and extremely important. Therefore, we assume that the compression and power processes proceed instantaneously so that they are reversible adiabatics, and the heat losses during the heat rejection process can be neglected. The heat loss through the cylinder wall is assumed to occur only during combustion and is further assumed to be proportional to the average temperature of both the working fluid and the cylinder wall. The results show that the effects of heat transfer loss and variable specific heats of working fluid on the cycle performance are obvious, and they should be considered in practice cycle analysis. Higher heat transfer to the combustion chamber walls lowers the peak temperature and pressure and reduces the work per cycle and the efficiency. The effects of other parameters, in conjunction with the heat transfer, including combustion constants and intake air temperature, are also reported. The results are of importance to provide good guidance for the performance evaluation and improvement of practical real engines.