Comparison of performances of air standard Atkinson and Otto cycles with heat transfer considerations

In this paper, the effects of heat transfer on the net output work and the indicated thermal efficiency of an air standard Atkinson cycle are analyzed. Comparisons of the performances of air standard Atkinson and Otto cycles with heat transfer considerations are also discussed. We assume that the compression and power processes are adiabatic and reversible and that any convective, conductive or radiative heat transfer to the cylinder wall during the heat rejection process may be ignored. 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 cylinder wall. It is found that the net output work versus efficiency characteristics, the maximum net work output and the corresponding efficiency bound are significantly influenced by the magnitude of the heat transfer. An increase in heat transfer to the combustion chamber walls decreases the peak temperature and pressure and, consequently, reduces the work per cycle and efficiency. The effects of other parameters, in conjunction with heat transfer, including combustion constants, compression ratio and intake air temperature are also reported. An Atkinson cycle has a greater work output and a higher thermal efficiency than the Otto cycle at the same operating condition. The compression ratios that maximize the work of the Otto cycle are always found to be higher than those for the Atkinson cycle at the same operating conditions. The results are of importance to provide good guidance for performance evaluation and improvement of practical Atkinson engines.