Thermodynamic Simulation of Performance of an Irreversible Otto Cycle with Engine Speed and Variable Specific Heat Ratio of Working Fluid

Abstract

The performance of an air standard Otto cycle is analyzed using finite-time thermodynamics. In the model, the linear relation between the specific heat ratio of the working fluid and its temperature, the friction loss computed from the empirical correlation, the internal irreversibility described using the compression and expansion efficiencies, and the heat transfer loss is considered. The relations between the power output and the compression ratio, between the power output and the thermal efficiency are derived by detailed numerical examples. The results show that the maximum power output, the working range of the cycle, the optimal compression ratio corresponding to maximum power output, the optimal power output corresponding to maximum thermal efficiency and the optimal thermal efficiency corresponding to maximum power output decrease with the increase of the value of the specific heat ratio. The results also show that if the compression ratio is less than a certain value, the power output increases with increasing engine speed, while if the compression ratio exceeds a certain value, the power output first increases and then starts to decrease with increasing engine speed. With further increase in the compression ratio, the increase in engine speed results in decreasing the power output. The results are of importance to provide good guidance for the performance evaluation and improvement of practical Otto engines.