A new comparative study on performance of engine cycles under maximum thermal efficiency condition

Abstract

As the parameters involved in the brake thermal efficiency of the engine cycles affect each other, the simultaneous investigation of the critical parameters, non-critical parameters, and the maximum temperature of the working fluid involved to optimize the brake thermal efficiency of the engine cycles is vital to implement the theoretical proposition into practical design perspectives. For this reason, a new method to compare the thermal efficiency of the engine cycles has been carried out with the simultaneous application of the optimal cycle design parameters (the geometric-compression ratio, expansion–compression​ ratio, and pressure ratio), the operating parameters (the heat-transfer loss, heat-release during combustion, friction loss, temperature-dependent specific heat ratio of the working fluid, and initial temperature of the working fluid), and the maximum temperature of the working fluid. The numerical examples show that the largest brake thermal efficiency is for the Dual-Miller cycle (46.32%) as compared to that for the classical-Otto (42.41%), classical-Diesel, classical-dual (40.22%), Otto-Miller (45.32%), Diesel-Miller (40.27%), Otto-Atkinson (42.31%), Diesel-Atkinson (32.19%), and dual-Atkinson (42.34%) cycles. When the heat-release during combustion and temperature-dependent specific heat ratio of the working fluid increased, the brake thermal efficiency of the Dual-Miller cycle rises and then begins to abate. As the heat-transfer coefficient, friction coefficient and initial temperature of the working fluid increased by about 20%, the brake thermal efficiency of the Dual-Miller cycle decreases by about 1.3%, 1.1%, and 3%, respectively. The numerical examples also show that the brake thermal efficiency of the Dual-Miller cycle first increases with increasing the maximum temperature of the working fluid, reaches its maximum value (43.01%) and then remains constant with further increases in the maximum temperature of the working fluid.