Performance optimization and comparative study on a novel coupled power cycle featuring isochoric and isothermal processes in internal combustion engine

Developing thermodynamic cycles with higher efficiency is a critical approach to addressing energy conservation, environmental protection, and emission reduction challenges in internal combustion engines. However, existing optimized cycles for internal combustion engines remain constrained by the fundamental limitations of classical thermodynamic cycles, such as the Otto, Diesel, and Brayton cycles, resulting in a bottleneck in cycle performance. Benefit from tension pistons based on advanced flexible materials, isothermal processes in internal combustion engine are flexible. In this paper, a novel cycle called coupled power cycle featuring isochoric and isothermal processes is proposed, increasing the average heat absorption temperature. A thermodynamic model is developed based on the fundamental laws of thermodynamics, and ideal thermal efficiency equation for ideal gas is derived. Results show that the thermal efficiency and the sustainability index are positively correlated to the pressure ratio in compressor and piston combustion chamber. By contrast, the power density will reach the top as a result of the increase in the pressure ratio in compressor while the power density will show the downwards trend with the pressure ratio in piston combustion chamber growing up. If the weights of thermal efficiency and power density are 0.5 and 0.5 differently, optimal performance is achieved at pressure ratios in compressor and piston combustion chamber at 46.00 and 1.56, respectively. Compared to the Diesel and Brayton cycles, the proposed coupled power cycle achieves at least a 30% and 10% improvement in thermal efficiency, respectively. Together with a specific swashplate, the proposed coupled power cycle is expected to realize a significant improvement in the performance of internal combustion engines.