Efficiency and power enhancement strategies for methane direct injection argon power cycle engines

Argon Power Cycle (APC) is a novel concept for high efficiency and zero carbon emissions, which replaces the air by an argon-oxygen mixture. Previous studies have experimentally demonstrated that APC has a great efficiency enhancement potential for methane-fueled engines. However, the port methane injection tends to cause knock and limits thermal efficiency gains. In addition, few researches have focused on the power of APC methanefueled engines. In this study, fundamental experiments are first conducted in a spark ignition methane direct injection engine with compression ratio = 9.6. Strategies such as lean combustion, dilution combustion (increasing argon mole ratio in argon-oxygen mixture), and boosting intake pressure are adopted. Results indicate that lean combustion has a more significant effect in efficiency enhancement under APC compared with air cycle due to the high specific heat ratio of argon. The combination of dilution combustion and boosting intake pressure is a feasible approach to further improve both efficiencies and powers. When argon mole ratio = 85 %, intake pressure = 0.12 MPa, and excess oxygen ratio = 1.37, the highest net indicated thermal efficiency reaches 59.0 % and the indicated mean effective pressure increases a factor of 1.61 than that at argon mole ratio = 79 % and intake pressure = 0.09 MPa. In this case, the ratio of gross indicated thermal efficiency to theoretical thermal conversion efficiency is 90.5 %. Based on thermodynamic calculation, when the ratio of gross indicated thermal efficiency to theoretical thermal conversion efficiency is assumed to be 90 %, a high net indicated thermal efficiency of 65 % is predicted to be obtained at compression ratio = 14.