Argon Power Cycle (APC) is an innovative future potential power system for high efficiency and zero emissions, which employs an Ar-O2 mixture rather than air as the working substance. However, APC hydrogen engines face the challenge of knock suppression. Compared to hydrogen, methane has a better anti-knock capacity and thus is an excellent potential fuel for APC engines. In previous studies, the methane is injected into the intake port. Nevertheless, for lean combustion, the stratified in-cylinder mixture formed by methane direct injection has superior combustion performances. Therefore, based on a methane direct injection engine at compression ratio = 9.6 and 1000 r/min, this study experimentally investigates the effects of replacing air by an Ar-O2 mixture (79%Ar+21%O2) on thermal efficiencies, loads, and other combustion characteristics under different excess oxygen ratios. Meanwhile, the influences of varying the methane injection timing are studied. Results indicate that by replacing air with an Ar-O2 mixture, thermal efficiencies and loads have a significant improvement, the operation boundary of excess oxygen ratio is extended from 1.73 to 2.91, the combustion duration period is shortened 3.5 - 7.0 °CA, and the cycling stability is significantly improved. For the APC, when the excess oxygen ratio is 1.34 and the methane injection timing is -130 °CA ATDC, the highest net indicated thermal efficiency of 51.1% and the gross indicated thermal efficiency of 53.1% are achieved, which are elevated by approximately 27.8% compared to the air cycle. Meanwhile, the maximum net indicated mean effective pressures of APC increases by 17.6% from 0.80 MPa to 0.68 MPa. The improvements in thermal efficiency are mainly attributed to the increase of thermal conversion efficiency. Moreover, compared to the air cycle, the higher in-cylinder temperature and pressure of APC allows for easier ignition and faster combustion, also leading to an increase in the thermal efficiency.
