Influence of injection timing and water-to-fuel ratio coupling on combustion characteristics and piston thermal load of jet fuel engines at variable altitudes

At high altitudes, reduced air density and oxygen concentration lead to decreased power output and increased piston thermal loads in compression-ignition (CI) aviation kerosene engines. Achieving coordinated optimization of combustion, indicated thermal efficiency (ITE), and piston thermal load under such conditions remains a key research challenge. This study systematically investigates the effects of fuel injection and water spray strategies at different altitudes on in-cylinder fuel-air mixing and flame development, revealing how combustion control parameters influence combustion behavior and piston thermal stress. Results show that when the injection timing (IT) is set at −6°CA ATDC, increasing altitude from 2000 m to 4000 m reduces the ITE from 37.17 % to 36.42 %. High-temperature regions are mainly located at the piston throat and crown recess, while peak thermal stresses occur at the top of the piston's internal cavity, both intensifying with altitude. Advancing the IT to −15°CA ATDC at 2000 m increases the ITE to 39.37 %, though peak in-cylinder pressure and maximum pressure rise rate exceed safety limits. Across altitudes, increasing the water-to-fuel ratio(WFR) generally reduces ITE, with a greater drop at higher elevations. However, at 2000 m and a WFR of 0.2, ITE slightly increases to 38.08 %. Additionally, a higher WFR effectively lowers piston temperature and thermal stress, reducing the peak values by 6.44 °C and 3.18 MPa, respectively, at an altitude of 4000 m.