Abstract: Abstract: There are 2 kinds of thermal fatigue failure modes of engine pistons. One is the high-cycle-fatigue failure mode caused by cyclic thermal shock loading in steady-state conditions. The other is the low-cycle-fatigue failure mode caused by thermal shock loading under transition conditions such as cold start, rapid acceleration, and fast deceleration. Although piston thermal loading has been widely studied by previous domestic and foreign researchers, the work focused on the thermal loading of steady-state conditions and overlooked the impact of the drastic variations of piston transient thermal loading on piston reliability and useful life, and the research result cannot reflect true realistic variations of piston thermal loading, and cannot accurately predict and evaluate the thermal fatigue life of the piston. In order to study the thermal loads of diesel engine pistons in different operating conditions, a non-road high-pressure-common-rail diesel engine was analyzed by using the method of thermal-mechanical decoupling. The finite-element simulation model of piston thermal loading under the steady-state condition of rate power and the above-mentioned transient condition was established. The simulation model was developed based on the experimental results of transient temperature measurements of the piston top. The model was successfully used to reveal the variation trends of the transient thermal loads of the pistons under these conditions. The analysis results showed that the time-dependent or crank-angle-dependent fluctuation of the piston thermal load under the steady-state condition of rated power was only limited to the piston top, the firing deck, and the first ring groove. As the fluctuation penetration distance measured from the piston top increased, the fluctuation amplitude decreased. The maximum fluctuation penetration distance of temperature was 3 mm, and the maximum fluctuation penetration distance of thermal stress was 5 mm. Under the transient conditions, the fluctuation amplitudes of the thermal loads were greater than those under the steady-state conditions, with the fluctuation of the cold start process being the greatest. Specifically, the maximum fluctuation amplitudes of the temperature, thermal stress, and thermal strain of cold start process were 200 ℃, 40 MPa, and 0.3 mm, respectively. During the process of rapid acceleration, although the maximum fluctuation amplitudes of the piston temperature and thermal strain were smaller than those of the cold start process, being 120 ℃ and 0.12 mm, respectively, the maximum fluctuation amplitude of the piston thermal stress reached the greatest, being 50 MPa. Such a large variation of stress had a great impact on piston durability life. During the process of rapid deceleration, the maximum fluctuation amplitudes of the piston temperature, thermal stress, and thermal strain were the smallest among all operating process, being 20 ℃, 10 MPa, and 0.02 mm, respectively. During the rapid deceleration process, the measured metal temperatures of the piston in various locations all increased shortly, then gradually decreased by a small magnitude, and finally reached stable after 200 s. The maximum fluctuation amplitudes of the piston temperature, thermal stress, and thermal strain during fast deceleration conditions were the smallest among all operating conditions, being 30 ℃, 10 MPa, and 0.02 mm, respectively. The research of this study can provide good guidance for the design of highly intensified aluminum-alloy pistons of diesel engines.