Multi-cycle thermal accumulation and stabilized temperature response of an exhaust valve in an ammonia-diesel dual-fuel marine engine

Ammonia-diesel dual-fuel (ADDF) engines have attracted increasing attention as a promising pathway for reducing carbon emissions in marine propulsion. The introduction of ammonia significantly alters the in-cylinder combustion process and the gas-side thermal boundary conditions of hot-end components, thereby changing the thermal exposure and temperature response of exhaust valves. Meanwhile, the thermal accumulation and stabilized response of exhaust valves during continuous operation remain insufficiently understood, particularly for the cycle-to-cycle temperature changes governed by repeated gas-side heating and solid thermal inertia. In this study, a coupled computational fluid dynamics (CFD) and finite element (FE) framework is developed to investigate the multi-cycle thermal accumulation and stabilized response of an exhaust valve in an ADDF marine engine. The transient gas-side thermal boundary conditions extracted from validated representative-cycle CFD simulations are periodically mapped onto consecutive engine cycles and imposed on a transient heat conduction model of the exhaust valve. The results show that, at an engine speed of 1600 rpm, the exhaust valve undergoes pronounced thermal accumulation during the early stage of cyclic operation, with the most rapid temperature rise occurring within the first 1000 cycles, corresponding to approximately 75 s of actual operation, and reaches a cyclically stabilized thermal state after approximately 3000 cycles, corresponding to approximately 225 s. Throughout the multi-cycle process, the maximum temperature is consistently located near the junction between the valve-head underside and the outer rim, where gas-side heating is coupled with the valve-head conduction path. As the ammonia energy ratio (AER) increases from 20% to 80%, the stabilized maximum temperature decreases by approximately 50 K and the number of cycles required for thermal stabilization changes only slightly from 3078 to 3014 under the present valve geometry and operating condition. These results indicate that AER mainly affects the stabilized temperature level, and its influence on the thermal stabilization time scale remains limited because the valve geometry, material thermal inertia, and heat-dissipation path are unchanged across the cases. The findings clarify how AER-dependent gas-side thermal boundaries drive valve-temperature accumulation and stabilized thermal levels, and provide a basis for identifying thermally critical regions and temperature-relevant service boundaries of exhaust valves under ADDF operating conditions.