Abstract: To solve the problem of low efficiency of the after-treatment system caused by low exhaust temperature in diesel engines under low-speed and low-load conditions, a thermodynamic performance test bench was set up for a six-cylinder diesel engine, and a GT-Power simulation model was established. A self-developed fully hydraulic variable valve system （FHVVS） was used to achieve secondary opening of the exhaust valve, allowing both the intake and exhaust valves to open simultaneously during the intake stroke. This enables the recirculation of hot exhaust gases to improve exhaust thermal management in the diesel engine. Parameters such as in-cylinder pressure, exhaust temperature, and fuel consumption under different exhaust thermal management schemes were measured experimentally. Through simulation calculations, key design parameters such as the maximum lift, opening duration, and opening phase angle of the secondary exhaust valve opening were optimized. A comparative analysis of parameters such as exhaust temperature and effective fuel consumption rate under different exhaust thermal management schemes was conducted. The results show that the improved internal exhaust gas recirculation （IEGR） exhaust thermal management mode increased the exhaust temperature to above 200 ℃ across all test conditions of the diesel engine, and the effective fuel consumption rate was reduced by 3.1% compared to that of the original IEGR mode.