Generally, the turbulent-flame velocity of natural gas is significantly lower than diesel in the combustion process, which results in the thermal loads of natural gas engines being significantly higher than those of diesel engines under the same stoichiometric condition without EGR. In this study, a heavy-duty natural gas engine is taken as the research object, which is used to measure the temperatures to analyze the heat transfer characteristics in the cylinder head water jacket around the valve bridges, under different speeds and loads, as well as different coolant temperatures and pressures. Twelve thermocouples are inserted by drilling through the metal in the cylinder head with different heights to measure the metal temperatures at thermally critical areas such as the valve-bridge regions. Therefore, the local heat flux and the extrapolation to coolant wall temperatures are obtained by Fourier’s Law under different engine operating conditions. In addition, the thermal balance tests of the engine are also carried out, and the energy distributions are analyzed in different parts of the engine. The results of the research show that: a) the engine cooling condition has a direct impact on the engine cylinder head temperature. If the cooling temperature is low, the temperatures of the cylinder head’s measuring points have the same increases as the increasing coolant temperature. When the coolant temperature is high, the measuring temperatures have hardly any difference from the increases in cooling temperature. With increasing cooling pressure, the temperature increase at all measuring points, and the temperature of the measuring points varies substantially under high load conditions compared with the low load condition. The results indirectly indicated that local nucleate boiling appeared in the water jacket. b) The heat transfer characteristic curve of the water jacket was obtained from the processing of experimental data. Wall heat flux increases with increasing load, and the relationship between wall heat flux and wall temperature is no longer linear. The heat transfer characteristic curve indicates that the convective heat transfer and boiling heat transfer both appeared in the cooling water jacket. c) With the decrease of engine load, the percentage of crankshaft power in the combustion heat gradually decreases, then the percentage of the heat taken away by the cooling water increases gradually. At the same time, the percentage of the heat taken away by exhaust has changed little. d) The engine cooling temperature has a substantial influence on the engine thermal balance, and the cooling pressure has little effect on the engine thermal balance. With increasing cooling temperature, the heat taken by the cooling water decreased, which lead to an increase in the proportion of crankshaft power. It can be concluded that properly increasing the coolant temperature of the engine can improve the fuel economy of the machine.

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