Abstract: Abstract: In order to investigate the energy, exergy loss, exergy change rules and to explain the change reasons from the perspective of micro-field in cylinder, the experiments, simulation, thermodynamics basic principles and theoretical calculation under various steady working conditions were carried out by taking a heavy-duty turbocharged diesel engine as the research object. In the working process of heavy-duty turbocharged diesel engine, there must be internal factors in working process which causing energy and exergy changes, especially in combustion process, such as diffusion and oxidation, mixing, heat transfer of different temperature layers, while these factors are closely related to the movement of mixture gas, the mixing uniformity of fuel and air, temperature distribution. The causes of energy, exergy and loss in turbocharged diesel engine working process were discussed from the aspects of velocity, equivalent ratio and temperature distribution in cylinder. Since the variation trend of the parameters were basically same under different working conditions, taking the B50 working condition as an example, the change causes and rules of energy flow, exergy flow and its loss in combustion process were analyzed from distribution of flow velocity, equivalent ratio and temperature distribution. The results indicated that, Firstly, the change of energy and exergy flow mainly occured in combustion process, the degree of change in other processes was not obvious. Second, parameters such as heat transfer exergy, accumulation burn fuel exergy, exhaust loss energy, CA90, exergy efficiency were positively correlated with engine load, the corresponding exergy efficiency were 34.7%, 38.9%, 40.8% at B25, B50, B75 working condition. Third, the proportion of internal exergy loss and heat transfer exergy to fuel exergy were negative correlated with engine load, the former were 31.3%、29.9%、28.3% at B25、B50、B75 working condition respectively, the thermal efficiency of diesel engine could be effectively improved by avoiding running at the low load, with the increasing of speed, the proportions of cumulative heat transfer and internal exergy losses to fuel exergy slightly increasing, while the proportions of exhaust exergy to fuel exergy increased more. Fourth, in the middle of the combustion process, the superposition of the flow motion and the high temperature zone near the cylinder wall aggravated the heat transfer loss. Fifth, the theoretical equivalent ratio region mostly concentrated in the medium velocity zone, the nonuniform distribution of fuel and air, rapid fuel oxidation rate contribute to a greater exergy loss rate caused by diffusion and oxidation, the theoretical equivalent ratio was closer to the cylinder wall caused the greater heat transfer loss, at B50 working condition, the maximum heat transfer ratio was about 11 J/°CA after top dead center 15 °CA when there was the maximum contact area between combustion chamber wall and in-cylinder gas. Sixth, the volume size of high and low temperature region and the temperature rise of low temperature region determined the exergy loss rate resulted from temperature difference in cylinder, the volume fraction of the low temperature region at the late stage of combustion process was about 15% at B50 working condition, the exergy loss caused by temperature difference was larger than other time. In the combustion process of a turbocharged diesel engine, the volume of high and low temperatures region in cylinder was closer, the slower the temperature raising in the low temperature region were, the greater the loss rate was caused by the heat transfer in the cylinder. Finally, the flow velocity distribution, homogeneity of equivalent ratio distribution, high and low temperature region volume and temperature raising in cylinder had certain effects on the internal exergy loss rate.