基于微观场的重型增压柴油机工作循环能量流及流仿真

Energy and exergy flow simulation in working process of heavy-duty turbocharged diesel engine based on microscopic field

为了探索重型增压柴油机工作循环中能量流、?流变化和损耗的规律并从缸内微观层面分析其原因,该文以CA6DL重型车用增压柴油机为研究对象,基于热力学基本原理、试验数据及仿真结果计算并分析了不同工况下工作循环中能量流、?流变化和损耗的规律。以B50工况为例,从缸内速度、当量比、温度分布的角度出发,对引起燃烧过程中能量流、?流变化和损耗的原因进行了分析。结果表明:增压柴油机工作循环中能量流、?流变化主要发生在燃烧过程;传热?、燃料累计?、排气损失、CA90、?效率等参数正相关于负荷,而内部?损失及传热?所占燃料?比例负相关于负荷,B25、B50、B75工况下内部?损失占燃料?比例分别为31.3%、29.9%、28.3%;理论当量比区域多集中在中等流速区;燃烧过程中期,缸内流场与高温区在缸壁附近的叠加作用加剧了缸壁的传热损失;当量比分布越不均匀,燃油氧化速率越快,高低温区体积占比差距越大,温升越缓慢,内部?损失率越大。

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.