压缩空气发动机工作过程分析

压缩空气发动机是一种采用压缩空气直接膨胀做功的"绿色"发动机。若将压缩空气发动机作为车辆动力源,可缓解传统内燃机车辆的尾气污染问题。然而,压缩空气发动机工作效率偏低,难以替代传统内燃机作为车辆主要动力。为提高压缩空气发动机工作效率,了解压缩空气发动机工作过程能量损失的产生机制以及其变化规律,该文应用热力学理论和分析方法对其工作过程损失分布进行了研究,并利用试验数据验证了压缩空气发动机效率变化规律。结果表明:由于节流以及高压、低温尾气排放等因素,压缩空气发动机工作过程损失主要存在于进气过程和排气过程,两者总共占进气总量的30%~40%;压缩空气发动机转速升高,进气损失和排气损失均增加;进气压力升高,进气损失减少,排气损失增加;进气温度升高,进气损失和排气损失均减少。从经济性考虑,压缩空气发动机应运行于低转速,且适当提高其进气压力和进气温度。

Exergy analysis on compressed air engine

Abstract: The compressed air engine is a new kind power machinery, which uses high-pressure air as its energy storage medium and working medium. The high-pressure air expands inside the cylinder to produce power. Compared to the conventional internal combustion engine, compressed air engine has many advantages in terms of simple structure, no pollutant emission and no fuel consumption, etc. Thus, the compressed air engine has attracted plenty of attention. The previous researchers almost concern about the prototype designing, working process studying and energy loss analyzing on some specific operating condition. However, it is still lack of a systematic study about the energy loss distribution of compressed air engine as well as their causes. In order to solve the issues mentioned above, the exergy loss distribution and its variation trend in the working process of a single-cylinder compressed air engine developed by Zhejiang University were studied in this paper, through applying the thermodynamic theory and exergy analysis method. At first, the mathematical model of the compressed air engine was established and explored in Matlab. After the validation, the mathematical model was used to analysis the difference between the ideal and actual working process of the compressed air engine. Then, the exergy balance model of compressed air engine was built, and was used to analyze the influences of the main operation parameters on the exergy loss distribution, containing of engine revolving speed, intake pressure and temperature. The simulation results of the analysis showed that there is a big difference between the actual working process and ideal working process of compressed air engine, which lead to a lower actual efficiency than the ideal efficiency. The exergy loss of compressed air engine mainly contains intake loss, heat transfer loss, exhaust loss and friction loss. The intake loss and exhaust loss are the two largest part of compressed air engine loss, which result from intake throttling and high pressure, low temperature gas exhausting, respectively. Added together, the intake loss and exhaust loss account for about 30%-40% of the total intake exergy. The raising of engine revolving speed or the falling of intake temperature both lead to an increase of exergy loss in the intake and exhaust processes, while the raising of the intake pressure leads to an increase of exergy loss in the exhaust process and a decrease of exergy loss in the intake process. From economic considerations, the compressed air engine need to run in low revolving speed with appropriate high intake pressure and temperature. In order to validate the variation trend of compressed air engine exergy efficiency, a test bench of compressed air engine was built. The actual exergy efficiency of compressed air engine under different operation parameters were tested and computed, including revolving speed, intake pressure and temperature. It is shown that the test results well agree with the simulation results, though there is a slight gap between them, which proved the accuracy of the built compressed air engine mathematical model.