发动机压缩空气再生制动理想热力循环分析

车辆制动能量的回收利用有利于改善整车的经济性,而基于传统内燃机的气动-内燃混合动力技术有望实现制动能量的高效、低成本回收利用.以城市路况运行车辆为应用对象,基于传统四冲程发动机提出了3种发动机压缩空气再生制动能量回收方案.通过建立3种方案共同的理想热力学循环,以可回收气体的最大压力、单位排量每循环回收气体质量、循环性能系数(COP)和循环平均指示压力为评价指标,对制动循环进行了分析.结果表明:增大压缩比、减小排气管缓冲腔容积与排量比或者减小排气门开启提前角均可以提高回收气体的最大压力,在机械结构允许的条件下,应尽可能减小排气管缓冲腔的容积;在制动过程中,减小排气门开启提前角可获得较高的循环平均指示压力和气体回收质量;随着气罐背压的增加,控制排气门开启提前角由大变小,可获得最佳的制动循环性能;理论上,二冲程制动循环COP与四冲程制动循环相同,但二冲程制动循环气体回收质量流量和制动功率为四冲程制动的2倍.

Thermodynamic Analysis of Engine Air Compression Cycle for Regenerative Braking

Braking energy regeneration is beneficial to the improvement of vehicle fuel economy. Among many re- generative braking approaches, air hybrid engine, which is based on conventional internal combustion engine, could realize braking energy recovery with high efficiency and low cost. Based on the conventional 4-stroke internal combustion engine, three engine compression regenerative braking approaches were proposed for air hybrid engines aiming to be applied to urban driving situations. The common ideal thermodynamic cycle was established, and the maximum regenerated air pressure, indicated mean effective pressure （IMEP）, coefficient of performance （COP）, and regenerated air mass per displacement were selected to analyze the cycle characteristics. The results show that higher compression ratio, smaller ratio of exhaust chamber to engine displacement or closer exhaust valve open- ing （EVO） angle before TDC （BTDC） could enhance the maximum regenerated air pressure. Considering the limitation of mechanical structure, the exhaust chamber volume should be designed as small as possible. During the process of regenerative braking, reducing the exhaust valve opening angle could obtain higher IMEP and larger compressed air quantity. With the increase of tank pressure, the EVO BTDC should be controlled from a big magnitude to a small one if the optimal COP is considered. 2-stroke regenerative braking has the same COP as 4-stroke case theoretically, while the regenerated air mass flow rate and the indicated power are twice as much as those of 4-stroke case.