不同峰值压力下压缩比对柴油机热效率的影响

Effects of Compression Ratio on Thermal Efficiency of Diesel Engine at Different Peak Firing Pressures

应用三维数值模拟软件CONVERGE研究了压缩比变化对重型直喷柴油机燃烧和排放性能的影响,并在不同峰值压力限制下研究了压缩比耦合过量空气系数提高热效率的潜力.结果表明,当过量空气系数较低时,随压缩比提高,指示热效率呈现出先增加后降低的趋势,当压缩比为21.5时可获得最高指示热效率.提高缸内峰值压力可显著改善热效率,与原机相比,当缸内峰值压力为20 MPa、25 MPa、30 MPa时,指示热效率分别可提高2.2%、3.5%和4.3%,对热效率改善幅度最明显的压缩比方案分别为17.5、20.5和23.5.这表明高峰值压力应与高压缩比燃烧室匹配,才能获得更高的热效率提升.当过量空气系数较低时,其值变化对热效率影响较大,应以提高过量空气系数作为改善热效率的主要手段;而当过量空气系数较高时,相比于提高过量空气系数,提高压缩比改善热效率的效果更加明显.

A three-dimensional numerical simulation software CONVERGE is used to study the effects of compression ratio on the combustion and emission performance of a heavy-duty DI diesel engine.The potential of increasing thermal efficiency by coupling excess air coefficient with compression ratio is also studied under different peak firing pressure(PFP)limits.The results show that when the excess air coefficient is low,the indicated thermal efficiency increases first and then decreases as the compression ratio increases.The highest indicated thermal efficiency can be obtained when the compression ratio is 21.5.Increasing the PFP can improve the thermal efficiency obviously.Compared with the original engine,when the PFPs are 20 MPa,25 MPa and 30 MPa,the indicated thermal efficiency can be increased by 2.2%,3.5%and 4.3%,respectively.The schemes with the compression ratios of 17.5,20.5 and 23.5 have the most obvious improvement in thermal efficiency.It means that the higher PFP should be matched with the higher compression ratio to obtain a higher thermal efficiency.When the excess air coefficient is low,it has a greater impact on thermal efficiency,and the increase of excess air coefficient should be used as a means of improving the thermal efficiency.When the excess air coefficient is high,increasing the compression ratio has a more obvious effect on thermal efficiency than increasing the excess air coefficient.