改善增压天然气发动机排放特性的途径

Approach of improvement in emission characteristic of turbocharged CNG engine

通过试验研究增压稀薄燃烧天然气发动机的排放特性,以及排气温度受过量空气系数λ的影响,研究结果表明:随着过量空气系数的增大,NOx与CO和HC的排放量变化趋势相反;压缩天然气发动机采用稀薄燃烧(λ>1.0)技术的源排放能达到国Ⅲ标准,在加装普通三效催化转化器(TWC)后,其排放也只能达到国Ⅲ标准。原因是普通三效催化转化器只有在λ≈1.0时,其转化效率最高;米勒循环发动机的膨胀比大于压缩比,这有利于降低排气温度和提高热效率。因此,本文提出天然气发动机达到国Ⅳ排放标准一种新的技术路线:基于当量比燃烧(λ=1.0)的米勒循环技术,通过连续可变气门正时(CVVT)机构来调节气阀的开启和关闭时刻。采用该技术可以适当地增大发动机的压缩比,从而保证发动机的动力性和提高热效率,又可有效地降低排气温度,实现当量比燃烧,极大地提高了排放污染物在三效催化转化器中的转化效率,使天然气发动机排放达到国Ⅳ排放标准。

The emission characteristic of turbocharged lean combustion natural gas engine and exhaust temperature influenced by excess air coefficient（λ） was studied by experiment.The results show that the trend of NOx with CO and HC are reverse with the increase of λ;source emissions of compressed natural gas（CNG） engine can meet States Ⅲ Emission Standard by adopting lean combustion technology,however its emissions only meet States Ⅲ Emission Standard with installing normal three-way catalytic converter（TWC）.This is because normal three-way catalytic converter achieves maximal conversion efficiency when the excess air coefficient is about 1.0.Expansion ratio is greater than compression ratio of Miller-cycle engine,which will help to reduce exhaust temperature and improve thermal efficiency.Therefore,a new method of CNG engine is proposed to meet States Ⅳ Emission Standard which is Miller cycle technology based on equivalence ratio combustion（λ=1.0）,through continue variable valve timing（CVVT） assembly to adjust valves opening/closing timing.Adopting this technology can not only properly increase engine compression ratio which ensures engine power output and improving thermal efficiency,but also effectively lower exhaust temperature and achieve equivalence ratio combustion,and greatly improve the three-way catalyst conversion efficiency for exhaust pollutants,which ensures the engine to meet States Ⅳ Emission Standard.