过量空气系数对柴油/甲醇RCCI发动机非常规排放特性的影响

为探究柴油/甲醇反应活性控制压燃(Reactivity Controlled Compression Ignition,RCCI)发动机非常规排放特性及影响机理,该研究对某高压共轨柴油机进气歧管进行改造,搭建了柴油/甲醇双燃料RCCI发动机专用试验台架,系统研究了不同甲醇替代率、过量空气系数对发动机非常规排放物的影响规律。结果表明:最大转矩转速(2 000 r/min)、不同负荷工况下,随甲醇替代率增大,柴油/甲醇RCCI发动机甲醇、甲醛、芳香烃碳氢化合物和SO_2排放量升高,非甲烷总烃、CO_2排放降低;25%负荷、甲醇替代率从0%增加到15%,CO_2排放量降低4.5%;100%负荷、30%甲醇替代率时,CO_2排放量较纯柴油模式减少6.8%。随过量空气系数减小,未燃甲醇、甲醛、非甲烷总烃和SO_2排放量降低,CO_2排放升高;25%负荷下,过量空气系数从3.48减小到3.05,5%、10%、15%替代率下甲醇排放量分别降低16.9%、12.7%和14.5%,甲醛排放量分别降低8.8%、10.8%和10.5%,非甲烷总烃排放量平均下降75%;100%负荷下,过量空气系数从1.6减小到1.38,10%、20%、30%替代率下甲醇排放量分别降低45.6%、45.9%和43.9%,非甲烷总烃排放分别降低18.2%、27.3%和60%,甲醛排放量平均减少34.4%;高负荷下芳香烃碳氢化合物随过量空气系数的减小而升高,低负荷下变化不明显;RCCI模式下,碳氢化合物的主要成分是未燃甲醇和甲醛,适当关小节气门开度,减小过量空气系数,对降低非常规污染物排放有利。外特性工况下,随发动机转速增加,未燃甲醇、非甲烷总烃、二氧化碳排放降低,甲醛排放增多;不同转速下随着甲醇替代率增加,非甲烷总烃和二氧化碳排放降低,未燃甲醇和甲醛排放量增加。研究结果可为柴油/甲醇双燃料RCCI发动机非常规排放物控制奠定理论基础。

Effects of excess air coefficient on non-regulated emissions of diesel/methanol RCCI engines

Abstract: Reactivity-Controlled Compression Ignition(RCCI) is widely expected as a promising dual fuel low-temperature combustion (LTC) strategy in recent engines. The potential strategy can control the in-cylinder fuel reactivity for the low NO and Particulate Matter (PM) emissions with high thermal efficiency. This study aims to investigate the effects of methanol fraction and excess air coefficient on the non-regulated emissions of diesel engines in RCCI mode under different operating conditions. A bench test was performed on a modified 4-cylinder high-pressure common-rail diesel engine under RCCI combustion. The results showed that the total hydrocarbon emissions increased sharply with the rise of methanol substitution rate, while decreased with the increase of load at 2000 r/min. The unburned methanol accounted for about 90% of total hydrocarbons (THC) emissions under the methanol/diesel dual fuel. The emissions of methanol, formaldehyde (HCHO), aromatic hydrocarbons (AHC), and sulfur dioxide (SO2) increased, whereas, the emissions of non-methane hydrocarbons (NMHC) and carbon dioxide (CO2) decreased, as the methanol substitution rate increased. The CO2 emission reduced by 4.5%asthe methanol substitution rate increased from 0% to 15% at 25% load, while dropped by 6.8% in the methanol proportion from 0% to 30% at 100% load. The emissions of unburned methanol, formaldehyde, NMHC and SO2 decreased, while the CO2 emission increased at different load rates with the decrease of excess air coefficient. Under 25% load, the emission of unburned methanol decreased by 16.9%, 12.7%, and 14.5%,while the emissions of formaldehyde reduced by 8.8%, 10.8%, and 10.5%, and the NMHC emissions reduced by 66.7% and 83.3% with 5%, 10% and 15% methanol substitution rate, as the excess air coefficient was reduced from 3.48 to 3.05.Under 100% load condition, the methanol emissions reduced by 45.6%, 45.9%, and 43.9%, and the formaldehyde emissions decreased by 36.5% and 33.3%, while the NMHC emissions dropped by 18.2%, 27.3%, and 60% with 10%, 20%, and 30% methanol, as the excess air coefficient decreased from 1.6 to 1.38. The emission of aromatic hydrocarbons rose with the decrease of excess air coefficient at high load, but the change was not obvious at low load conditions. Therefore, the relatively low excess air coefficient was beneficial to reducing the non-regulated emissions, where the valve opening was used to properly control the in-cylinder fuel reactivity during RCCI mode. The findings can provide a potential theoretical basis to balance the methanol/diesel dual-fuel RCCI combustion and pollutant emissions.