【作者】 李扬；

【导师】 郑尊清；

【作者基本信息】 天津大学 ， 工程硕士（专业学位）， 2021， 硕士

【摘要】 汽油压燃是一种高效清洁的燃烧方式,但其面临着低负荷燃烧稳定性差的瓶颈难题,关键原因在于汽油燃料活性低、低负荷热力学条件差,燃烧对边界条件非常敏感。火花点火可以大大降低汽油压燃低负荷燃烧对缸内热力学状态的严重依赖,为汽油压燃低负荷稳定燃烧的实现提供了一个潜在的可行途径。因此,本文基于重型发动机开展燃烧边界条件对火花辅助汽油压燃低负荷影响的试验研究,探索实现重型发动机低负荷火花辅助汽油压燃稳定、高效、清洁燃烧的技术途径。首先,结合CFD模拟进行了火花辅助压燃燃烧室结构和火花塞布置的设计,并进行了喷油和点火策略对火花辅助汽油压燃低负荷燃烧和排放影响的试验研究。研究表明,该低负荷工况条件下存在一个实现稳定燃烧的可控喷油和点火正时范围,且点火正时需要与喷油正时保持合适的配合才能保持稳定燃烧。随喷油压力提高,可实现稳定燃烧的喷油、点火正时范围扩大,40MPa时喷油和点火正时范围分别为为-34至-28°CA ATDC、-14至-26°CA ATDC,60MPa时喷油和点火正时范围分别为-34至-22°CA ATDC、-12至-28°CA ATDC,80MPa时喷油和点火正时范围分别为-34至-20°CA ATDC、-12至-28°CA ATDC。受活塞导流结构的影响,每个喷油压力下存在使得滞燃期最短的喷油时刻,随喷射压力提高这一时刻滞后,40MPa为-32°CA ATDC、60MPa为-30°CA ATDC、80MPa为-28°CA ATDC,稳定燃烧范围内最高热效率点处于该喷油时刻下的较晚点火时刻,即该喷油时刻保证了火花塞附近合理的混合气分布,可以在较晚的点火时刻下实现稳定燃烧,使燃烧相位推迟到上止点后更适合的位置,降低压缩负功,提高热功转换效率。适当提高喷油压力在扩大低负荷稳定运行控制范围的同时,配合合理的喷油和点火正时,最高热效率有所改善,但由于放热速率提高,最高效点的NOx排放有所增加。在67.7k Pa进气压力、40℃进气温度、80MPa喷油压力时采用合理的喷油和点火正时可以实现33.89%的指示热效率。进一步开展了进气边界条件对火花辅助汽油压燃低负荷影响的研究。提高进气压力由于可以降低泵气损失、缸内温度和传热损失,指示热效率得到较明显改善,NOx排放降低,但同时过量空气系数增大会缩小稳定燃烧的可调喷油和点火正时范围。在进气温度为40℃时,通过喷油和点火策略优化,在102k Pa进气压力下可实现37.83%的最高指示热效率,NOx为8.57g/(k W·h),同时保持95.53%的燃烧效率。在进气量固定时,提高进气温度虽然由于增加了传热损失,对指示热效率不能起到改善作用,但可以提前燃烧相位,提高自燃放热比例,显著扩大稳定燃烧时的喷油和点火正时范围,也可以降低THC的生成从而提高燃烧效率。引入EGR由于降低缸内平均温度和氧浓度,起到显著降低缸内NOx生成的作用,同时传热损失的减小也对热效率起到一定改善作用,但其提高热效率的效果相对提高进气压力而言较小,同时EGR也使得稳定燃烧的喷油和点火正时范围减小。在此基础上,进行了通过适当提高进气压力、温度和EGR率改善低负荷火花辅助汽油压燃热效率及NOx排放的探索研究。最终结果表明,在进气温度为60℃、进气压力为133k Pa时可实现39.47%最高指示热效率的稳定燃烧,NOx可降低至4.16 g/(k W·h),燃烧效率为94.68%,最高压升率为0.24MPa/~oCA;在98k Pa进气压力、80℃进气温度、EGR率为30%条件下,可以实现37.6%的指示热效率,NOx仅为2.33g/(k W·h),同时燃烧效率为96.84%,最高压升率低为0.16MPa/~oCA。这一研究对于改善火花辅助汽油压燃低负荷燃烧稳定性和实现高效、清洁燃烧具有重要的参考价值。更多还原

【Abstract】 Gasoline compression ignition is an efficient and clean combustion method,but it faces the bottleneck of unstable combustion at low load.The key reasons are low reactivity of gasoline fuel,and unfavorable thermodynamic condition in the cylinder at low-load.What’s more,this combustion mode is sensitive to boundary conditions.The spark ignition is a feasible and potential way for the stable gasoline compression ignition under low-load conditions,which can greatly reduce the heavy dependence on the thermodynamic state in the cylinder.Therefore,based on the experimental study of the effect of combustion boundary conditions on the spark-assisted gasoline compression ignition,this paper explores the technical approach to stable,efficient and clean gasoline compression ignition combustion of of heavy-duty engines under low-load conditions.First,combined with CFD simulation,the spark-assisted compression ignition combustion chamber structure and spark plug position is designed,and then the experiment is carried out,mainly to study the effect of fuel injection and spark strategies on the low-load combustion and emissions of spark-assisted gasoline compression ignition.Studies have shown that there is a controllable range of injection timing and spark timing for stable combustion under this low-load operating condition,and the spark timing needs to be properly matched with injection timing to maintain stable combustion.As the fuel injection pressure increases,the variant range of injection timing and spark timing for stable combustion can be expanded.The start of injection and spark timing ranges are-34 to-28°CA ATDC and-14 to-26°CA ATDC at fuel injection pressure of 40MPa,-34 to-22°CA ATDC and-12 to-28°CA ATDC at fuel injection pressure of 60MPa,-34 to-20°CA ATDC and-12 to-28°CA ATDC at fuel injection pressure of 80MPa.Affected by the diversion of the piston profile,each injection pressure has a special injection timing at which the combustion has the shortest ignition delay.This special timing is postponed when the injection pressure increases,and it is-32°CA ATDC for 40MPa,-30°CA ATDC for 60MPa,-28°CA ATDC for 80MPa.At this special injection timing,matched with a later spark timing within the controllable range for stable combustion,the highest thermal efficiency point is achieved.Because this injection timing ensures a reasonable mixture distribution near the spark plug,and the combustion phase is delayed to a more suitable position after the top dead center,hence the negative compression work is reduced,and the thermal efficiency is improved.Except for the effect of expanding controllable range,the thermal efficiency is improved by increasing injection pressure together with a suitable injection and spark timing.When the intake gauge pressure is 67.7k Pa,intake temperature is 40℃and injection pressure is 80MPa,a thermal efficiency of 33.89%is achieved.Due to the increased heat release rate,the NOx emissions at the most efficient point increases.Study on the effect of the intake boundary conditions on spark-assisted gasoline compression ignition was further carried out under low load conditions.The cylinder temperature,the loss of heat transfer and pumping loss are reduced by increasing intake pressure,thus the thermal efficiency is significantly improved,and NOx emissions are reduced.But at the same time,the increase in excess air coefficient will narrow the controllable range of injection timing and spark timing for stable combustion.When the intake air temperature is 40℃,through reasonable and optimized fuel injection and spark strategies,the maximum indicated thermal efficiency of 37.83%can be achieved at 102k Pa intake pressure,and the NOx is 8.57g/(k W·h)while maintaining a combustion efficiency of 95.53%.When the intake air volume is fixed,although increasing the intake air temperature cannot improve the indicated thermal efficiency due to the increased loss of heat transfer,it can advance the combustion phase,increase the compression-ignition heat release ratio,reduce the unburned hydrocarbons and improve combustion efficiency,and significantly extend the controllable range of injection timing and spark timing for stable combustion.The introduction of EGR reduces the average temperature and oxygen concentration in the cylinder,which significantly reduces the NOx production in the cylinder.At the same time,the reduction of the loss of heat transfer also improves the thermal efficiency to a certain extent,but its effect on improving thermal efficiency is relatively weaker than that on increasing the intake pressure.On this basis,an exploratory study was conducted to improve the thermal efficiency and NOx emission of spark-assisted gasoline compression ignition by appropriately increasing intake pressure and temperature and introducing EGR under low-load conditions.The final results show that when the intake air temperature is 60℃and the intake pressure is 133k Pa,a stable combustion with the highest indicated thermal efficiency of 39.47%can be achieved,NOx emission is reduced to 4.16g/(k W·h),the combustion efficiency is up to 94.68%,and the highest pressure rise rate is down to 0.24MPa/~oCA.Under the conditions with 98k Pa intake pressure,80℃intake temperature,and EGR rate of 30%,an in dicated thermal efficiency of 37.6%can be achieved,as well as NOx is only 2.33g/(k W·h),the combustion efficiency is up to 96.84%,and the highest pressure rise rate is down to 0.16MPa/~oCA.This research has important value as a reference for improving the combustion stability of spark-assisted gasoline compression ignition and approaching efficient and clean combustion under low-load conditions.更多还原