Study On Ignition And Flame Development Characteristics Of Highly Premixed Charge Compression Ignition Combustion

The concept of highly premixed charge compression ignition combustion is proposed for the operation of reactivity controlled compression ignition(RCCI)at high engine load conditions.At present,optical diagnostic studies on this combustion model is limited,and the knowledge of in-cylinder combustion process is not well understood.The main task of this paper is to study the heat release,ignition and flame development of RCCI combustion at highly premixed condition.Further discussions aim to explore the flame development mode of RCCI under different strategies.First,by changing the direct-injection(DI)fuel and direct-injection timings,a comparative analysis of heat release and flame characteristics was conducted.The results provide insight into the chosen of the DI fuels suitable for highly premixed charge compression ignition.Firstly,a two-stage high-temperature heat release(HTHR)features the RCCI combustion with DI of PODE and n-hexadecane,but the two-stage HTHR for RCCI with DI of n-heptane is not distinct due to the relatively lower cetane number that produces lower reactivity gradient of the mixture in the combustion chamber.Secondly,under highly premixed condition,the mass of DI fuel is so low that,the ignition delay of early DI strategy is mainly determined by the cetane number.However,the effect of spray atomization quality on ignition delay get more important at late-injection condition.Thirdly,PODE is suitable for RCCI operation at high engine loads.When the mass fraction of port-injection is 93.8%,there is almost no soot formation for PODE,which is much better than that of n-heptane and n-hexadecane.Then,PODE is chosen as the direct-injection fuel,the highly premixed charge compression ignition combustion model is studied using high-speed imaging,natural flame emission spectroscopy and formaldehyde PLIF techniques.The results show that early highly premixed charge compression ignition combustion presents a single-stage HTHR.The flame development in the low-reactivity region is a hybrid of flame front propagation and auto-ignition after the auto-ignition in the high-reactivity region.Formaldehyde PLIF results prove the coexistence of flame front propagation surrounding the region and auto-ignition kernels in the low-reactivity region.While,late highly premixed charge compression ignition combustion presents two-stage HTHR.The first stage is due to the auto-ignition of PODE and entrained isooctane,and the second stage is due to the flame propagation in the low-activity region.The separation of the combustion reduces the peak PRR in RCCI combustion.In addition,the flame propagation in low-reactivity regions of highly premixed charge compression ignition is different from the flame propagation of spark-ignition(SI)engine.The lowtemperature reaction of RCCI takes place in the low activity area before the arrival of the flame front.But there is no low-temperature reaction in unburned area of SI engine.The mass of direct-injection fuel is the key factor that determines the flame development patterns in the low reactivity region of RCCI.If the mass of directinjection fuel is relatively low,flame propagation prevails;otherwise sequential autoignition dominates.The ignition and flame development characteristics of highly premixed charge compression ignition combustion are revealed through this research.It has certain guiding significance for the further understanding of the flame development modes of RCCI,and is helpful for the enrichment and development of the combustion reaction path control theory.