An Experimental Investigation On The Effects Of Injection Strategy On Combustion And Emission Characteristics Of Gasoline Compression Ignition (GCI)

Gasoline compression ignition(GCI)has been regarded as one of the potential novel combustion concepts to achieve clean and high efficiency combustion.However,GCI encounters auto-ignite issues at low loads due to the low in-cylinder temperature and low reactivity of gasoline,while at high loads,excessively high pressure rise rate and high soot emission limit GCI high load extension.It has been recognized that the air-fuel mixing and combustion processes can be controlled by injection strategies,and the controllability of the mixing process and concentration stratification can be improved by using early pilot and port fuel injection(PFI)strategy,consequently the combustion rate and premixed combustion ratio can be effectively controlled,and more stable GCI combustion,lower soot emission and better controllability of combustion process can be achieved.In this study,an experimental investigation on the effects of injection strategies on the GCI combustion and emission characteristics in a modified single-cylinder test engine has been conducted.Firstly,the potential of a multiple-injection strategy(an early pilot injection coupled with a main injection)and a compound injection strategy(PFI coupled with direct injection)in improving the GCI performances of combustion and emissions was explored.Compared with gasoline HCCI,higher thermal efficiency,lower soot emission and more stable combustion could be achieved by using these injection strategies.Then,the effects of PFI and early pilot on GCI combustion and emissions was conducted under the same conditions.Compared with single injection strategy,lower NO_x and soot emissions and less sensitivity of NO_x to CA50 could be obtained.However,THC and CO emissions and MPRR were increased.It was also found that the strategy of PFI coupled with direct injection could better control the concentration stratification and GCI combustion more effectively than the other strategy.Then,the effects of the compound injection strategy on thermal efficiency,NO_x and soot emissions at higher loads were studied.It is seen that MPRR could be reduced with pilot injection by applying small pilot-main injection interval.However,soot became more sensitive to CA50.Higher combustion efficiency and lower THC and CO emissions could be obtained as the load increased.However,both indicated thermal efficiency and soot emission were penalized due to lower PFI proportion,mainly limited by the MPRR and in-cylinder pressure during load extension.In addition,low NO_x and soot emissions could be achieved with EGR and higher injection pressure.In this study,the effects of the multiple injection strategy and the compound injection strategy on the combustion and emission characteristics were systematically demonstrated.The potential of these two injection strategies in improving the controllability of GCI combustion,thermal efficiency and emissions were revealed and discussed in detail.An indicated thermal efficiency of 46.4~48.9%and soot emission of 0.0025~0.005g/kWh could be achieved when GCI was operated at40~80mg/cycle load condition.Finally,soot of about 0.03g/kWh and indicated thermal efficiency of about 45%can be obtained as the load was further extended to16.32 bar.