Combustion Stability Improvement And Knock Characteristics And Control Research On Gasoline Compression Ignition (GCI) Engine

Gasoline compression ignition(GCI)is a promising combustion concept owing to its high thermal efficiency and low pollutant emissions.However,under low-load conditions,low reactivity of gasoline fuel leads to long ignition delay and retarded combustion phase,which result in bad combustion stability and even the occurrence of misfire.On the other hand,under high-load conditions,too concentrated heat release caused by the premixed compression ignition leads to rough combustion and even knocking combustion.Therefore,low-load condition combustion instability and cold start problem as well as rough/knocking combustion in heavy load are two major challenges faced by GCI.The present study aims to propose strategies to improve the combustion stabilities of low-load conditions,and to conduct investigations on knocking characteristics,mechanisms and the optimal control strategies.Firstly,the influence of internal EGR rate on combustion characteristics of GCI under different engine loads is studied under steady-state conditions.The competition between heating effect and dilution & heat capacity effect brought by internal EGR is discussed.Stable idling with low fuel consumption of GCI is achieved by high internal EGR rate.The results show that under part load conditions,the heating effect plays a dominant role in burning rate when the internal EGR rate is low.While the influence of dilution and heat capacity effect is highlighted with high internal EGR rate,which suppresses combustion.However,under idling conditions with low fuel supply,the burning rate and the onset timing of combustion are dominated by the heating effect,which dominates that high internal EGR rate is beneficial to increase the burning rate,advance the start of combustion and improve combustion stability under idling conditions.In experiments of the present work,a stable idling condition with fuel supply of 4.98 mg/cycle is realized by high internal EGR and proper injection strategy,in which the fuel consumption per displacement is the same as that of the current commercial engines.Secondly,based on the above strategy of high internal EGR rate,the tests with lower temperature boundary conditions,that concerning the conditions of cold idling and cold start are carried out.Results show that the NVO injection strategy is most appropriate for achieving cold start of a GCI engine.The initial in-cylinder thermodynamic state,the unburned mixture reactivity and the rate of in-cylinder heat accumulation are the decisive factors for the cold start of a GCI engine.NVO injection causes fuel reforming reactions,which increases the fuel reactivity and thus enhances heat accumulation rate,shortening the time of the cold start process.However,when the coolant and the intake air are both at room temperature,the lack of chemical reaction and heat accumulation in the cylinder leads to the failure of the engine start-up.Under such cold boundary condition,the method of intake preheating is used to improve the ignition capacity of the engine,and the rapid cold start of GCI can be achieved.Then,based on statistical methods,wavelet transform and combustion analysis,the knocking characteristics of GCI were studied in experiments.The results show that the in-cylinder pressure oscillation is a ubiquitous phenomenon in the GCI combustion,even under normal combustion conditions,there is still a slight pressure oscillation.Unlike the pressure oscillation generated by random auto-ignition of the end mixture in SI combustion,the GCI pressure oscillation is caused by the excessive local combustion rate in combustion chamber,which is not randomly happened,and the oscillation intensity can be adjusted by adjusting the injection timing.Due to the difference in combustion temperature,the frequency of GCI pressure oscillation is significantly lower than the same order frequency of SI pressure oscillation.In addition,the fuel distribution in the cylinder can be adjusted through double injection strategy.The proper proportion of fuel premixing improves the GCI combustion,and its combustion stability and torque output are also significantly increases.However,the excessive premix ratio brings about “over-premixing” phenomenon,which results in uncontrollable knocking combustion,which means knock cannot be suppressed by delaying the main injection timing.With such a high premixing ratio,late injection timing and high coefficient of variation(Co V)might lead cycles with excessive concentrated heat release and extremely high knock intensity,which damages to the engine.Finally,based on heavy load conditions of the GCI engine,the effects of injection pressure,fuel concentration,negative valve overlap(NVO),multiple injections and Atkinson cycle on engine performance and combustion process were studied.The results show that the injection pressure,injection timing and fuel amount play vital roles in GCI combustion.Low injection pressure leads to insufficient mixing of air and fuel,resulting in lower indicated mean effective pressure(IMEP)and higher Co V.Properly increasing the injection pressure is beneficial to improve the fuel-air mixing process and the IMEP,and reduce the Co V simultaneously.However,if the injection pressure is too high,“over-mixed” fuel make the combustion process extremely sensitive to the changes of the injection timing.The change of NVO has a direct influence on the effect of scavenging and the amount of residual hot gas in the cylinder,but under high load conditions,it is not appropriate to adopt excessive NVOs in order to ensure sufficient power output.At the same time,in order to avoid too long ignition delay caused by scavenging effect,the valve timing with almost zero NVO is selected under the heavy load condition.When adopting double injection strategy,significantly better engine performance on thermal efficiency and Co V is obtained,due to proper fuel distribution and reduction of local rich fuel zone.And in order to avoid uncontrollable combustion,the pre-mix ratio should not be too high.The use of the Atkinson cycle on the basis of double injection strategy further enhances engine performance.The improve of engine performance by Atkinson cycle is based on the fact that the it reduces the effective compression ratio of the engine,thereby reducing the compression negative work,while the expansion work of the combustion can be effectively maintained,thus increasing the amount of work by the engine.However,the Atkinson cycle reduces the thermodynamic state of the cylinder,so it is necessary to control the injection timing more reasonably to avoid unstable combustion.Overall,focusing on the combustion stability and knocking combustion process,the competitive relationship between heating effect and dilution & heat capacity effect brought by internal EGR was investigated,the technical method for realizing stable idling condition was proposed,fast cold start process was achieved by combining internal fuel reforming and intake pre-heating in the present work.In addition,the characteristics and combustion process of GCI and SI were analyzed,and the technical method to improve engine performance by combining split injection and Atkinson Cycle was proposed.The present work provides important information for combustion optimization and real application of GCI engines.