Investigation On Stratified Flame And Multi-point Micro-flame Induced Multi-point Auto-ignition Combustion Processes In Gasoline Engines

Controlled Auto-Ignition(CAI)combustion is effectively in improving the fuel economy of spark ignition(SI)gasoline engines and simultaneously achieving ultra-low NOx emissions.However,CAI combustion can only be applied to the production engines in a narrow load-speed region.In order to solve these problems,the concept of stratified fame ignited(SFI)and multi-point micro-flame ignited(MFI)hybrid combustion were proposed in order to expand the operating range of the gasoline engine with high thermal efficiency.In the case of SFI combustion,rich gasoline-air mixture to be easily ignited is formed around the central spark plug with the help of direct injection.As a result,stratified flame is stably formed after spark ignition in order to promote the combustion of diluted mixture left.In the case of MFI combustion,small amount of dimethyl ether(DME)is directly injected into the cylinder,and multi-point micro-flame is generated with the auto-ignition of DME-air mixture and/or spark ignition for the combustion control of highly-diluted gasoline-air mixture.Firstly,the effect of fuel injection and spark ignition strategies on the combustion and emission characteristics of SFI and MFI combustion were experimentally investigated in a single-cylinder four-stroke thermodynamic engine.In the meantime,the effect of spark ignition on the relationship between flame propagation and auto-ignition in the case of SFI combustion was simulated with a three-dimensional(3-D)software.Finally,the formation mechanism of multi-point micro-flame in the MFI combustion was investigated in an optical engine with high-speed photography.In order to extend the operating range of the gasoline engine with high thermal efficiency,the combustion concepts mentioned above were applied to a poppet-valved two-stroke gasoline engine.The effect of boosting system and anti-knock approaches on the performances of the poppet-valved two-stroke gasoline engine at full loads were simulated with a one-dimensional(1-D)code in order to meet vehicles'requirements for power and fuel economy.The results can be drawn as follows:In SFI combustion at stoichiometry,advanced spark timing results in earlier ignition timing and shorter combustion duration.With the decrease of direct injection fraction of gasoline in a cycle,ignition timing changes slightly,but combustion duration decreases.Advanced spark timing or decreased the fraction of direct injection gasoline could alter the SFI events from flame propagation-dominated combustion to auto-ignition-dominated one.With 3-D simulation,it is found that uneven distribution of fuel in the cylinder results in the difference in different directions of flame propagation towards the cylinder wall.The mixture prior to the flame front with low speed is heated and compressed by the flame front,resulting in the occurrence of auto-ignition.In the spark induced MFI combustion,the heat release process can be divided into three stages.Stage I is the one from low temperature oxidation reactions of DME,and stage II is the one of high temperature reactions of DME and flame propagation after spark ignition.Stage III is the one from multiple auto-ignition of premixed gasoline mixture far from central spark plug and flame propagation.Increased the amount of direct injection DME increases the heat release rate in stages I and II,and advances the combustion phasing,resulting in faster heat release rate in stage III and shorter combustion duration.In the case of lower amount of direct injection DME,advanced spark timing increases the heat release rate in stages II and III,resulting in advanced combustion phasing and shortened combustion duration.In the MFI combustion without spark ignition,the combustion processes have three types of features,i.e.,ramp type,double-peak type,and trapezoid type.Ramp type MFI combustion can achieve higher thermal efficiency and ultra-low NOx emissions lower than 1 g/k W·h.When net indicated mean effective pressure(IMEPn)increases from 0.2 MPa to 0.7 MPa and excess air coefficient changes from 1.5 to 2.7,ramp type MFI combustion could achieve stable combustion processes with cycle-to-cycle variation coefficient lower than 3.5%,simultaneously resulting in high thermal efficiency and ultra-low NOx emissions.The maximum indicated thermal efficiency reaches around 44%at 2000 r/min and 0.7 MPa IMEPn.With an optical engine,it is found that the multi-point micro-flame widely distributes in the cylinder,and there is a large time difference among the sequential micro-flame sites in the case of ramp type MFI combustion.As for double-peak type MFI combustion,the multi-point micro-flame mainly distributes in the center of cylinder,and they appears in very short time intervals.The 1-D simulation results show that,when a 0.7×10^-3 m³ two-cylinder two-stroke gasoline engine is match a two-stage serial boosting system including a supercharger with a large corrected mass flow rate and a downstream compressor with low corrected mass flow rate to supply charge air,it could replace a 1.6×10^-3 m³naturally aspirated four-stroke gasoline engine.To obtain the brake power of 80 k W at3000 r/min,the boosting system needs to supply the intake with 0.48 bar at the mass flow rate of 0.156 kg/s.At the full loads in the range from 2000 r/min to 3000 r/min,the brake specific fuel consumption of the two-cylinder two-stroke gasoline engine is around 222 g/k W·h when water injection is used to suppress knock.