Compression Ignition Characteristics And Emission Control Of Low Octane Gasoline

Gasoline compression ignition mode can significantly reduce particulate matter（PM）emissions while maintaining low NOemissions and high thermal efficiency.Compared with commercial gasoline,stable combustion can be achieved over full operating condi-tions with low-octane gasolines without modifications on the original diesel engine hard-ware or combustion strategies.Thus,low-octane gasoline is a relatively ideal fuel for compression ignition engines.In this thesis,the ignition characteristics of a low-octane gasoline,as well as its combustion and emission characteristics in a heavy-duty compres-sion ignition（CI）engine were investigated.Meanwhile,the piston bowl geometry of the diesel engine for ultra-low emissions was optimized,and its adaptability to low-octane gasoline was studied to explore the potential for lower emissions.The advantages of low-octane gasoline compression ignition are demonstrated by comparing the combustion and emission characteristics of various gasoline-like fuels.The design principle is also proposed for the physical and chemical properties of ideal compression ignition engine fuels.Based on a rapid compression machine（RCM）and0-D simulation of combustion chemical kinetics,the compression ignition characteristics of the low-octane gasoline were investigated,and a multi-component gasoline surrogate（MCGS）was formulated.It can be concluded that MCGS reveals a more pronounced negative temperature coefficient（NTC）behavior than primary reference fuel（PRF）and toluene reference fuel（TRF）,so that can more accurately capture the ignition character-istics of the low-octane gasoline.Furthermore,an analysis method is proposed to quanti-tatively weigh the contribution of different parameters to the second-stage ignition delay time increase within the NTC range.It is found that the difference in the reactivity of the non-fuel specific reaction at the start of second-stage ignition is the dominant factor for a more pronounced NTC behavior of MCGS than PRF.The influence of EGR rate and injection pressure on the combustion and emission characteristics of the low-octane gasoline and diesel were compared in a heavy-duty CI engine under stable operating points,and the combustion strategies over full operating conditions were optimized.Results show that the low-octane gasoline can break the trade-off relationship between NOand PM,and the PM emissions of the low-octane gasoline are less sensitive to EGR rate and injection pressure than diesel.Under high EGR rates,accumulation mode particles are dominant for both low-octane gasoline and diesel.How-ever,under low EGR rates,diesel produces much more nucleation mode particles than the low-octane gasoline.With a 40 MPa lower injection pressure than diesel,lower NOand PM emissions are achieved over a wide operating range with the low-octane gasoline.Furthermore,compared with diesel,the composite CO₂emission decreases by 4.2%under the cold and hot Federal Transient Procedure（FTP）cycle with the low-octane gasoline while meeting the limits of the California Air Resources Board’s regulation for Model Year 2027.The piston bowl geometry of the baseline CI engine was optimized based on 3-D Computational Fluid Dynamics（CFD）simulation.It is found that the optimized design can simultaneously reduce NOand soot emissions of diesel with no loss of efficiency compared with the baseline.Such effects are achieved by reducing the oxygen-rich area surrounded by the high temperature combusted mixture in the early and middle stage of combustion to reduce NOproduction,and guiding the air-fuel mixture moving towards the center of the cylinder in the late combustion stage to promote soot oxidation and reduce heat transfer loss.The experimental results show that,with the optimized piston bowl ge-ometry,lower fuel consumption is achieved compared with the baseline at the same level of NOemissions.However,the application of the baseline spray angle hinders the soot emission from reducing.Meanwhile,simulation results validate that the optimized design is adaptable to the low-octane gasoline in terms of decreasing NOand soot emissions.