Investigation on mixture formation and combustion characteristics of a heavy-duty SI methanol engine

Due to the simplicity of the exhaust gas aftertreatment system, the heavy-duty natural gas spark ignition (SI) engines play an important role in achieving the macro targets of the reduction of petroleum consumption, carbon and harmful emissions. Considering that power-to-methanol is economically more feasible than power-tomethane, methanol is proposed to continue the advantages of natural gas engines. Based on the fuel properties of methanol and our researching experience, methanol mixture preparation and flow within the cylinder are the most significant. A novel high-pressure port fuel injection (PFI) system is utilized to promote atomization and evaporation of methanol. The effects of the injection strategy, including injection pressure (pinj) and injection timing (alpha inj), and excess air-fuel ratio (lambda) on the mixture formation were simulated and then studied by engine experiments. Three-dimensional simulation and engine tests were conducted under the condition of 1500 rpm and the brake mean effective pressure (BMEP) of 0.854 MPa. pinj varied from 8 MPa to 24 MPa by 4 MPa, alpha inj was from 240 degrees CA BTDC to 400 degrees CA BTDC by 40 degrees CA and lambda was set to be 1.1,1.2 and 1.3, respectively. Simulation results indicate that the mixture homogeneity can be promoted by the optimized injection strategy. The mixture is the most homogeneous at alpha inj = 340 degrees CA BTDC and pinj = 8 MPa and the most inhomogeneous at alpha inj = 400 degrees CA BTDC and pinj = 24 MPa when lambda = 1.2. The experimental results indicate that the more homogeneous mixture can improve engine brake thermal efficiency by shortening CA0-10 and CA10-90 as well as methanol lean-burn. The optimized injection strategy can improve the brake thermal efficiency by 2.6%, 2.7% and 2.8% when lambda = 1.1, 1.2 and 1.3, respectively.