Research On Combustion And Emission Characteristics Of Diesel/Natural Gas Dual Fuel Engine

Internal combustion engines, as the most widely used thermal machinery, consume enormous amount of fossil fuels every year and cause serious environmental pollution. Faced with the issues incurred by internal combustion engines, natural gas has become the most promising alternative fuel for its advantages of abundant reserves, low cost as well as reduced emissions, which, however, is the major motivation for the research focus on dual fuel natural gas engines. This paper incorporated CFD numerical simulation with experimental method to investigate the in-cylinder combustion process of a diesel/natural gas dual fuel engine and made comparative analysis for the effects of substitution rate, pilot injection timing and engine load on the combustion and emission characteristics based on the simulation results.In the present work, the combustion model for combustion simulation of a WP10 diesel/natural gas dual fuel engine was selected and the boundary and initial conditions were defined. A full set of experiments were conducted and the combustion model was calibrated with the recorded in-cylinder pressure data.Combustion process of the test engine was simulated based on the verified simulation model and the effects of substitution rate, pilot injection timing and engine load were analyzed based on the simulations; the results indicated that:(1) With substitution rates of 30%,50%,70% and 80%, the in-cylinder pressure and temperature decrease with the increase of substitution rate and when substation rate raises from 30% to 80%, the in-cylinder pressure decreases by about 28% with a 17% reduction in in-cylinder temperature; the propagation distance of laminar flame and the flame region are reduced due to the reduced pilot injection quantity at higher substitution rate; besides, the mass fractions of NO and SOOT reduce by 84% and 68.42% respectively as substitution rate varies from 30% to 80%, implying that the emission performance is significantly improved at higher substitution rate.(2) When pilot injection timing varies from 19°BTDC to 7°BTDC, the in-cylinder pressure and temperature decrease with the retardation of pilot injection timing; in fact, the maximum in-cylinder pressure is reduced by 26.69% and the maximum in-cylinder temperature is reduced by 157K; in addition, NO emissions at the pilot injection timing of 7°BTDC is 75.09% lower than that of the pilot injection timing of 19°BTDC while SOOT emissions reach the highest value at the pilot injection timing of 7°BTDC, reduce by 57% when pilot injection timing advances toll°BTDC and change slightly when pilot injection timing further advances to 19°BTDC.(3) At engine torques in the range of 300 N-m to 800N-m, both in-cylinder pressure and in-cylinder temperature show an increasing trend with the increasing engine torque; the in-cylinder pressure increases by 24% and in-cylinder temperature raises by 461K accompanied with increased laminar flame speed, larger flame region as well as less NO and SOOT formation with engine torque increasing from 300N·m to 800N·m; It should be noted that the emissions of NO and SOOT at engine torque of 800N·m is four and twenty-one times as much as that at the engine torque of 300N-m.