Experimental Study And Numerical Simulation Of High Substitution Ratio Natural Gas Dual-fuel Engine

Natural gas spark ignition engine had been widely used.Natural Gas Dual-fuel(NGDF)engine could achieve much higher efficiency compared with natural gas spark ignition engine,but which still suffered from some technical bottlenecks.At low load,the ultra lean natural gas-air mixture led to incomplete combustion;at high load,rough combustion caused high Maximum Pressure Rise Rate(MPRR).Natural gas substitution ratio was limited due to the above reasons,which caused that the industrial application of NGDF technology had not been realized.In this paper,experimental study and numerical simulation were carried out aiming at the technical challenges.Based on the 90% natural gas substitution ratio limit of China VI standard,the combustion control strategies under different loads were investigated on a single-cylinder engine test bench.It was found that using throttle valve to increase equivalence ratio could significantly promote combustion efficiency at low load,hot Exhaust Gas Recirculation(EGR)was used to reduce the negative effects of NOx emissions and pumping loss.At high load,Homogeneous Charge Induced Ignition(HCII)strategy could achieve lower MPRR,while higher Indicated Thermal Efficiency(ITE)was observed at Reactivity Controlled Compression Ignition(RCCI)strategy.It was also found that NGDF could achieve low Soot and NOx emissions simultaneously even at extremely low pilot fuel injection pressure(20 MPa).In order to further improve the combustion and emission characteristics of NGDF engine,pilot fuel with higher cetane number named Polyoxymethylene Dimethyl Ethers(PODE)was used to ignite natural gas.Experiments were conducted to investigate the effects of PODE blended.The results showed that PODE could significantly promote the ignition characteristics of n-heptane and lower critical compression ratio.With the proportion of PODE increasing,total ignition delay and physical ignition delay were significantly shortened while chemical ignition delay had no obvious changes,which indicated that the effects of PODE low temperature reaction were dominated in ignition process.The results of engine test bench experiments showed that PODE could achieve lower THC,CO,NOx,Soot emissions and higher ITE in comparison with diesel.To better understand the combustion process of NGDF engine,a three-dimensional numerical model was built.The effects of PODE mixing were investigated.In comparison with n-heptane,PODE blended fuel could ignite more methane-air mixture in the beginning of combustion process,which was beneficial to reduce THC and CO emissions.It was also found that the favorable conditions were different between pilot fuel and natural gas combustion,which led to incomplete combustion and high THC emissions.Spark ignition assisted NGDF mode was put forward to solve the above problems.The results showed that spark ignition assisted NGDF mode in conjunction with low-pressure injection strategy could significantly improve the combustion characteristics and achieve approximately 100% combustion efficiency at low load with no cost increasing.