Construction Of Combustion Mechanism And Study On Fuel Injection Strategy For Diesel/Natural Gas Dual Fuel Engine

Energy shortage and environmental pollution are the biggest challenges facing mankind at present.Because of its high thermal efficiency and low emission potential,more and more attention has been paid to the research on diesel/natural gas dual fuel engine.At present,the influence mechanism of diesel injection strategy on the main reaction path of diesel/natural gas dual fuel engine combustion is not clear.In this paper,the combustion process and harmful emissions generated mechanism of diesel/natural gas dual fuel engines under different diesel injection strategies are investigated by coupling engine bench test and 3D numerical simulation.First,the combustion mechanism of diesel/natural gas dual fuel engine was constructed.Using n-hexadecane to replace diesel and using a mixture of methane,ethane and propane to replace natural gas.The detailed mechanism of n-hexadecane was simplified by DRG,DRGEP,sensitivity analysis,ROP analysis and isomer lumping.The kinetic parameters of key reaction for reduced mechanism were optimized by brute force sensitivity analysis.Coupling the reduce n-hexadecane sub-mechanism into the natural gas sub-mechanism,a diesel/natural gas dual fuel reduced mechanism with 155 components and 645 reactions was constructed.The results of comparison between the simulated and experimental values of ignition delay,species concentration,laminar flame speed and combustion of actual dual fuel engine show that the simplified mechanism can simulate the combustion process of diesel/natural gas dual fuel engine.Then,the diesel single injection strategies（injection pressure and injection timing）of dual fuel mode were explored.The results show that,under high load condition,the BTE significantly increased with the injection pressure increased or the injection timing advanced.At the medium load condition,the BTE is increased when the injection timing is advance,while the effect of injection pressure on BTE is small.Compared to the pure diesel mode,the dual fuel mode can achieve lower CO₂ emissions.One of the reasons is that the main reaction path for CO₂ generation in the dual fuel mode is CH₂O→HCO→CO→CO₂,while the main reaction path for CO₂ generation in the pure diesel mode is CH₂CHO→CH₂CO→HCCO→HCO→CO→CO₂.Finally,the effects of multiple injection strategies on combustion and emission of diesel/natural gas dual fuel engines were investigated.The results show that,the addition of pilot injection can improve BTE and reduce THC and CO emissions compared with single injection.When the pilot injection timing is –25 –10 °CA ATDC,as the main injection timing advance,the flame propagation speed rise,BTE and NOX emission increase,while the THC and CO emissions decrease.The in-cylinder combustion shows obvious two-stage exothermic phenomenon,the first stage heat release rate is dominated by the diffusion combustion of diesel and the premixed combustion of natural gas,and the second stage heat release rate is dominated by the premixed combustion of natural gas.Energy loss from dual fuel engines mainly comes from THC emissions.When the pilot injection timing is too forward（over –25 °CA ATDC）,the main injection timing has little effect on the performance of diesel/natural gas dual fuel engine,and the energy loss mainly comes from heat transfer loss and friction loss.