Effects Of Injection Strategy And Combustion Chamber Shape On Combustion Characteristics Of Natural Gas Engine

In order to meet the stricter second-stage regulations for marine engines,the equivalent ratio+TWC method is expensive for marine engines;lean combustion is another way to achieve high performance and low emissions for engines.However,the engine using multi-point injection can reduce the methane escape caused by the valve overlap angle,natural gas itself has the weakness of slow flame propagation,especially when using lean combustion.In addition,lean combustion will also increase the combustion instability,the combustion of mixed gas in the cylinder will become worse,and the emissions will worsen.In order to optimize the combustion characteristics of multipoint injection natural gas engine with premixed combustion,efficient combustion of natural gas in engine cylinder can be realized For the intake process of natural gas engine,injection strategy（gas injection position and direction,gas injection timing,and gas injection rate）affects the mixture of natural gas and air and the distribution of mixed gas after entering the cylinder.In addition,the flow movement in the cylinder also plays a vital role in the distribution of mixture and flame propagation.In this study,two methods,engine bench test and 3D numerical simulation,were adopted to optimize the efficient combustion of mixture in cylinder and select the medium and low load conditions with poor engine combustion effect.The influence of different nozzle with different gas inlet mixture uniformity and different gas injection rules on engine combustion and emission was investigated experimentally.Numerical simulation is used to analyze the effects of the change of injection timing and the combustion chamber structure on the combustion characteristics of lean-burn natural gas engines.This study has important reference value for the realization of more efficient and clean combustion of natural gas engines and the development of a natural gas engine combustion system applicable to the soon-to-be implemented marine two-stage emission regulations.First of all,the engine bench test results with different nozzle shapes and injection rate show that under the condition of 50%load,the nozzle with side wall perforation can improve the uniformity of intake gas mixing by increasing the disturbance effect of natural gas and air.When A is 1.42,the maximum combustion pressure P_max in the cylinder increases by 7.1%（from 41.84bar to 44.79bar）,the combustion retardation period is shortened by 2°CA,the peak heat release rate is relatively increased,the mixture burns efficiently,the combustion phase is advanced,and the COV_IMEP changes from 5.37%to 3.89%,which is conducive to the expansion of the lean combustion limit.CH₄and CO emissions were reduced by 9.6%and 8.5%respectively.At low and medium loads,the combustion effect in cylinder is better when higher injection pressure of 7.27bar is adopted.Under 50%load:with higher gas injection pressure,the maximum combustion pressure P_max increased by 4.8%（from 48.89bar to 51.36bar）,and CH₄ and CO emissions decreased by 13.6%and 25%,respectively.In addition,reducing injection pressure will enlarge the injection pulse width.Considering the escape of scavenging natural gas caused by early injection time and the residual inlet caused by late injection time,the range of change of injection time under low injection pressure will be severely limited.Therefore,high gas injection pressure（7.27bar）should be adopted for natural gas under medium and low load conditions.Subsequently,based on the thermal fluid software CONVERGE,we introduces the convergence of launching 3D simulation model,selects reasonable sub-models and calculation methods,determines the initial and boundary conditions,and verifies the accuracy of the model.Under 50%load condition,the evolution and combustion process of the mixture concentration field distribution in the cylinder under different gas injection time were analyzed.Simulation results show that with the gas time delay lead to natural gas into the cylinder within the time delay,the mixture concentration area gradually to be near the upper cylinder,60°CA after the check point on inlet injection,the ignition point,along the mixture concentration distribution in cylinder cylinder axis is obviously layered effect,the maximum combustion pressure up to51.04 bar,increased by 12.52%,the combustion phase advance,significantly shorten the combustion duration.Finally,on the basis of the original combustor,CATIA is used to design the combustor with the reduced mouth shape and the combustion chamber with the open shape for the simulation calculation,keeping the same compression ratio and comparing with the original machine,to study the influence of the combustor structure change on the in-cylinder combustion characteristics.The simulation results show that,for marine lean-burn natural gas engines,shrink-mouth combustor has a stronger squeezing effect from the aspect of combustion,the natural gas in the fire bank clearance is reduced,the mixture in the cylinder is more concentrated,and the average turbulent kinetic energy in the cylinder is higher than that of the original engine.In the later stage of compression stroke,it forms a rolling zone conducive to flame propagation.The mixture burns fast,the maximum combustion pressure Pmax is49.15bar（an increase of 8.4%）,the peak phase is 3°CA earlier,the combustion duration is shortened,and the shrinkable combustion chamber is a more ideal combustion chamber.The open combustion chamber did not obtain the reasonable mixture distribution as expected,and the extrusion effect was weak,the turbulent kinetic energy in the cylinder was low,and the mixture combustion effect was poor.