Experimental Study On Combustion Cycle-by-Cycle Variations Characteristics Of Hydrogen Direct Injection Engine Based On EGR

Energy crisis and environmental pollution have become the important factors that restrict the sustainable development of society at present.It is a hot research direction and an important topic in automobile field to seek the clean alternative fuels for engines.As a green renewable clean fuel,hydrogen has faster flame propagation speed,lower ignition energy,wider range of combustion,shorter quenching distance and reduces combustion cyclic variations.Therefore,the gasoline blended with hydrogen has been paid more and more attention.However,there are still some shortcomings of hydrogen-blended engines,among which the most prominent problem is the remarkable increase of NOX emission after hydrogen addition.The introduction of EGR can significantly reduce the NOX emissions of hydrogen-blended engines,but it will cause the deterioration of combustion and lead to increase of cyclic variations.Combustion cycle-by-cycle variation is a irregular combustion phenomenon of spark ignited engines,which greatly restricts the improvement of engine performance.Therefore,it is of great significance and practical value to study the effects of EGR on combustion cyclic variations characteristics of hydrogen direct-injection engine.Based on the project from National Natural Science Foundation of China "Study on Energy-saving Mechanism of HEV Equipped with Lean-burn Gasoline Engine with Hydrogen Direct Injection System and with Local Hydrogenous Mixture",this thesis completed the construction of experimental bench for hydrogen direct injection/gasoline intake injection dual fuel engine with EGR system.Based on d SPACE system,the engine control model and the EGR valve control model were established by MATLAB/Simulink.The effects of EGR and hydrogen direct injection on the combustion cyclic variations and emission characteristics of engine with different excess air ratios,ignition timings,hydrogen fractions and hydrogen injection timings were studied.The test results and main conclusions are as follows:(1)The effect of ignition timing on the combustion cyclic variations was studied.The results show that COVIMEP decreases firstly and then increases with the advance of ignition timing,and there is a spark advanced angle(MCV)corresponding to the minimal coefficient of cyclic variation.For a limited ignition timing range,the peak cylinder pressure increases monotonously with advanced ignition timing,while COVPmax decreases.COVIMEP and COVPmax increase with the increase of excess air ratio,the average maximum cylinder pressure decreases gradually,and the MCV increases gradually.After hydrogen addition,with the increase in hydrogen fraction,COVIMEP and COVPmax both decrease,the mean value of the peak cylinder pressure increases gradually,and the MCV continuously decreases.(2)The effect of hydrogen fraction on the combustion cyclic variations was studied.With the increase of hydrogen fraction,the cyclic variations of the indicated mean pressuredecreases,the distribution of it tends to be concentrated,and the interdependency between the indicated mean pressure and the crank angle corresponding to the maximum cylinder pressure is enhanced.COVIMEP increases with the increase of hydrogen fraction.The peak cylinder pressure and the maximum rate of pressure rise decrease,and the peak cylinder pressure and the maximum pressure rise rate and their corresponding crank angles show a good linear interdependency.Meanwhile,the mean value of the peak cylinder pressure and the maximum rate of pressure rise increase with hydrogen fraction,and COVPmax decreases.Additionally,flame development period and fast burning duration are shortened,while the central combustion crank angle is advanced after hydrogen addition.In lean burn conditions,the cycle distribution of the indicated mean pressure value becomes dispersed,and the interdependency between the indicated mean pressure and the crank angle corresponding to the peak cylinder pressure is weakened and COVIMEP increases.And the peak cylinder pressure and the maximum rate of pressure rise increase,while the interdependency between them and their corresponding crank angle become worse.The cyclic variations of the peak cylinder pressure and the maximum rate of pressure rise slightly decrease and COVPmax increases accordingly.With the increase of excess air ratio,the flame development period and fast burning duration are prolonged,the central combustion crank angle is postponed,and the central combustion crank angle can be all achieved at the optimum range with large hydrogen fraction.(3)The effect of hydrogen injection timing on the combustion cyclic variations was studied.With the advance of injection timing,the indicated mean pressure increases firstly and then decrease.The cycle distribution is dispersed at earlier and later hydrogen injection timing.The distribution between indicated mean pressure and the crank angle corresponded to the peak cylinder pressure shows that more cycles with the highest or lowest indicated mean pressure correspond to the same crank position,while COVIMEP decreases and then increases.The cyclic variations of the peak cylinder pressure and the maximum rate of pressure rise first decrease and then increase,and the interdependency between them and their corresponding crank angle indicates a good linear correlation at each injection timing,respectively.Additionally,COVPmax first decreases and then increasse at different excess air ratios and hydrogen fractions.Whilst,the flame development periodand the fast burn duration are both first shoretened and then prolonged,and the central combustion crank angle is first advanced and then postponed.When at lean burn operation,these parameters are more obvious with varied injection timing.Under different injection timings,with the increase of EGR rate,COVIMEP and COVPmax increase gradually.While,the flame development period and the fast burning duration are prolonged,and the central combustion crank angle is retarded.The interdependency between the indicated mean pressure and the crank angle of peak cylinder pressure,the peak cylinder pressure and its crank angle,the maximum pressure rise rate and its crank angle demonstrate a good injection timing with a good correlation at small EGR rate.The changes of these parameters are all obvious at large EGR rate.(4)The effect of EGR on the combustion cyclic variations was studied.The results show that,with the increase of EGR rate,the indicated mean pressure value decreases and the interdependency between the indicated mean pressure and the crank angle corresponding to the peak cylinder pressure becomes worse and the distribution area is enlarged and dispersed,while the range of the indicated mean pressure enlarges and COVIMEP increases.Meanwhile,the cyclic variations of the peak cylinder pressure and maximum rate of pressure rise increase.The interdependency between the peak cylinder pressure and maximum rate of pressure rise and their corresponded crank angle become worse,respectively.The distribution areas are enlarged and the mean values decrease.Also,COVPmax increases and the crank angle corresponded to the peak cylinder pressure and the maximum rate of pressure rise increase.The flame development period and the fast burning duration are extended,while the central combustion crank angle is retarded.After hydrogen addition,these parameters with EGR rate become more gentle,and COVIMEP and COVPmax decrease.With different excess air ratios,the cyclic variations of the indicated mean pressure,the peak cylinder pressure and the maximum pressure rise rate and distributions and the flame development period,the fast burning duration and the central combustion crank angle show a similar law with the increase of EGR rate,while cyclic variations of them are obvious for larger excess air ratio and larger EGR rate.(5)The effects of hydrogen addition and EGR on emissions were studied.The results show that with the advance of ignition timing,HC emission increases gradually,CO emission increases first and then decreases,and NOX emission increases.With the increase of hydrogen fraction,HC and CO emissions decrease obviously,but NOX emission sharply increases.With the increase of excess air ratio,HC emission slightly decreases first and then increases,CO emission decreases gradually and NOX emission increases first and then decreases rapidly.With the advance of hydrogen injection timing,the emissions of HC,CO and NOX increase first and then decrease,but they vary a little with large hydrogen fraction.After the introduction of EGR,with different hydrogen fractions,excess air ratios and hydrogen injection timings,HC emission increases with the increase of EGR rate,the emission of CO increases first,then slightly decreases and then sharply increases,and NOX emission remarkably decreases.With large hydrogen fraction,HC and CO emissions keep at a low level,while the effect of reduced NOX emission is obvious.For larger EGR rate,HC and CO and NOX emissions vary gently with the advance of hydrogen injection timing,in which NOX emissions remain low.Above all,hydrogen addition greatly changes the combustion cyclic variations under different boundary conditions.Hydrogen addition can effectively reduce the combustion cyclic variations and enhance combustion stability,but NOX emissions increase sharply.The introduction of EGR can significantly reduce NOX emissions,but the combustion cyclicvariations remarkably increase.The combination of EGR and hydrogen addition can make full use of their advantages.The synergistic effect of EGR combined with hydrogen can reduce the cyclic variations while guaranteeing lower emissions and enhancing the combustion stability.