Experimental And Simulation Investigation On Combustion And Knock Characteristics Of Hydrogen Engine

Burning renewable zero-carbon fuels is an important way to achieve the goal of carbon neutrality in engines.Hydrogen can be produced from renewable energy sources,achieving carbon neutrality throughout its life cycle,high mass-energy density,wide flammability limits,and fast flame propagation.However,at the same time,hydrogen has many limitations,such as low volume calorific value,easy pre-ignition and abnormal combustion phenomena such as engine knocking and even super knocking,which affects thermal efficiency and seriously damages key engine components.In order to propose targeted suppression methods for different intensities of hydrogen engine knocking,it is necessary to conduct in-depth research on the detonation combustion process and impact mechanism of hydrogen engines.In this paper,based on the optical engine test platform combined with multi-dimensional numerical simulation,the combustion performance and knock characteristics of the engine under the conditions of port injection and in-cylinder direct injection of hydrogen are studied.Firstly,the influence of key parameters on the combustion characteristics and knocking of hydrogen engine was studied based on the optical machine.The results show that due to the short duration of hydrogen combustion,advancing the ignition timing makes the engine performance decrease while the cyclic variation increases,and the knock probability first increases and then decreases with the advance of the ignition timing;increasing the equivalence ratio makes the hydrogen The performance of the engine is enhanced,and at the same time,the detonation intensity and probability are increased;the occurrence of pre-ignition and flashback causes the cylinder structure to generate local high temperature and induce spontaneous combustion,which in turn leads to the occurrence of detonation,and the violent heat of detonation and super detonation The release makes the next cycle more prone to pre-ignition and flashback.In addition,the knock intensity of the hydrogen engine showed a trend of first increasing and then decreasing with the advance of the pre-ignition time.Secondly,based on multi-dimensional numerical simulation,the research on the correlation between multi-cycle variation and knocking is carried out.The results show that there is a strong negative correlation between the IMEP and the duration of the initial combustion stage in conventional knocking conditions,and there is a positive correlation between the peak pressure and the knock intensity.In addition,it is found that the earlier the knock initiation time,the stronger the knock intensity.Through the analysis of the initial field distribution and flame structure,it is found that the initial flow field distribution of the high-knock cycle is more turbulent,the flame propagation anisotropy is smaller,and the initial flame propagation speed is faster.Through the analysis of the combustion process in the cylinder,it is known that the spontaneous combustion flames in the conventional knocking condition are all slow-burning flames,and the rapid combustion of multi-point spontaneous combustion is the cause of the strong pressure oscillation;Coupling phenomenon of pressure waves.Finally,the effect of hydrogen direct injection strategy on the combustion characteristics and knocking of hydrogen engine was studied.The research shows that with the delay of the injection time,the inhomogeneity of the initial flow field and the concentration field distribution increases,and the existence of the local over-rich region makes the time of spontaneous combustion earlier and also increases the probability of spontaneous combustion,which shortens the combustion duration of the engine.IMEP decreases,knocking gradually evolves from conventional knocking to super knocking;with the increase of injection pressure,the initial flow field velocity and in-cylinder mixture uniformity increase,the combustion duration of the engine first increases and then decreases,IMEP It first increased and then decreased,and the knock intensity gradually decreased with the increase of injection pressure,but the sensitivity to the change of injection pressure was weak.