| It is important for the internal combustion engine industry to develop natural gas engines with high efficiency and low emissions.Compared with gasoline,the ignition temperature of natural gas is higher,and the flame propagation speed is slower.Lean-burn technology can improve the thermal efficiency of the engine while reducing the emissions of nitrogen oxides.In addition,the lower combustion temperature of lean burn is beneficial to reduce the knock tendency,which makes it possible to achieve higher compression ratios and high brake mean effective pressure.However,lean combustion is also faced with challenges,such as difficult ignition,slow combustion speed,large cycle variation,and so on.By initiating large area and multi-point ignition,the pre-chamber jet ignition can improve the ignition reliability of the lean mixture and accelerate the combustion speed,which is favorable to improving engine performance.Enhancing the ignition energy of the pre-chamber jet and improving the jet ignition stability are helpful to further improve the lean-burn performance of natural gas engines.This dissertation took the pre-chamber natural gas engine as the research object,aiming at enhancing the pre-chamber jet energy and improving the combustion stability under lean-burn conditions.Based on a research-based single-cylinder engine,the correlation between cyclic variation and pre-chamber jet energy was analyzed,and then the influence of auxiliary fuel injection and ignition timing was investigated for further analysis.The difference in ignition performance between the passive and active pre-chamber was compared and analyzed by 3D CFD numerical simulation.Based on the above works,this paper further explored the effect of spark ignition position and pre-chamber auxiliary hydrogen injection on jet characteristics.The main work and results are as follows:Firstly,a single-cylinder natural gas engine test platform with a cylinder bore of 190 mm was built for in-cylinder combustion analysis,and an auxiliary injection device was developed to control the mixture concentration inside the pre-chamber.Based on the platform,the combustion characteristics of a natural gas engine were tested,and then the effect of j et energy on the cycle-to-cycle variations under lean-burn conditions was conducted.A correlation analysis was carried out to evaluate the correlation degree between some parameters of the pre-chamber combustion and the main chamber maximum combustion pressure(and its crank angle).The results indicated that with the increase of the pre-chamber jet energy and stability,the combustion heat release rate of the main chamber is accelerated,and the combustion stability is enhanced.The pre-chamber maximum pressure and its crank angle have a moderate or higher correlation with the main chamber maximum combustion pressure and its corresponding crank angle.These two pre-chamber combustion parameters are suitable for characterizing the cycle-by-cycle variations of pre-chamber combustion and reflect the variations in the jet energy and jet ignition timing.The influence of cyclic variations in the pre-chamber combustion on the cycle-by-cycle variations of the main chamber combustion increases with the increase of the pre-chamber jet energy,which is helpful to reduce the influence of the initial combustion conditions.Secondly,a method for estimating mixture concentration inside the pre-chamber at ignition timing was proposed.Based on the single-cylinder natural gas engine platform,experiments were conducted to analyze the effects of the end time of auxiliary fuel injection and ignition timing on the engine performance.The results indicated that,at a fixed spark ignition timing of 20℃A BTDC,compared with the ending injection at the middle of the intake stroke,the mixture concentration inside the pre-chamber at ignition timing is increased when the end time is retarded to the early stage of the compression stroke so that the pre-chamber combustion speed is accelerated and the pre-chamber jet energy and stability are improved.When the intake pressure is 0.18 MPa and the excess air ratio is 1.9,the coefficient of variation of IMEP is reduced to 1.36%,and the IMEP increases by 5%.When the intake pressure is 0.223 MPa and the excess air ratio is 2.0,the coefficient of variation of IMEP decreases to 1.56%,and the IMEP increases by 3%.Adjusting the spark ignition timing,on the one hand,affects the amount of the main chamber lean mixture flowing into the pre-chamber,on the other hand,affects the thermodynamic state of the mixture in the two chambers and the phase of combustion.The experimental results indicated show that at a fixed end time of auxiliary fuel injection,with the delay of ignition timing the IMEP remains relatively stable and the coefficient of variation of IMEP is less than 4%if the CA50 is within 5℃A ATDC and combustion duration CA5-90 is less than 30℃A.The emissions of NOx and CO decrease with the delay of ignition timing.When the ignition timing is further retarded,the CA50 moves backward to 10℃A ATDC and CA5-90 is greater than 30℃A.The variation coefficient of IMEP increases to nearly 10%limit and the IMEP decreases by nearly 5%.A significant decrease in engine combustion stability and power is observed.Thirdly,a numerical model of the pre-chamber natural gas engine was established and the operating characteristics of the passive and active pre-chamber jet ignition systems under leanburn conditions were studied.The charge exchange volume of the passive pre-chamber is small because of the throttling effect of the connecting channel,while the active pre-chamber achieves effective scavenging and enrichment through auxiliary fuel injection.In the compression stroke,a compression tumble flow around the horizontal axis of the pre-chamber is formed inside the pre-chamber by the lean mixture from the main combustion chamber guided by the structure of the pre-chamber.The mixture concentration distribution in the pre-chamber is controlled by this flow field.Under lean-burn conditions,the in-cylinder flame propagation in the axial direction of the connecting channel is mainly driven by the jet momentum,while the propagation perpendicular to the axial direction is attributed to the coupling effect of jet entrainment and turbulent flame propagation.Finally,the effects of spark ignition location and the pre-chamber auxiliary hydrogen injection on the jet characteristics and engine performance were studied through numerical simulation.By locating the spark at the lower part of the pre-chamber,such as the point between the neck and flow-guide region,the injection time of the hot jet is advanced.Moreover,the mixture concentration near the ignition location is beneficial to the formation and development of the flame kernel.The compressed airflow at the ignition location promotes flame propagation toward the upper part of the pre-chamber.As a result,more gases are wrapped into the flame front and participate in the heat release of the pre-chamber.Therefore,the pre-chamber combustion is accelerated and the jet energy is enhanced,which improves the lean burn inside the main chamber.The pre-chamber combustion process,jet characteristics,and main chamber combustion process were compared under three pre-chamber strategies,namely,passive prechamber,active pre-chamber with methane injection and with hydrogen injection.The results indicated that the injection of auxiliary hydrogen strengthens the mixture reaction in the prechamber and significantly increases the pre-chamber combustion rate.The injection velocity and temperature of the pre-chamber jet rise with the increase in hydrogen injection duration.Therefore,the main chamber combustion rate is accelerated and the combustion duration is reduced,which expands the lean burn limit. |
PDF Full Text Request