| Pre-combustion chamber turbulent jet ignition is an emerging combustion mode that can expand the lean combustion limit of the engine and ensure stable engine operation under lean conditions.Compared with the traditional spark plug ignition,the pre-combustion chamber jet ignition can burn faster under the lean mixture and can improve the engine thermal efficiency.At present,most of the research on pre-combustion chamber jet ignition is focused on natural gas engines,while relatively little research and analysis has been done on pre-combustion chamber jet ignition systems for dilute gasoline engines.Therefore,this paper adopts a combination of bench test and numerical simulation to systematically study the influence of structural parameters such as main and pre-combustion chamber shapes,pre-combustion chamber cross-sectional ratio,volume,nozzle diameter and pre-combustion chamber injection parameters on the combustion,jet and main combustion chamber combustion characteristics of gasoline engine,laying a theoretical and data foundation for the design and development of jet ignition rarefied combustion gasoline engine,and providing a direction for subsequent bench performance optimization tests.This study will provide a theoretical and data basis for the design and development of the jet ignition rarefied combustion gasoline engine,and provide a direction for the subsequent bench performance optimization tests.In this paper,firstly,an existing single-cylinder diesel engine was modified,and the active pre-combustion chamber jet ignition system was designed and matched,and the potential of the active pre-combustion chamber jet ignition system in improving the combustion rate of the main combustion chamber was verified through tests.Then a three-dimensional CFD model of the engine was built using Converge software,and the three-dimensional simulation model was calibrated through tests,and the test results were controlled within the error tolerance with the simulation results to ensure the reliability of the simulation model.Then,based on the calibrated model,the effects of the main and pre-combustion chamber shapes on the mixture formation,jet and combustion characteristics of the pre-combustion chamber were studied in simulation.The results show that: different main combustion chamber shapes will change the intensity of the vortex ratio inside the main combustion chamber,which in turn affects the distribution of the mixture inside the pre-combustion chamber;the overall distribution of the pre-combustion chamber mixture is uniform,and there is no large thick area near the spark plug,which is more favorable to the ignition and rapid flame propagation inside the pre-combustion chamber;at the same time,it is necessary to match a suitable main combustion chamber shape to avoid the phenomenon of the jet flame hitting the wall,otherwise it will greatly reduce the jet energy.The suitable shape of the pre-combustion chamber is beneficial to improve the intensity of the tumble flow in the pre-combustion chamber and further improve the mixture state in the pre-combustion chamber.The results show that the flat top design of the pre-combustion chamber is more effective than the circular design,and it can avoid the narrow area at the top of the pre-combustion chamber,which makes the mixture too concentrated and leads to incomplete combustion.Based on the simulation study of the shape of the combustion chamber,the single variables such as pre-combustion chamber structure parameters(cross-section ratio,volume,aperture diameter)were further studied on the influence law of the combustion characteristics of the jet and the main combustion chamber by using the controlled variable method.The results of the study on the cross-section ratio parameters show that: when the cross-section ratio is too small,the overall height of the pre-combustion chamber is low,the high-temperature jet enters the main combustion chamber earlier,and the jet strength is lower and the continuous disturbance ability to the main combustion chamber is insufficient;when the cross-section ratio is too large,the overall height of the pre-combustion chamber becomes higher,which is more favorable to the mixture distribution near the spark plug and beneficial to the ignition,but the flame propagation distance inside the pre-combustion chamber becomes longer,which will lead to a slower flame propagation speed.Comprehensive results,the choice of cross-sectional ratio of0.2 is more appropriate.The results of the pre-combustion chamber volume parameters show that: the smaller the volume of the pre-combustion chamber,the worse the pre-combustion chamber mixture distribution,the less combustible mixture components,the weaker the jet energy,the worse the main combustion chamber combustion effect;and the larger the precombustion chamber volume,the more combustible mixture components in the precombustion chamber,the greater the jet strength,the better the overall mixture distribution,but too large pre-combustion chamber volume will make the pre-combustion chamber internal surface-to-volume ratio increases,as well as the increase in heat loss.resulting in increased heat loss,as well as lowering the overall compression ratio,resulting in slower combustion in the main combustion chamber.Combined with the above results,the choice of 4.5% of the precombustion chamber volume is more appropriate.The results of the study of the parameters of the jet diameter show that: the larger the diameter,the lower the average jet velocity of the jet,the shorter the duration of the pre-combustion chamber jet,the less continuous disturbance time to the main combustion chamber,but more high-temperature hot jet products per unit of time,the greater the ignition energy into the main combustion chamber;the smaller the diameter,the higher the average jet velocity of the jet,the more disturbance to the main combustion chamber,but the long duration of the pre-combustion chamber jet,the unit Crankshaft angle within the pre-combustion chamber of high-temperature jet products into the main combustion chamber is relatively small,the energy is lower.Combined with the above results,it is more appropriate to choose an orifice diameter of 1.0mm.The above simulation results were integrated to select a set of optimized main and precombustion chamber layout schemes,and the effects of injection parameters in the precombustion chamber on the combustion characteristics of the pre-combustion chamber and the main combustion chamber were studied in depth.The results show that the pre-combustion chamber has the best combustion effect and the shortest combustion duration in the main combustion chamber when the excess air coefficient in the pre-combustion chamber is between0.9 and 0.98,while the mixture is too rich or too lean will lead to the slow combustion rate in the main combustion chamber.The fuel injection timing is 240°CA before the upper stop point,which can ensure that the fuel in the pre-combustion chamber has enough time to mix with fresh air and avoid excessive fuel in the pre-combustion chamber from entering the main combustion chamber through the injection holes. |
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