Research On Lean Limit Expansion Of Gasoline Engine Based On Pre-chamber Ignition

With the growing energy shortage and environmental pollution problems,many countries have introduced several regulations to limit the fuel consumption and pollutant emissions of vehicles.Due to the constraints of insufficient clean energy,slow construction of charging infrastructure and high costs,the gasoline engine will remain the main power source of passenger cars in the form of pure internal combustion engine or hybrid power for a long period of time in the future.Therefore,there is still a need to continue to develop high-efficiency,near-zero-emission gasoline engines to improve engine thermal efficiency and reduce CO2 and harmful emissions as much as possible,so as to cope with the increasing energy and environmental problems.Improving the theoretical thermal efficiency can be achieved by increasing the compression ratio and improving the specific heat ratio of the working mass.Lean combustion can not only improve the specific heat ratio of the work mass,but also suppress the problems such as the tendency of knock caused by high compression ratio.Therefore,high compression ratio combined with lean combustion is an effective technical path to improve engine thermal efficiency.However,an increase in combustion dilution rate can introduce problems such as unstable ignition and slow combustion,which leads to more exhaust losses.Besides,misfires,late combustion and other abnormal combustion phenomena tend to occur more frequently,which reduced the thermal efficiency.Therefore,the lean limit of combustion cannot be expanded indefinitely.How to promote the formation of flame kernel and fire propagation under lean combustion conditions is the core technical problem to expand the lean combustion limit of gasoline engines.The development of new ignition technology is an effective way to increase ignition energy and promote flame development.The new ignition technology includes both traditional hot plasma ignition and cold plasma ignition.The core difficulty of cold plasma ignition is the instability of ignition under high load with high back pressure.Also,with the high cost,it is difficult to be applied in mass production vehicles in a short period of time.Hot plasma ignition contains capacitive/inductive discharge and their derived ignition methods,such as multielectrode ignition,high-energy ignition,active/passive pre-chamber ignition,etc.However,the performance comparison of different ignition methods still lacks quantitative evaluation,and the research on the mechanism and characteristics of prechamber ignition is still not very clear at the current stage.With the expansion of the lean burn limit,misfires,late combustion and other abnormal combustion phenomena are more likely to occur.How to achieve real-time diagnosis and control of abnormal combustion based on the use of new ignition methods is an inevitable problem for further expansion of the lean burn limit.This thesis takes the constant-volume combustion bomb as the test platform to compare the ignition and flame propagation performance of several different new ignition methods;the pre-chamber ignition is selected as the focus of this thesis,and an empirical formula for the variation of flame jet penetration distance with time for a single-hole pre-chamber is proposed.A research platform for high-compression-ratio gasoline engine was built,and the effects of passive pre-chamber geometry parameters on engine combustion conditions were investigated experimentally and combined with two-stage high-energy ignition to further expand the lean burn limit and improve engine thermal efficiency.The active pre-chamber and its fuel supply system are designed and constructed,and the effects of three types of parameters on the engine combustion conditions,namely the chamber structure,fuel control boundary conditions,fuel type of the active pre-combustion chamber,are analyzed,and the highest indicated thermal efficiency and maximum lean burn limits are achieved using active pre-chamber fueled with pure methane.Finally,the ion current signal in the pre-chamber under normal combustion,misfire,and knocking conditions is studied,and the characteristic parameters are extracted for misfire and knock diagnosis;the causes of the ion current signal characteristics under knocking conditions are explained using three-dimensional simulation.The ion current-based neural network knocking diagnosis model further improves the accuracy of diagnosing candidate knocking cycles.The work and main findings of this thesis are summarized as follows.(1)Development of test platform and novel ignition system.Two test platforms of a constant-volume combustion bomb and a direct injection engine were adopted to study the ignition and combustion characteristics of different ignition methods.For the constant-volume combustion bomb platform,the injection/ignition timing control system is developed,which is applicable to the control of various ignition methods;for the engine test platform,the gasoline engine control system is developed,which realizes the simultaneous acquisition of multi-sensor signals and the individual control of multiple actuators.Finally,the active/passive prechamber ignition system is designed and optimized.In particular,an additional fuel supply system was developed for the active pre-chamber system,which is capable of air pressurization and dehumidification,fuel premixing,mixture heating and injection,and other functions.(2)Study on the formation of the flame kernel and flame propagation characteristics under different ignition modes in a constant-volume combustion bomb.Two schlieren image processing algorithms were developed for the calculation of the combusted area;several parameters to evaluate the combustion performance based on the combusted area increment and the in-bomb pressure were established.The effects of different ignition methods on the formation of the flame kernel and flame propagation of steady-state constant-volume combustible gas mixtures were studied.The results show that two-stage high-energy ignition and three-electrode ignition both have a strong promotion effect on flame kernel formation but cannot promote laminar flame propagation in the middle and late stages of combustion or expand the lean burn limit.For the working condition of λ=1.1,it takes 67.9 ms to reach the measurement endpoint(half of the initial unburned area)from the ignition of a normal spark plug,the passive pre-chamber needs 31.3 ms,and the active pre-chamber takes only 11.8 ms.Therefore,passive/active pre-chambers not only increase the ignition energy but also promote the turbulent flame propagation in the late stages of combustion.Based on the circular free turbulent jet model,the development law of single-hole pre-chamber flame jet in the stationary flow field is summarized,and the empirical formula of flame jet penetration distance is derived and verified.(3)Combustion and emissions characteristics of gasoline engine with passive prechamber ignitionBased on the test platform of a high compression ratio engine with a compression ratio of 16,the influence of different geometric characteristics of passive pre-chambers,such as the number of orifices,orifice size,orifice direction,spark plug electrode position,and pre-chamber volume on the combustion characteristics of the engine at different speeds and loads,was studied.By optimizing the structure of the passive prechamber,the lean burn limit is expanded from λ=1.3 of the ordinary spark plug to λ=1.5 at the medium to high load conditions,and the maximum indicated thermal efficiency reaches 36.9%,improved by 10.9% compared with 33.3% of ordinary spark plug ignition under equivalent ratio condition.The steady-state emission characteristics of ordinary spark plug and passive pre-chamber ignition were compared,and the results showed that more unburned gases such as HC and CO were emitted from the exhaust gas under passive pre-chamber ignition,while NOX emissions increased or decreased in a load-dependent manner.The effects of the combined ignition methods of ordinary spark plug and passive pre-chamber combined with two-stage high-energy ignition on combustion and emission were explored,and the results showed that high-energy ignition under small load conditions can significantly shorten the ignition delay and combustion duration,reduce cycle fluctuations,reduce unburned HC emissions,and further improve thermal efficiency,but will lead to an increase in NOX emissions;while the combined high-energy ignition has less impact on combustion and emissions under heavier loads.(4)Study on the combustion and emissions characteristics of gasoline engine with active pre-chamber ignitionBased on the test platform of a high compression ratio gasoline engine,the effects of active pre-chamber fuel control parameters,such as the injection timing,injection pulse width,and injection pressure,on the combustion characteristics of the engine are studied,and the fuel control parameters are optimized to optimize the effect of expanding the lean combustion limit and improving the thermal efficiency.The effects of different geometric characteristics such as the number of injection orifices,the diameter of injection orifices,the orifices direction,and the volume of the pre-chamber on the combustion characteristics are investigated so that the pre-chamber geometry can be optimized.Finally,the effects of pure methane gas and gasoline/air premixed gas as the active pre-chamber supply fuel respectively on the engine combustion and emission characteristics were also compared,and the results showed that the active prechamber supplied with gasoline mixture achieved stable combustion of λ=1.8 with an indicated thermal efficiency of 38.8% under medium and large load conditions.In contrast,the active pre-chamber supplied with methane can achieve λ=2.0 stable combustion;the indicated thermal efficiency reaches 39.1%,which is 18.8% higher than the normal spark plug equivalent ratio working condition;it achieves the ultra-low NOX emission with the exhaust gas concentration of only 21×10-6,and the HC and CO concentrations in the exhaust gas are also lower than other ignition methods.(5)Study of ion current characteristics inside the pre-chamber of gasoline engineBased on a high compression ratio engine testbench,ion current signals were collected from the active/passive pre-chamber and compared with those collected from the normal spark plug,and it was found that under lean burn conditions,the correlation between the ion current signal in the active pre-chamber and combustion condition in the main combustion chamber is stronger than that of the ordinary spark plugs and the passive pre-chamber conditions.Using the passive pre-chamber as a typical case,the integrated value of ion current can be used to diagnose misfire cycles.With a guaranteed misfire diagnosis sensitivity of 100%,the misfire diagnosis accuracy of the ion current in the passive pre-chamber can reach 99.7%,which is significantly improved compared with 89.6% of the normal spark plug condition.The existence of a knock indication peak in the ion current signal within the passive pre-ignition chamber under detonation operating conditions was found.If a combustion cycle with a maximum pressure rise rate greater than 0.2 MPa/°CA is defined as a candidate knocking cycle,99.7%sensitivity and 95.7% accuracy can be achieved using the knocking indication peak to diagnose candidate knocking cycles.A black box model for knock diagnosis based on ion current parameters was established by means of a neural network to further improve the diagnostic accuracy of candidate knocking cycles to 98.4%.The causes of the characteristic waveform of ion current in the pre-chamber under the knocking condition are explained with the help of a three-dimensional simulation method.The main reason is the large pressure rise rate in the main combustion chamber caused by knock,which generates a strong return flow and drives the combustion mixture to carry many ions and electrons to the vicinity of the ion current sensor.