| Currently,mankind is faced with the severe energy crisis and environmental pollution problems,which put forward the requirements of high efficiency,clean and high energy density for many power systems.As a rotary-type internal combustion engine,Wankel engine has the inherent advantages of simple structure,small space proportion,strong fuel adaptability and high power-to-weight ratio,and also needs to meet this development requirement.As known,now Wankel engine is mainly used in the field of military unmanned aerial vehicle(UAV).With the rapid development of new energy vehicles,it also has potential applications as a power source for the range extender in the field of civil electric vehicle.This makes the application of Wankel engine technology on new power system become a research hotspot at present.However,the combustion chamber of the Wonkel engine is narrow and long,and during its working process there are some problems,such as slow flame propagation speed,flame easy quenching and unstable combustion,which makes it diff-icult for prototypes using traditional combustion technology to meet the high market standards.Therefore,in order to expand the application to the range extender in civil electric vehicle field and so on,it is very important to carry out a series of in-depth research on its combustion characteristics and new combustion technology development.In addition,in view of the fact that the research and development level of the Wankel engine in China lags behind that of foreign developed countries obviously,it is of great strategic significance to seize the opportunity to tackle its key core technologies.Therefore,to explore the combustion process in-cylinder of the Wankel engine and improve its performance,this paper carried out systematic research on its working process,new combustion mode and multi-fuel characteristics.Meanwhile,considering the advantages of diesel Wankel engine,such as low fuel consumption rate,high thermal efficiency and good safety performance,this paper focuses on the combustion characteristics of diesel Wankel engine and its new combustion mode exploring.The key information and their evolution law during the entire working cycle of a diesel Wankel engine are studied in-depth by adopting the experimental test and numerical simulation methods.Firstly,a constant volume bomb experimental setup under the operating conditions of diesel Wankel engine is built.Secondly,a 3D-dynamic simulation model coupled with chemical reaction kinetics of diesel Wankel engine is established.Then,a novel gaseous/liquid dual-fuel combustion mode for diesel Wankel engine is proposed.Finally,the influence of key impact factors on engine combustion and emission characteristics under different combustion mode is systematically investigated.The obtained research achievement have some academic value and practical significance,which are as follows:(1)Fuel spray characteristics experimental setup under the diesel Wankel engine operating conditions is built successfully.Hence,the basic fuel spray characteristics of the prototype under its in-cylinder working environment are obtained and the effects of ambient pressure,injection pressure and injection width on the diesel spray characteristics are investigated.The experimental results show that the ambient pressure is the most important factor due to its greatest influence degree on the fuel spray characteristics.This provides some useful basic experimental data for the subsequent simulation model establishment,and also clears that the low and middle injection pressure and suitable injection timing should be chosen to avoid the fuel excessive impact on the rotor wall under high injection pressure.(2)A 3D-dynamic simulation model of diesel Wankel engine is established and a step by step verification method is adopted.Based on the experimental data,the reliability of the simulation model is evaluated in detail.Subsequently,key information during the flow and combustion process in-cylinder is obtained.Simulation results indicate that,at assisted-ignition timing the fuel distribution and concentration around assisted-spark plugs in the combustion chamber have significant influence on the following combustion process.Diesel fuel stratified charge in front of the combustion chamber can make combustion reaction mainly occur in this area,and fully utilizing the forward flame propagation law can realize the controllability of the combustion zone.(3)The fuel injection and assited-ignition strategy of diesel Wankel engine is optimized to improve its combustion efficiency.As for injection strategy,when the leading and trailing assited-spark plugs are used,at assisted-ignition timing the fuel should distribute between them and more fuel should concentrate around the trailing assited-spark plug,which is better to the combustion process.Delaying injection timing,the fuel distribution area becomes narrower and more concentrated.As for assisted-ignition strategy,the engine combustion performance under dual assisted-spark plugs is quite better than that of single assisted-spark plugs,and the centralized and symmetrical arrangement of dual assisted-spark plugs along the chamber width is more conducive to the improvement of combustion efficiency.Delaying assited-ignition timing,the peak pressure change little due to the little difference in fuel distribution in the combustion chamber.(4)A novel natural gas-diesel combustion mode is proposed,and the combustion and emission characteristics of the dual-fuel Wankel engine are analyzed.During mixture formation process,the natural gas movement is susceptible to the in-cylinder airflow motion,while diesel is mainly determined by the wall impinge position and high-speed unidirectional flow.At assited-ignition timing,natural gas distributes in the whole combustor chamber while diesel concentrates in the middle and front of the combustor chamber.Increasing natural gas blending ratio,the distribution area of natural gas varies little,however diesel becomes narrow obviously.Due to natural gas's good diffusion characteristics,wider flammability limit and faster flame propagation speed after successful ignition,engine combustion and emissions performance are obviously improved.(5)The influence of different operating parameters on natural gas-diesel Wankel engine performance is further explored.Dlayng dual-fuel injection timing,mixture became richer,and the pressure in-cylinder and combustion rate increase.Howerver,the richer air-fuel mixture finally increases emissions.Similarly,under the dual-fuel combustion mode,the combustion performance is better by adopting dual assisted-spark plugs.Changing assisted-ingniton strategy,the natural gas distribution around the assisted spark plugs changes little at assisted-ingniton timing,but diesel distribution change obviously.After the combustion begins,more mixture concentrated near the assisted-spark plugs is beneficial to the following combustion process.Besides,the improvement of natural gas combustion process plays a leading role in improving the overall combustion rate.This paper provides theoretical guidance for improving diesel Wankel engine performance,and also has some reference for expanding the application of other liquid fuel and new gaseous/liquid combustion modes in the Wankel engine and its new technology development. |
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