Combustion Process Optimization Of Natural Gas Engine For Heavy-duty Vehicles Based On Active Control Of Heat Release Rate Profiles

The development of natural gas(NG)engine technology has a positive impact on the management of energy saving and emission reduction in the field of heavy-duty vehicles(HDVs)in China.The stoichiometric operation NG engine with EGR and three-way catalyst(TWC)can meet the regulatory requirements,but it faces the pain point of mutual constraints between "low gas consumption" and "easy detonation".In order to balance the above contradiction,the experiments were conducted on a 13 L inline six-cylinder heavy-duty stoichiometric operation NG engine.In this study,a bench testing and numerical simulation platform were built.The optimization of combustion process of NG engine of HDVs based on active control of heat release rate(HRR)profiles were studied.From the perspective of improving ignition,organizing airflow distribution,improving compression ratio(CR),and suppressing detonation,this paper intends to carried out the following three parts of research.Firstly,the impact mechanism of combustion boundary conditions on the combustion process were carried out.Secondly,the adjustable range of HRR profiles for collaborative matching of multiple combustion boundary conditions was analyzed.Lastly,a collaborative matching strategy that can achieve efficient combustion of the engine was proposed.The brake thermal efficiency(BTE)was improved from 38.0%to 41.5%.The main contents and conclusions are as follows:(1)In order to effectively suppress engine knock,a design method of high-low pressure EGR(HLP-EGR)system was proposed.Through the coordinated control of high and low pressure EGR circuits,the introduction ability of EGR under high engine load was greatly improved.The HLP-EGR system was designed and developed in this study based on the original high-pressure EGR(HP-EGR)circuit,with the addition of a low-pressure EGR(LP-EGR)system with intercooler.When the intake of HP-EGR circuit increases gradually,the maximum intake of LP-EGR circuit first increases and then decreases,and the total intake of EGR in cylinder gradually increases,too.Through the optimization design of parameters such as the structural size of the LP-EGR circuit,and when the HP and LP EGR valves were fully opened,the EGR introduction amount of the HLP-EGR system was the highest.For example,under the working condition of 1200 r/min and 80% load,the EGR rate increased from 17.1% to 21.1%.(2)The impact mechanism of key combustion boundary conditions on incylinder heat release process was explored,which provided theoretical basis for improving combustion speed.The results show that the corresponding time and the burned mass fraction have a significant impact on the combustion process when the flame propagates to the squeeze zone.In this test,early spark timing(ST),high ignition energy,using the combustion chamber(CC)with eccentric and bottom rib can enhance the turbulence intensity in the cylinder before reaching the TDC,and improving CR can promote positive feedback between turbulence and combustion.The application of the above boundary conditions could advance the corresponding time when the flame propagates to the squeeze zone,increasing the burned mass fraction at this moment.Making the HRR profiles closer to TDC,CA50 moved forward,and the DOC was shortened.An efficient and rapid combustion can be realized.(3)Through a wide range of coordinated control of key combustion boundary conditions,the adjustable range of HRR profiles of NG engine under different working conditions was expanded,which provides path support for its efficient and clean combustion.Firstly,adopting ST cooperated with HLP-EGR technology.The developed HLP-EGR system improved the EGR introduction.The operating range of ST was broadened,so the adjustable range of HRR profiles broadened,too.Secondly,adopting IE cooperated with ST and HLP-EGR technology.High IE could improve ignition process,so the adjustable range of HRR curve was expanded overall under medium load,while it moves forward overall under high load.Thirdly,adopting CC shape cooperated with ST and HLP-EGR technology under the IE was 105 m J.Adding ribs at the bottom of the piston bowl at low speeds helped to achieve earlier and faster heat release.At high speeds,the EHACC could achieve early heat release,and the combustion of the ACC was lagging behind BCC.The controllable range of HRR profiles moved forward overall at low speeds,and expanded overall at high speeds.Lastly,adopting high CR cooperated with ST and HLP-EGR technology under the IE was 105 m J and EHACC.By improving CR itself could enhance the introduction ability of HLP-EGR,and significantly improved combustion speed and stability.The controllable range of HRR profiles moved forward overall at low speeds,and expands overall at high speeds.Taking the economic working condition of the target prototype(1200r/min,100% load)as an example,through the cooperative matching of multiple combustion boundary conditions,the HRRmax(347～681J/℃A),the CA50(8.8～25.5℃A ATDC)and the combustion duration(16.3～32.7℃A)were realized.(4)Based on the active control of HRR profiles,a cooperative matching strategy of combustion induction and knock suppression was proposed.By improving the EGR introduction ability of the engine,the occurrence of engine detonation phenomenon was effectively suppressed.The efficient and rapid combustion induction of the engine was realized by cooperative control of parameters such as CR.The process of active control of heat release process by cooperative matching of parameters was as follows: by using the advanced ST,although the HRR profile can be moved forward and concentrated.But its advance range was limited by detonation under high load.Therefore,a cooperative strategy between ST and EGR was proposed.Increasing EGR rate could greatly make ST advance and advance combustion heat release,but prolong ignition delay time and DOC.Subsequently,it is proposed that high IE technology can improve the ignition process,but with less improvement in the DOC.Then,the CC shape was optimized,and the EHACC was adopted to enhance the turbulence intensity in the cylinder,which effectively improves the combustion speed and isotropy at large EGR rate.At this point,the BTE was improved from 38.0%to 40.1%.Finally,high CR technology was proposed.The combination of high CR,high EGR rate,and earlier ST could effectively improve combustion speed,making the HRR curve more forward and concentrated,and realized efficient and rapid combustion process in the cylinder.Under the economic working condition of the target prototype,the technology path with an EHACC with the CR of 12.5,the IE of105 m J,HLP-EGR rate of 16.5%,and ST of 28℃A BTDC was adopted.CA50 and DOC were advanced to 8.8℃A ATDC and 16.3℃A,respectively.The BSFC was decreased to 184.7g/(k W·h),the BTE was increased to 41.5%,and the improvement was 9.2% higher than the original machine.