Investigation The Cycle Variability Of The Premixed Si Natural Gas Engine Under Lean-burn Combustion And Hydrogen

As the emissions regulations become more and more stringent,internal combustion engines are strictly required to further improve fuel economy and greatly reduce the emissions.Heavy-duty natural gas spark ignition(SI)engine employs the lean-burn combustion and low-carbon alternative fuel,which is conducive to achieving clean combustion.However,lean-burn combustion could greatly increase the cycle-to-cycle variations(CCV)in cylinder.Specifically,the fast burning cycles can release heat in a short time and trigger knocking combustion due to faster-than-average cycles over advanced spark timing,while the slow burning cycles can lead to partial burn and even misfire,and significantly increase the HC emissions and fuel comsumption.Consequently,the fast and slow cycles result in combustion instability,and thereby leading to the speed and output power instability,engine operating unsteadily,shaking,misfire,power dropping,high fuel consumption,and even affecting the drivability and conformability issues in the vehicles.Generally,if the CCV in the cylinder could be eliminated,the fuel conversion efficiency could enhance 10-20% as well as reduce the exhaust emissions.This study is manly divided into experimental sector and numerical simulation sector.In experimental sector: the steady-state experiments were comprehensively conducted on the lean-bun premixed natural gas SI engine with different speeds,loads,compression ratios and hydrogen energy fractions.The influences of operating/design parameters and hydrogen enrichment on CCV were completely investigated,and thereby obtaining the fundamental reasons and performance.In numerical simulation sector: initially,the laminar flame structure,laminar flame speed calculation and measurement,natural gas combustion mechanism were introduced in detailed,and one dimensional laminar flame premixed flame was calculated by numerical simulation.The effects of the initial temperature and pressure,dilution gas,hydrogen enrichment and hydrogen coupled with EGR were comprehensively simulated on the purified liquefied natural gas/air laminar flame speed.Additionally,the influences of high EGR,low EGR and their combination,internal EGR were also completely simulated on the combustion,performance and emissions characteristics in a hydrogen enriched natural gas SI engine.Finally,the full-size 3-D model was built according to the real combustion chamber.The consecutive cycle method and parallel perturbation method were used to simulate the CCV in the lean-burn premixed natural gas SI engine.Subsequently,the influences of the in-cylinder flow and turbulence kinetic energy distribution,initial spark kernel formation,growth and development,turbulent flame propagation on the CCV in the lean-burn premixed natural gas SI engine,and thereby pointing the direction of reducing the CCV.The main conclusions in this dissertation are summarized as follows:(1)As the engine speed increases,the cylinder pressure distribution tends to concentrate in the lean-burn premixed natural gas SI engine,the CCV of the in-cylinder pressure decreases;the coefficients of variation of the indicated mean effective pressure(IMEP),peak combustion pressure(PCP)and combustion duration decrease.In addition,there is a strong linear correlation between the SOC and PCP.At low-load condition,the in-cylinder pressure distribution tends to be concentrated with the increase of compression ratio(CR).However,with further increase of the CR,its effect on the in-cylinder pressure distribution decreases.At high-load condition,increasing CR has little effect on the in-cylinder pressure distribution.In addition,the decrease magnitude of the coefficients of variation of the IMEP is decreased with increasing the CR in the lean-burn premixed natural gas SI engine.(2)As the hydrogen energy share increases,the cylinder pressure distribution tends to concentrate;the coefficients of variation of the IMEP and PCP decrease.The PCP increases and its location advances with increasing the hydrogen energy ratio.The SOC and 50% combustion location advance with increasing the hydrogen energy ratio,and thereby reducing the coefficients of variation of the CA50 and combustion duration;there is a strong linear correlation between the CA50 and PCP.(3)The laminar flame speed of liquefied natural gas(LNG)increases with increasing the initial temperature,while decreases with increasing the initial pressure.In addition,the laminar flame speed of LNG1/ air is slightly higher than LNG2/air due to the higher ethane concentration.Moreover,a small percentage of other alkanes have influences on the laminar flame speed,but the affects are limited.The LNG2/air is increased with increased the hydrogen fraction,and thereby extending the lean-burn limitation,and increasing the adiabatic combustion temperature.LNG 2/air/hydrogen coupled with EGR can not only reduce excessive NOx emissions,but also make a contribution to increase laminar flame velocity.Moreover,hydrogen has a great influence on the formation of O,H and OH radicals,which remarkably affect the chain reactions in the combustion processes.(4)The peak combustion pressure of the in-cylinder pressure decreases,the ignition delay period prolongs,and the 50% combustion location retards with the increasing of the EGR ratio either using the HP EGR or adopting the LP EGR,even combined the HP and LP EGR systems.Thereinto,with total 15% EGR ratio,combining 10% HP EGR ratio with 5% LP EGR ratio was achieved the highest indicated thermal efficiency(ITE = 34.9%)compared with other types of EGR.The internal EGR reduces with decreasing the valve overlap,while the PCP and peak heat release rate slightly increase with decreasing the valve overlap.In addition,the volumetric efficiency and indicated thermal efficiency are firstly increased and then decreased with decreasing the valve overlap.(5)The consecutive cycle method takes a long time to simulate the cyclic variability,while the parallel perturbation method significantly reduces the wall clock time compared with the former method.As the initial spark kernel transforms into a turbulent flame,the in-cylinder turbulence flow interacts violently with the flame front,and the flame front will be further stretched,distorted and wrinkled.The disordered turbulence flow in the cylinder dramatically increases the deformation of the flame front,which further entrains the unburned gas and forms an isolated island or block combustion area,and thereby greatly increasing the combustion area and accelerating the combustion rate.In addition,the turbulent flame propagation velocity in the high pressure cycle is significantly higher than that in the low-pressure cycle.The main goals of this current dissertation aim at to thoroughly analyze the causes,mechanism and changing rules of the CCV in the lean-burn natural gas SI engine by experiments and numerical simulations,which in order to further reduce the combustion variability,improve the operation stability and smoothness,reduce the noise and vibration caused by combustion variability,and reduce the fluctuation of output torque and power in the lean-burn natural gas SI engine.Reducing the CCV in the in the lean-burn natural gas SI engine,which is beneficial to appropriately advance the ignition timing and improve the CR,and thereby further enhancing the effective thermal efficiency and fuel conversion economy,expanding the lean-burn limitation,reducing the partial combustion,decreasing hydrocarbon emissions,so as to achieve further optimize the lean-burn premixed natural gas SI engine,and show a bright prospect in engineering application.