Study On The Combustion Technology Of Diesel Methanol Dual Fuel For Meeting The Emission Regulation Of China Ⅵ Without Urea After-treatment

The development of internal combustion engines(ICE)today is facing with the double pressures of shortage from oil resources and strict emission regulations.The utilizations of new combustion technologies and new fuels are effective ways to achieve efficient and clean combustion of ICE.In recent years,the dual-fuel low-temperature combustion technology in which low-reactive fuel is injected into intake port and highactive fuel is directly injected in cylinder has become a research hotspot.The technology of diesel methanol compound combustion(DMCC)has great potential in simultaneously reducing oil consumptions and harmful emissions.Previous studies have shown that DMCC engine could meet the China V emissions regulation for heavy duty diesel engine without urea after-treatment.However,with increasingly strict emissions regulations of China,it is an important challenge for DMCC engines to meet the strict China Ⅵ emissions requirements in coming year.The diesel methanol dual fuel(DMDF)combustion technology to meet the requirements of the China Ⅵ emissions regulation without urea selective catalytic reduction(SCR)has been systematically studied on the high-pressure common rail DMCC engines with premixed methanol in the intakes and directly injected diesel in the cylinders.The main research contents and conclusions are as follows:(1)The goal of main emissions for DMCC engine to meet the China Ⅵ emissions regulation without urea SCR was achieved.The original weighted NOx emission of 0.38g/k Wh under the World Harmonized Steady-state Cycle(WHSC)could meet the China Ⅵ emission regulation.With the assistance of an after-treatment of DOC +CDPF consisting of diesel oxidation catalyst(DOC)close-coupled with catalytic diesel particulate filter(CDPF),the weighted emissions of NOx,PM and carbon monoxide(CO)are 0.36 g/k Wh,0.0077 g/k Wh and 0.04 g/k Wh,respectively,all of which could meet the requirements of China Ⅵ emissions regulation.Due to the limitations of the after-treatment,the weighted total hydrocarbons(THC)emissions were slightly higher than the China Ⅵ limits.In addition,the weighted brake thermal efficiency(BTE)of the DMCC China Ⅵ engine is 3.7 % higher than that of the baseline engine.(2)The diesel injection strategy suitable for extremely low emissions and high thermal efficiency of DMDF combustion was proposed.It was found that the combustion mode gradually changed from quasi-homogeneous charge compression ignition(QHCCI)to premixed stratified compression ignition(PSCI)with the advance of the diesel main injection timing due to the change of the stratification of mixture activity and concentration in cylinder.Adopting an appropriate diesel "early injection" strategy could achieve near-zero emissions of PM and high thermal efficiency.The requirements for diesel injection pressure were relatively low in DMDF mode due to the small amount of diesel injection and long mixing time.The multiple injection strategies of diesel could optimize the DMDF combustion process.A single injection strategy at medium and light loads could avoid overmixing and help improve the combustion efficiency.The twice pulse injections needed to be adopted under high loads.The introduction of a high proportion of pilot injection could effectively improve the activity of methanol mixture,and greatly increase the combustion efficiency and brake thermal efficiency(BTE).In the case with high PM emissions,the introduction of a small amount of diesel at the latter period of main combustion could reduce the accumulated PM emissions(up to 28.8%),while the brake specific fuel consumption and the NOx remained almost unchanged.(3)The optimal proportion between methanol and diesel in DMDF combustion and emission control was studied.The increasing methanol energy ratio(MER)could greatly reduce the EGR demands to run low-temperature combustion mode,which was helpful to break the trade-off relationship between NOx and PM emissions.In medium and heavy loads,the highly premixed methanol could reduce the activity of in-cylinder mixture and delay the combustion phase,which was beneficial to suppress the "rough burning" and then achieve controlled dual fuel low temperature combustion.However,a relatively low MER(30% to 60%)should be adopted in low loads to prevent the reduction of combustion efficiency caused by excessively high MER.In DMDF modes,the MER should be gradually increased as the load and speed increase.(4)It was found that the intake state was an important factor affecting the combustion and emissions of DMDF.Increasing the intake air temperature could significantly reduce CO and THC emissions and then improve the combustion efficiency and combustion stability at light and medium loads.The resulting small increase in NOx emissions could be controlled by higher EGR rate.However,excessively high intake air temperature might cause the maximum of pressure rise rate to exceed the limit and "rough combustion" at high loads.A slight intake throttling could enhance the intake air vortex and flame propagation rate,which is helpful to improve combustion stability of DMDF.However,excessive throttling would reduce BTE due to the obviously increased pumping loss.(5)The EGR control strategies for DMCC engine to achieve ultra-low emissions under whose operating conditions were proposed.High-pressure EGR(HP-EGR)and low-pressure EGR(LP-EGR)had distinct effects on DMDF combustion.Compared with LP-EGR,HP-EGR had significant advantages in high thermal efficiency.The DMCC engine did not introduce EGR at idling conditions.In the PSCI mode at loads with 1.25 MPa BMEP and below,adopting composite EGR strategy with the priority of high-pressure EGR could achieve higher BTE on the premise of meeting the ultralow NOx emission requirements.The EGR rate range of PSCI combustion was controlled at 40% to 56%,and the lower the load,the greater the excess air coefficient,and the higher the EGR rate demand.The control strategy of composite EGR was as follows: According to the needs of NOx control,firstly opened the high-pressure EGR valve and then the low-pressure EGR valve and finally closed the back pressure valve gradually.In the QHCCI mode at higher loads,the LP-EGR strategy and a lower EGR rate(<25%)should be adopted to avoid the PM boom.(6)The efficient and clean combustion modes of DMCC engine for different loads under whose WHSC operating conditions were systematically studied.Among them,the combustion mode of idling conditions should be consistent with the baseline engine.The premixed stratified compression ignition(PSCI)combustion mode could be operated under 70% load(1.25 MPa BMEP)and below conditions,which could achieve extremely low original NOx emissions(<0.4 g/k Wh)and PM emissions(close to zero)and high thermal efficiency(up to 43% BTE).At higher loads,limited by the peak pressure and the maximum of pressure rise rate,it was suitable to operate the quasihomogeneous compression ignition(QHCCI),which could control the NOx emissions to a quite low level without the penalty of PM emissions and BTE.