Extending The Operating Range Of Ultra-low Emissions From DMCC Engine

Reducing energy consumption and hazardous emissions is the only way to sustainable development for internal combustion engine industry,which is due to energy crisis and environmental pollution problems.Conventional engine is quite difficult to simultaneously achieve increasing BTE and reducing emissions.Thus,engine manufacturers are facing with many difficulties in the development of next generation engine,and new combustion mode engine fueled with low carbon clean fuel attracted researchers’attention.DMCC is a typical representative of the new combustion strategy.Previous experimental results show that this combustion strategy could not only increase BTE,but also reduce NOx and soot emissions.However,high efficient combustion region of this combustion mode is relatively small.Meanwhile,the emission level is difficult to meet the China VI emission regulations.For DMCC strategy,extending the operating boundary needs to be worked out urgently.In this work,extending the operating boundary of DMCC engine was studied on the modified electronic-controlled turbocharged intercooled diesel engine,which could conduct DMDF combustion mode.Experimental research and simulation research were used.Based on the understanding of DMDF combustion mode,the operating boundary of DMDF engine is extended.Methods to suppress methanol autoignition were proposed.Finally,based on the analysis of the reasons for DMCC engine to achieve high efficiency and clean combustion,the mechanism of achieving high efficiency and clean combustion of DMDF engine is illustrated.The main conclusions of this article are as follows:（1）Several typical combustion modes of DMDF were found.There are four typical combustion modes for DMDF combustion,which is divided by combustion timing of two fuels and the shape of HRR curve.The first one is DMDF mode1,diesel and premixed methanol were simultaneously compression ignited in this combustion mode.The shape of HRR curve is n-type.The second one is DMDF mode2,premixed diesel fuels were firstly compression ignited,then the premixed methanol fuels were quickly ignited.The shape of HRR curve is m-type.The third one is DMDF mode 3,premixed methanol fuels were firstly compression ignited,then diesel fuels were ignited.The shape of HRR curve is M-type.The fourth one is DMDF mode 4,this combustion mode is similar with conventional diesel combustion and the shape of HRR curve is h-type.DMDF mode1 and mode2 could simultaneously achieve high efficiency and clean combustion.DMDF mode1 and mode2 are the directions of DMDF combustion optimization.（2）The specific measures to suppress auto-ignition of methanol were worked out.Mixture temperature is the decisive factor of methanol autoignition.Methanol autoignition process could be effectively suppressed by reducing cylinder temperature.EGR could suppress methanol autoignition process,which is due to that H₂O and CO₂could reduce cylinder temperature.Oxygen concentration has little effect on the combustion of methanol.Another important factor of methanol autoignition is the equivalence ratio.With the increase of methanol equivalence ratio,the ignition timing of methanol autoignition was advanced,the HRR peak of methanol autoignition was increased,and cumulative heat release was increased.Thus,the cylinder pressure was increased.During methanol autoignition process,reducing methanol equivalence ratio can ensure that the engine is within the safety design limit.（3）The operating boundary of DMCC engine was extended.Compared with original operating boundary,there are knock-free area and low efficiency area in extended operating boundary,and the area of the high-efficiency zone has grown.The results show that under new operating conditions missfire and low efficiency combustion are the main reason to limit extending the operating boundary of DMDF engine.Extended boundary point of DMDF mode has higher BTE,which is compared with the same boundary point of diesel mode.Meanwhile,NOx and soot emissions were reduced,the drop in soot emissions is much higher than the drop in NOx emissions the highest drop in soot emissions is 99.38%,which is 33.33%for NOx emissions.（4）BTE of DMDF mode is higher than diesel mode,which is caused by many reasons.First,the effective work of DMDF mode is higher than diesel mode;In the intake and compression stroke,methanol gasification process will absorb large amount of heat,which could reduce the temperature of intake charge,thus the negative compression work is reduced.Then,methanol evaporated in the intake manifold and cylinder will absorb large amount of heat.This will reduce coolant temperature and engine temperature.Thus,the heat loss will be reduced.Finally,the energy losses in exhaust gases in DMDF mode is lower than that of diesel mode.Under DMDF mode,the exhaust temperature is obviously lower than that of diesel mode.These factors work together to make the BTE of the DMDF mode significantly higher than that of pure diesel mode.（5）The methods of DMCC engine with high efficiency and clean combustion were obtained.Under DMDF high efficient and clean combustion mode,diesel was premixed combustion,methanol mixture was quasi-homogeneous compression ignition.The quasi-homogeneous premixed combustion mode provides a basis for DMDF to achieve high efficiency and clean combustion.The key to achieve quasi-homogeneous premixed combustion is combustion process control.In addition,high MSP is also the another key to realize diesel premixed combustion.Under high MSP conditions,the combustion process of diesel fuel will make a substantial change.All of diesel fuels will be burned in the way of premixed combustion.Cylinder temperature could promote the transformation of methanol combustion mode.Increasing cylinder temperature can effectively improve the combustion efficiency of methanol.At the same time,increasing the activity of premixed methanol can also effectively improve the combustion efficiency of methanol.