Study Of Combustion Process And Emissions On A Diesel Engine With Directly Injected Methanol And Pilot Diesel

As an alternative fuel for engines,methanol has a lot of advantages,such as wide sources,it can be easily prepared from coal and coke oven gas,and it can reduce Soot and nitrogen oxide emissions simultaneously,etc.Among several alternative fuels,methanol has important research value and bright application prospects.Nowadays,the application of methanol on diesel engines mainly includes three methods:direct mixture,port injection,and direct injection.Compared with the other two methods,direct injection has a lot of advantages,such as it can avoid inefficient combustion at low load and knocking at high load.It can achieve high methanol substitution rate and low pollutant emissions.Therefore,it is of significant to study this method.Direct injection contains three injection strategies,namely,D/M and M/D modes are that diesel injection is earlier and later than methanol injection respectively;M/D/M mode is that methanol is injected once before and after diesel injection.In this paper,a three-dimensional simulation model of a diesel/methanol dual-fuel engine is built on a turbocharged inter-cooled diesel engine,the following researches are carried out:（1）The differences of combustion characteristics,engine performance and emissions of port injection and direct injection are investigated;（2）The effects of key control parameters of D/M mode on combustion characteristics,engine performance,and emissions are studied;（3）M/D and M/D/M modes are investigated;（4）The optimal settings of different injection strategies at different loads are obtained,then the optimal injection strategy at different loads are obtained;（5）Two kinds of direct injection methods of single-channel injector and dual-channel injector are proposed,and a comparative study is carried out.The influence of key injection parameters of single-channel injector is studied.The main conclusions are as follows:The R_mof the port injection method is restricted by inefficient combustion at low load,while the R_mof the direct injection method is not restricted,even if it is as high as 90%,it can still maintain good fuel economy.The R_mof the port injection method is restricted by knocking at high load,while the R_mof the direct injection method is not restricted,and it can prevent knocking even if it is up to 95%.Compared with port injection,Soot emission decreases by 94%and NO_xemission decreases by 57%at low load,Soot emission decreases by 98%and NO_xemission decreases by 66%at high load,and THC,CO and CH₂O are close to zero emissions.Peak in-cylinder pressure decreases with the delay of diesel injection timing and the increase of dwell.Too small dwell leads to misfire or knocking combustion,and too large dwell is detrimental to fuel economy.With the advancing of diesel injection,equivalent indicated specific fuel consumption（EISFC）,Soot,THC,CO and CH₂O emissions decrease,but ringing intensity（RI）and NO_xemission increase.Too small dwell or too large dwell may lead to high THC,CO and CH₂O emissions.Higher initial temperature is detrimental to fuel economy,and reducing initial temperature in order to improve fuel economy.With the increase of initial pressure,EISFC reduces and fuel economy is optimized,and the effect of initial pressure on EISFC is more obvious at high load.Initial swirl ratio has little effect on EISFC,higher initial swirl has higher RI value.As the nozzle diameter increases and the injection pressure decreases,the RI decreases continuously,and the EISFC shows a decrease first and then an increase.The results show that 0.36mm+27.4MPa can always maintain good fuel economy and prevent knocking.The effect of diesel injection pressure on EISFC is not obvious,but it can affect RI.The results show that diesel injection pressure of 55MPa can maintain good fuel economy and prevent knocking.The spatial interval angleα=-2?andβ=15?can achieve best fuel economy and lowest pollutant emissions.D/M mode can control combustion velocity and suppress combustion noise effectively,and help to achieve complete combustion.It is not advisable to adopt M/D mode at high load,because not only knock is easy to occur,but also THC,CO and CH₂O emissions are high.In M/D/M mode,a higher first methanol injection ratio is better at low load and medium load,while a lower first ratio is better at high load.It is found that M/D/M mode is optimal at high load and M/D mode is optimal at low load and medium load.In the single channel injector,diesel ignition delay is prolonged by front methanol,and this effect can be weakened by increasing the diesel amount.The presence of front methanol has little effect on fuel economy,but it will increase ringing intensity.Too small or too large diesel amount may lead to knocking combustion.Too small nozzle diameter paired with too large injection pressure is easy to knocking combustion,and too large nozzle diameter paired with too small injection pressure is not good for fuel economy.The results show that 0.36mm+27.4MPa can obtain the optimal configuration and can achieve a same level performance compared with dual-channel injector.