Study On Ignition Core Formation And Combustion Process In A Diesel Pilot Ignited Direct Injection Natural Gas Engine

Natural gas is a perfect alternative fuel for diesel engine due to its low C/H ratio and C/LHV ratio,which help to decrease soot and CO₂ emissions.Diesel pilot ignited natural gas engine is widely used as it maintains good fuel economy of diesel engine and has a good controllability of ignition timing.Traditional diesel pilot ignited NG engine has high UHC emissions at low load caused by slow flame propagation.Moreover,it usually has a lower maximum load than equivalent diesel engine due to limited charge efficiency and high ringing intensity at high load.Natural gas direct injection enables mixture stratification,which controls ignition core formation and flame propagation speed for combustion optimization.This paper conducted a numerical study on ignition core formation and combustion process in a diesel pilot ignited natural gas direct injection engine.Firstly,ignition core formation and the effects on combustion process were studied under different natural gas and pilot diesel fuel distributions.It was found that ignition core formed on the rich region of natural gas is not beneficial to ignition core formation due to a lack of oxygen and high equivalence ratio.Ignition core formed on the relative leaner region adjacent to the rich region of natural gas is beneficial both to the pilot fuel ignition process and following combustion process which is enhanced by the increased flame propagation speed in the adjacent rich region.Multiple ignition cores formation could help to expand the reaction zones across the combustion chamber,which would enhance the combustion.Secondly,NG split injection strategy was proposed to optimize NG equivalence ratio distributions.The effects of NG stratifications and boost pressure on ignition core formation and flame propagation were investigated.Compared with single injection,NG split injections could reduce the NG mass flow to the squish volume,which helps to decrease UHC emissions.NG distributions have great impacts on ignition core development.High percentage of rich mixture(0.6<?_NG?0.85)around the ignition core could increase the overall combustion rate.The over rich mixtures(?_NG>0.85)gathered in the chamber center causes fast flame propagation,which would increase ringing intensity.At a certain load,increasing boost pressure would decrease NG equivalence ratio and rich zones of NG.As a result,initial flame development is faster while the later flame propagation is slower,which increases combustion loss.As heat transfer loss keeps decreasing on the other hand,indicated thermal efficency increases first and then decreases with boost pressure increasing.Moreover,the turning point of indicated thermal efficiency is related to stratifications of natural gas equivalence ratio.Finally,combustion processes under low load and high load were further studied in a diesel pilot ignited direct injection natural gas engine.It was revealed that retarded NG injection and lower NG injection pressure could help to create appropriate mixture stratifications at low load,which is beneficial both to fast formation of ignition cores and fast combustion of NG.Under high load,large amounts of rich mixtures and over rich mixtures will be formed easily,which characterize a bulk ignition.In this situation,micro-pilot ignition is attainable.With high boost pressure and large EGR,high load could be obtained with unacceptably high peak cylinder pressure.However,RIVCT is a good method to solve this problem.With RIVCT strategy,ignition core forms slower and initial flame propagation speed decreases.As a result,heat release becomes soft,which is beneficial to extend high load limit of pilot ignited engine.