Experimental And Numerical Investigations On Ethanol/diesel Dual-fuel PCI-LTC Combustion Strategy

Ethanol/diesel dual-fuel premixed compression ignition(PCI)combustion strategy can simultaneously reduce soot and NOx emissions while maintaining high thermal efficiency.Moreover,dual-fuel PCI combustion can achieve better control of combustion phasing and heat release rate,which is benefit for extension to high load condition.Therefore,dual-fuel PCI combustion has been proven a very promising combustion strategy to meet the strict emission regulations in the future.However,ethanol/diesel dual-fuel operation suffers the issues of high UHC(unburned hydrocarbon)and CO emissions as well as high cyclic variations,and also the specific mechanism for auto-ignition evolution is not quite clear.In this paper,experiments and simulations have been conducted to study the combustion and emission characteristics of ethanol/diesel dual-fuel operation,and also the specific mechanism of auto-ignition and combustion process was analyzed.Firstly,the ignition delay of ethanol/n-heptane mixtures was investigated based on a shock tube,and also the effect of in-cylinder fuel stratification in dual-fuel engines on ignition delay gradient was evaluated.The experimental results showed that the fuel stratification of dual-fuel operation could result in large gradient of ignition delay.Meanwhile,for ethanol/n-heptane blend with fixed equivalent ratio of ethanol(0.3),increasing n-heptane content could significantly decrease the ignition delay when the equivalence ratio was larger than 1.0.However,the ignition delay was only slightly decreased with increasing n-heptane content when the equivalence ratio was larger than 1.0.Secondly,in dual-fuel PCI combustion,premixed ethanol/air atmosphere could affect the ignition delay of direct injected fuel as well as the following soot formation.In this paper,simulations were performed to investigate the combustion characteristics of n-heptane spray under premixed ethanol/air and iso-octane/air atmosphere in a combustion vessel.The effects of equivalence ratio of premixed fuel,ambient temperature and cooling effect of premixed fuel vaporization were investigated.Under premixed ethanol/air atmosphere,the results showed that ignition of mixture first occurred in the regions near stoichiometric ratio,rather than regions with largest n-heptane content(i.e.highest fuel reactivity).This is mainly because the cooling effect of n-heptane vaporization restricted the low temperature reaction.Meanwhile,the equivalence ratio of regions where ignition pockets first occur was mainly determined by ambient temperature and pressure.Thus,under higher equivalence ratio of premixed ethanol,the mixtures near stoichiometric ratio exhibited lower fuel reactivity,which consequently resulted in prolonged ignition delay.It was also found that premixed ethanol could inhibit the soot formation of n-heptane spray at low ambient temperatures while contributed to significantly increased soot emission at high ambient temperatures.Furthermore,the simulated results indicated that the soot formation of n-heptane spray under premixed ethanol/air and iso-octane/air atmosphere was mainly determined by ignition delay,while the oxygen in ethanol molecule exhibited relatively smaller effects.Thirdly,the effects of port fuel properties and port fuel proportions on the combustion and emission characteristics of dual-fuel operation were investigated.The experimental results show that premixed ethanol exhibits larger effects on the combustion phasing of dual-fuel operation.Meanwhile,ethanol/diesel dual-fuel operation could achieve relatively moderate combustion even under higher load,which is benefit for high load extension.However,compared with gasoline/diesel dual-fuel operation,ethanol/diesel dual-fuel operation presents higher UHC and CO emissions as well as larger cyclic variations.This is mainly because the combustion of premixed ethanol was sensitive to fluctuations of initial charge temperature.Finally,the fuel stratification characteristic of ethanol/diesel dual-fuel engine,and the effects of ethanol proportion,direct injection strategy and nozzle-hole number on fuel stratification as well as auto-ignition and combustion process were investigated.The results showed that reduced nozzle-hole number could decrease the peak pressure and peak pressure rise rate.This is mainly because fewer nozzle holes can produce fuel stratification gradients in both circumferential and radial directions of the cylinder,thus the development of raction zone is slowed down and consequently results in lower heat release rate.Meanwhile,experimental results show that fuel stratification could greatly affect the cyclic variability of dual-fuel operation,and stable auto-ignition is essential to maintain the combustion stability of dual-fuel operation.Under the test conditions,with same combustion phasing and peak pressure,it is found that higher ethanol proportion coupled with retarded main injection timing could contribute to stable auto-ignition,which consequently results in lower cyclic variations.