Experimental Research On The Effects Of Biofuels On Different Low Temperature Combustion Modes

Fuel property optimization is an effective way to achieve highly-efficient and clean combustion in diesel engines.Due to the renewable and extensive sources,biofuels can be a promising option.In the current study,four biofuels,including biodiesel,n-butanol,2,5-dimethylfuran(DMF),and ethanol,were investigated in two low temperature combustion modes: the blended fuel combustion mode,wherein the blends of biodiesel/n-butanol,biodiesel/DMF,biodiesel/ethanol blends were injected directly into the cylinder,or the dual-fuel combustion mode,wherein the lower reactivity biofuels(n-butanol,DMF,and ethanol)were injected into the intake port while biodiesel was directly injected into the cylinder.In addition,the combustion and emission characteristics of these two low temperature combustion modes were compared and discussed.Firstly,the low temperature combustion achieved using blended fuels was studied.Experimental results revealed that,compared with neat biodiesel,the addition of 20%(vol.)n-butanol,DMF,and ethanol in biodiesel can improve the combustion performance and reduce the soot emission,which can be attributed to the molecule oxygen content,higher volatility,and lower reactivity to prolong the ignition delay.The soot reduction ability of these three blends can be sequenced by E20 > DMF20 > Bu20.However,the hydrocarbon(HC)and CO emissions of E20 were the highest,resulting in a slightly lower indicated thermal efficiency(ITE).Compared with diesel and biodiesel,the ITEs of the blends show no advantage at low loads while had significant advantage at high loads and high EGR rate operating conditions.At high loads,high soot emissions were observed when fueled with the blends with 20% blending ratio,while the soot emission can be significantly reduced when the blending ratio was increased up to 50%.Soot-free emission can be achieved with DMF50,however,with the penalties of higher max pressure rise rate(MPRR)and lower combustion stability.Further experiments were conducted to explore the performance of these fuels in dual-fuel combustion mode.Experimental results showed that,at 40% EGR rate,a good trade-off between NOx emission and ITE for different fuels can be achieved.The soot reduction ability of these three dual-fuel cases can be sequenced by n-butanol > DMF > ethanol,the highest soot emission of n-butanol/biodiesel case can be controlled within 0.2 FSN.For NOx emission,both n-butanol/biodiesel and ethanol/biodiesel cases can be controlled within 0.4 g/(kW?h),while still maintaining relatively high ITEs.The n-butanol/biodiesel dual fuel combustion can achieve the highest ITE.Comparisons between the above two low temperature combustion modes indicated that,at low and medium load conditions,the blended fuel combustion can achieve better fuel economy and lower emissions,while at high load conditions,both two combustion modes can achieve relatively high thermal efficiency.Due to the lower MPRR in dual-fuel combustion mode,higher premixed ratio of the lower reactivity fuel can be used,which can extend its potential of soot reduction and the higher load extension.