Effect Of Pre-injection Timing And Pre-mixing Ratio On Combustion And Emission Characteristics Of Methanol/P20 Dual Fuel Engine

Commercial vehicles with diesel as the main fuel in China,often lead to a large diesel consumption,and the diesel consumption of motor vehicles in our country is showing an increasing trend every year.However,the oil reserves in the region where our country is located are not very rich,and the dependence on foreign imports has reached more than 70%.Our country is facing a severe oil shortage problem,and it is urgent to looking for clean and alternative fuels.As a fuel with high oxygen content,methanol is environmentally friendly and low in cost.However,the methanol is difficult to dissolve with diesel.So diesel engines generally adopt dual-fuel combustion mode to achieve a higher methanol premix ratio.The polymethoxy dimethyl ether(PODE)is miscible with diesel,and has a higher oxygen content and cetane number.In order to further optimize the performance of the diesel engine.In this paper,20%PODE by volume ratio is added into diesel,namely P20.Meanwhile adopts the method of inlet port injection of methanol and direct injection of P20 in the cylinder.The effects of pre-injection timing(-16° CA,-18° CA,-20° CA,-22° CA)and methanol premix ratio(10%,20%,30%,40%)on the characteristics of dual-fuel engine were studied by engine bench test In addition,the combustion and emission characteristics of methanol/diesel dual-fuel engine and methanol /P20 dual-fuel engine are compared and analyzed.The results show that:The ignition delay period is prolonged with the methanol premixing ratio increases.Moreover,methanol is an oxygen-rich fuel,which promotes combustion and shortens combustion duration.At small loads,with the increase of methanol premix ratio,the peak pressure,the peak pressure rise rate and the peak heat release rate in the cylinder all decreased to a certain extent due to the endothermic heat of methanol vaporization.Compared with P20,the peak pressures in the cylinders of M10、M20、M30 and M40 are reduced by 1.4%、2.6%、3.8% and 6.6% respectively,the peak pressure rise rate is reduced by 5.1%、7.5%、9.7% and23.8% respectively,and the peak heat release rate is reduced respectively 1.4%、7%、11.1%and 12% respectively.At medium and heavy loads,the pressure peak in the cylinder,the peak pressure rise rate and the peak heat release rate all increase due to the high oxygen content of methanol.Specially,the growth is most significantly at the high loads.The peak cylinder pressures of the four dual-fuel engines have increased by 3.3%、6.4%、8.9% and 13.9%,respectively,and the peak pressure rise rates have increased by 4.3%、23.4%、59.5% and112.7%,respectively.The peak rate increased by 5.4% 、 23.4% 、 53.3% and 112.1%respectively.At low and medium loads,the average cylinder temperature of the four dual-fuel engines decreases with the increase of methanol premixed ratio;At heavy loads,the oxygen content in the cylinder increases,the combustion is more sufficient,and the average temperature in the cylinder increases as the methanol premixing ratio increases.In addition,as the methanol premixing ratio increases,the intake air temperature gradually decreases,the instability of the combustion process increases,and the cyclic variation coefficient of the in-cylinder peak pressure increases.Compared with P20,the cyclic change coefficient of peak cylinder pressure of M40 increases by 0.26、0.22 and 0.18,respectively.With the increase of the methanol premixed ratio,the specific fuel consumption of the dual-fuel engine increases and the effective thermal efficiency decreases at low loads.The NOx emissions under different loads are significantly decreased with the increase of methanol premixing ratio.Compared with P20,the NOx emissions of M10、M20、M30 and M40 under medium loads are reduced by 19.6%、27.7%、33.1% and 38.6% respectively.With the increase of methanol premix ratio,the number concentrationof particulate matter emission in methanol /P20dual-fuel engine emissions in the range of 0.3～1μm number of particulate matter emission concentration increased gradually.Especially at heavy loads,the increase of particle number concentration is the most obvious.Compared with P20,the number concentration of particulate matter in M10、M20、M30 and M40 increased by 24%、39%、52% and 64%,respectively.The main reason is that the intake pipe in the winter test injected methanol at a single point,which lead to a sharp drop in intake temperature and lower cylinder temperature,resulting in insufficient fuel combustion and increased particulate emissions.When the preinjection timing of the direct injection fuel in the cylinder is advanced from-16° CA to-22° CA,the ignition delay period of the four fuels under different loads was prolonged,and the combustion duration increased first and then decreased.With the increase of the pre-spray advance angle,the peak pressure of four different dual-fuel combustion increases first and then decreases.In addition,the peak cylinder pressure of M10、M20 and M30 is higher when the pre-injection advance angle is-18°CA under low and medium loads.However,due to the longer ignition delay period of M40 fuel,the peak cylinder pressure is larger when the pre-injection advance angle is-20° CA.The peak value of pressure rise rate increases first and then decreases with the advance of pre-injection timing at small loads.The change of pre-injection timing has little effect on the peak value of pressure rise rate at medium and large loads due to the shortening of ignition delay period.At low loads,increasing the pre-injection advance angle will cause the peak heat release rate to increase first and then decrease.But at medium and heavy loads,increasing the advance angle of pre-injection will lead to a slight decrease of peak heat release rate.The peak temperature in the cylinder increases first and then decreases with the advance of pre-injection timing.With the advance of pre-injection timing,the NOx emission increases first and then decreases,and when the pre-injection advance angle is-18°CA,the NOx emission is relatively high.Moreover,at small loads,with the increase of pre-injection advance angle,the particulate emissions of methanol /P20 dual-fuel engines with different premixed ratios showed a downward trend.However,in the case of medium and large loads,the timing of pre-injection has little effect on the number concentration of particulate matter.The study of methanol/diesel dual fuel and methanol /P20 dual fuel showed that the peak cylinder pressure,peak heat release rate and peak pressure rise rate of methanol /P20 dual-fuel engine are all higher than those of methanol/diesel dual-fuel engine.Under different loads,the engine burning methanol /P20 dual fuel has smaller cycle variation coefficient and better combustion stability.Compared with methanol/diesel dual fuel,when the engine uses different premix ratios of methanol/P20 dual fuel,NOx emissions are slightly reduced,and particulate matter emissions are significantly reduced.Therefore,the comprehensive performance of methanol /P20 dual-fuel engine is better than that of methanol/diesel dual-fuel engine.