| Clean alternative fuel can effectively improve the performance of power and economy of the engine,and reduce the emission of pollutants simultaneously,which has become a research hotspot in the field of engines.Methanol as a highly oxygenated fuel is conducive to reducing diesel particulate emissions.However,due to the poor miscibility of methanol and diesel,it is difficult to blend with diesel directly.As a result,the dual-fuel mode is usually adopted to achieve a higher replacement rate on diesel engine.PODE has a high cetane number,good ignitability,and good miscibility with diesel as a coal-based alternative fuel.Therefore,this paper study the effects of PODE blending ratio and EGR rate on the combustion and emission characteristics of methanol-PODE/diesel dual-fuel engine by using the engine bench test,which adopts the method of port injection of methanol and direct injection of PODE/diesel mixed fuel in the cylinder.The experiment is carried out on an electrically controlled high-pressure common rail diesel engine.The engine speed is unchanged at 1600r/min,and control the average effective pressure in the cylinder is 0.28MPa,0.7MPa and 0.92MPa,respectively,namely under three conditions of low load,medium load and heavy load.Meanwhile,the replacement rate of port-injected methanol remains unchanged at 20%,and the mixing proportion of PODE in the PODE/diesel mixed fuel of direct injection in the cylinder is adjusted to 10%,20%,30%,and 40%,and adjust the EGR rate of the engine to be 4%,8%,12%,respectively.Moreover,the effects of PODE blending ratio and EGR rate on engine combustion characteristics,cycle variation,NOx emissions,soot and particle number concentration were measured under different engine operating conditions.The experimental research results show that:Compared with the methanol-diesel dual fuel P0 without PODE,the ignition delay period and combustion duration of P10,P20,P30 and P40 were shortened to different degrees under three different conditions of low load,medium load and heavy load,and the ignition delay period was shortened most obviously at low load.The reason is that PODE has a high cetane number and a fast burning rate.In addition,compared with P0 at low load,the ignition period of P10,P20,P30 and P40 are shortened by 5.5%,8.5%,9.2%and 14.4%,respectively.The combustion duration of P10,P20,P30 and P40 was shortened by 4.8%,2.4%,2.4%and 9.6%,respectively.Moreover,the peak pressure in the cylinder,the peak heat release rate,and the peak pressure rise rate increase with the increase of the PODE blending ratio,and they all increase most obviously at low load.Compared with P0,the peak pressure in the cylinder of P10,P20,P30 and P40 increased by 1.5%,4.5%,7.5%and 12.1%respectively,and the peak heat release rate increased by 10.8%,17.7%,27.9%and 43.1%,respectively,and the peak pressure rise rate increased by 7.8%,17.8%,29.4 and 45%,respectively.Furthermore,with the increase of the PODE blending ratio,the combustion heat release in the cylinder is more concentrated,and the average temperature in the cylinder also increases due to the acceleration of the combustion rate.Finally,the temperature in the cylinder increases more obviously when the load is small.Compared with P0,the peak temperature in the cylinder of P10,P20,P30 and P40 increases by 33.9K,65.8K,92.6K and 146.4K,respectively,and the P40 has the best combustion stability under three loads.Compared with P0,the MaxP-COV numbers of P40 are reduced by 0.14,0.09 and 0.10 at small load,medium load and high load,respectively.Due to the low calorific value of PODE,it is easy to lead to the increase of specific fuel consumption.Thus,when the PODE is mixed,the fuel thermal efficiency decreases slightly.In addition,With the increase of the PODE blending ratio,the shortening of the combustion duration leads to a reduction in NOx emissions,and the increase in the oxygen content in the cylinder leads to a reduction in smoke and particulate emissions.Among them,NOx,smoke and particulate matter emissions decrease most significantly under heavy load.Moreover,compared with P0,the NOx emission of P10,P20,P30 and P40 decreased by 4.6%,4.4%,7.4%and 6.1%,respectively.The exhaust smoke was decreased by 17%,17%,17%and 33.3%,respectively.Furthermore,the number concentration of total particulate matter of particle size in the range of 0.3μm to 1.0μm was decreased by 62.2%,78.9%,85.7%and 87.5%,respectively.With the increase of EGR rate,due to the influence of the circulating exhaust gas on the combustion process in the cylinder,the ignition delay period and combustion duration are prolonged,the cylinder pressure and heat release rate peaks decrease,and the pressure rise rate peaks decrease,and the influence of EGR rate is most obvious when the load is heavy.Compared with no EGR,when the EGR rate is 12%,the combustion durations of P0,P10,P20,P30 and P40 increased by 3℃A,4℃A,2.5℃A,3.5℃A and 6.5℃A,respectively.Moerover,the peak pressure in the cylinder decreased by 0.4MPa,0.1 MPa,0.3MPa,0.2MPa and 0.3MPa,respectively.The peak heat release rate decreased by 6.7J/℃A,2.7J/℃A,2.4J/℃a,0.7J/℃A and 3.1J/℃A,respectively.In addition,the peak pressure rise rate decreased by 0.069MPa/℃A,0.052MPa/℃A,0.049MPa/℃A,0.03MPa/℃A and 0.042MPa/℃A,respectively.Under heavy load,increasing the EGR rate can effectively control the combustion temperature in the cylinder and significantly reduce NOx emissions.Compared with no EGR,the NOx emissions of P0,P10,P20,P30 and P40 were reduced by 63.3%,65.2%,67.3%,63.9%and 66.9%when the EGR rate was 12%,respectively.In addition,When the engine load is low,the particulate matter does not change significantly with EGR due to the heavy excess air coefficient.However,the particulate matter increases with the increase of the EGR rate at the medium load and heavy load. |
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