Study Of Combustion And Emission Characteristics Of Diesel Methanol Dual Fuel Engine At High Altitude

With the increasing concern of environmental protection,more attention has been paid to the emission performance of diesel engine,and more stringent emission regulations have formulated by countries to limit the emission pollutants of diesel engine at high altitude.As an oxygenated fuel,methanol can achieve clean combustion of diesel engine,which is of great significance for energy saving and emission reduction.However,most of the researches on diesel methanol dual fuel（DMDF）engine,which can achieve high methanol substitution percentage（MSP）,are concentrated in plain areas,and few researches are carried out at high altitude.Therefore,there is important academic value and practical significance to study the performance of DMDF engine at high altitude.In this paper,based on an electronically controlled high pressure common rail diesel engine,the DMDF engine was modified by adding an electronically controlled methanol injection system to the intake manifold.The combustion and emission characteristics of DMDF engine at high altitude were studied by using the internal combustion engine high altitudes simulation system designed by the research group.High speed（2200r/min）,medium speed（maximum torque speed 1800r/min）,low speed（1200r/min）and corresponding heavy load（75%load）,medium load（50%load）and light load（25%load）of each speed were selected as working conditions to test the influence of MSP on DMDF engine performance at different altitudes（10m,700m,1670m and 2400m）.The results indicated that:with the increase of MSP,the constant volume of combustion increased,the in-cylinder pressure decreased at light load and increased significantly at medium and heavy load,the ratio of premixed combustion increased,the peak value of premixed combustion increased and the combustion duration was shortened.Although the coefficient of variable of peak pressure(COV_PP)and the coefficient of variable of indicated mean effective pressure(COV_IMEP)become larger,the two parameters could be controlled between 0.6%and 3.5%.Meanwhile,the exhaust temperature was decreased,and the brake thermal efficiency（BTE）was increased 0.64%～2.48%at medium-heavy load.Moreover,the nitrogen oxides（NO_X）and soot emission at most working conditions were decreased with the increment of MSP.The deterioration of trade-off relationship between NOx and soot emissions at high altitude could be effectively alleviated,with the use of DMDF mode.However,with the increment of MSP,HC emissions increased and the CO emissions increased significantly.Taking 1800r/min-50%load with 30%of MSP as test conditions,the influences of diesel fuel injection parameters（injection pressure and injection timing）on the performance of DMDF engine at different altitudes were tested.The results showed that:with the increment of diesel injection pressure at different altitudes,the maximum in-cylinder pressure and instantaneous heat release rate increased slightly,and the ignition delay period,combustion duration and CA50 all displayed a downward trend.Higher the altitude was,smaller the variation range was.The BTE increased with the increment of diesel injection pressure.For emission characteristic,with the increment of diesel injection pressure,the emissions of soot,CO and HC decreased while the emissions of NO_Xincreased.A compromise emission level could be obtained when the diesel injection pressure was102MPa.When the diesel injection timing was delayed,the maximum values of in-cylinder pressure,heat release rate and in-cylinder temperature at different altitudes were decreased,the combustion process gradually changed from premixed combustion to diffusion combustion.Meanwhile,the ignition delay period was shortened,the combustion duration was prolonged,the CA50 was delayed and the BTE decreased.The postponement of diesel injection timing was beneficial to the decrement of NO_X and HC emissions,but it would lead to the increment of soot and CO emissions.The diesel injection timing should be postponed to-7 or-5°CA ATDC.At different altitudes,when the engine was run at light load,the increment of MSP would lead to the decrease of power performance and brake thermal efficiency.As a result,it is inappropriate to apply DMDF mode at light load.At high altitude,the application of DMDF mode at medium and heavy load could improve the combustion process in cylinder,achieve higher brake thermal efficiency,and realize the efficient and clean combustion of the engine.Based on DMDF mode,the increment of diesel injection pressure could further improve the power performance,economy and emission characteristics of the engine.The postponement of diesel injection timing could effectively reduce NO_X and HC emissions.