Combustion And Emission Characteristics Of Methanol/Diesel Dual Fuel Engines At Different Altitudes

Methanol,as a low carbon fuel with high oxygen content and fast combustion speed,can achieve efficient and clean combustion of diesel engines when combined with a new combustion mode,namely reaction controlled compression ignition(RCCI).Therefore,methanol/diesel dual-fuel RCCI combustion mode has great potential in improving the adaptability of plateau diesel engines.Based on this,this research adopts the dual fuel control system independently developed by the laboratory,and selects four typical operating conditions: 1400r/min-25% load,1800r/min-50% load,1800r/min-75% load and 3200r/min-75%.The combustion and emission performance of methanol/diesel dual-fuel engines at different altitudes were systematically studied.The main conclusions are as follows:(1)Combustion characteristics: with the increase of altitude,the peak value of cylinder pressure and instantaneous heat release rate gradually decreased,while the peak value of cylinder average temperature and exhaust temperature gradually increased.At medium and high loads,the introduction of methanol can accelerate the combustion rate in the cylinder,improve the combustion process in the plateau engine,increase the cylinder pressure and the peak value of the instantaneous heat release rate,and reduce the exhaust temperature.The introduction of EGR slowed down the combustion rate of the dual-fuel engine,reduced the cylinder pressure and the peak value of the instantaneous heat release rate,and increased the exhaust temperature.Increasing the excess air coefficient appropriately can promote the combustion process of dual-fuel engine,increase the peak pressure in cylinder,and reduce the peak average temperature in cylinder and exhaust temperature.(2)Economy: at medium and high loads,under the condition of no fixed excess air coefficient,the effective thermal efficiency of the engine decreases gradually with the increase of altitude,and decreases by 0.3%-0.9% at 2000 m altitude.At high load,the introduction of methanol can improve the effective thermal efficiency of plateau engine.When the methanol substitution rate increases to 15%,the effective thermal efficiency of engine at 2000 m altitude is equivalent to that of the original engine on plain.However,at medium and small load,when the methanol substitution rate exceeds 5%,the effective thermal efficiency of dual-fuel engine begins to decline.Under medium and small loads,increasing the EGR rate and excess air coefficient appropriately can improve the effective thermal efficiency of the dual-fuel engine,and then improve the economy of the plateau engine.(3)Emission characteristics: under the condition of unfixed excess air coefficient,except for small load,with the increase of altitude,THC,CO,NOx,unburned methanol and formaldehyde emissions of dual-fuel engine gradually decrease,while particulate matter PN and soot emissions gradually increase.After the introduction of methanol,engine NOx,soot emission and particulate matter PN all showed a trend of decreasing at the same time.Under the load of 1800r/min-75%,when the methanol replacement rate increased to 15%,soot emission at 2000 m altitude was similar to that of the original engine on the plain.With the increase of EGR rate and excess air coefficient,NOx emission can be further reduced.Under 1800r/min-75% load,when the EGR rate reaches 25% and the methanol replacement rate is 20%,the NOx emission can be reduced by 82.5%.Reducing the excess air coefficient or increasing the EGR ratio can reduce unconventional emissions of dual-fuel engines.Comparatively speaking,reducing the excess air coefficient is more effective in reducing unconventional emissions.Under 1800r/min-50% load,when the excess air coefficient decreases from 3.5 to 2,the maximum reduction of unburned methanol and formaldehyde emission is 57.8% and 24.5%,respectively.