Investigation of the Effects of Injection Timing on Thermo-Atmosphere Combustion of Methanol

The effects of various injection timing of methanol on thermo-atmosphere combustion of methanol by port injection of dimethyl ether (DME) and direct injection of methanol were experimentally investigated. The experiment results show that, as injection timing is at 6 degree before TDC, the combustion process comprises three stages: low temperature heat release of DME, high temperature heat release of DME and diffusion combustion of methanol. As injection timing increases, premixed combustion proportion of methanol is increased and diffusion combustion proportion is decreased. As injection timing increases to 126 degree before TDC, diffusion combustion of methanol disappears. At this time, the combustion process shows typical two stages heat release of HCCI combustion. As injection timing increases, required DME rate is increased, combustion efficiency and indicated thermal efficiency all first increase and then decrease. With increased injection timing, HC emissions and CO emissions first decrease and then increase, while NO_x emissions first increase and then decrease. When injection timing is at 126 degree before TDC, NO_x emissions decrease to almost zero. In conclusion, better integrated performance can be achieved by later injection. CFD simulation results indicate that there is intensive concentration stratified near the combustion chamber wall at later injection timing. At early injection timing the mixture in the chamber becomes more homogeneous. It is noted that the spray will impinge on piston top or cylinder wall at early injection timing so that HC and CO emissions increase obviously due to plenty of fuel deposited in the crevice between the piston and cylinder wall. Therefore the nozzle cone angle should be improved to match the piston movement in the case of early injection.