The Study Of Combination Combustion Mode For DME Injected Into Intake Pipe And Methanol Injected Directly Into Cylinder

Diesel engines have the advantages of high efficiency, an economical fuel, low HC and CO. But NOx and soot are high, and it is very difficult to meanwhile low them. Gasoline engines have low efficiency, high NOx and outcomes of imperfect combustion such as CO and HC. The results are high consumption of fuel and serious pollution. With oil becoming less and less and emission regulations becoming more and more strict, one important choice that solves the problems of pollution and the lack of fuels is that we should seek for new combustion models. One of the models is the combination of DME injected into intake pipe and methanol injected directly into cylinder.A series of experiment are carried out on a refit four-cylinder engine. The fuels are supplied by an electric controlled injection system. By adjusting the methanol injection timing, it can be realized different combustion mode. The effects of combustion mode on combustion, performance and emissions have been investigated. The results show that, as injection timing is near before TDC (BTDC), the combustion process comprises three stages: low temperature reaction of DME, high temperature reaction 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 -125 degree ATDC, diffusion combustion of methanol disappears. At this time, the combustion process shows typical two stages heat release of homogenous charge compression ignition (HCCI) combustion. As injection timing increases, required DME rate is increased, IMEP,combustion efficiency and indicated thermal efficiency all first increase and then decrease. Whether methanol can combust normally of not has nothing to do with the concentration of it, and it is up to the concentration of DME at an late injection timing. That is, the normal combustion of DME follows that of methanol. At an early or middle injection timing concentration of DME should be increased if wanting methanol to combust normally. With the increase of load, the necessary concentration of DME for a normal combustion will be decreased. With increased injection timing, HC and CO emissions first decrease and then increase, while NO_x emissions first increase and then decrease. NO_x emissions decrease to almost zero at HCCI combustion mode. In conclusion, the best integrated performance can be achieved by adjusting injection timing in different engine mode.