Effect Of Dimethyl Ether On The Combustion Processes Of An Optical Access Engine With Methane-air Mixture

Shortening combustion duration of a lean-burn natural gas engine can improve its thermal efficiency.For this purpose,dimethyl ether（DME）,as an active additive,was direct injected into the cylinder to promote the combustion of lean natural gas-air mixture formed through intake port injection.The effect of DME direct injection on the combustion characteristics of a methane-air mixture in the conditions of spark ignition and compression ignition was investigated in an optical engine with high-speed photography at 600 rpm and the excess air coefficient of 1.4.The results show that:In the case of spark ignition,at given ignition timings and the total fuel heat released in a cycle,ignition timing advances,initial heat release rate with crank angle,flame area and its luminosity increase with the increase of the energy ratio of DME in the tatol energy released in a cycle(r_DME).Delayed DME injection timing can accelerate the ignition process and early heat release rate with crank angle while the;later combustion duration increases.With the delay of DME injection timing and the increase of r_DME,flame propagation speed increases.Advanced DME injection timing results in bright and uniform flame images close to round.At an injection timing of DME,advanced ignition timing decreases ignition delay.At a given ignition timing,ignition shortens with delayed DME injection timing.In the case of compression ignition,the combustion processes are mainly controled by DME direct injection timings.The curve of the heat release rate has one peak when DME injection timing is in the range of 60⁴0°CA before top dead center（BTDC）,and the combustion advances with delayed DME injection timing.When the DME injection timing is in the range of 30¹5°CA BTDC,the curve of heat release rate has two peaks,and the flame images have the feature of auto-ignition and flame propagation in the cylinder.When DME is injected into the cylinder at 60°CA BTDC and 50°CA BTDC,there are some auto-ignition zones with small area,and the mixture in the cylinder is mainly consumed by the means of flame propagation.At the DME injection timing in the range 40¹5°CA BTDC,the spontaneous combustion occurs in an extensive area.The initial flame areas rapidly increase while flame propagation appears in the later stage.In addition,DME injection timing affects the site of auto-ignition.With the delay of DME injection timing,the auto-ignition sites are more concentrated.Compared with pure methane in the case of spark ignition,DME direct injection coupled with methane-air mixture can greatly increase initial heat release rate with crank angle,shorten combustion duration,and enhance the growth rate of flame development.