Premixed Combustion Of DME-ethanol Blend In An Electronic-Controlled And Turbocharged CI Engine

Dimethyl ether(DME), as a kind of alternative coal-based fuel, is in favor of easing shortage of petroleum resources, guaranteeing national energy security and realizing sustainable development of economy. Meanwhile, according to fuel design, the low cetane number and high vaprozation latent heat of ethanol are beneficial to realize premixed combustion of DME-ethanol blends. Premixed combustion is able to reduce vehicle emissions and remit the urban pollution.In this thesis, the effect of ethanol-DME blending ratio on combustion characteristics is investigated on an electronic-controlled and turbocharged DME engine. Simultaneously, the premixed combustion of DME-ethanol blends is realized by injection strategy and exhaust gas recirculation(EGR).An engine test bench was set up to study the combustion and emission characteristics of DME-ethanol blends. The test bench includes a 6-cylinder, electronic-controlled and turbocharged DME engine, an eddy current dynamometer, combustion analyzer, exhaust emission measurement system, electronic scale and so on.The effect of ethanol-DME blending ratio on engine combustion characteristics was studied under different speeds and loads. The results showed that with engine speed or load increasing, the combustion of each fuel gradually changed to diffusion combustion. Mixing ethanol increased premixed combustion ratio, but as load increased, the effect of ethanol weakened. The max pressure rise rate of DME-ethanol blends was significantly larger than that of pure DME, which restricted the blending ratio. Compared to pure DME, DME-ethanol blends not only realized free-smoke combustion but reduced NOx emissions. As the load increased, the NOx emissions of DME-ethanol blends further reduced and the biggest drop reached 20.3%. However, the HC, CO emissions and brake specific fuel consumption of DME-ethanol blends were larger than those of pure DME.The effects of muti-injection strategy, including main injection timing, pilot injection timing and pilot injection quantity, on combustion and emission characteristics of DME-ethanol blends were investigated. The results showed that with main injection timing delaying, the in-cylinder pressure curve changed from unimodal to bimodal and NOx emissions reduced remarkably. However, the HC, CO emissions, brake specification fuel consumption and exhaust gas temperature increased. Also, the effect of ethanol on NOx emission reduction was more significant. However, excessive delay of main injection timing may worse combustion. By using pilot injection,three-section heat release curve emerged, which consisted of low temperature reaction and high temperature reaction of pilot injection fuel and diffusion combustion of main injection fuel. The burning of pilot injection fuel was premixed combustion, which was helpful to decrease NOx emissions. With the pilot injection timing advancing, NOx emissions decreased firstly and then rised. With the pilot injection quantity increasing, NOx emissions remained unchanged after a rapid decline at first. An optimal injection strategy was concluded to enable the lowest NOx emissions. Compared with pure DME, blended fuels could help to reduce NOx emissions with any injection strategy, while the pressure rise rate, HC and CO emissions of blended fuels increased.The effect of EGR on the premixed combustion of DME-ethanol blends was studied. The results showed that increasing EGR rate reduced in-cylinder pressure but increased premixed combustion ratio. At the same time, it reduced NOx emissions, but increased the max pressure rise rate, HC and CO emissions, and specific brake fuel consumption. With the combination of fuel design, injection strategy and EGR, the premixed combustion of DME-ethanol blends was achieved, where NOx emission was lower than 100 ppm. Larger the ethanol content, lower EGR rate was needed to achieve premixed combustion model.