Characteristics Study On HCCI Combustion And Exhaust Emission Of DME

With the increasingly strict regulations of the engine emission and the significantly greater economical demands for engine, HCCI combustion mode technology has been drawn more and more attention because of its potential in saving energy and reducing emission, and also because dimethyl ether (DME) is an alternative fuel of high-efficient clean diesel. Homogeneous charge compression ignition (HCCI) is a new combustion mode. Different from combustion in spark ignition (SI) and compression ignition(CI) engine, HCCI has the advantage of higher thermal efficiency but low PM and NOx emission.Nowadays, many domestic and foreign researchers have experimented and simulated on DME HCCI combustion model of the chemical kinetics and mechanism of combustion. This paper is based on previous researchers' achievements,with Chemkin software to do the simulating research on the characteristics of DME HCCI combustion mechanism and the emission.It's found that the combustion process of DME can be divided into four stages: solicitation stage of series reaction;active accumulation stage;lower temperature reaction (LTR);high temperature reaction (HTR).At the first stage, the reaction chains are induced and only a little heat is released. At the second stage, the producing rate is faster than the consuming one and a large amount of energy was released, resulting in the rise of in-cylinder pressure and temperature. At the third and fourth stage, all kinds of reacting rates are very fast and at the same time, the oxidation of DME and CO creates HO2, and the released heat reaches its summit. DME was cracked by two combustion ways: DME split: CH₃OCH₃ (+M)→middle product and DME dehydrogenation Reaction: CH₃OCH₃ + HO₂→middle and product CH₃OCH₃ + H→product.Based on the mechanistic research on DME oxidation, the Chemkin software was used again to simulate the impact of the combustion-border conditions such as inlet temperature, excess air coefficient, rotating speed and air pressure on HCCI combustion of DME fuel. Experiment was also made on the characteristics of DME HCCI model engine's emission. It shows that inlet temperature, excess air amount and rotating speed can change the time and pressure it costs. Although the pressure of inlet air can change the combustion pressure and improve engine performance, it can not change the combustion time, so it isn't an effective way to control the comb -ustion time. Results show that DME HCCI combustion model can reduce NOx and soot, but CO and CH significantly increased.The ZS1100 diesel engine have been reconstructed to perform the pure diesel combustion test and the mixed fuels(diesel and DME) test and the pure DME HCCI combustion test. The five-exhausted-componect analyser and Gas Chromatography (GC) have been used to detect and analyze the emission of Engine test-bed. The GC on-line analyses proved only formaldehyde but no other trace of minim material in the emission. Thus further improvements of collecting and inspecting exhaust emission are needed.