A numerical study was conducted to investigate combustion and emission characteristics in a high-speed direct-injection engine with a common-rail injection system under various operating conditions. In order to analyse the combustion characteristics, several models were used in this study. They were the renormalization group k-e model, the hybrid Kelvin-Helmholtz (wave) and the Rayleigh-Taylor model, the shell auto-ignition model, and the laminar and turbulent characteristic timescale combustion model. The prediction of exhaust emissions was conducted using nitrogen oxide NO x formation with an extended Zel'dovich mechanism and Hiroyasu soot formation with the Nagle-Strickland-Constable oxidation model respectively. Experimental combustion and emission characteristics were compared with calculated results under various operating conditions, such as injection timing, injection pressure, fuel mass, and engine speed. The calculated results show similar patterns to the experimental results in the cylinder pressure and the rate of heat release. In the emissions characteristics, NO x emission decreased as injection timing was retarded and the NO x and soot amounts increased with the increase in the injected fuel mass. The calculated soot trends for various injection timings showed different patterns from the experimental trends as the injection timing were retarded.
The purpose of this study is to investigate the effect of injection parameters on the injection and spray characteristics of dimethyl ether and diesel fuel. In order to analyze the injection and spray characteristics of dimethyl ether and diesel fuel with employing high-pressure common-rail injection system, the injection characteristics such as injection delay, injection duration, and injection rate, spray cone angle and spray tip penetration was investigated by using the injection rate measuring system and the spray visualization system. In this work, the experiments of injection rate and spray visualization are performed at various injection parameters. It was found that injection quantity was decreased with the increase of injection pressure at the same energizing duration and injection pressure In the case of injection characteristics, dimethyl ether showed shorter of injection delay, longer injection duration and lower injected mass flow rate than diesel fuel in accordance with various energizing durations and injection pressures. Also, spray development of dimethyl ether had larger spray cone angle than that of diesel fuel at various injection pressures. Spray tip penetration was almost same development and tendency regardless of injection angles.
The characteristics of spray behavior and combustion of DME (dimethyl ether) were investigated using experimental and numerical approaches. For experiments, injection rates and macroscopic spray characteristics were investigated at various injection parameters by using an injection rate system and a spray visualization system. The combustion and emission characteristics were also obtained from the modified engine for DME fuel and emission measurement equipment. For numerical approaches, the combustion characteristics of DME fueled engine were predicted by a 3D-CFD code, the KIVA code coupled with the CHEMKIN (KIVA-CHEMKN) and spray behavior and evaporation were calculated by considering the thermo-chemical properties of DME. In order to calculate the fuel oxidation and emission formation such as NOx, a detailed chemical kinetic mechanism which was composed of 83 species and 360 reaction paths was considered. To simulate soot emission, two-step phenomenological model was applied. Both experimental and numerical results indicate that injection delay, ignition delay, and combustion duration of DME are shorter than that of diesel because of good evaporation and mixing characteristics. The pressure history predicted by the KIVA code agrees well with the measurements from the test engine. The amount of NOx emission was predicted by the reduced NOx mechanism shows good agreements to the experiments.
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