Spray combustion and emission characteristics in a compression ignition engine fueled with dimethyl ether by using reduced chemical mechanism

Abstract

In this study, dimethyl ether (DME) fuel was applied to a diesel engine through the experimental and numerical approaches to analyze the injection and spray characteristics including nozzle flow characteristics and combustion and emission characteristics. In particular, the reduced DME reaction mechanism was developed to investigate the ignition and combustion characteristics and it can reduce the calculation time with good calculation accuracy comparing with the comprehensive mechanism. In a diesel engine, the fuel injection and spray characteristics can play a significant role in the combustion and emission characteristics. Therefore, injection and spray characteristics were investigated by the experimental results from the injection rate meter and spray visualization system under various operating conditions. The fuel quantity of DME was lower than diesel at the same energizing duration, which led to the lower peak injection rate of DME. The injection delay of DME was shorter than that of diesel due to the lower compressibility of DME. In addition, the possibility of cavitation occurrence was identified through the application of the experimental injection rate into the nozzle flow model. In particular, DME fuel was assumed that cavitation was easily generated and lasted longer compared with the diesel fuel because of higher vapor pressure of DME. The non-evaporating spray visualization at the high ambient pressure was acquired from the spray visualization system and the spray development process of DME was nearly similar to that of diesel. The homogeneous prediction model for spray tip penetration was applied to the estimation of spray tip penetration of DME and diesel. This model had much better prediction ability than the previous model suggested by Hiroyasu et al. because this included the fuel properties and experimental results of spray cone angle to determine the model constants. The multi-fluid model for the internal nozzle flows including the cavitation phenomena was used to validate the accuracy through the comparison of benchmark results. The automated two-dimensional body-fitted grid generator was used to generate the calculation meshes. The optimal initial volume fraction of gas phase which was combination of fuel qualities, initial number density and initial radius was also suggested for multi-fluid model. The three types of inlet shape nozzles can be applied to the experimental and numerical results. Even though the very low operating conditions, the cavitation in the nozzle appeared and in the case of tapered nozzle, the distribution of cavitation region was smallest among three types because it gradually reduced the flow passages. The reduced mechanism was developed by the systematic mechanism reduction methods. The detailed DME mechanism (79 species and 351 reversible reactions) was firstly modified to the irreversible reactions (79 species and 702 irreversible reactions) for systematic mechanism reduction. Through the peak molar concentration analysis, the 10 important species which was the initial values of next Jacobian and principal component analysis were selected by the 1.0×10-2 of threshold value. The Jacobian analysis of concentration sensitivity and was applied to the sensitivity analysis from the SENKIN program under 24 calculation conditions and obtained the list of species classified by three types: important, necessary, and redundant. The reduced mechanism (35 species and 220 irreversible reactions) was selected through the 3.0% of the ignition delay error criterion. The principal component analysis of rate sensitivity was further applied to the sensitivity results from the obtained reduced mechanism. Therefore, the final reduced mechanism (35 species and 159 irreversible reactions) was developed. The numerical analysis was conducted by using the KIVA-CHEMKIN code under various operating conditions and compared with the experimental one. To achieve the DME fueled engine simulation, there were two main modifications of multi-dimensional CFD code that was the incorporation of the fuel properties of DME into the fuel library and the application of the detailed and reduced DME mechanism. Through the comparison of combustion characteristics and emission characteristics, numerical simulation applied the detailed DME mechanism had good agreements with experimental results. In addition, the numerical approaches with the obtained reduced mechanism were compared with those with the detailed mechanism. From the comparison of combustion characteristics and emission characteristics according to the injection timings, the reduced mechanism had similar results to the detailed mechanism by quantitative and qualitative ways. The reduced mechanism can reduce the calculation time about 70% of the detailed mechanism. Using the reduced DME mechanism could achieve the objective of this study that can keep the prediction accuracy of ignition delay, temperature, concentration of the major species with significant reduction of calculation time.