Combustion And Emission Characteristics Performance Optimization Of An Engine Fueled With Dimethyl Ether

The development of China's automotive industry is facing major challenges of energy supply and environmental protection. It is of great importance to develop clean and alternative fuels for internal combustion engines and vehicles. Dimethyl ether (DME) is expected as a promising alternative fuel for its smoke-free combustion and rich resources. Promotion of DME utilization can not only relieve the current crisis of energy, but also effectively reduce air pollution. The study on combustion and performance of DME engine in this dissertation is summarized as follows:The DME fuel supply and injection system tailored for DME fuel was developed for a turbocharged diesel engine. An experimental study on performance and emission characteristics was conducted on the engine fueled with DME. The results show that, at full load, the brake specific fuel consumption (BSFC) of DME engine at low engine speed is lower than that of diesel engine, and NOx emissions of the DME engine decrease remarkably. HC emissions reduce while CO emission slightly increases. The combustion of DME engine is free of smoke at all test engine loads.Engine power output and torque decrease with increasing DME fuel supply temperature. Through the parameter optimization of DME fuel injection system, it is found that increasing the plunger diameter and effective stroke can ensure the constant engine power output at different fuel temperature. At the same engine load, using a nozzle orifice with a smaller diameter, NOx emission level is higher, and BSFC is lower. Under the required engine power output, a good compromise for DME fuel consumption and NOx emissions can be obtained using P7100 pump with the plunger diameter of 13mm and the stroke of 12mm matched with a injector of 6×0.40mm, or using P8500 pump with the plunger diameter of 12mm and the stroke of 14mm matched with a injector of 6×0.43mm. The results of ESC test show that, by means of optimization of injection system, emissions of DME engine can meet the requirements of EURO-Ⅲwithout employing EGR and aftertreatment. Using a CFD KIVA-3V model, combustion process and NOx formation of both diesel fuel and DME on a turbocharged diesel engine at the rated power were investigated. The chemical kinetic mechanism of DME consists of 78 species participating in 336 reactions, and n-heptane mechanism is used to present diesel fuel combustion including 65 species participating in 248 reactions. It is found that the calculated in-cylinder pressure and heat release rate are in good agreement with the measured results for both fuels. The ignition delay period of DME is shorter than that of diesel fuel. At initial period of combustion, the high temperature region locates at rich side of diesel fuel spray and propagates along the direction perpendicular to the spray development under the effect of in-cylinder gas flow. For DME, the ignition position occurs near the nozzle. At initial period of DME combustion, the temperature at lean side of DME spray is higher than that of diesel fuel. With the development of DME spray, the high temperature region appears as a narrow belt along the DME spray from the nozzle to the tip of the spray. In comparison to diesel fuel, the local maximum temperature of DME combustion is lower than that of diesel fuel, and in-cylinder temperature gradient of DME engine is small. The selected mechanism of NOx formation with 9 reactions can well predict NOx emission level of the engine.In order to reduce emissions of the DME engine,the high-pressure loop exhaust gas recirculation (HP-EGR) system was developed on a turbocharged DME engine,and the experimental study of emissions reduction was conducted by means of EGR and oxidation catalyst converter (DOC). In this HP-EGR system, exhaust gas before turbine flows through an EGR valve and an EGR cooler, and then enters the air intake pipe after the inter-cooler. The results indicate that,by means of EGR, inlet air temperature, exhaust gas and fuel consumption increase, and the mass air flow decreases. EGR shortens the ignition delay period, and increases the combustion duration. With EGR, a substantial reduction in NOx emissions without smoke is obtained while fuel consumption and CO emission tend towards deterioration, and HC emissions show no remarkable change. HC and CO emissions are significantly reduced by means of DOC. Through the parametric optimization, at the fuel delivery advance angle of 3°CA BTDC, ESC test results show that emissions of DME engine meet the requirements of EURO-IV. Meanwhile, there is a good fuel economy in comparison to the base engine.In order to further reduce the NOx emissions, second type of HP-EGR system was developed. In this HP-EGR system, exhaust gas before turbine flows through an EGR valve and EGR coolers, mixing wih fresh air, and then enters into the compressor of turbocharger. The effect of EGR rate on combustion and performance was investigated. The results show that, using this type of HP-EGR, a higher EGR rate can be obtained, and a remarkable reduction in NOx emissions can be observed. The results of ESC emission test show that, at the fuel delivery advance angle of 3°CA BTDC, emissions of DME engine can meet the requirements of EURO-V by means of EGR and DOC. In comparison to fuel delivery advance angle of 3°CA BTDC, a reduction in NOx emissions is obtained while the fuel economy is improved by using two-stage cooling EGR technology at the fuel delivery advance angle of 5°CA BTDC.Taking advantage of homogeneous charge compression ignition (HCCI) combustion and in-cylinder direct injection combustion, a new concept combustion system, namely compound charge compression ignition (CCCI) combustion system by port aspiration and direct injection of DME, is proposed. The effect of premixed fuel ratio and fuel delivery advance angle on CCCI combustion was investigated. In order to further reduce emissions and improve the fuel consumption in a CCCI engine, fuel design concept and port aspiration of CO2 were employed. The experimental results show that CCCI combustion exhibits a multi-stage combustion mode including HCCI combustion , pre-mixing combustion and diffusion combustion. The heat release pattern mainly depends on premixed fuel ratio and fuel delivery advance angle. Compared with HCCI combustion mode, CCCI combustion can extend the operating range with little change in NOx emissions and a considerable reduction in HC and CO emissions.