Post injection in a compression ignition engine fueled with dimethyl-ether

摘要

The effects of post injection on engine combustion characteristics and emission reduction were investigated at low speed and low load conditions. Combustion performance and exhaust emissions were tested by varying the post injection timings and quantities. The main injection was fixed at 2 crank angle degree after top dead center. Post injection timing varied from 12 to 50 crank angle degree after top dead center. The fuel quantity of post injection varied from 1 to 3 mg, and the fuel quantity from main injection was adjusted to maintain a constant load. The fuel quantity from main injection with post injection was smaller than that of single injection because the fuel injection was split into two. This reduced the pressure rise rate and the heat release rate during main combustion. The effects of post injection on main combustion diminished as post injection timing retarded. Hydrocarbon and carbon monoxide emissions were reduced with post injection close to the main injection. As the post injection timing was retarded, the post combustion temperature was lowered due to volume expansion during the expansion stroke. This increased the hydrocarbon and carbon monoxide emissions for late post injection timing. Despite decreased in-cylinder gas temperature with retarded post injection, hydrocarbon and carbon monoxide emissions declined at 20 crank angle degree post injection, and minimum fuel consumption was noted at 25 crank angle degree post injection. The fuel spray was placed on the top outside corner for post injection at 20 and 25 crank angle degree, and this improved the remaining air usage after main combustion. Oxides of nitrogen emissions were reduced with a post injection. The post combustion reduced the main fuel quantity by the split injection. This reduced oxides of nitrogen production during the main combustion. Post combustion occurred at low in-cylinder gas temperature and released small amount of heat. Therefore, post combustion did not contribute in the production of oxides of nitrogen emissions.