In this study, an experimental investigation was conducted using a direct injection single-cylinder diesel engine equipped with a test common rail fuel injection system to clarify how dimethyl ether (DME) injection characteristics affect the heat release and exhaust emissions.
For that purpose the common rail fuel injection system (injection pressure: 15 MPa) and injection nozzle (0.55 × 5-holes, 0.70 × 3-holes, same total holes area) have been used for the test. First, to characterize the effect of DME physical properties on the macroscopic spray behavior: injection quantity, injection rate, penetration, cone angle, volume were measured using high-pressure injection chamber (pressure: 4MPa). In order to clarify effects of the injection process on HC, CO, and NOx emissions, as well as the rate of heat release were investigated by single-cylinder engine test. The effects of the injection rate and swirl ratio on exhaust emissions and heat release were also investigated.
Those two parts of the study (injection test and single-cylinder engine test) showed that the 5-hole injection nozzle has faster atomization and quicker vaporization around the spray core. It promotes premixed combustion but results in greater HC and CO emissions when a large quantity of fuel is injected. With the 3-hole injection nozzle, at the beginning of the spray development, showed a little longer spray penetration, wider spray angle and bigger spray volume than that of the 5-hole nozzle. It takes time to form the fuel atomization, which inhibits premixed combustion and results in an increase of unburned components at small quantity of fuel injection.
With the 3-hole injection nozzle, a higher injection rate results in a longer ignition delay, allowing spray to be distributed in a combustion chamber more extensively. A higher injection rate shows shorter combustion duration and improved cycle efficiency. When a large quantity of fuel is injected, an increase in swirl ratio enhances spray distribution and air-fuel mixture, therefore contributing to the reduction of HC and CO emissions with the 5-hole injection nozzle in particular. Due to the difference in injection and combustion process of the nozzles, at a large quantity of fuel injection, the 3-hole injection nozzle with an equivalent energy consumption rate of the 5-hole injection nozzle results in less NOx emissions.