Analysis of combustion process and emission performance test for diesel engine using internal exhaust gas recirculation

With the sustained development of economy and technology, small non-road diesel engines are extensively applied in industrial and agricultural production. The single-cylinder engines, in particular, plays a crucial role in modern agriculture, working as the main power source for small-to medium-sized tractors and mini-farming machinery etc. Due to current situation of fuel issue in China and structure limitation of single-cylinder diesel engine, a new method is presented to improve combustion and emission performance using internal exhaust gas recirculation (IEGR) in terms of engine internal purification, that is, by adding advance intake profile to intake cam shaft. As for the small-sized non-road diesel engine, an optical engine equipped with the AVL Visio scope consisting of a charge coupled device (CCD) camera Pixel-Fly VGA, an endoscope, illumination device and the AVL-Thermo Vision software was used in order to capture combustion images. In addition, the AVL-Thermo Vision software was applied in the measurement of temperature and soot distribution of diffusion flame. The resolution and frequency of CCD camera were 640×480 pixel and 10 Hz respectively. By means of images collecting and processing for combustion process, together with pressure collecting and emission performance testing, the influences of IEGR on the diesel engine's combustion process and emission performance were analyzed. The results showed that under the conditions of 1760 r/min and 50% load rate, in-cylinder pressure peak decreased from 5.49 to 5.43 MPa, the start of heat release was delayed by 0.5°CA and the maximum instantaneous heat release rate was reduced from 85.7 to 82.4 J/deg when IEGR was introduced. Furthermore, the average temperature of combustion flame was reduced, centering between 1900 and 2100 K. The area of high-temperature intense radiation was reduced, in which the area percent of temperature higher than 2200 K dropped from around 30‰ to below 10‰. That was beneficial to controlling NOx emissions. The average value for KL factor was obviously higher than that for the original engine within the entire range of combustion, the peak of which rose from 40.5 to 67.4. At the speed of 1760 r/min, the NOx emissions decreased within the entire range of load rate, especially under 50% load rate (by 19.6%) when IEGR was introduced. Yet, the soot emissions increased as the load rate was increased and the growing rate became even larger at higher load rate. The soot emissions rose by 49.33% when full load rate was achieved. Another countermeasure, i.e. the fuel supply advance angle, was adjusted to collaboratively optimize engine performance. NOx and soot emissions could be improved simultaneously. When the fuel supply advance angle was extended longer from 8 to 12°CA, for example, NOx emissions rose up but was still improved in relation to that produced from the original engine, while soot emissions were decreased substantially thus better than that of the original in the load range from small to medium. Also, brake specific fuel consumption could get to a relatively low level at partial load rate. Therefore, it is validated that this method has the potential to improve engine performance comprehensively and the present work can provide theoretical basis for the application of IEGR on small-sized non-road diesel engine. ©, 2015, Chinese Society of Agricultural Engineering. All right reserved.