Future emission legislation will require substantial reductions of NOx and particulate matter (PM) emissions from diesel engines. The combustion and emission formation in a diesel engine is governed mainly by spray formation and mixing. Important parameters governing these are droplet size, distribution, concentration and injection velocity. Smaller orifices are believed to give smaller droplet size, even with reduced injection pressure, which leads to better fuel atomization, faster evaporation and better mixing.
In this paper experiments are performed on a single cylinder heavy-duty direct injection diesel engine with three nozzles of different orifice diameters (Ø0.227 mm, Ø0.130 mm, Ø0.090 mm). Two loads (low and medium) and three speeds were investigated. The test results confirmed a substantial reduction in HC and soot emissions at lower loads for the small orifices. At medium loads, the soot and CO emissions increase which makes the smaller orifice a disadvantage compared with the reference nozzle (Ø 0.227 mm). This could be caused by a lower local air-fuel ratio in the combustion zone, since more fuel is injected at increased loads and smaller orifices have a shorter fuel penetration leading to decreased air utilization.
It has also been observed that the smaller the orifice, the shorter the ignition delay. The smaller orifices also improve the mixing, which is shown by shorter combustion duration. This results in reduction of the heat and time losses, resulting in a higher thermal efficiency, i.e. lower fuel consumption. One of the nozzles with smaller orifices (Ø 0.130 mm) has improved combustion for all of the test cases, resulting in an increase in fuel conversion efficiency compared to the reference nozzle (Ø 0.227 mm).
The results from the experimental tests show that the benefits of a well distributed spray with small droplets produced by smaller orifices can be substantial in diesel combustion.