Analysis of Nozzle Orifice Size Effect on Smoke in a Diesel Engine
Masahiro ISHIDA*, Daisaku SAKAGUCHI* and Hironobu UEKI*
ABSTRACT
The effect of nozzle orifice size on smoke emission was examined experimentally in a turbocharged DI diesel engine, and was analyzed theoretically by using the two-zone model. The nozzle orifice size was varied from 0.26 to 0.18 mm under various engine operation condition. The rates of NO-formation, soot-formation and soot-oxidation were analyzed based on the measured combustion pressure time history, the measured exhaust NOx concentration and the measured exhaust smoke. As results, the changes due to orifice size in the excess air ratio of the burned zone, the soot particle size and the number of soot particles were estimated. It is shown that the soot size is primarily dependent on the orifice size and secondarily but more markedly on the spray penetration for the ignition delay period, and the number of soot particles is primarily dependent on the amount of fuel injected and secondarily on the excess air ratio of the burned zone.
To meet increasingly stringent emission standards in marine diesel engines as well as automotive small diesel engines, extensive research has been carried out to explore various ways to reduce NOx and particulate emissions from diesel engines. In various methods for reducing NOx and/or particulate, the utilization of water such as water-emulsified fuel (Tsukahara, et al. [1]), port water injection (Ishida, et al. [2]), stratified water-fuel injection (Takasaki, et al. [3]), and exhaust-water-selective recirculation (Fukuda, et al. [4]) has been reexamined, and a marked effect of water was shown especially on reduction in NOx. On the other hand, smoke can be reduced, as is well known, by an increase in fuel injection pressure as well as a decrease in nozzle orifice size because the fuel droplet size is reduced significantly [5]. One of the most popular semi-empirical models of soot formation in a Diesel engine suggested by Hiroyasu & Kadota[6] includes only two steps; (1) soot formation linked directly to the concentration of initial fuel; and (2) soot oxidation related to the concentration of molecular oxygen. This model and its modifications were successfully applied in a number of studies. However, It was also shown that two-step models have rather limited range of applicability. In addition, these models do not provide the information regarding the size of soot particles. Therefore, multistep phenomenological model of soot formation and oxidation was currently proposed by Kazakov & Foster [7], which could predict reasonably well the in-cylinder soot concentration, particle number density and partical size. However, the information with respect to the size of soot particle is still one of the issues of practical interest.
The objective of this study is to clarify semi-empirically physical factors which affect the size of soot particle and the number of soot particle in diesel engines, The amount of fuel injected, nozzle orifice size, ignition delay, fuel spray penetration, the excess air ratio of the burned zone as well as the fuel injection pressure are taken into consideration as the factors in the present analysis. The experimental work was conducted in a turbocharged DI diesel engine in which the nozzle orifice size was varied from 0.26 to 0.18 mm every 0.02 mm under various engine operation conditions.
