Smoke from DI Diesel Engines

Abstract

Although DI Diesel engines are becoming popular because of their fuel economy as well as low exhaust emissions, they emit higher amount of visible exhaust termed as smoke. The characterization of this smoke has remained a challenge in engine development and modelling work. A new phenomenological model is explained in this chapter encompasses the spray and the wall interaction by a simple geometrical consideration. A dimensionless factor was introduced to take care of the nozzle hole manufactured by hydro-erosion (HE) as well as the conical shape of the nozzle hole (k-Factor) in case of valve-closed-orifice type of nozzles. The smoke emitted from the wall spray formed after wall impingement is the major contributor to the total smoke at higher loads. As the fuel spray impinges upon the walls of the combustion chamber, its velocity decreases. This low-velocity jet contributes to the higher rate of the smoke production. Therefore, the combustion bowl geometry along with injection parameters plays a significant role in the smoke emissions. While considering the smoke, interestingly the chemical kinetics, which is very fast compared to the mixing phenomenon in the spray entraining the surrounding air, is dropped. In addition, the thesis considers the mixing at the wall in detail. The latter enables explanation of sudden rise in the rate of increase in smoke at about 50% load for many types of engines at different speeds. Therefore, smoke, the result of incomplete combustion (chemistry) has been treated by ignoring the fast chemistry, as the slow physical mixing seems to be controlling. Though it appears paradoxical, it is the truth. There are mainly two types of smokes from a DI diesel engine, cold and hot smoke. Cold smoke is white in appearance under direct illumination, consisting of a mixture of fuel and lubricating oil particles in an un-burned or partly burned state. This form of smoke is sometimes referred to as liquid smoke or fog. Hot smoke is black in appearance, consisting of solid particles of carbon from otherwise complete combustion of fuel. This form of smoke is often referred as hot or solid smoke. In the present work, the study is focused on black smoke coming out of diesel engine under hot conditions.