Diesel engines have been progressing a lot during recent years. Beside the driving pleasure due to high torque and the low fuel consumption, it is more and more important to reduce combustion noise and pollutant emissions such as smoke and NOx.
The exhaust after-treatment systems, in particular Diesel Particulate Filter (DPF), have therefore become as a standard for Euro5 applications and for some of the heaviest vehicles for Euro4.
The DPF requires periodic regenerations under high temperature conditions in order to burn out the soot cumulated inside. One of the simplest strategies consists of one or several late injections. Typically, retarding the main and adding an early post injection generate high exhaust temperature while a very late one with a suitable quantity provides hydrocarbons, which burn inside the Diesel Oxidation Catalyst (DOC), and ensures the required temperature upstream the DPF.
Unfortunately, as the late injection occurs in unfavorable thermodynamic conditions, the fuel spray impinges on the cylinder liner wall even with small injected quantities and leads to the contamination of the lub oil.
A CFD study using STARCD with ECFM-3Z combustion model showed the importance of the interaction between consecutive injections and allowed to understand the dilution - regeneration trade-off.
For instance, it is shown that the very late injection sprays can penetrate in the hot burnt gases due to the combustion of the previous injection. It allows to reduce the fuel impingement on the cylinder wall. However, this phenomenon has to be optimized in order to obtain just the vaporization of the lately injected fuel but without its combustion and to keep enough hydrocarbons for the DOC.