Needle lift of injector nozzle with pre- and post-injection
1 Needle lift
2 Pilot injection
3 Crank angle
4 Main injection
5 Advanced post-injection
6 Retarded post-injection
For vehicles with a diesel particulate filter system two post-injections are employed during the regeneration process, in addition to the pre- and main injections, depending on the requirements.
Advanced post-injection is initiated in certain load/speed ranges immediately after main injection. Fuel is then injected during the ongoing combustion.
The main purpose of this advanced post-injection is to raise the exhaust gas temperature during the regeneration process of the particulate filter. In addition, some of the diesel particulates produced during regeneration are after-burned.
Retarded post-injection only occurs shortly before BDC (Bottom Dead Center) and also serves to raise the exhaust gas temperature.
In contrast to the previous injections, during retarded post-injection the fuel is not burnt, but evaporates due to the residual heat in the exhaust gas. This exhaust/fuel mixture is delivered to the exhaust system by the exhaust troke.
In the oxidation catalytic converter, the fuel vapor reacts with the residual oxygen (above a certain temperature) and burns. This provides sustained heating of the oxidation catalytic converter, which supports the regeneration of the particulate filter.