Oxidation phenomena of diesel post fuel in the expansion stroke

Post fuel is injected to avoid plugging of diesel particulate filters (DPF). However, at advanced post fuel injection timings, the post fuel is partially oxidized in the cylinder under the high temperature conditions and the hydrocarbon supplied to DOC is not sufficient to regenerate the DPF. In this paper, partial oxidation phenomena at advanced post fuel injection timings were investigated with an engine and a high pressure-temperature optical constant volume chamber. In the engine experiments, the oxidation ratio of the post fuel was calculated from the exhaust gas composition. When the post fuel is injected at 30 CA ATDC, 83% of the post fuel is oxidized, however when the injection timing is 60 CA ATDC, the oxidation ratio of the post fuel becomes 2.2% and there is a 40.9% fuel loss mostly due to oil dilution. The reaction speed, in-cylinder temperature, and oxygen concentration (mol/m³) at the post fuel injection timing are related to the partial oxidation ratio of the post fuel. In the experiments with the constant volume chamber, the spray behaviors at the post fuel injections were observed. The ignition delay becomes longer with lower in-cylinder temperature and lower oxygen concentration, and the spray dilution of the post fuel during the ignition delays affects the heat release and the location where the partial oxidation in the chamber occurs. Based on the experimental data of engine speed, oxygen concentration, and in-cylinder temperature at the start of post fuel injection, an experimental formula for the post fuel oxidation ratio based on the Arrhenius equation was suggested by multiple regression analysis of the engine test data.