Due to legal requirements, it is necessary to reduce the soot emissions of diesel engines in motor vehicles. For this purpose, diesel particulate filters are used in which the soot emissions are captured. The deposited soot must be removed from the filter at regular intervals during a so-called regeneration, since during vehicle operation the build-up of the soot layer in the filter increases the back pressure of the exhaust gas and thereby decreases the efficiency of the engine. By means of engine measures (post-injection), the temperature during regeneration is increased so that the soot in the filter can completely oxidize. If the regeneration is carried out incompletely, it is named partial regeneration, which is the subject of the investigation of the present thesis.
The aim of this investigation was to assess whether it is possible to load the diesel particulate filter again with soot from the engine directly after a partial regeneration with a remaining amount of soot. Furthermore, it should have been determined which consequences such a reloading has on the subsequent operation of the diesel particulate filter. The objective of this thesis was to evaluate, based on fundamental investigations on an engine test bench (stationary tests), whether the application of partial regeneration makes sense, and if necessary, how an adaptation of the vehicle operating strategy would have to be carried out. In addition, a simulation model was used to make the assessment more comprehensive, especially with regard to a worst-case regeneration. A worst-case regeneration means that the engine is switched to idle during regeneration, for example when a vehicle has to be stopped at a red traffic light during regeneration. This worst-case regeneration is usually associated with a high temperature development due to very good conditions for soot oxidation (low mass flow, high oxygen concentration). This high temperature development may result in damage to the diesel particulate filter due to thermal stress in the filter material.
The engine test bench investigations provided insights into pressure loss, soot and flow distribution and temperature distribution in the diesel particulate filter. It was observed that the application of partial regenerations causes significantly lower pressure losses of the filter for reloads than for a corresponding load from the empty state. Based on this finding it has been possible to derive a correction function which ensures load detection via pressure loss for the stationary tests conducted. The deep bed filtration effect could be identified as the reason for the lower pressure loss level, which occurs to a much lesser extent, when reloaded after a partial regeneration. To determine the soot distribution, a specially developed method was used to disassemble diesel particulate filters from the engine test bench in different states (loaded, partially regenerated and reloaded) in such a way that the soot layer thicknesses could be determined by optical analysis methods, such as light and scanning electron microscopy. As a result, it was found that after a partial regeneration the soot layer thicknesses are significantly thinner in the radial center of the filter than on the outside area close to the canning. This phenomenon was detected in a somewhat attenuated form even in the state after reloading. This means a significant difference to the load from the empty state, in which the soot layer thicknesses are more or less constantly distributed over the entire cross section. This inhomogeneous distribution of soot layer thicknesses has a direct influence on the flow and temperature distribution in the diesel particulate filter. Due to the lower soot layer thicknesses in the filter center, higher flow velocities and temperatures were detected in this area. The interaction between soot, flow and temperature distribution is critical for high temperatures which can arise during regeneration and especially in a worst-case regeneration in a diesel particulate filter due to the exothermic soot oxidation. This interaction is significantly influenced by the jump to idle in a worst-case regeneration.
The final evaluation whether critical temperatures could cause damage to the diesel particulate filter was conducted with a simulation parameter study. The aim of this study was to determine the impact of the idle jump timing on the temperature behavior in the diesel particulate filter. The study showed that under the investigated experimental conditions no critical temperatures arose, which could have caused damage to the diesel particulate filter.
The results of this thesis show that the partial regeneration as an operating strategy for a car seems possible and reasonable. The limitation of this study is that the tests were carried out only on the engine test bench under stationary conditions. It would furthermore be necessary to re-test the results under transient conditions for example using a test vehicle in real-life conditions.