On soot oxidation in heavy duty diesel engine combustion
Soot emissions from diesel engine combustion are harmful both for the health and the environment. Soot is also a limiting factor in the work to lower the emissions of nitrogen emissions (NOx) and to improve the fuel consumption. In the present work, experimental and simulation methods were used to study soot processes during the combustion. In the combustion chamber, soot is first formed but then to a large extent oxidized again. The final exhaust emissions level is a results of a competition between these two processes. Actions to lower the soot formation during the fuel injection period by for instance high fuel injection pressure tend to increase NOx. One efficient method to lower NOx is to use cooled exhaust gas recirculation (EGR). A major problem with this method is that the availability of oxygen decreases for efficient fuel and soot oxidation. It is therefore needed to find methods capable to lower both soot and NOx. Additionally, general knowledge about soot emissions processes is useful to adapt the combustion to new non-fossil fuels.
The work shows that soot oxidation is equally important as soot formation. Control of the soot oxidation late in the combustion cycle can be done with a comparatively small increase of NOx. Using an approach to systematically break down the combustion history in a number of events, the soot oxidation process was divided into three phases: during the fuel injection period, directly after the end of injection and finally, oxidation of rich zones late in the cycle. The last phase was identified to be important to study more in detail. Studies of flame to wall interactions indicated that the turbulent mixing after wall impingement takes place on a longer time-scale than in the free part of the flame, which is an effect that can have an impact on the local rate of soot oxidation.
The work was partly funded by the Swedish Energy Agency (project 30754-1&2).