Application and Development of Optical Soot Diagnostic Techniques

Internal combustion engines (ICE) have been a vital part of society ever since their inception more than a hundred years ago. While initially it seemed as if the replacement of horse and carriage had solved the issue with pollutant emissions, it became evident in the 60’s that emission from ICEs is a major concern. The first emissions in focus were the nitrogen oxides (NOx). Lately, however, focus has shifted towards emissions which affect the global warming trends. The two worst pollutants, with respect to global warming, are carbon dioxide and soot.

The main focus of this thesis is the study of soot processes in optical diesel engines as well as development of optical soot diagnostics. While the diesel engine is favoured due to its high fuel efficiency, and therefore low CO2 emissions, it does suffer from higher soot emissions than the spark ignition engine. Soot emission is the net result of two competing processes; soot formation and soot oxidation. Soot oxidation processes have previously been shown to determine the trends of soot emissions for conventional diesel combustion and for that reason this thesis puts more focus on these.

Another known fact is that injections of fuel after the main injection, so called post injections, have been shown to reduce soot emissions. However, exactly how this works is not clear. In this thesis we elucidate the mechanisms of post injection soot reduction with the use of a novel soot diagnostic technique called diffuse back-illumination extinction imaging. Using this technique it is revealed that one of the reasons behind the soot reduction of post injections is that most or all of the soot that is formed by a short post injection is also oxidized, thus leading to no net emissions. The oxidation process is especially enhanced due to the increased mixing which occurs in the jet as the injector closes.

Lastly, a new soot thermometry technique based on the DBI technique is developed. By directly measuring the amount of soot within a flame with an active technique such as DBI, many of the uncertainties associated with conventional optical thermometry techniques such as two color pyrometry can be avoided.