On particulate characterization in a heavy-duty diesel engine by time-resolved laser-induced incandescence

This dissertation describes the results of soot measurements acquired in the combustion chamber of an optically accessible heavy-duty Diesel engine. The Diesel engine is the most efficient internal combustion engine. Pollutant emissions from the engine, such as soot and NOx, however, form a tremendous problem. Soot is mainly formed during an inefficient combustion process and plays a role in climate change and affects human health. NOx, however, is mainly produced during an efficient combustion process and is formed in a high temperature environment. It is one of the substances that contributes to the formation of acid rain and plays a role in the destruction of ozone. As a consequence, authorities are posing stricter regulations for the emissions from engines. The smallest and probably most harmful soot particles are not trapped by soot particulate filters. Consequently, the formation of soot has to be reduced in the combustion chamber. The formation process of soot has not yet been fully understood. This dissertation describes the development of laser-induced incandescence (LII) as a measurement technique enabling the investigation of the soot formation process in-situ. In this technique soot is heated to temperatures at which its thermal radiation is visible, by a shift of the Planck’s emission curve. The intensity of the soot emission is dependent on the soot volume fraction and on the temperature of the soot. After the laser pulse the soot particles cool down, causing a gradual decrease in LII intensity, that can be measured with a photo-multiplier tube (PMT), a light-sensor with a high temporal resolution. This so-called time-resolved LII (TR-LII) is dependent on the particulate size. A smaller particulate cools down faster than a bigger one. With TR-LII and the assumption of a physical cooling model, the soot particle size can be determined by fitting the model to the measured TR-LII curve. In the model, used in this dissertation cooling effects by conduction, vaporization and radiation are taken into account. Soot is a composition of unburned carbon molecules. It is formed mainly at the location in the engine where the fuel-air mixture is too rich. The air-flow in an internal combustion engine is highly turbulent. Besides that, the Diesel fuel is injected into the cylinder at such high pressures that the break-up behavior of the fuel spray is also influenced by turbulence, making it difficult to reproduce an experiment. Consequently, it is necessary to use simultaneously as many optical measurement techniques as possible, in order to extract reliable information about the Diesel combustion process. In order to find out more about the influence of one physical process on another one, part of the measurements has been performed in a constant volume high pressure high temperature combustion cell, which had easier optical access. This is a combustion chamber in which the Diesel combustion process can be investigated without the moving piston, improving the reproducibility of the experiments. The dissertation describes the issues which should be considered during LII measurements in an engine environment. One of the conclusions is that the time resolution of the detection system is crucial in the interpretation of the measurements and that the available measurement systems are not able to measure the signal in an engine environment in a correct way. A tool to extract the correct information from the measurements is given. Measurements have been performed for several engine loads. No influence of the engine load or injection pressure on the primary particle size has been observed. During the engine cycle, first, an increase of the primary particle size has been observed and later on during the combustion cycle a decrease of the primary particle size. A correct application of LII in an engine can be used to improve the efficiency of engines and as a consequence the reduction of soot in the exhaust