Abstract
Despite the diesel engine's popularity, its emission of high levels of nitric oxide (NO) is problematic. Reduction of pollutant emissions requires a profound understanding of the combustion process and in particular the exact NO formation pathways. Since the latter are not entirely understood, accurate quantitative experiments are necessary to provide new insights and to validate existing combustion models. To date, reliable experimental techniques for in-cylinder NO concentration measurements are scarce. This thesis is dedicated to the development of the laser-induced fluorescence (LIF) technique to a level at which quantitative, in-cylinder concentration measurements of NO can be realised in a heavy-duty diesel engine. A number of laser techniques are investigated, allowing a correction for the severe attenuation that is experienced by both the laser beam and the induced fluorescence in the combustion chamber. Additionally, a spectroscopic model accounting for the pressure and temperature dependence of the fluorescence intensity per molecule is discussed. By applying these two corrections, quantitative in-cylinder NO concentrations are obtained. An important observation is that, during the combustion stroke, significant amounts of NO are observed before the most important NO formation mechanism (the Zeldovich mechanism) can come into play, suggesting that other mechanisms are involved in the early NO formation. Furthermore, measured in-cylinder NO concentration histories are compared with calculated concentrations. Although the agreement between experiment and calculation suggests that the Zeldovich mechanism is a major contributor to the formation of the observed NO, the contribution of other mechanisms early in the combustion cannot be excluded.
