Laser Diagnostics of HCCI and Partially Premixed Combustion

Sammanfattning: The work presented in this thesis deals with measuring in-cylinder combustion species and events using different laser-based diagnostic methods. A variety of engine operating modes, like HCCI and partially premixed diesel combustion, have been investigated. In the very first measurements, in-cylinder flow-fields were compared to CFD and steady-state blow rig results. After that, the ignition and combustion process of HCCI was investigated using laser-induced fluorescence (LIF) of formaldehyde and hydroxyl. In the low-temperature reactions that precede the main combustion, formaldehyde (HCOH) was seen to form homogeneously over the viewed area. Hydroxyl, OH, is formed in the high temperature regions that mark the main combustion; it was determined that OH is formed in areas from which formaldehyde had disappeared. By using different start of injection timings, different degrees of homogeneity could be obtained for HCCI combustion and the effects of this were examined using the above mentioned laser-technique. The engine-out NOx level was monitored to see what in-homogeneity level could be tolerated before getting too much NOx. Going from early injections towards late, a distinct change in the homogeneity was seen with injection at 70 CAD and around 50 CAD NOx levels started to increase. Later, LIF measurements were performed on combustion modes other than HCCI, these studies also included the use of exhaust gas recirculation (EGR). The modes that were examined and compared with port and DI HCCI were UNIBUS combustion using two fuel injection events and low-temperature diesel combustion with one injection 8 crank angles before top dead centre. The feasibility of using formaldehyde, or other partially-oxidized fuel elements, as a naturally occurring fuel tracer were investigated by comparing the distribution of those species with that of the common fuel tracer toluene. The distributions of the two species are similar to each other in HCCI meaning that formaldehyde could be a tracer candidate. In low-temperature diesel, it seems like a good tracer; however more research is needed on the impact of polyaromatic hydrocarbons. In the last part, high-dilution low-temperature diesel combustion was studied and during this time the first planar flow measurements in a firing diesel engine were obtained. Furthermore, LIF measurements of partially-oxidized fuel and LII of soot were performed. The initial distribution of partially-oxidized fuel was found to correlate well with regions of heat release that were identified from the flow field divergence. In the later stages of combustion, soot and partially-oxidized fuel were found to be concentrated in the cylinder centre and no bulk flow exist, at least not for our piston design, that transport this fluid to regions where oxidation could take place. Partially-oxidized fuel was also found in the squish region which can be a source of emissions of CO and unburned fuel. It was also seen that single-cycle measurements show good similarity to the mean results. In the very last study that is presented, the flow structures of two low-temperature diesel operating conditions are compared at two swirl ratios. Differences are seen in the reverse-squish flow as well as in the fluid motion near the bowl rim. The main influence of decreasing the swirl was that the fluid motion at the bowl rim was altered.