The effects of injection pressure and swirl on in-cylinder flow pattern and combustion in a compression-ignition engine

In-cylinder swirl flow is well known to influence emissions behaviour in diesel engine combustion. The post-oxidation part of the combustion stands for typically 25 - 45 % of total Heat Release and is of paramount importance for engine out particulate matter (PM) emissions, especially during an engine transient, at low lambda. To investigate the link between in-cylinder flow and engine out emissions, single-cylinder and optical engine measurements were performed. Injection pressure, swirl and tumble were varied, and emission data, together with high speed photography of in-cylinder flow field during the injection-and the post-oxidation events, were measured. Particle image velocimetry (PIV) software was used to evaluate the flame luminescence images and to calculate the flow field in the cylinder, crank angle resolved during combustion. The glowing soot structures from the combustion were used as tracers. Single cylinder tests with an active valve train were used, which gave a controlled variation in swirl number, 0.4 to 6.7, and tumble number, 0.5 to 4.0. The main findings is that the injection pressure strongly affects the flow field in the cylinder, both before and during the post-oxidation period. Correlations between measured engine out soot emissions and changes in in-cylinder flow has been found and was coupled to the changes in swirl and injection pressure. The observed swirl vortex in the post-oxidation period deviates strongly from solid body rotation. The unsymmetrical rotation was found to be a function of injection pressure. This deviation is concluded to affect the soot oxidation in form of increased turbulence during the post-oxidation period.