After-oxidation in Heavy Duty (HD) diesel combustion is of paramount importance for emissions out from the engine. During diffusion diesel combustion, lots of particulate matter (PM) is created. Most of the PM are combusted during the after-oxidation part of the combustion. Still some of the PM is not, especially during an engine transient at low lambda. To enhance the PM oxidation in the late engine cycle, swirl together with high injection pressure can be implemented to increase in-cylinder turbulence at different stages in the cycle. Historically swirl is known to reduce soot particulates. It has also been shown, that with today's high injection pressures, can be combined with swirl to reduce PM at an, for example, engine transient. The mechanism why the PM engine out is reduced also at high injection pressures is however not so well understood. In this work flow field data during combustion and after-oxidation together with soot and temperature measurements was combined to examine how flow field affects soot formation and oxidation.Swirl number was varied together with injection pressure and the engine tests were done in a HD optical engine. The load was set to 10 bar & 20 bar IMEP at low lambda without EGR, typically transient load points. A high speed colour camera captures picture of the combustion seen through a glass piston-bowl. The flow field was extracted with combustion image velocimetry (CIV) that traces the glowing soot particulates (or the light luminosity difference) by cross correlation between two pictures from the high speed colour camera. From the same pictures the KL factor and flame temperature were simultaneously calculated with the 2-colour method. Both CIV and the 2-colour method are line of sight optical methods that catches flow, soot and temperature from the light observed through the piston.It was found that in the after-oxidation part of the cycle, the flow in the piston bowl deviates strongly from solid body rotation (that can be assumed to be the case before injection). With increased injection pressure this deviation from solid body rotation increased at constant swirl number. When swirl number was increased, the deviation from solid body rotation increased even further. This seems to be an important factor during the after-oxidation part of the combustion by amplifying the turbulence. The flame temperature together with KL factor (a measure of soot density inside the cylinder) was also influenced when the flow field in the cylinder was changed. With increased injection pressure, from 500 bar to 1000 bar, the maximum KL was amplified during combustion with 50%, but the measured tail pipe soot was decreasing from 1.22 FSN to 0.49 FSN. This together with increased solid body deviation for the 1000 bar case, at the after-oxidation part of the combustion, leads to the conclusion: The flow field during the late part of the cycle has strong impact on tail pipe soot emissions. What was created during the diffusion combustion has less impact on the tail pipe soot compared to the flow field effects during after-oxidation.