Combination of flow field measurements, shown in this paper, give new information on the effect of engine run parameters to formation of different flow fields inside piston bowl. The measurements were carried out with particle image velocimetry (PIV) technique in optical engine. Good set of results was achieved even though the feasibility of this technique in diesel engines is sometimes questioned. Main challenge in diesel engines is background radiation from soot particles which is strong enough to conceal the PIV signal. Window staining in diesel engine is also a problem, since very high particle image quality is needed for velocity analysis.All measurements were made in an optical heavy-duty diesel engine. Optical design of engine was Bowditch type . The engine was charged and equipped with exhaust gas recirculation (EGR). The exhaust gas level was monitored by oxygen concentration and the level was matched to former soot concentration measurements.At first part of the measurement campaign, images were taken in motored engine. Imaging was done to measure in-cylinder swirl after top dead center and without fuel injection. It was possible to measure the swirl inside the piston bowl, since there was a glass piston top. Swirl flow measurements were also a good way to find out optimal seeding density of flow tracer particles.In the second part of measurement campaign, the effect of injection pressure to the flow field inside the piston bowl was measured. The length of liquid core of fuel spray was also measured. Fuel injection pressure was varied from 1500 bar to 2500 bar. It is assumed that the flow field of spray and the initial swirl motion in the cylinder will enhance mixing of late cycle (spray) combustion.