In an internal combustion engine comprising a piston (1) having a combustion bowl (3) recessed in the crown of the piston and a plurality of arcuate recesses (7) formed in the side wall (6) of the bowl spaced apart by arcuate intermediate portions of the side wall, swirl means to cause the inlet air to rotate about the axis of the bowl, and fuel injection means (2) that serves to direct a plurality of fuel jets radially of the bowl each towards a corresponding recess (7), the piston is characterised in that the ratio of the maximum distance by which each recess (7) extends radially into the side wall (6) compared with the radius of the side wall (6), lies within the range 0.10 to 0.20.