An internal combustion engine comprises a chamber having a longitudinal centreline, inlet valving operable to admit constituents of a combustible mixture into the chamber for combustion therein to provide a pressure increase and outlet valving operable to release an outflow of liquid from the chamber under the influence of that pressure increase as an energy output of the chamber. The chamber has a flowpath defined therein for flow of the liquid towards the outlet valving. The flowpath diverges from centreline in a downstream direction thereof and has a cross-section area in a direction perpendicular to the centreline that remains substantially equal or decreases in the downstream direction of the flowpath.