A geometry for an IC combustion chamber is disclosed that increases the air-fuel mixing efficiency within the chamber. The piston head includes a squish surface portion for generating a squish jet, and a bowl portion that cooperatively with the cylinder head defines a combustion volume. A horizontal channel extends between the squish surface portion and the bowl portion, and is adapted to provide a high-momentum jet into the bowl portion. A transverse channel may also be provided that directs the squish flow toward the horizontal channel, thereby generating a toroidal vortex therein. A premix fuel injector may inject fuel near the upstream end of the horizontal channel. A converging nozzle portion near the distal end of the horizontal channel may increase the flow speed into the bowl portion. A chamber fuel injector is disposed near the downstream end of the horizontal channel.