An internal combustion engine of reciprocating-piston type with injection of liquid or gaseous hydrocarbon fuel, provided with catalytic means for initiating the combustion of the air/fuel charge in the combustion chamber of the or each cylinder. The major part of the combustion chamber of each cylinder is afforded by a recess formed in the piston crown, and a catalytic element of mesh, grid, perforated or sintered or other construction, is carried by the piston in a position at least partly overlying the said recess, the fuel being injected into the said recess to contact and pass through the catalytic element, being ignited by its contact with the catalytic element.