An internal combustion engine, and a method of operating an internal combustion engine, of the kind in which recycled exhaust gases are reacted with a hydrocarbon fuel in a combustion space (1C) to produce a reformed fuel of enhanced calorific value by cracking molecules of the hydrocarbon fuel, the invention providing for means (14, 17) for the supply of air to the combustion space (1 C) in the cylinder (1 B) and separate means (15, 16) for the supply of the cracked and reformed fuel enabling an efficient engine operation to be achieved with economical use of fuel.