With reference to FIG. 1, the present invention provides an internal combustion engine which operates repeatedly a multi-stage combustion process, the engine having a combustion chamber, supply means (12) for supplying fuel and air to the combustion chamber and exhaust means (16) for exhausting combusted gases from the combustion chamber. During a first stage of combustion (FIG. 1(a)- FIG. 1(c)) the supply means (12) supplies fuel and air to the combustion chamber and the supplied fuel and air are combusted by a spark ignition Otto process or by a compression ignition Diesel process. Then (FIG. 1(d)) at least a majority of the combusted gases resulting from the first stage combustion are retained in the-combustion chamber, additional air is supplied to the combustion chamber(FIG. 1(e)) and the resulting mixture is combusted in a second combustion stage (FIG. 1(e) and FIG. 1(f))by homogeneous charge compression ignition. The mixture of fuel and air during the first stage combustion is a rich mixture (with an air-fuel ratio λ less than 1) and the mixture of fuel and air and combusted gases during the second stage of combustion is a weak mixture (with an air-fuel ratio λ greater than 1).