Engine system comprising a compression ignition engine (20) including at least one combustion chamber and a cold flame vaporizer (40) in which a fuel is partially oxidized in preheated air to form a cold flame gas. The cold flame vaporizer (40) is in fluid communication with the combustion chamber of the compression ignition engine (20). There is further provided means (50) for supplying air such that the cold flame gas can be mixed with the additional air before being injected into the combustion chamber, and means (22) for injecting a pilot fuel into the combustion chamber, thereby producing a pilot flame in the combustion chamber which ignites the mixture of cold flame gas and air. There is also provided a method for a substantially NOx-free combustion of diesel fuel in a compression ignition engine (20).