For the purpose of this Regulation , 2.. `` Approval of an engine '' means the approval of an engine type or family with regard to the level of emission of gaseous and particulate pollutants by the engine ; 2.. `` Compression ignition ( C.I . ) engine '' means an engine which works on the compressionignition principle ( e.g . diesel engine ) ; 2.. `` Engine type '' means a category of engines which do not differ in such essential engine characteristics as defined in paragraphs 1 to 4 of Annex 1A to this Regulation ; 2..