Abstract: Internal combustion engine having a main combustion chamber (26) and a subsidiary combustion chamber (30) interconnected by a passageway (32) having a flow control element (34) arranged therein to cyclically vary the extent of communication provided between the main and subsidiary combustion chambers. In use a subsidiary air fuel charge is first ignited in the subsidiary combustion chamber (30) with the control element (34) substantially blocking communication between the two combustion chambers (30,26). Then the control element (34) is moved to permit the igniting air fuel charge in the subsidiary combustion chamber (30) to move through the passageway (32) to ignite a main air fuel charge in the main combustion chamber (26). The control element (34) is reciprocable having a head portion (82) which is moved from within the passageway (32), into the subsidiary combustion chamber (30) to effect opening of communication between the combustion chambers (26,30).