intake valve 24 will close and only the intake valve 25 will be opened so that a rich fuel air mixture will be delivered to the chamber only through the intake passage 23 the operation of the invention of this embodiment may be best understood by reference to fig3 and 4 during the initial intake stroke the intake valve 24 will be opened and an intake charge of pure air from the intake passages 22 and 35 will be drawn into the combustion chamber 17 as indicated by the open arrows in these figures as has been noted any overlap between the opening of the exhaust valve 19 and the intake valve 24 will therefore not result in the loss of any fuel through the exhaust valve 19 as a result of this valve overlap as the piston 14 reaches an intermediate position in its intake stroke the intake valve 25 will open and a fuel air mixture will be admitted to the chamber 17 during the initial stage of this opening both valves 24 and 25 will be opened and there will be both pure air and fuel air mixture admitted to the engine at the completion of the intake stroke however the intake valve 24 will be closed and only a rich fuel air mixture will be admitted to the chamber.