A variation of the two cycle engine, known as the "three port" type, is illustrated by Fig. 12. The general characteristics and operation are the same as the regular two cycle type, with the exception of the meanes for admitting the vapor. Instead of the vapor being admitted through an opening in the side of the crankcase, a third port is provided, which opens into the cylinder just below the piston when the latter is at the top of its stroke; the carburettor is connected with this port. The piston covers this port except when it is at the top of its stroke, and at this time it is opened into the base.

The piston on its up stroke creates a partial vacuum in the base, and when the third port is uncovered at the top of the stroke, the vapor rushes in. The gas is thus admitted in a sort of puff instead of during the entire up stroke, as in the ordinary type, and is, therefore, more energetic and positive at high speeds. Since the piston covers the port except during the admission, a non return valve on the vapor inlet is not necessary, and the admission of vapor is less obstructed.

For these reasons the three port engine can be run at a higher rate of speed, and practically all engines intended to run at a speed greater than 500 revolutions per minute are now built on this principle. The external appearance is, with the above exception, the same as the ordinary two cycle engine.