Johnson Evinrude Outboard Motors 1956-1970 1.5-40hp repair manual., Page 42Get this manual
POWERHEAD with each revolution of the crankshaftCompare this system with four-cycle engineA stroke of the piston is required to accomplish each phase of the cycle and the power stroke occurs on every other revolution of the crankshaftStated another way, two revolutions of the four-cycle engine crankshaft are required to complete one full cycle, the four phasesPhysical Laws The two-cycle engine is able to function because of two very simple physical lawsOne: Gases will flow from an area of high pressure to an area of lower pressureA tire blowout is an example of this principleThe high-pressure air escapes rapidly if the tube is puncturedTwo: If gas is compressed into smaller area, the pressure increases, and if gas expands into larger area, the pressure is decreasedIf these two laws are kept in mind, the operation of the two-cycle engine will be easier understoodActual Operation Beginning with the piston approaching top dead center on the compression stroke: The intake and exhaust ports are closed by the piston; the reed valve is open; the spark plug fires; the compressed fuel-air mixture