1971 Evinrude 40HP outboards Service Manual, Page 40Get this manual
DESCRIPTION The power head consists of the cylinders, pistons, rods, crankshaft, and crankcaseThe power head has two horizontally mounted cylindersAlternate firing order is used so that each cylinder delivers one power impulse per crankshaft revolutionSee Figures 5-1 and 5-2
E XHAUST PORT EXHAUST GASES
INTAKE PORT
THEORY OF OPERATION
Two-cycle engines used on outboard motors require only two piston strokesone up, one down, to effect crankshaft revolution and to complete the exhaustintake-compression-ignition sequence that produces powerIn two-cycle engine, ignition of the fuel-air mixture occurs as the piston reaches the top of each strokeThe explosion drives the piston downwardToward the end of the downward stroke, ports which lead ,to the exhaust system are uncoveredThe exhaust gases flow into these ports, thus reducing the pressure in the cylinder At almost the same time, intake ports are openedThese ports connect with the crankcase where fuel and air mixture has been induced by carburetionThe downward motion of the piston compresses this mixture in the crankcase and forces it through the intake ports into the cylinderThe inrushing charge of the fuel-air mixture helps in ejecting the last of the exhaust gases from the cylinderSee Figure 5-1A, Fuel Intake and ExhaustAs the piston begins its upstroke, it closes the intake and exhaust ports and begins to compress the fuel and air mixture trapped in the cylinderThe upward travel of the piston also reduces the pressure in the crankcase compartmentThe resulting suction opens leaf valves which admit additional air and fuel from the carburetor into the crankcase, thus preparing the next cylinder chargeSee Figure 5-1B, Compression Stroke