2   STROKE   VS.    4   STROKE   Engines 

In picture 1 fuel and air are mixed in the crankshaft (below piston) and previous fuel is compressed above the piston, that fuel is then ignighted (picture 2), which then pushes the piston downward and the exhaust valve is opened (picture 3). As the piston moves downwards it also pushes the new fuel upward through the transfer port, where the exhaused port is closed in appropriate timing. The engine is considered a two stroke engine because the piston undergoes one upward stroke and one downward stroke for each combustion cycle.