Two stroke
A
reciprocating engine cycle in which the
piston takes over some of the valve functions in order to obtain a
power stroke each revolution of the
crankshaft. This involves the use of
ports in the
cylinders which are covered and uncovered by the movements of the piston. As the piston moves down, it clears these ports so that the
exhaust gases can exit and a fresh charge of mixture can enter at the same time. In a typical two-stroke engine the
fuel-air mixture enters the
crankcase through a
reed valve. When the piston is at the bottom of the cylinder a port is uncovered. As prior movement of the piston has compressed the mixture in the
crankcase it flows into the cylinder. Further
compression in the cylinder starts as soon as the piston reverses and covers the ports. At the same time
compression is occurring in the cylinder, movement of the piston has created a
vacuum in the
crankcase which draws a fresh charge of mixture from the
carburetor into the
crankcase. The compressed charge is fired as the piston reaches
top dead center. As
expansion of the burning charge forces the piston downward, the reed valve in the
crankcase closes and the mixture in the
crankcase is compressed. As the piston uncovers the ports at the bottom of the stroke, compressed mixture from the
crankcase enters the cylinder again and is deflected by a
baffle on the
piston head into the outer end of the cylinder. This incoming fresh mixture then assists in pushing the burned gases out of the cylinder and the cycle is repeated.