2-Cycle and 4-Cycle Principles
This diagram shows the two steps (strokes) in a complete power cycle in a 2-stroke-cycle engine.
A. The crankshaft has rotated awat from bottom dead center (BDC), moving the piston toward
the cylinder head and closing the transfer and exhaust ports. Trapped above the piston is the
air fuel charge inducted from the previous cycle. Below the piston, the rotary disk has started
to uncover the induction port that leads to the carburetor and the outside atmosphere.
Because the volume of the crankcase beneath the piston is increasing, its pressure relative to
the outside air is decreasing. This pressure differential causes outside air to rush into the
carburetor where it mixes with available fuel before continuing through the induction port
and rotary disk, into the crankcase.
B. As the piston nears top dead center (TDC) on the secondary compression operation, the air
fuel mixture reaches the ignition point. Ignition is achieved by the heating effect of compression
and the heat retained by the glow plug element from the previous cycle, among other things.
Combustion occurs betweeb the top of the piston and the cylinder head. This volume is known
as the combustion chamber. As combustion progresses and the gas increases, pressure rises
proportionally. The momentum of the rotating engine components carries the piston beyond
TDC in the direction of crankshaft rotation. The rotary disk is almost closed as pressure caused
by combustion (several 100 pounds per square inch), drives the piston away from the cylinder
head on the power event.
C. Diagram C shows the piston moving to a point at which the power (or expansion) event has
just ended with the opening of the exhaust port. The period of crankshaft rotation necessary to
move the piston from exhaust opening to transfer opening is called exhaust lead. Exhaust lead
is necessary in most engines to provide time for the relatively high exhaust-gas pressure to
blow down to atmospheric pressure. If exhaust lead were eliminated, exhaust gas would
probably back-flow through the transfer port, contaminating the cool, fresh air-fuel mixture
in the crankcase. Notice that the rotary disk valve is still closed, allowing the decending
piston to continue compressing the fresh air-fuel mixture in the crankcase (primary
compression), as it awaits the opening of the transfer port.
D. Diagram D depicts the exhaust and transfer ports open, with cylinder and crankcase
scavenging taking place. The previously compressed fresh air-fuel mixture is flowing through
the bypass channel and transfer port-chasing the tail-end exhaust gases out of the exhaust port.
4-CYCLE PRINCIPLE
This diagram shows the four steps (strokes) in a complete power cycle in a 4-stroke-cycle engine.
(Click the image for a larger view)
1. On the downstroke, the intake valve is opened and the fuel/air mixture
is drawn into the combustion chamber.
2. On the upstroke, the intake valve has closed and the fuel/air mixture undergoes compression.
3.Ignition takes place by means of the glow plug (not shown); the fuel/air mix explodes, and the
piston is forced down for the second time in the cycle. This is called the "power stroke".
4. The exhaust valve is open and the piston moves up for the second time in the cycle, and the
burned gases are expelled out through the exhaust valve into the muffler.