1. Reference:
http://www.animatedengines.com/otto.html

2. •The piston descends from the top of the cylinder to the
bottom of the cylinder, reducing the pressure inside the
cylinder.
• A mixture of fuel and air, or just air in a diesel engine, is
forced by atmospheric (or greater) pressure into the
cylinder through the intake port. The intake valve(s) then
close.
•The volume of air/fuel mixture that is drawn into the
cylinder, relative to the volume of the cylinder is called,
the volumetric efficiency of the engine.

3. During the intake stroke,
the piston moves
downward, drawing a
fresh charge of vaporized
fuel/air mixture. The
illustrated engine
features a poppet intake
valve which is drawn
open by the vacuum
produced by the intake
stroke.

4. •The piston returns to the top of
the cylinder compressing the air,
or fuel-air mixture into
the combustion chamber of the
cylinder head.

5. As the piston rises, the
poppet valve is forced
shut by the increased
cylinder pressure.
Flywheel momentum
drives the piston
upward, compressing
the fuel/air mixture.

6. •While the piston is close to Top Dead Center, the compressed
air–fuel mixture in a gasoline engine is ignited, usually by
a spark plug, or fuel is injected into the diesel engine, which
ignites due to the heat generated in the air during the
compression stroke.
•The resulting massive pressure from the combustion of the
compressed fuel-air mixture forces the piston back down
toward bottom dead centre.

7. At the top of the
compression
stroke, the spark
plug fires, igniting
the compressed
fuel. As the fuel
burns it expands,
driving the piston
downward.

8. •The piston once again returns to top dead center
while the exhaust valve is open.
•This action evacuates the burnt products of
combustion from the cylinder by expelling the
spent fuel-air mixture out through the exhaust
valve(s).

9. At the bottom of the
power stroke, the
exhaust valve is
opened by the
cam/lifter
mechanism. The
upward stroke of the
piston drives the
exhausted fuel out of
the cylinder.