To understand the basic idea behind how a reciprocating
internal combustion engine works, it is helpful to have a good
mental image of how "internal combustion" works.
Say that you
took a big piece of plastic sewer pipe, maybe three inches in
diameter and three feet long, and you put a cap on one end of
it. Then say that you sprayed a little WD-40 into the pipe, or
put in a tiny drop of gasoline. Then say that you stuffed a
potato down the pipe and ignite the fuel air mixture. Like
this:
The potato
cannon uses the basic principle behind any reciprocating
internal combustion engine: If you put a tiny amount of
high-energy fuel (like gasoline) mixed with air in a small,
enclosed space and ignite it, an incredible amount of energy
is released in the form of expanding gas. For example, if you
can create a cycle that allows you to set off explosions like
this hundreds of times per minute, and then if you can harness
that energy in a useful way, what you have is the core of an
internal combustion reciprocating engine!
Almost all
reciprocating aircraft engines currently use what is called a
four-stroke combustion cycle to convert gasoline into
motion. The four-stroke approach is also known as the Otto
cycle, in honour of Nikolaus Otto, who invented it in
1867. The four strokes are illustrated below. They are:
The intake
stroke. The
compression stroke. The
combustion stroke The exhaust
stroke.
The
piston is connected to the crank shaft by a
connecting rod. Here's what happens as the engine
goes through its cycle:
The
piston starts at the top, the intake valve opens, and
the piston moves down to let the engine take in a
cylinder full of air and gasoline. This is the intake stroke. Only the tiniest drop of gasoline
needs to be mixed into the air for this to work.
Then
the piston moves back up to compress this fuel/air
mixture. Compression makes the explosion more
powerful.
When
the piston reaches the top of its stroke, the spark
plug emits a spark to ignite the gasoline. The
gasoline charge in the cylinder explodes,
driving the piston down.
Once
the piston hits the bottom of its stroke, the exhaust
valve opens and the exhaust leaves the cylinder
to go out the tail pipe.
Now the
engine is ready for the next cycle, so it intakes
another charge of air and gas. ...
Notice
that the motion that comes out of an internal combustion
engine is rotational. In an engine the linear
motion is converted into rotational motion by the crank
shaft. The propeller serves as a flywheel for the
engine, smoothing out the irregularities of the motion.
