Introduction
to How Car Engines Work
Internal
Combustion
Parts
of an Engine
What
Can Go Wrong
Engine
Subsystems
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. One good example is an old
Revolutionary War cannon. You have probably seen
these in movies, where the soldiers load the cannon
with gun powder and a cannon ball and light it.
That is internal combustion, but it is hard to imagine
that having anything to do with engines.
A more relevant example might be this: Say that
you took a big piece of plastic sewer pipe, maybe
3 inches in diameter and 3 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. Like this:
I
am not recommending that you do this! But say you
did... What we have here is a device commonly known
as a potato cannon. When you introduce a spark,
you can ignite the fuel. What is interesting, and
the reason we are talking about such a device, is
that a potato cannon can launch a potato
about 500 feet through the air!
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)
in a small, enclosed space and ignite it, an incredible
amount of energy is released in the form of expanding
gas. You can use that energy to propel a potato
500 feet. In this case, the energy is translated
into potato motion. You can also use it for more
interesting purposes. For example, if you can create
a cycle that allows you to set off explosions like
this hundreds of times per minute, and if you can
harness that energy in a useful way, what you have
is the core of a car engine!
Almost
all cars 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 honor of Nikolaus Otto,
who invented it in 1867. The four strokes are illustrated
in Figure 1.
They are:
Figure
1
You can see in the figure that a device called a
piston replaces the potato in the potato cannon.
The piston is connected to the crank shaft
by a connecting rod. As the crankshaft revolves,
it has the effect of "resetting the cannon."
Here's what happens as the engine goes through its
cycle:
Notice
that the motion that comes out of an internal
combustion engine is rotational, while
the motion produced by a potato cannon is linear
(straight). In an engine the linear motion is
converted into rotational motion by the crank
shaft. The rotational motion is nice because we
plan to turn (rotate) the car's wheels with it
anyway.
Two
other things that are good to note:
There
are different kinds of internal combustion engines.
The gas turbine engine is another form of
internal combustion engine. A gas turbine engine
has interesting advantages and disadvantages, but
its main disadvantage right now is an extremely
high manufacturing cost (which means it costs more
than the piston engine used in cars today). Click
here for more information on gas turbines.
There
is such a thing as an external combustion
engine. A steam engine in old-fashioned trains
and steam boats is the best example of an external
combustion engine. The fuel (coal, wood, oil, whatever)
in a steam engine burns outside the engine to create
steam, and the steam creates motion inside the engine.
It turns out internal combustion is a lot more efficient
(takes less fuel per mile) than external combustion,
plus an internal combustion engine is a lot smaller
than an equivalent external combustion engine. This
explains why we don't see any cars from Ford and
GM using steam engines.
Almost all cars today use a reciprocating internal
combustion engine because this engine is:
