There are some fairly simple, yet important events
that take place in an engine. One of these is the compression of
the fuel/air mixture.
Why Compression is Necessary
There are three things required for burning to take
Air (specifically oxygen)
If any of these are reduced or removed, the fire
will slow down or go out.
Picture a campfire burning strongly. Air is flowing
freely to the flame. There is enough fuel (wood) to burn. The heat
from the fire keeps itself going. Each piece of wood, as it burns,
warms itself and the wood nearby.
Someone stumbles by the fire, and kicks the wood,
scattering it. The fire starts to go out. Why?
There is still oxygen. The amount of fuel available
is adequate, but the fuel is too far apart for the burning pieces
to continue to heat each other. Each stick of wood is not heating
its neighbor as well as it did when they were close together in
the center. If the wood is pushed to the center again, the fire
resumes its strong flame.
The Example Applied
The same principle is applied in a cylinder. The
air (oxygen) and fuel (gasoline) are well mixed in the carburetor.
They enter the cylinder, but fuel particles and oxygen are far apart.
When the piston comes up in the cylinder, the air molecules and
fuel particles are forced close together. When one or two droplets
are ignited by the spark plug, the chain reaction is set off.
Combustion is thorough and quick. When a gas is compressed,
as in a cylinder, heat is generated as the molecules collide much
more in tighter space. This gets the fuel/air mixture close to the
burning point even before the spark ignites them.
Every engine has a piston that compresses the fuel
and air. Every piston has rings that seal around the sides of the
piston preventing gasses from escaping. Some pistons have two rings,
others three. Piston rings are made of very hard steel that slide
up and down in the cylinder walls. When piston rings wear, the result
is compression loss that results in great power loss. The rings
are a very important part of an engine. They are also the first
to be damaged if there is improper lubrication.
In a two-cycle engine where the gas and oil are mixed,
oil in the gas ensures the lubrication of the upper cylinder walls.
In a four-cycle engine, lubrication of cylinder walls
is from the oil pump and oil splashed by the crankshaft churning
in the oil pan.
In a diesel engine, there is splash and oil pump
lubrication of the cylinder walls by oil in the crankcase, but diesel
fuel is, by nature, a better lubricant than gasoline. The fuel itself
lubricates the upper cylinder walls.
In the late 60s I worked at Red Devil mine. The boss
told me to fuel up the diesel tractor. It had two engines: a gasoline
engine that started the big diesel engine. It had two fuel tanks.
Through ignorance, I put gasoline in the diesel tank and almost
got fired. As it was, my mistake was discovered soon enough, but
I contaminated fifty-five gallons of fuel by mixing gasoline and
diesel fuel in the same tank. Gasoline would have burned in the
diesel engine, but it would have destroyed it through friction on
the cylinder walls. The old rust bucket tractor had no labels on
The cylinder walls must be hot enough to promote
combustion, but cool enough that they dont melt or warp. This
is why the cooling system in all engines is so important.
Heat in a material is the sum total of all the kinetic
energy of all the molecules. When we say something is hot
we are actually saying that there are frequent collisions of the
atoms and molecules as they vibrate in a material. When we say that
something is cold we are really saying that the collisions
decreased and the molecules have slowed down.
When we increase the pressure of a gas, we are increasing
the temperature. The molecules are closer together and collide more.
The kinetic energy of the piston is converted to heat energy.
Gasoline Engines and Spark Plugs
Standard atmospheric pressure is 14.7 psi1. If the
compression ratio is 8:1, the pressure in the cylinder is 8 x 14.7
psi or 117.6 psi. At a ration of 10:1 the pressure in the cylinder
is 10 x 14.7 psi or 147 psi. Under that pressure, the temperature
is raised considerably.
engines have no spark plugs to ignite the fuel. In the cylinder,
the pressure is so great the temperature is very high. The pressure
is so great (16:1 or 234 psi) that the temperature becomes high
enough to ignite the fuel without a spark plug.
Compression brings fuel particles together in an
engine, heating them, and giving them the opportunity to burn more
rapidly. The principle of compression is easy to understand, but
is worth mentioning as there is considerable power loss and inefficiency
resulting from decreased compression from bad piston rings or head
Get an old piston that still has rings. How many rings does
this piston have? Compress the rings. Can you see how they would
seal the piston in the cylinder? Is the groove in the piston
a tight fit for the rings? Is there a post in the groove that
keeps the piston ring from turning around in the groove? Why
do you think this is so?
Ask some of the local mechanics why cylinders are honed before
installing new rings. Ask them to demonstrate how to get the
piston and rings in a cylinder. What caution must be exercised?
File a old piston ring. Is it hard or soft? File the piston.
Is it hard or soft?
Pull the spark plug from an engine (like a chainsaw). Put
your finger over the spark plug hole, and pull the starter rope.
Can you feel the compression? If you can get a compression tester,
test the pressure in the cylinder. Some compression gauges give
pressure but dont indicate the ratio. If a cylinder
has 105.8 psi, what is the compression ratio?
Make a campfire with good dry wood. Push the sticks close
together. Pull them apart. Does the fire burn faster if the
wood is closer?
Draw a piston in a cylinder at the bottom of the stroke and
the top of the stroke. Measure the volume in each position.
What is the compression ratio? Now draw a piston in a cylinder
that has a high compression ratio.
Get a hand pump and pump a bicycle tire. Is it hot? Where
does the heat come from?
If you can get a simple compression tester, test the compression
in a snowmachine, outboard, four wheeler, and chainsaw. What
is the difference between them?
Some engines have a head gasket and others do not. Ask a
local person who does mechanics which local machines do and
which dont. How can he tell if the head gasket is damaged?
Where is it most often damaged? Can you use any gasket material
for a head gasket? Why?
Talk to the local power plant operator about the compression
in a diesel engine. How does the fuel get into the engine if
the pressure is so great? Does a diesel engine have a carburetor?
Research how compression is achieved in a jet turbine engine.
What three things are necessary for something to burn?
If a campfire is burning too slowly, what can you do to make
it burn faster besides adding more wood?
Why is compression necessary?
What is the purpose of piston rings?
Draw a cylinder where the fuel is not compressed.
Draw a cylinder with the fuel compressed.
What is the approximate compression ratio of a gasoline engine?
What does psi mean?
What can cause compression loss?
If the compression ratio is 9:1 and atmospheric pressure
is 14.7 psi, how many psi is there in the cylinder when the
piston is at the top of the cylinder?
If the compression ratio is 16:1 in a diesel engine, what
is the pressure in psi?
The compression ratio in a chainsaw is supposed to be 7:1,
but the rings are bad and there is a 15% compression loss. What
is the psi in the cylinder?
The compression in a diesel engine is 17:1. How much pressure
must the fuel pump generate if the fuel is injected when compression
is at its greatest? Greater than ______________.