# Compression

 Standards A 1, 8a, 15 B 3 D 1, 3 Concepts Forms of energy Surface area
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 place:

• Heat
• Fuel
• Air (specifically oxygen)

If any of these are reduced or removed, the fire will slow down or go out.

## Example

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.

Compressing

Compressed

## 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.

Wood closeWood scattered

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.

## Piston Rings

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.

## 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 tanks.

## Heat Retention

The cylinder walls must be hot enough to promote combustion, but cool enough that they don’t melt or warp. This is why the cooling system in all engines is so important.

## Heat

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.

Diesel Engines

Diesel 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.

Conclusion

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 gasket leakage.

## Activities

1. 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?
2. 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?
3. File a old piston ring. Is it hard or soft? File the piston. Is it hard or soft?
4. 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 don’t indicate the ratio. If a cylinder has 105.8 psi, what is the compression ratio?
5. 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?
6. 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.
7. Get a hand pump and pump a bicycle tire. Is it hot? Where does the heat come from?
8. 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?
9. Some engines have a head gasket and others do not. Ask a local person who does mechanics which local machines do and which don’t. 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?
10. 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? Why?
11. Research how compression is achieved in a jet turbine engine.

## Student Response

1. What three things are necessary for something to burn?
2. If a campfire is burning too slowly, what can you do to make it burn faster besides adding more wood?
3. Why is compression necessary?
4. What is the purpose of piston rings?
5. Draw a cylinder where the fuel is not compressed.
6. Draw a cylinder with the fuel compressed.
7. What is the approximate compression ratio of a gasoline engine?
8. What does psi mean?
9. What can cause compression loss?

## Math

1. 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?
2. If the compression ratio is 16:1 in a diesel engine, what is the pressure in psi?
3. 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?
4. 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 ______________.