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Teacher
Edition Contents
Skill,
Tools, & Craftsmanship
Cutting
& Drying Fish
Sharpening
Nails,
Pegs, & Lashings
Falling
Trees &
Small-Scale
Logging
Guns
Chainsaw
Clutch & Chain
Ice
Pick
Shelters
Wood
Stoves
Wall
Tents
Steambaths
Insulation
& Vapor Barriers
Gas
Lamps & Gas Stoves
Travel
Piloting
A Boat
Boat
Design
Magnetos
& Spark Plugs
Carburetors
Compression
Outboard
Motor Lower Unit
Outboard
Motor Cooling System
Dogsleds
Snowmachine
Tracks
Snowmachine
Clutch
Snowshoes
Winter
Trails
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Activities
- 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?
Most pistons have at least two rings. The rings are split
so they can expand and seal against the cylinder walls. Note that
there is a post that keeps each ring from turning. Note that the
split in one ring is no directly above/below that of another.
This is to keep the gasses from escaping past them.
- 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?
Cylinders are honed before putting new rings in because the
cylinder walls are glazed to the shape of the old rings. Honing
removes the glaze and gives the new rings a chance to fit themselves
to the cylinder wall. Exercise caution putting the piston in the
cylinder or the rings can be broken if they aren’t aligned
correctly.
- File a old piston ring. Is it hard or soft? File the piston.
Is it hard or soft?
The rings are very hard, much harder than the file. The piston
is probably soft aluminum.
- Pull the sparkplug from an engine (like a chainsaw). Put your
finger over the sparkplug 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?
Atmospheric pressure is approximately 14.7 psi. Round it off
to 15 psi. 105.8 divided by 15 is about 7.
- 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?
The fire burns much faster when 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?
All air pumps get hot in operation. The heat comes from compressing
the air. This understanding is preparing students for Boyle’s
and Charles law.
- If you can get a simple compression tester, test the compression
in a snowmachine, outboard, four-wheel ATV, 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 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?
Head gasket material is special because it must withstand heat
and pressure. A blown headgasket will reduce the compression in
a cylinder, and cause the engine to run poorly or stop.
- 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?
Diesel engines don’t have a carburetor. Each cylinder
has a fuel injector that sprays the fuel in at the proper time
under great pressure.
- Research how compression is achieved in a jet turbine engine.
Student Response
- What three things are necessary for something to burn?
Air (oxygen), fuel, heat
- If a campfire is burning too slowly, what can you do to make
it burn faster besides adding more wood?
Move the sticks of wood closer together.
- Why is compression necessary?
To move the fuel droplets close together so they can ignite
each other easily.
- What is the purpose of piston rings?
To seal the pressure in the cylinder. They keep gasses from
escaping.
- Draw a cylinder where the fuel is not compressed.
The fuel drops are far apart.
- Draw a cylinder with the fuel compressed.
The fuel drops are close together.
- What is the approximate compression ratio of a gasoline engine?
7:1 to 11:1
- What does psi mean?
Pounds per square inch
- What can cause compression loss?
Worn piston rings.
Math
- 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?
132.3
- If the compression ratio is 16:1 in a diesel engine, what is
the pressure in psi?
235.2
- 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?
87.5
- 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 .
250 psi
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