Whether you are a
homeowner cleaning up a few trees and limbs in the yard, a
farmer cutting firewood and keeping the pasture clear, or a
firefighter cutting a firebreak for a forest
fire, you know how handy a chain saw can be!
Chain saws are also great when you are interested in
getting up close and personal with gasoline power. If you want
to see a basic two-stroke
engine in its simplest application, then a chain saw is
the best place to start! In this edition of HowStuffWorks,
we'll learn about chain saws and two-stroke engines by
completely dismantling a typical 16-inch chain saw. You will
also learn how magnetos, centrifugal clutches and carbs work
in the process!
Here is a basic 16-inch chain
This chain saw has the following features:
One of the most
amazing things about the engine is the horsepower
that it develops for its weight. It approaches 1 horsepower
per pound of engine weight! This is possible because the
engine is air-cooled (eliminating the weight of the
radiator, water pump and water) and two-stroke. A two-stroke
engine, as explained in How
Two-stroke Engines Work, has no valves or cams and
generates power on every rotation of the crankshaft (twice as
often as a four-stroke
engine). This feature of the two-stroke engine means that
it can generate twice the power of a four-stroke engine of the
- It has a 16-inch (40.6-cm) cutting bar. The chain runs
in a groove around this bar, and on the chain are the
- It has an air-cooled two-stroke
- The chain is driven by a centrifugal clutch.
When the engine is idling, the clutch is disengaged. When
the engine speeds up, the centrifugal plates in the clutch
spin outward to engage the clutch, and the chain begins
- The engine displaces 3 cubic inches (49 cc).
- The engine, consisting of the cylinder, piston,
connecting rod, crankshaft, carburetor and magneto, weighs
just 4 pounds (1.8 kg).
- The engine develops approximately 3 horsepower.
- The engine is started with a pull starter.
- The ignition is powered by a magneto connected to a spark
- Gas/oil/air are mixed with a carburetor
and drawn into the engine by crankcase vacuum.
- Exhaust flows out through a simple spark arrestor.
- There is a small oil pump and a separate chain-oil
reservoir to oil the chain and keep it from binding on the
- The gas tank holds about 1 pint (0.5 liters) of gasoline
mixed with two-stroke engine oil.
In the following photo of the same saw, you can see the
pull-cord used to start the engine, the air intake for both
the cooling blower and the carb, the fuel tank cap, the kill
switch (red) and the throttle trigger, as well as the choke,
throttle lock and carb adjusting screws.
The following photo shows the saw from another angle:
Under the Covers
Here is the right side
cover plate for the saw:
When you remove the cover plate, you expose the centrifugal
You can also see the spark arrestor and the spark plug:
Also visible is one end of the gas tank.
On the other side of the engine you find the pull-cord and
the air intakes:
In the above photo, the air intake to the left of the
pull-cord handle supplies cooling air to the engine, and the
two smaller intakes to the right of the pull-cord handle
supply air for the carburetor. Removing the plate covering the
carb reveals a paper air filter, and then the choke plate and
the carb's venturi.
There's nothing to the choke -- it
is just a plate that obstructs air flow.
This short video of the
choke in operation (0.5 MB) shows just how simple the
choke really is. The section later in the article explaining
the carb also explains the choke in detail.
Removing the pull-cord assembly reveals a self-contained
unit that holds the cord's spool and spring:
Its center shaft fits into two spring-loaded pawls that
engage when you pull the cord to start the engine, but
otherwise spin freely. The pawls are mounted on the flywheel,
In the above photo, the flywheel is the aluminum
disc on the left with the blades. These blades suck in air and
force it back and around the fins on the combustion chamber to
cool it. The flywheel also contains the magnets that power the
magneto -- the white block on the lower right side of the
flywheel. The hole above the magneto is where the carb
At this point, you can see that we have stripped the saw
down to the engine and a top, bottom and front cover plate.
Let's look a little more closely at the clutch, magneto and
carb before moving on to the engine itself.
The Centrifugal Clutch
clutch is the link between the engine and the chain. The
clutch's purpose is to disengage when the engine is idling so
that the chain does not move. When the engine speeds up
(because the operator has pulled the throttle trigger to begin
cutting), the clutch engages so that the chain can cut. You
can see the clutch in the following photo:
The clutch consists of three parts:
The center shaft and weights spin
as one. If they are spinning slowly enough, the weights are
held against the shaft by the spring. If the engine spins fast
enough, however, the centrifugal force on the weights
overcomes the force being applied by the spring, and the
weights are slung outward. They come in contact with the
inside of the drum and the drum starts to spin. The drum,
weights and center shaft become a single spinning unit because
of the friction between the weights and the drum. Once the
drum starts turning, so does the chain.
- An outer drum that turns freely - This drum includes a
sprocket that engages the chain. When the drum turns, the
- A center shaft attached directly to the engine's
crankshaft - If the engine is turning, so is the shaft.
- A pair of cylindrical clutch weights attached to the
center shaft, along with a spring that keeps them retracted
against the shaft
There are several advantages to a centrifugal clutch:
- It is automatic. (In a car with a manual transmission,
you need a clutch pedal. A centrifugal clutch doesn't.)
- It slips automatically to avoid stalling the engine. (In
a car, the driver must slip the clutch.)
- Once the engine is spinning fast enough, there is no
slip in the clutch.
- It lasts forever.
The magneto creates the
electrical charge needed to fire the spark plug, and the spark
plug creates the spark inside the combustion chamber to ignite
the gasoline (see How
Two-stroke Engines Work for details). The job of the
magneto is to create a surge of high voltage (between
10,000 and 20,000 volts) at just the right moment during each
revolution of the crankshaft. This voltage arcs across the tip
of the spark plug to ignite the gasoline.
The magneto is the white block in the following photo:
The idea behind a magneto is simple. It is basically an electrical
generator that has been tuned to create a periodic
high-voltage pulse rather than continuous current.
An electrical generator (or a magneto) is the reverse of an
In an electromagnet, there is a coil of wire around an iron
bar (the armature). When you apply current to the
electromagnet's coil (with a battery,
for instance), the coil creates a magnetic field in the
armature. In a generator, you reverse the process: You move a
magnet past the armature to create electric current in the
This magneto consists of five parts:
You can see the two magnets in the
- An armature (In this magneto, the armature is shaped
like a capital "U." The two ends of the U point toward the
- A primary coil of perhaps 200 turns of thick wire
wrapped around one leg of the U
- A secondary coil of perhaps 20,000 turns of very thin
wire wrapped around the primary coil
- A simple electronic control unit that commonly goes by
the name "electronic ignition"
- A pair of strong permanent magnets embedded in the
When the magnets fly past the U-shaped armature, they
induce a magnetic field in the armature. This field induces a
small amount of current in the primary and secondary coil.
What we need, however, is extremely high voltage. Therefore,
as the magnetic field in the armature reaches its maximum, a
switch in the electronic control unit opens. This switch
breaks the flow of current through the primary coil and causes
a voltage spike (of perhaps 200 volts). The secondary coil,
having 100 times more turns than the primary coil, amplifies
this voltage to approximately 20,000 volts, and this voltage
feeds to the spark plug.
The carburetor on a chain saw
is pretty simple, as carbs go, but it's not entirely
uncomplicated. The job of the carb is to accurately meter
extremely tiny quantities of fuel and mix it with the air
entering the engine so that the engine runs properly.
If there is not enough fuel mixed with the air, the engine
"runs lean" and either will not run or potentially gets
damaged (in a two-stroke engine, the fuel also supplies the
engine's lubricant). If there is too much fuel mixed with the
air, the engine "runs rich" and either will not run (it
floods), runs with a lot of smoke, runs poorly (bogs down,
stalls easily) or, at the very least, wastes fuel. The carb is
in charge of getting the mixture just right.
The carb on a chain saw is simpler than most carbs because
it really has only three situations that it has to cover:
one operating a chain saw is really interested in any
gradations between idle and full throttle, so incremental
performance between these two extremes is not very important.
In a car, the many gradations are important, and this is why a
car's carb is a lot more complex.
- It has to work when you are trying to start the engine
- It has to work when the engine is idling.
- It has to work when the engine is wide open.
You can see the carb for the chain saw in the following two
Photo 1: This is the side that connects to
Photo 2: This is the side that receives the
outside air through the air
video (5.1 MB) takes you on a quick tour of the carb.
Here are the parts of a carb:
the video for a better look at these parts.
- A carburetor is essentially a tube.
- There is an adjustable plate across the tube called the
throttle plate, which controls how much air can flow
through the tube. You can see this circular brass plate in
photo 1 above.
- At some point in the tube there is a narrowing, called
the venturi, and in this narrowing a vacuum is
created. The venturi is visible in photo 2.
- In this narrowing there is a hole, called a jet,
that lets the vacuum draw in fuel. You can see the jet on
the left side of the venturi in photo 2.
The carb is operating "normally" at full throttle. In this
case, the throttle plate is parallel to the length of the
tube, allowing maximum air to flow through the carb. The air
flow creates a nice vacuum in the venturi, and this vacuum
draws in a metered amount of fuel through the jet. You can see
a pair of screws on the top-right of the carb in photo 1. One
of these screws (labeled "Hi" on the case of the chain saw)
controls how much fuel flows into the venturi at full
When the engine is idling, the throttle plate is nearly
closed (the position of the throttle plate in the photos is
the idle position). There is not really enough air flowing
through the venturi to create a vacuum. However, on the back
side of the throttle plate there is a lot of vacuum (because
the throttle plate is restricting the airflow). If a tiny hole
is drilled into the side of the carb's tube, fuel can be drawn
into the tube by the throttle vacuum. This tiny hole is called
the idle jet. The other screw of the pair seen in photo
1 is labeled "Lo" and controls the amount of fuel that flows
through the idle jet.
When the engine is cold and you try to start it with the
pull-cord, the engine is running at an extremely low rpm. It
is also cold, so it needs a very rich mixture to start. This
is where the choke plate comes in.
When activated, the choke plate completely covers the
this video of the choke plate to see it in action). If the
throttle is wide open and the venturi is covered, the engine's
vacuum draws a lot of fuel through the main jet and the idle
jet. Usually, this very rich mixture will allow the engine to
fire once or twice, or to run very slowly. If you then open
the choke plate, the engine will start running normally.
With all of the cover plates
removed, what you have is the naked engine, as seen in these
Top view of the engine
Bottom view of the
What you can see in these photos is that a chain saw is
really just an engine with some cover plates bolted onto it, a
handle to hold it, and a chain and bar to cut with. The engine
is the main structural member that everything bolts onto!
In the photos above you can see all the major components:
- The engine itself, visible mainly as the aluminum
combustion chamber covered with cooling fins
- The flywheel, which also contains the blower to cool the
engine and the magnets for the magneto
- The magneto
- The air intake (which connects to the carb)
- The spark plug
- The spark arrestor/muffler
- The centrifugal clutch
- A small oil pump that lubricates the chain
These three videos are helpful in understanding the engine:
You can remove the flywheel,
magneto, spark plug, muffler and centrifugal clutch and what
you are left with is the combustion chamber, piston,
connecting rod and crankshaft. If you remove the crankshaft
cover, you expose the crankshaft, as seen in this photo:
The two ends of the crankshaft run on roller bearings, as
If you pull on the crankshaft, the piston comes with it:
This piston is 1.5 inches (38.1 mm) in diameter and 1.5
The connecting rod connects the piston to the crankshaft. A
set of roller bearings keeps everything smooth at the
So there you have it -- a complete chain saw! It is a
miracle of modern manufacturing technology that you can buy
one of these, containing several hundred parts, for about
$100. And it will run really well for years with little or no
For more information on chain saws and related topics,
check out the links on the next page!
Lots More Information!
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Chain Saw Manufacturers