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 article, 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!
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 running.
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 ignition is powered by a magneto connected to a spark plug.
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 bar.
The gas tank holds about 1 pint (0.5 liters) of gasoline mixed with two-stroke engine oil.
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 same displacement.
Under the Covers
Here is the right side cover plate for the saw:
When you remove the cover plate, you expose the centrifugal clutch:
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, shown here:
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 attaches.
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
The centrifugal 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:
An outer drum that turns freely - This drum includes a sprocket that engages the chain. When the drum turns, the chain turns.
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
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.
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 electromagnet. 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 coil.
This magneto consists of five parts:
An armature (In this magneto, the armature is shaped like a capital "U." The two ends of the U point toward the flywheel.)
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 flywheel
You can see the two magnets in the following photo:
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:
It has to work when you are trying to start the engine cold.
It has to work when the engine is idling.
It has to work when the engine is wide open.
No 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.
You can see the carb for the chain saw in the following two photos:
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 throttle.
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 venturi (watch 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 two photos:
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 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 seen here:
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 inches high.
The connecting rod connects the piston to the crankshaft. A set of roller bearings keeps everything smooth at the crankshaft end.
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 maintenance.
For more information on chain saws and related topics, check out the links on the next page.