What is a chainsaw?
The clue is in the name! A chainsaw has two main parts: a saw blade built into a chain, wrapped around a long metal guide bar, and a small, one-cylinder gasoline (petrol) engine (sometimes an electric motor powered by a cord or battery pack). The chain is a bit like a bicycle chain, running around sprockets (gear wheels designed to turn a chain) only with about 30 or so sharp teeth (made from a hardened steel alloy) mounted around it at intervals. Inside the engine, as the piston moves in and out of the cylinder, it pushes a connecting rod that turns a crankshaft. The crankshaft turns gears that are connected (through a centrifugal clutch, explained below) to one of the sprockets on which the chain is mounted—and the chain spins around.
What happens inside a chainsaw?
Yes, crudely speaking, that's what a chainsaw does: in scientific terms, it converts the chemical energy locked in gasoline into mechanical energy you can use to "do work," turning a tree into logs, sawdust, noise, and heat. Here's a very simplified explanation:
The fuel you put in a chainsaw's gas tank contains, in chemical form, all the energy you'll consume cutting down and chopping up logs. To keep it nice and light, a typical chainsaw tank holds just 0.5 liters (1.1 US liquid pints) of gas (a car's gas tank holds maybe 45–55 liters or 12–15 US liquid gallons, which is roughly 100 times more).
The fuel feeds through a carburetor to mix it with air.
The air-fuel mixture passes into a cylinder, which works much like the ones in a car engine but with only a simple push-pull (two-stroke) action instead of the more complex (four-stroke) cycle used in a car. Inside the cylinder, the air-fuel mix is ignited by a spark (sparking) plug, burns, releases its energy, and pushes a piston back and forth. The piston in a chainsaw engine has a bore (diameter) of about 45mm (1.75 in) and a stroke (traveling distance) of about 33mm (1.3 inches)—so it's less than half the size of a typical car engine piston and moves only half as far.
A connecting rod and crank convert the back and forth motion of the piston to rotary motion.
A drive shaft takes power to the centrifugal clutch.
A chainsaw engine runs all the time, but you don't want the chain spinning unless you're actually cutting wood, like using it as a brush cutter machine: that's dangerous and it wastes energy. The clutch solves this problem. As explained in more detail below, the centrifugal clutch connects the engine and the chain when the engine speed is fast (when the operator pulls on the throttle) and stops the chain from spinning when the engine speed is low (when the chainsaw is just idling).
