How a Chain Saw Engine Works

Ever wonder how a chainsaw's two-cycle engine operates without valves, a cam, or lifters? Read on and you'll become knowledgeable about your pro saw's engine. Have a look at the image below. We'll be referring to these parts by name. And yes, we know it is a "napkin-quality" image. (What can we say? In school, we took shop classes -- not art.)

Two-Cycle Basics

Lets start our lesson with an explanation of a "cycle" or "stroke." These words refer to the same thing: a single trip a piston makes up or down a cylinder. It is as simple as that. Now, it takes two strokes to rotate the crankshaft one time. The animation below shows when the piston travels down the cylinder, the crankshaft rotates a half of a turn and when the piston travels back up, it makes another half-rotation. But this is not what determines whether an engine is a two-cycle or four-cycle. This is determined by power strokes.

Power Strokes

A power stroke is the trip a piston makes when it is being pushed by a combustion event. This is what gives every engine its power. So, a two-cycle engine has a power stroke every two cycles. A four-cycle engine has a power stroke every four cycles. This means, a two-cycle engine has a power stroke every time the crankshaft makes a full rotation and a four-cycle engine has a power stroke every other crankshaft rotation. Now that you know this, it's not hard to figure out why two-cycle engines perform so well. They have twice as many power strokes as a four-cycle engine.

A Short Video on How a Two-Cycle Engine Works

One of the first things the narrator of the video says is that two-cycle engines are simple and low-cost engines. While this is generally true, the primary reason professional chain saw engines are two-cycle is because of power-to-weight performance.

How Air Flows Through a Two-Cycle Engine

If want a deeper understanding of how a two-cycle engine works or just want to punish yourself with more technical details, read on. As you have already learned from the video above, a two-cycle engine, or any engine for that matter, is really just a type of air pump. If you understand how air flows through it, you have a good understanding of how it works.

OK, once the air filter cleans some air, it flows to the carburetor. Inside the carburetor, fuel enters the air stream. The air (and fuel mixture) then travels to the intake port on the cylinder. When the port opens, the air/fuel charge is drawn into the crankcase. All of this air flow occurs because of a vacuum inside the engine's crankcase. (You'll learn more about how the vacuum got there in a minute.) Once the crankcase is filled with air (and fuel), the vacuum is neutralized.

When the piston reaches the top of its stroke, it reverses direction and begins to travel back down the cylinder. Part way down the cylinder, its skirt (the bottom part) closes the intake port. As the piston continues to retreat, it pressurizes the air/fuel mixture that was just drawn into the case. Now pressurized, the air/fuel mixture starts to flow through the transfer ports and into the empty combustion chamber.

When the piston reaches the bottom of its stroke, it turns around again and starts to travel back up the cylinder. Along the way, it closes the transfer ports, trapping the air/fuel mixture in the combustion chamber. This trapped air continues to be compressed as the piston travels up the cylinder. (As the top of the piston compresses the air/fuel mixture, underneath the piston, the volume of space in the crankcase area is expanding. This expansion is what creates the vacuum that draws the next air/fuel charge into the crankcase area.)

Now, when the piston gets almost to the top of its stroke, the spark plug ignites the air/fuel mixture. The burning mixture creates heat, which causes the pressure in the combustion chamber to increase dramatically. This pressure pushes the piston back down the cylinder, producing power. A rod connected to the piston rotates the crankshaft.

The story is not quite over, because part way down the cylinder, the piston opens the exhaust port. This allows the hot air and burned fuel to escape into the muffler. Then the process starts all over again.

If your head doesn't hurt too much from learning how a two-cycle engine works, the following tidbit will amaze you: A saw running at 13,500 rpm will do the above operation 225 times every second. Wow!

Got questions or comments? Call or stop in.