Two-Cycle Engines

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 above. 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.)

Lets start our lesson with an understanding 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. 

What is a power stroke anyway? A power stroke is the trip a piston makes when it is being pushed by a combustion event. This is what gives an 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.

How A Chainsaw Engine Works

The best way to understand how an engine works is to follow the air (and fuel) through the engine. With the help of the animation below, let's begin the journey at the air filter. Once the filter cleans the air, it flows to the carburetor. Inside the carburetor, fuel enters the air stream. It then travels to the intake port on the cylinder. When the port opens, the air/fuel charge is drawn into the crankcase. 

As the piston travels down the cylinder, its skirt closes the intake port. After the port is closed, the piston continues its retreat, pressurizing the air/fuel mixture in the case. This pressure causes the air/fuel mixture to flow through the transfer ports and into the combustion chamber. 

When the piston reaches the bottom of the stroke, it turns around and travels back up the cylinder. Along the way,  it closes the transfer ports. Once these ports are closed, the air fuel mixture is trapped in the combustion chamber. The piston continues to travel up the cylinder. This compresses the air/fuel mixture further. When the piston gets almost to the top, the spark plug ignites the air/fuel mixture. As the mixture burns, the heat causes the pressure to increase dramatically. 

The pressure forces the piston down the cylinder. As the piston travels down the cylinder, it rotates the crankshaft. Part way down the cylinder, the piston opens the exhaust port. This allows the hot air to escape into the muffler. 

Now you know how a two-cycle engine works.

Here is another tid-bit of knowledge you can apply to what you just learned:  A saw running at 13,500 rpm would do the above operation 225 times every second. Wow!

Got questions or comments? Call or stop in.