The four-stroke cycle engine develops one power stroke for every four movements of the piston (two up and two down). This type might seem to be a waste of motion as well as parts, for it requires many more parts. However, it has many advantages, particularly in larger engines where compactness is not as significant a factor.

The four-stroke engine does not have a reed, and the air-fuel mixture does not pass through the crankcase. Instead, there are two valves, as in l-l, one that opens and closes a passage from the carburetor, another that opens and closes a passage to the exhaust system. The valves are operated by the camshaft, a shaft with teardrop-shaped lobes that push the valves

open, and at appropriate times, allow springs to close them. The camshaft has a gear at one end, which meshes with a gear on the crankshaft. The gear on the camshaft has twice as many teeth as the crankshaft gear, so that for every complete revolution of the crankshaft, the camshaft turns 180 degrees. This means that each valve opens and closes just once during

two revolutions of the crankshaft, which is exactly what’s needed for a four-stroke cycle. The valves in the typical four-stroke lawn mower or snow blower engine are located in the block. This is an antiquated automotive design, but it’s good enough for mowers and blowers. There are a few four-strokes with valves in the cylinder head, a popular automotive design, shown in l-4. In this case the camshaft lobes push on a long rod, called a pushrod, which pivots a see-saw-like part called a rocker arm.

The operation of any four-stroke cycle engine, regardless of valve location, is the same. Let’s look at 1-5, with the piston going down in what is called the intake stroke. The dropping of the piston creates a void in the cylinder, and the cam-shaft opens the intake valve. Air rushes through the carburetor to fill that void, pulling fuel droplets with it, into the cylinder. When the piston is near the bottom of the cylinder, the camshaft closes the intake valve. The piston begins rising, and when it reaches the top, it has compressed the air-fuel mixture into the little recess above the piston, the combustion chamber. This upward movement, shown in 1-6, is called the compression stroke.

In l-7 the spark plug ignites the mixture, which explodes, forcing the piston down in what is called the power stroke. When the piston rises, as in 1-8, the camshaft opens the exhaust valve and the piston pushes the exhaust into the exhaust system.