Camshaft Notes

The camshaft is probably the single most important component when it comes to determining the power characteristics of your engine. No other single component has as much influence on how your engine performs. The camshaft controls the opening and closing of the intake and exhaust valves in the cylinder head, either indirectly through push rods, rocker arms, or followers, or sometimes directly
your engine is just a big air pump. The more air you can pump into and out of your engine, the more potential you have to make power. The camshaft not only opens and closes your valves to let air in and out, but determines when and for how long the valves remain open. With this in mind, let’s talk about what happens as the engine spins. What follows next is a basic explanation of four-cycle engine operation, described in relation to the four valve events. For each rotation of the cam, we have four valve events. The crankshaft rotates twice for each revolution of the camshaft, so four valve events happen for every two revolutions of the engine.

Event 1 - Intake valves opening

The camshaft opens the intake valve, and the piston moves down the cylinder. As the pressure drops in the cylinder, the air starts moving past the intake valve to fill the cylinder. This period of the engine cycle is known as the intake stroke.

Event 2 - Intake valves closing

At some point, usually after the piston reaches the bottom of the intake stroke, the intake valve closes. The piston moves up the cylinder, beginning the compression stroke and compressing the fuel/air mixture within. At some point, usually before the piston reaches the top of the compression stroke, the spark plug ignites the mixture, causing it to burn and expand rapidly. The crankshaft has rotated once at this point.

Event 3 - Exhaust valves opening

After the piston reaches the top of the compression stroke, pressure from the burning, expanding mixture pushes the piston back down the cylinder. The exhaust valve starts to open, usually before the piston is all the way down, allowing some of the burnt gasses to exit the cylinder. This is commonly referred to as the blowdown phase. The piston begins to move back up, forcing the rest of the hot gas out of the cylinder.

Event 4 - Exhaust valves closing

As the piston moves back up the cylinder, the exhaust valve remains open, usually until slightly after the piston reaches the top of the cylinder. We refer to this as the exhaust stroke. As the piston reaches the top again, the intake valve begins to open again, often before the exhaust valve is fully closed, and the whole cycle begins anew. The period when both valves are open simultaneously is referred to as “overlap.” The crankshaft has now gone around twice.

In a nutshell, here’s how it all happens.

Overlap
Intake stroke
Compression stroke
Blowdown
Exhaust stroke
Repeat

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