How Internal Combustion Engines Work

Winter is a good time for taking the time to learn new skills or delve deeper into topics that interest you. Well, at least up here in the northlands where below zero temps make a cup of coffee at the workbench inside very appealing. I suppose if you live in Hawai’i the only difference in seasons is which side of the island has better waves. But there’s no snow to shovel, so still extra time for learning new things.

Anyway, something I find very interesting is that many people do not even have the most rudimentary understanding of the workings of items they use daily. I guess my family was a bit unusual in that as children we were encouraged to take things apart and/or build things. Well, actually the adults did that, too. And, since my parents were children of the Depression and WWII rationing, they fixed everything rather than buying new. I actually still have my grandmother’s pressure cooker with the home-built replacement handles.

My eighth grade science class had a unit on internal combustion engines, complete with a full-size clear Lucite engine that could be hand cranked to move the camshaft, pistons, and valves. To this day I think that unit was the most useful of my K-12 experience. Even though I don’t pull my car apart on a regular basis, it is in a general way helpful to understand how it all works. The single cylinder, four-stroke engine of your gasoline lawn mower is the same internal combustion engine as your twelve cylinder Jaguar, without all the clutter and computerization and eleven extra cylinders. Small Engines and Outdoor Power Equipment has the perfect primer on how small engines work, which is exactly how big engines work. Here’s the very clear, excellent description of the 4-stroke cycle.

The 4-Stroke Cycle

How Internal Combustion Engines Work

Small Engines and Outdoor Power Equipment

The Intake Stroke

During the intake stroke, a mixture of air and fuel is introduced to the combustion chamber. The intake valve is open and the piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC).

To understand what happens next, think of the suction produced like a syringe drawing liquid. This happens because as the plunger inside slides toward the handle, it creates a low-pressure area at the tip. A piston performs the identical task. As the piston moves toward BDC, it creates a low-pressure area in the cylinder and draws the air-fuel mixture through the intake valve. The mixture continues to flow, due to inertia, as the piston moves beyond BDC. Once the piston moves a few degrees beyond BDC, the intake valve closes, sealing the air-fuel mixture inside the cylinder.

The Compression Stroke

Compression occurs as the piston travels toward TDC, squeezing the air-fuel mixture to a smaller volume. The air-fuel mixture is compressed for a more efficient burn and to allow more energy to be released faster when the mixture is ignited. Think about the warning label on pressurized spray cans: Keep contents away from fire. This is not only because the contents are flammable, but because pressurization makes them potentially explosive. If an engine has to perform so much work just to bring the air-fuel mixture to the point of combustion, where does it find the ability to perform work? This ability derives from the fact that the energy required for compression—and stored in the flywheel—is still far less than the force produced during combustion. In a typical small engine, compression requires one-fourth the energy produced during combustion. The surplus drives the power stroke.

The Power Stroke

The engine’s intake and exhaust valves are now closed. At approximately 20 degrees before TDC, the spark plug initiates combustion, creating a flame that burns the compressed air-fuel mixture. The hot gases produced by combustion have no way to escape, so they push the piston away from the cylinder head. That motion is transferred through the connecting rod to apply torque to the crankshaft.

The Exhaust Stroke

As the piston reaches BDC during the power stroke, the power stroke is completed. The exhaust valve opens, allowing the piston to evacuate exhaust as it moves, once again, toward TDC. With the chamber cleared of exhaust, the piston reaches TDC. An entire cycle is complete.


For more information about this topic check out Small Engines and Outdoor Power Equipment

Small Engines and Outdoor Power Equipment

A Care & Repair Guide for: Lawn Mowers, Snowblowers & Small Gas-Powered Imple

  • Peter Hunn (editor)

A practical, hands-on guide for repairing and maintaining small gas engines and the things they power: lawnmowers, snowblowers, chain saws, power washers, generators, portable lawn care equipment, and more.

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