Single Overhead Cam Internal Combustion Engine

SPECIAL NOTE: We have two very similar internal combustion engine models: this single overhead cam model, and a dual overhead cam model that is somewhat more expensive. Which should you get? If you want the model that most clearly and simply illustrates how these engines work, get the version on this page. It allows you to see all three pistons operating simultaneously from a single view, and is easier to assemble. If you are specifically interested in the dual overhead cam design (which is a higher-performance racing style of engine), then get the dual overhead cam model.

Here you can see the model in action being driven by its build-in “starter motor” (actually a small electric motor positioned where the starter motor would be, driving the flywheel).

Looking at the side of the engine you can follow the 4-stroke cycle simultaneously for all three cylinders. Focusing on one cylinder at a time, and starting with the piston at top dead center with both valves closed, the strokes are as follows:

1. Intake stroke: the blue intake value is open, and as the piston moves down a mixture of fuel and air is sucked into the chamber.

2. As the piston starts to move back up both valves are closed and the fuel-air mixture is compressed.

3. Just before the piston reaches top dead center again, the spark plug fires, igniting the mixture. This begins the power or combustion stroke, when energy is delivered to the crank shaft by the gas pressure pushing down on the piston.

4. When the piston reaches the bottom the exhaust valve opens and the piston returns to the top, completing the exhaust stroke, and leaving the engine where it was at the beginning of step one.

The engine can run on its own, or you can disengage the starter motor and crank the engine by hand (a three-position switch lets you select between off, running by itself, or supplying power to the spark plugs only). In this video I try to crank the engine as fast as I can, and reach maybe 500-600rpm (revolutions per minute), which is at the low end of the idle speed of a typical real engine. When these machines are operating at a typical speed of 2000-4000rpm, the pistons, valves, and spark plugs are operating at an almost unimaginable speed. (At 3000rpm each spark plug is firing 50 times per second, for a total of 150 explosions per second in a 3-cylinder engine!)

Notice that the crankshaft makes two complete revolutions during a single 4-stroke sequence. That’s why the timing gear at the top of the engine, which drives the distributor and operates the valves, is twice as big as the timing gear on the crank shaft. There is a 2-1 gear reduction between the crank shaft and the cam shafts.

Shown here is a close up of the distributor (with the green camshaft barely visible behind it). As the big gear rotates, the brass wiper arm of the distributor makes contact with each of the three contact points arranged around the circle. At each point of contact an electric circuit is completed and one of the spark plugs fires (or, in the case of this model, one of the LEDs lights up briefly). Also visible on the gear are engraved marks that indicate the positions of the high points of each of the six cams on the camshaft. (Because this kit comes with a single-piece camshaft, you only need to align the first of the marks and the rest are automatically correct.)