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The Otto engine was a large stationary single-cylinder internal combustion four-stroke engine designed by Nicolaus Otto. It was a low-RPM machine, and only fired occasionally since it was a hit and miss engine.
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Open-frame mechanical design
Like many engines of the early 1900's, the Otto engine used an open-frame design with all operating cams and levers mounted on the outside of the cylinder head. While this design permits very easy access and observation of all moving components, it also subjects the mechanisms to any dust and moisture in the environment. Consequently this type of machine needs regular cleaning and greasing by an engine operator to keep running correctly, or the levers and gears will eventually seize up from the lack of oiling and dust collecting in the oil to form a sticky and abrasive scum.
By contrast, modern internal combustion engines operate with all mechanical components in a closed pressurized oil bath, protected from the elements by an outer casing surrounding all mechanical systems. Modern engines can run for months between maintenance, which primarily consists of just changing the engine oil and the oil filter. However, due to the enclosed and protected design, it is difficult to appreciate the technical details of operation or the synchronized rhythmic functions of modern engines.
Spark plug firing
The Otto is one of the first engines to use a spark plug, which is a device that produces a small electric spark to ignite the fuel charge. Otto engines were equipped with a number of different mechanism designs to trigger sparking. This usually consisted of a pivoting trip-arm that briefly grabs a power switch lever and gives it a quick pull. The switch lever is then released and allowed to snap back to its original position in preparation for the next cycle. This system requires an external electric battery, ignition coil, and electric charging system similar to modern automobile engines.
Later Otto engines employed a small magneto directly on the engine. Rather than tripping a switch, the spark plug firing arm applies a quick rotation to the magneto rotor, which then snaps back under spring tension. This quick rotation of the magneto coil produces a very brief current flow that fires the spark plug and ignites the fuel. This design has the advantage of requiring no external battery, and is how modern portable gas engines operate, incorporating the magnet portion of the magneto into the flywheel. Modern portable engines excite the magneto with every flywheel rotation, and so use a cam-operated electric switch to prevent plug firing except for the power stroke of the engine (see wasted spark).
Engine speed regulation
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This is a demonstration of how the speed regulation works in the Otto engine. The spinning balls are the centrifugal governor, and as the machine runs slower the small wheel moves to the left, inserting the rod into the nearby roller and pushing it up to trigger the intake of fuel to fire the engine for one revolution.
If the machine is under load and still running too slowly, the cam continues to stay inserted and makes the engine fire repeatedly for each ignition cycle. When the engine speed increases, the governor pulls the small wheel to the right and the machine coasts without injecting any fuel, though the spark plug continues to fire with no fuel in the cylinder.
Cylinder Cooling
Otto engines use a flowing water jacket around the cylinder wall, similar to modern engine cooling systems. These stationary Otto engines on display at the Western Minnesota Steam Threshers Reunion all share a single large heat radiator outside the building. This centralized distant heat dissipation system also helps to keep the engine building cool.