The Four-Stroke and Its History Explained

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The four-stroke spark-ignition internal combustion engine, invented by Nikolaus Otto in 1876, was a big step forward for transportation because it was small, light in weight, and could operate on fast-evaporating gasoline fuel. Its underlying principle is something very simple and familiar to us: Adding heat to a gas raises its pressure. Find a way to let that pressure act against a load and you have the elements of an engine.

Nikolas August Otto. (Public Domain/)

On a clear day, the sun heats the land and the air above it, raising its pressure. The heated body of air expands, becoming wind. We know this is an “engine”—a device capable of doing work—because high winds uproot trees and blew Dorothy and Toto to Oz.

In the steam engine—which had been the dominant form of heat engine during the 19th century—water is heated inside a sealed container by burning fuel under it. Sufficiently heated, water becomes a gas and, being in our example confined inside a sealed container, its pressure rises.

If we make a cylinder, closed on one end and containing a close-fitting but movable piston, we can cause the piston to move quite forcibly by admitting steam to that cylinder. The expansion of the steam drives the piston, which can be connected to a load.

Because rotary motion is present in many kinds of machinery, a way was found to convert the back-and-forth motion of the piston to rotary motion. The simplest way is to link the piston to a rotating crank. One end of this link, or “connecting rod,” is hinged to the piston, and the other end is bored to fit over the “handle” of the crank (called the “crankpin”). One end of this “connecting rod” moves in a straight line with the piston, while its other end moves in a circle with the crank. A flywheel is usually added to make these motions smooth and continuous.

To keep this motion going we need some simple “housekeeping” functions: valves—one that will admit steam to the cylinder when the piston is nearest the cylinder’s closed end, and another that will release the expanded steam once the piston has been pushed to the far end of the cylinder. Mechanism is provided to open and close these valves in such a way as to repeat this power-producing cycle indefinitely.

The “choo-choo” sound of a steam locomotive is the intermittent outrush of spent steam from the working cylinder.

When inventors of the mid-to-late 19th century looked at a steam locomotive, its largest elements were the cylindrical boiler, many feet in length, and the firebox at one end. The parts converting expanding hot, high-pressure steam into mechanical power—the steam cylinders at the front of the engine, with their long connecting rods extending to drive the road wheels through crank action—were a small fraction of the whole.

Inventors asked themselves, “What if we burn the fuel inside the cylinders themselves? Then we’d have no need for the boiler or firebox.” The result would be smaller and lighter, perhaps enough so to power road vehicles.

Throughout the 19th century steam engines burned solid fuels—the coal that powered England’s Industrial Revolution, the wood that drove river steamers in the still-young United States—could not be burned inside cylinders.

But there was a brand-new fuel that could be: city-illuminating gas. The reduction of iron from its oxide ores required carbon, but at first, coal couldn’t be used because it contained a lot besides plain old carbon. Coal was therefore roasted in large retorts in the absence of air to become coke, which worked well in iron smelting. Roasting coal evolved gases and liquids that were also combustible. Lights burning this coal gas were much brighter and cleaner than oil lamps, so major cities were quickly plumbed to deliver this gas to streetlights, households, and businesses.

Inventors saw that this gaseous fuel could be ignited by spark or flame, and after 1860 many proprietary gas engines were created. They were feeble because the inventors, their minds prisoner to steam ideas, were trying to make engines that would produce a power stroke on every revolution of the crank. This forced them to try to fit too many functions into too little time, resulting in little power.

Nikolaus Otto, working in Germany, designed an engine which provided a separate piston stroke (a move from one end of the cylinder to the other) for each of the necessary four functions. As in the case of the steam engine, the stored kinetic energy of a flywheel kept operation smooth and continuous.

1) Intake: A valve in the head (closed end) of the cylinder opens and the piston moves away from the head. A combustible mixture of air and fuel rushes in from a mixing device (carburetor).

Intake Stroke. (Illustration by Robert Martin and Ralph Hermens/)

2) Compression: The intake valve closes and the piston is driven back toward the head, compressing the gas-air mixture in the cylinder.

Compression Stroke. (Illustration by Robert Martin and Ralph Hermens/)

3) Power: The compressed mixture is ignited by some means while the piston is close to the head. Mixture combustion releases heat, greatly raising the pressure of the combustion gas. That pressure, transmitted through the piston to the connecting rod, spins the crankshaft.

Power Stroke. (Illustration by Robert Martin and Ralph Hermens/)

4) Exhaust: As the piston approaches its far point, an exhaust valve in the head opens, releasing the expanded combustion gas into an exhaust pipe. As the piston rises toward the head, it pushes residual combustion gas out of the cylinder.

Exhaust Stroke. (Illustration by Robert Martin and Ralph Hermens/)

The cycle repeats.

Because Otto provided a separate and dedicated piston stroke for each of these functions, they were highly effective.

Unfortunately, Otto had chosen the degree of compression (No. 2 above) to be quite high, so that when his experimental engine fired for the first time, it did so with great violence. He was shaken by this. Fortunately, his wife Anna was present. She assured him he would feel better after dinner and a night’s sleep, and would surely solve any problems the following morning.

Mrs. Otto was proved correct, and the Otto Four-Stroke Cycle continues to be the dominant prime mover for motorcycles, cars, and light trucks to this day—147 years later.

Each of these four functions deserves a separate explanation to follow soon.

Otto’s 1876 four-stroke engine. (Wikimedia Commons/)

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[Bianco2564]

That's a particularly poor illustration of the 4 stroke cycle,  valve opening and closing not shown, no form of ignition and it implies only one valve sharing the inlet and exhaust ports.