If not for the piston engine, the majority of adults in modern society would have a difficult time getting to where they need to be on an everyday basis. Anyone who drives or rides in a conventional motor vehicle is the beneficiary of such an engine (electric cars do not have pistons, instead being powered purely by motors.)
Also known as a reciprocating engine, the chief hallmark of these engines that they translate pressure into rotational motion. This rotational motion – in other words, movement about a physical or conceptual axis – can be converted into translational and other forms of motion with ease, as with the tires of your car rolling you and the rest of the vehicle suspended above them down the road.
Various types of piston engines exist, the most familiar of which was just described – the internal combustion engine, which includes gas-powered auto engines and other subtypes. Among the other piston engine varieties are the external combustion engine and the Stirling engine.
You'll learn, among other things, that nuclear power plants have more in common with Old West locomotives than you might think, and in general gain an appreciation for how need and human ingenuity have yet again combined to produce something remarkable and transforming.
The Piston and Cylinder Assembly
For whatever reason, pistons seem to get more attention from everyday folks then the very thing that makes them functional, which is the cylindrical chamber that houses them. Regardless of notoriety, the piston-and-cylinder is at the heart of the single device that has arguably changed the world more than any single machine, and that is the internal combustion engine.
A piston is itself a cylinder with a closed or solid head that moves back and forth within a larger cylindrical case, which is the cylinder on the name is based. The piston may move against fluid pressure or be moved by fluid pressure. In a steam engine, the piston is closed at both ends; a rod passes through the center, but the joint is tightly sealed. In a gasoline engine, it is open at one end to allow for the oscillation (back and forth movement) of other moving parts within the engine.
How a Piston Engine Works
The movements of a piston engine are tightly coordinated and orchestrated. An engine can consist of a single piston, although this is uncommon. Various configurations including multiple piston-and-cylinder combinations are possible, including rows, "vee" shapes, and "zig-zag" combinations of these.
The number of individual pistons aside, all of these engines behave in the same general way, regardless of how much power they can generate or what fuel serves the source of pressure within the cylinder.
The classic four-stroke cycle of a reciprocal engine features four steps, or processes:
Intake: In the first step of the four-stroke cycle, a fuel of some kind is forced into the cylinder through an intake port at the top, which pushes the piston to the bottom of the cylinder.
Compression: The piston is then pushed back to the top, which compresses the fuel and ignites it via a spark plug in most engines. In diesel engines, sufficiently compressing the fuel is enough to ignite it (loosely speaking, in physics, pressure and temperature increase together.)
Ignition: The ignition of the fuel pushes the piston downward once more, thereby generating useful work (a quantity in physics akin to usable energy) to the engine. This "stroke" is alternatively known as the combustion or power step.
Exhaust: The waste chemicals from the combustion of the fuel are emitted through an exhaust port, and the cycle repeats. Despite the seemingly thorough nature of the four strokes, the cycle efficiently repeats thousands of times per minute in standard automobiles – about 50 to 100 times per second.
- You may be fully appreciating for the first time at this point why your engine strictly requires a lubricant, or motor oil; even in a perfectly tuned top-end engine, that's a lot of inevitable friction that must be addressed and dissipated somehow.
The External Combustion Piston Engine
The foregoing describes the world you live in, where automobiles are virtually universal. It wasn't always this way, of course, even in relatively recent human history.
The French military engineer Nicolas-Joseph Cugnot was behind one of the first attempts to get a fluid of some kind to drive a piston inside a cylinder for the purpose of powering a vehicle. (A fluid is a gas or a liquid, such as steam or water, the former being the gaseous form of the latter.) In 1769, Cugnot built a clumsy three-wheeled "steam wagon" that was meant to carry cannons and could manage about 3 miles per hour (5 kilometers per hour) but had a tendency to go out of control and crash.
By the middle of the 19th century, steam power was in such widespread use that the attendant technological gains allowed for vast improvements. The steam locomotive train is a great example of a (now obsolete) external combustion engine: External because coal that was ignited and burned outside the engine (in a furnace) was used to boil large amounts of water, generating steam that was then pumped into the cylinders inside the engine.
The Internal Combustion Piston Engine
In 1826, the American Samuel Morley secured the first patent for a kind of engine that placed the ignition of the fuel and the expansion of the cylinder owing to the resulting pressure boost in the same physical locus. Not until 1858, however, did Morley produce a three-wheeled wagon fitted with an internal combustion engine that ran on "coal gas" and made a journey of 50 miles.
A key advance in the construction of internal combustion engines was the ability to compress the gas before igniting it, making it easier for the fuel to undergo combustion; the pressure and temperature of a gas tend to rise in concert, whereas reducing the volume of a gas (that is, compressing it) increases its pressure.
As soon as the internal combustion engine began to approach a remotely compact size, engineers and dreamers immediately started dreaming of how to use them to power the first flying machines.
By the 1880s, daring inventors were experimenting with, if not flying machines, "hopping machines" that used steam- or gas-powered piston engines, some making it as far as 150 feet, but many others being destroyed in the struggle to advance human observational horizons and travel frontiers.
The Wright brothers, Orville and Wilbur, are famous today, but they were actually somewhat late entrants into the late-1800s version of the "space race" that would unfold over half a century later between the United States and the Soviet Union. In 1899, they did their due diligence and experimented a great deal with gliding machines before trying to equip them with engines, thereby learning more about the underlying aerodynamics.
Since the Wright brothers' first triumphant flight in 1903 in Kitty Hawk, North Carolina, the combustion engine has come a long way. While jet engines are used today in large commercial and other high-powered aircraft, most smaller and private airplanes are still built using propellers and internal combustion engines.
- You may often see reciprocating engines for aircraft called heat engines, but all internal combustion engines are heat engines, with external combustion engines being the other primary category of heat engines.
- U.S. Department of Energy: Internal Combustion Engine Basics
- National Aeronautics and Space Administration: Internal Combustion Engine
- NASA: Pushing the Envelope: A NASA Guide to Engines
- University of Calgary Energy Education: Reciprocating Engine
- SKYbrary: Piston Engine
- University of Calgary Energy Education: Heat Engine
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.