Engine

An engine is a machine that converts an energy source into mechanical energy, often called work or movement. Engines are central to everyday machines, including automobiles, buses, and airplanes. Engines can broadly be divided into three types: thermal, electrical, and physical engines. Technically, engines rely on combustion and motors run on electricity, but the terms are often used interchangeably, and as electric vehicles continue to advance, motors are increasingly replacing engines in the transportation industry. Both use energy to produce motion, but the chief difference in practical terms is that an engine contains its own fuel source, while a motor relies on an external source for its energy.

Background

Prehistoric humans used simple machines such as oars and levers to perform work. Later, people learned to harness the power of animals, water, and wind using simple engines and tools such as ropes and pulleys. For example, a capstan is a wheel with handles affixed to a vertical axis that was used on early ships to maximize human effort when hauling ropes and cables. While early engines typically relied on human power, animals were later used for many of these applications. The first known water-powered mill was developed in the first century BCE in Iran and from there later spread throughout the Roman Empire.

Heron, a genius of the Roman Empire, built the first steam engine almost 1,800 years before the Industrial Revolution began. Heron was also known in Alexandria as Mechanikos, or “machine man,” for his awe-inspiring hydraulic and steam-powered machines. Priests hired him to build singing birds, moving statues, and other mechanical wonders that would draw and inspire followers. His inventions included the first coin-operated vending machine, automatons, and a fire engine. Though most of his writing was lost when the Library of Alexandria was destroyed, several volumes survived. One includes diagrams and an explanation of an invention he called the aeolipile, meaning “wind ball.” It is the first known steam engine or reaction steam turbine. The aeolipile involves a boiler and a hollow sphere mounted on a pivot so, as steam escapes spouts on opposite sides and aimed in opposite directions, the sphere spins. Although Heron’s manuscript does not include information about its use, the aeolipile was likely used, as most of his other machines were, to entertain audiences. It would not have been efficient enough to make other uses practical and was designed at least 1,500 years before the technology existed to build a boiler that would hold significantly more pressure. However, the diagram indicates the basic principles of reactive force. His writings also cover the principles of levers, pulleys, wedges, and screws to lift heavy objects.

Translations of Heron’s manuscripts became available in the 1540s, prompting inventors to build upon his devices. English inventor Thomas Savery created the first modern steam engine in 1689, and two decades later Thomas Newcomen improved upon his design. Newcomen’s steam engine was used to power mills and factories and as a water pump. James Watt improved upon the Newcomen steam engine in 1769, ushering in the era of steam power and launching the Industrial Revolution in Great Britain and the United States.

Ancient peoples also developed water-raising systems that relied on the principles of engines, and during classical antiquity and the Middle Ages, armies used siege engines such as huge mechanical crossbows, battering rams, and catapults. These were based on simple mechanisms such as counterweights, momentum, and tension. Many relied on humans for their power; for example, a battering ram uses the physics of momentum to increase the impact of a swinging log against a door or gate. Catapults and crossbows converted stored potential energy in devices such as springs to launch objects such as huge rocks.

The use of fuel as a propellant developed in ancient times in Asia. Black powder, later called gunpowder, was invented in China by about 200 BCE. By about 600 CE, the Chinese had learned to launch fireworks and other explosive charges into the sky, and in 1232 they were launching rockets as weapons against the Mongols. These “fire arrows,” as they were called, carried flammable materials or poison to the enemy. For centuries, variations of these rockets were used in warfare; for example, cannons replaced such siege weapons as catapults and firearms replaced crossbows. The principles of the rocket engine were refined as airplanes and rockets became essential in transportation, military use, and space exploration.

Thermal Engines

Thermal engines convert a heat source into motion. The three types of thermal engines are external combustion (EC), internal combustion (IC), and reaction engines. The most common are internal combustion engines, which power traditional machines such as cars, lawnmowers, and some ships. An IC engine burns fuel, usually with an oxidizer, in a combustion chamber. Combustion generates exhaust that has a greater volume than the fuel and oxidizer. This expansion provides the motion through mechanical parts such as pistons and crankshafts. IC engines vary according to cycle, type of fuel, number and array of the cylinders, total displacement, power, and power-to-weight output.

The fuel and exhaust products are kept separate in steam engines and other types of external combustion engines. Fuel is burned in one chamber, while a heat exchanger or the wall of the engine transfers heat to fluid in the engine. The fluid dilates and contracts or changes phases, such as gaseous or liquid, but unlike combusted fuels in IC engines, the chemical composition of the fluid does not change. Some EC engines, known as open-cycle engines, may exhaust the fluid. Others, called closed-cycle engines, or recirculate the fluid. EC engines are typically bulky and heavy and less fuel-efficient than IC engines, but are still widely used in power plants and in naval vessels. Boiling-water nuclear reactors are external heat engines. Nuclear fission in the reactor core produces heat, which causes water—the coolant—to boil and produce steam. The steam drives a turbine before it cools in a condenser. Once it returns to its liquid form, it continues to circulate through the reactor system. The turbine is connected to a generator, where spinning blades convert the mechanical or kinetic energy to electrical energy.

Reaction engines are also called jet engines. They rely on Newton’s Third Law of Motion, which holds for every action or force in nature there is an equal and opposite reaction. Reaction engines use this force to generate thrust. Ordinary reaction engines, or airbreathing jet engines, use turbines to feed air to the engine and compress it into the combustion chamber. Ramjet engines use speed to feed air to the engine. The concentrated air is mixed with fuel and ignited, generating exhaust and thermally expanding the gas, which powers the turbine and forcefully exits the back of the engine. This exit point is the propelling nozzle, which is narrower than the air intake and boosts the thrust of the jet engine. Rocket engines are non-airbreathing jet engines. Rather than use air from outside the engine, rocket engines rely on oxidizers that are included in the solid fuel the engines require.

Electrical Engines

Electrical engines (or motors) include magnetic, electrostatic, and piezoelectric varieties. Magnetic engines, which are the most common, use the flow of electricity and a magnetic field to create rotation. The outer part of the motor, called the stator, does not move. Magnets are usually embedded into the stator. The stator is separated by an air gap from the rotor inside it. The conductor is commonly wrapped around the rotor, which spins.

Electrostatic and piezoelectric engines are less commonly used than magnetic drives because they have some larger drawbacks. Electrostatic engines use like electric charges, which repel one another. This generates rotation in a rotor. These engines require high voltages. Piezoelectric engines use electricity to generate ultrasonic vibrations. The materials required for this process, (often lead zirconate titanate or lithium niobate) are expensive.

Physical Engines

Physical engines such as clockwork, hydraulic, and pneumatic engines use stored mechanical energy. These types of engines, which include battering rams and catapults, are the earliest developed by humans. However, they are not efficient and cannot operate on large energy reserves. For example, clockwork engines must be wound regularly, typically daily. They rely on elastic energy stored in springs. Hydraulic and pneumatic engines rely on large tubes of compressed fluids.

Fuels

Gasoline and diesel are among the most commonly used engine fuels. Other energy sources include biodiesel, compressed natural gas, electric batteries, ethanol, hydrogen, and propane. Many types of engines can be used with renewable energy such as water, solar, and wind power. For example, people have used wind and water energy for millennia, but in the twenty-first century, the technology to store this energy has made the process more efficient and affordable. Construction processes have enabled countries to build enormous hydroelectric dams that can power cities. Solar panels have become smaller and more affordable. The sun is being used to power vessels and companies are working to develop efficient solar-powered cars.

Bibliography

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