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internal-combustion engine

internal-combustion engine, one in which combustion of the fuel takes place in a confined space, producing expanding gases that are used directly to provide mechanical power. Such engines are classified as reciprocating or rotary, spark ignition or compression ignition, and two-stroke or four-stroke; the most familiar combination, used from automobiles to lawn mowers, is the reciprocating, spark-ignited, four-stroke gasoline engine. Other types of internal-combustion engines include the reaction engine (see jet propulsion, rocket), and the gas turbine. Engines are rated by their maximum horsepower, which is usually reached a little below the speed at which undue mechanical stresses are developed.

Reciprocating Engines

The most common internal-combustion engine is the piston-type gasoline engine used in most automobiles. The confined space in which combustion occurs is called a cylinder. The cylinders are now usually arranged in one of four ways: a single row with the centerlines of the cylinders vertical (in-line engine); a double row with the centerlines of opposite cylinders converging in a V (V-engine); a double zigzag row somewhat similar to that of the V-engine but with alternate pairs of opposite cylinders converging in two Vs (W-engine); or two horizontal, opposed rows (opposed, pancake, flat, or boxer engine). In each cylinder a piston slides up and down. One end of a connecting rod is attached to the bottom of the piston by a joint; the other end of the rod clamps around a bearing on one of the throws, or convolutions, of a crankshaft; the reciprocating (up-and-down) motions of the piston rotate the crankshaft, which is connected by suitable gearing to the drive wheels of the automobile. The number of crankshaft revolutions per minute is called the engine speed. The top of the cylinder is closed by a metal cover (called the head) bolted onto it. Into a threaded aperture in the head is screwed the spark plug, which provides ignition.

Two other openings in the cylinder are called ports. The intake port admits the air-gasoline mixture; the exhaust port lets out the products of combustion. A mushroom-shaped valve is held tightly over each port by a coil spring, and a camshaft rotating at one-half engine speed opens the valves in correct sequence. A pipe runs from each intake port to a carburetor or injector, the pipes from all the cylinders joining to form a manifold; a similar manifold connects the exhaust ports with an exhaust pipe and noise muffler. A carburetor or fuel injector mixes air with gasoline in proportions of weight varying from 11 to 1 at the richest to a little over 16 to 1 at the leanest. The composition of the mixture is regulated by the throttle, an air valve in the intake manifold that varies the flow of fuel to the combustion chambers of the cylinders. The mixture is rich at idling speed (closed throttle) and at high speeds (wide-open throttle), and is lean at medium and slow speeds (partly open throttle).

The other main type of reciprocating engine is the diesel engine, invented by Rudolf Diesel and patented in 1892. The diesel uses the heat produced by compression rather than the spark from a plug to ignite an injected mixture of air and diesel fuel (a heavier petroleum oil) instead of gasoline. Diesel engines are heavier than gasoline engines because of the extra strength required to contain the higher temperatures and compression ratios. Diesel engines are most widely used where large amounts of power are required: heavy trucks, locomotives, and ships.

Rotary Engines

The most successful rotary engine is the Wankel engine. Developed by the German engineer Felix Wankel in 1956, it has a disk that looks like a triangle with bulging sides rotating inside a cylinder shaped like a figure eight with a thick waist. Intake and exhaust are through ports in the flat sides of the cylinder. The spaces between the sides of the disk and the walls of the cylinder form combustion pockets. During a single rotation of the disk each pocket alternately grows smaller, then larger, because of the contoured outline of the cylinder. This provides for compression and expansion. The engine runs on a four-stroke cycle.

The Wankel engine has 48% fewer parts and about a third the bulk and weight of a reciprocating engine. Its main advantage is that advanced pollution control devices are easier to design for it than for the conventional piston engine. Another advantage is that higher engine speeds are made possible by rotating instead of reciprocating motion, but this advantage is partially offset by the lack of torque at low speeds, leading to greater fuel consumption.

Engine Operation

The Four-Stroke Cycle

In most engines a single cycle of operation (intake, compression, power, and exhaust) takes place over four strokes of a piston, made in two engine revolutions. When an engine has more than one cylinder the cycles are evenly staggered for smooth operation, but each cylinder will go through a full cycle in any two engine revolutions. When the piston is at the top of the cylinder at the beginning of the intake stroke, the intake valve opens and the descending piston draws in the air-fuel mixture.

At the bottom of the stroke the intake valve closes and the piston starts upward on the compression stroke, during which it squeezes the air-fuel mixture into a small space at the top of the cylinder. The ratio of the volume of the cylinder when the piston is at the bottom to the volume when the piston is at the top is called the compression ratio. The higher the compression ratio, the more powerful the engine and the higher its efficiency. However, in order to accommodate air pollution control devices, manufacturers have had to lower compression ratios.

Just before the piston reaches the top again, the spark plug fires, igniting the air-fuel mixture (alternatively, the heat of compression ignites the mixture). The mixture on burning becomes a hot, expanding gas forcing the piston down on its power stroke. Burning should be smooth and controlled. Faster, uncontrolled burning sometimes occurs when hot spots in the cylinder preignite the mixture; these explosions are called engine knock and cause loss of power. As the piston reaches the bottom, the exhaust valve opens, allowing the piston to force the combustion products—mainly carbon dioxide, carbon monoxide, nitrogen oxides, and unburned hydrocarbons—out of the cylinder during the upward exhaust stroke.

The Two-Stroke Cycle

The two-stroke engine is simpler mechanically than the four-stroke engine. The two-stroke engine delivers one power stroke every two strokes instead of one every four; thus it develops more power with the same displacement, or can be lighter and yet deliver the same power. For this reason it is used in lawn mowers, chain saws, small automobiles, motorcycles, and outboard marine engines.

However, there are several disadvantages that restrict its use. Since there are twice as many power strokes during the operation of a two-stroke engine as there are during the operation of a four-stroke engine, the engine tends to heat up more, and thus is likely to have a shorter life. Also, in the two-stroke engine lubricating oil must be mixed with the fuel. This causes a very high level of hydrocarbons in its exhaust, unless the fuel-air mixture is computer calculated to maximize combustion. A highly efficient, pollution-free two-stroke automobile engine is currently being developed by Orbital Engineering, under arrangements with all the U.S. auto makers.

Cooling and Lubrication of Engines

Most small two-stroke engines are air-cooled. Air flows over cooling fins around the outside of the cylinder and head, either by the natural motion of the vehicle or from a fan. Many aircraft four-stroke engines are also air-cooled; larger engines have the cylinders arranged radially so that all cylinders are directly in the airstream. Most four-stroke engines, however, are water-cooled. A water jacket encloses the cylinders; a water pump forces water through the jacket, where it draws heat from the engine. Next, the water flows into a radiator where the heat is given off to the air; it then moves back into the jacket to repeat the cycle. During warm-up a thermostatic valve keeps water from passing to the radiator until optimum operating temperatures are attained.

Four-stroke engines are lubricated by oil from a separate oil reservoir, either in the crankcase, which is a pan attached to the underside of the engine, or in an external tank. In an automobile engine a gear pump delivers the oil at low pressure to the bearings. Some bearings may depend on oil splashed from the bottom of the crankcase by the turning crankshaft. In a two-stroke engine the lubricating oil is mixed with the fuel.

Environmental Considerations in Engine Design

In order to meet U.S. government restrictions on exhaust emissions, automobile manufacturers have had to make various modifications in the operation of their engines. For example, to reduce the emission of nitrogen oxides, one modification involves sending a certain proportion of the exhaust gases back into the air-gasoline mixture going into the engine. This cuts peak temperatures during combustion, lessening the amount of nitrogen oxides produced. In the stratified charge piston engine two separate air-fuel mixtures are injected into the engine. A small, rich mixture that is easily ignited is used to ignite an exceptionally lean mixture that drives the piston. This results in much more efficient burning of the gasoline, further reducing emissions. Another device, the catalytic converter, is connected to the exhaust pipe; exhaust gases travel over bars or pellets coated with certain metals that promote chemical reactions, reducing nitrogen oxide and burning hydrocarbons and carbon monoxide.

For many years engine knock (rapid uncontrolled burning that sometimes occurs when hot spots in the cylinder preignite the mixture causing loss of power) was fought through the introduction of lead into gasoline. However, concern over air pollution and lead's destructive effect on catalytic converters forced its removal. The state of California, with the worst air pollution in the United States, has instituted a series of measures designed to reduce automobile emissions; these include special gasolines, different air-gas mixtures, and higher compression ratios. All cars, trucks, and gasolines sold in California must comply with these regulations.

Evolution of the Internal-Combustion Engine

The first person to experiment with an internal-combustion engine was the Dutch physicist Christian Huygens, about 1680. But no effective gasoline-powered engine was developed until 1859, when the French engineer J. J. Étienne Lenoir built a double-acting, spark-ignition engine that could be operated continuously. In 1862 Alphonse Beau de Rochas, a French scientist, patented but did not build a four-stroke engine; sixteen years later, when Nikolaus A. Otto built a successful four-stroke engine, it became known as the "Otto cycle." The first successful two-stroke engine was completed in the same year by Sir Dougald Clerk, in a form which (simplified somewhat by Joseph Day in 1891) remains in use today. George Brayton, an American engineer, had developed a two-stroke kerosene engine in 1873, but it was too large and too slow to be commercially successful.

In 1885 Gottlieb Daimler constructed what is generally recognized as the prototype of the modern gas engine: small and fast, with a vertical cylinder, it used gasoline injected through a carburetor. In 1889 Daimler introduced a four-stroke engine with mushroom-shaped valves and two cylinders arranged in a V, having a much higher power-to-weight ratio; with the exception of electric starting, which would not be introduced until 1924, most modern gasoline engines are descended from Daimler's engines.

Bibliography

See E. F. Obert, Internal Combustion Engine (1950); C. F. Taylor and E. S. Taylor, The Internal Combustion Engine (1984); and J. B. Heywood, Internal Combustion Engine Fundamentals (1988).

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Internal-Combustion Engine

Internal-combustion engine

The invention and development of the internal-combustion engine in the nineteenth century has had a profound impact on human life. The internal-combustion engine offers a relatively small, lightweight source for the amount of power it produces. Harnessing that power has made possible practical machines ranging from the smallest model airplane to the largest truck. Lawnmowers, chainsaws, and electric generators also may use internal-combustion engines. An important device based on the internal-combustion engine is the automobile.

In all internal-combustion engines, however, the basic principles remain the same. Fuel is ignited in a cylinder, or chamber. Inside the sealed, hollow cylinder is a piston (a solid cylinder) that is free to move up and down and is attached at the bottom to a crankshaft. The energy created by the combustion, or burning, of the fuel pushes down on the piston. The movement of the piston turns the crankshaft, which then transfers that movement through various gears to the desired destination, such as the drive wheels in an automobile.

Basic principles

The most common internal-combustion engines are the piston-type gasoline engines used in most automobiles. In an engine, the cylinder is housed inside an engine block strong enough to contain the explosions of fuel. Inside the cylinder is a piston that fits the cylinder precisely. Pistons generally are dome-shaped on top, and hollow at the bottom. In a four-stroke engine, the piston completes one up-and-down cycle in four strokes: intake, compression, power, and exhaust.

The first stroke, the intake stroke, begins when the piston is at the top of the cylinder, called the cylinder head. As it is drawn down, it creates a vacuum in the cylinder. This is because the piston and the cylinder form an airtight space. This vacuum helps to draw the fuel-air mixture into the cylinder through an open intake valve, which closes when the piston reaches the bottom of the cylinder.

On the next stroke, called the compression stroke, the piston is pushed up inside the cylinder, compressing or squeezing the fuel-air mixture into a tighter and tighter space. The compression of the mixture against the top of the cylinder causes the air to heat up, which in turn heats the mixture. Compressing the fuel-air mixture also makes it easier to ignite and makes the resulting explosion more powerful. There is less space for the expanding gases of the explosion to flow, which means they will push harder against the piston in order to escape.

At the top of the compression stroke, the fuel-air mixture is ignited by a spark from a spark plug placed in the cylinder head, causing an explosion that pushes the piston down. This stroke is called the power stroke, and this is the stroke that turns the crankshaft.

The final stroke, the exhaust stroke, takes the piston upward again, expelling the exhaust gases created by the explosion from the cylinder through an exhaust valve. When the piston reaches the top of the cylinder, it begins the four-step process again.

Development of the internal-combustion engine

In 1824, French physicist Nicholas Carnot (17961832) published a book that set out the principles of an internal-combustion engine that would use an inflammable mixture of gas vapor and air. Basing his work

on Carnot's principles, another Frenchman named Jean-Joseph-Étienne Lenoir (18221900) presented the world with its first workable internal-combustion engine in 1859. Lenoir's motor was a two-cycle (two-stroke), one-cylinder engine with slide valves that used coal gas as a fuel. A battery supplied the electrical charge to ignite the gas after it was drawn into the cylinder. In 1862, another Frenchman, Alphonse-Eugène Beau de Rochas (18151893), designed a four-stroke engine that would overcome many problems associated with the gas engines of that time.

Two-stroke engines eliminate the intake and exhaust strokes, combining them with the compression and power strokes. This allows for a lighter, more powerful enginerelative to the engine's sizerequiring a less complex design. But the two-stroke cycle is a less efficient method of burning fuel. A residue of unburned fuel remains inside the cylinder, which hinders combustion. The two-stroke engine also ignites its fuel twice as often as a four-stroke engine, which increases the wear on the engine's parts. Two-stroke engines are therefore used mostly where a smaller engine is required, such as on some motorcycles and with small tools.

An internal-combustion engine can have anywhere from one to twelve or more cylinders, all acting together in a precisely timed sequence to drive the crankshaft. Automobiles generally have four-, six-, or eight-cylinder engines, although two-cylinder and twelve-cylinder engines are also available. The number of cylinders affects the engine's displacement, that is, the total volume of fuel passed through the cylinders. A larger displacement allows more fuel to be burned, creating more energy to drive the crankshaft.

In the case of an engine with two or more cylinders, however, the spark from the spark plugs must be directed to each cylinder in turn. The sequence of firing the cylinders must be timed so that while one piston is in its power stroke, another piston is in its compression stroke. In this way, the force exerted on the crankshaft can be kept constant, allowing the engine to run smoothly. The number of cylinders affects the smoothness of the engine's operation: the more cylinders, the more constant the force on the crankshaft and the more smoothly the engine will run.

In addition to piston-driven, gas-powered internal-combustion engines, other internal-combustion engines have been developed, such as the Wankel engine and the gas turbine engine. Jet engines and diesel engines are also powered by internal combustion.

[See also Diesel engine; Jet engine ]

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internal combustion engine

internal combustion engine Engine in which fuel is burned inside, so that the gases formed can produce motion, widely used in automobiles. An internal combustion engine may be a two-stroke engine or a four-stroke engine. In the most common type of engine, a mixture of petroleum vapour and air is ignited by a spark. The gases produced in the explosion usually drive a piston along a cylinder. A crankshaft changes the reciprocating (to-and-fro) movement of the pistons into rotary motion. In the Wankel rotary engine, the gases produced in the explosions drive a triangular rotor. Nikolaus Otto built the first practical internal combustion engine in 1867. Rudolf Diesel designed the first diesel engine in 1897. Because combustion is incomplete and reliant on fossil fuels, internal combustion engines are a major cause of air pollution.

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Cummins, Albert Baird

Albert Baird Cummins, 1850–1926, U.S. Senator from Iowa (1909–26), b. Green co., Pa. He studied law in Chicago and in 1878 joined his brother in practice in Des Moines. As governor of Iowa (1901–8), Cummins worked to break up railroad domination in politics and to inaugurate progressive policies in the state. He was elected (1908) to the U.S. Senate and was coauthor there of the Esch-Cummins Transportation Act of 1920.

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internal combustion engine

in·ter·nal com·bus·tion en·gine • n. an engine that generates motive power by the burning of gasoline, oil, or other fuel with air inside the engine, the hot gases produced being used to drive a piston or do other work as they expand.

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