internal-combustion engine


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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 propulsionjet propulsion,
propulsion of a body by a force developed in reaction to the ejection of a high-speed jet of gas. Jet Propulsion Engines

The four basic parts of a jet engine are the compressor, turbine, combustion chamber, and propelling nozzles.
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, rocketrocket,
any vehicle propelled by ejection of the gases produced by combustion of self-contained propellants. Rockets are used in fireworks, as military weapons, and in scientific applications such as space exploration.
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), and the gas turbineturbine,
rotary engine that uses a continuous stream of fluid (gas or liquid) to turn a shaft that can drive machinery.

A water, or hydraulic, turbine is used to drive electric generators in hydroelectric power stations.
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. 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 enginediesel engine,
type of internal-combustion engine invented by the German engineer Rudolf Diesel and patented by him in 1892. Although his engine was designed to use coal dust as fuel, the diesel engine now burns fuel oil.
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, 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).

Internal-Combustion Engine

 

a heat engine in which the chemical energy of fuel burning in the combustion chamber is converted into mechanical work. The first practical and usable gas internal-combustion engine was designed by the French mechanical engineer E. Lenoir in I860. In 1876 the German inventor N. Otto built an improved four-stroke gas engine.

An internal-combustion engine is simpler than a steam engine, since one stage of power conversion—the steam-boiler system—is eliminated. This improvement resulted in greater compactness of the internal-combustion engine, lower weight per unit of power, and more economical operation; however, fuel of a higher quality (gas or oil) was needed for it.

In the 1880’s, O. S. Kostovich in Russia built the first gasoline-powered carburetor engine. In 1897 the German engineer R. Diesel, working to improve the efficiency of the internal-combustion engine, proposed an engine with compression ignition. The improvement of this internal-combustion engine at the L. Nobel plant in St. Petersburg (now the Russkii Dizel’ plant) in 1898–99 made possible the use of heavy oil as a fuel. As a result, the internal-combustion engine became a most economical stationary heat engine. The first tractor with an internal-combustion engine was developed in the USA in 1901. The further development of internal-combustion engines for motor vehicles enabled the brothers O. and W. Wright to build the first airplane with an internal-combustion engine, which began its flights in 1903. In the same year, Russian engineers installed an internal-combustion engine in the ship Vandal, thus producing the first motor ship. The first practical diesel locomotive was developed in Leningrad in 1924 on the basis of designs by Ia. M. Gakkel’.

Internal-combustion engines are classified as operating on liquid or gas fuel, as four-stroke or two-stroke according to the manner in which the cylinder is filled with a fresh fuel mixture. and as having internal or external mixing of the fuel. Engines with external mixing of the fuel include carburetor engines, in which a mixture of liquid fuel and air is formed in a carburetor, and gas-mixing engines, in which a fuel mixture of gas and air is formed in a mixer. In internal-combustion engines with external mixing the working fuel mixture is ignited in the cylinder by an electric spark. In engines with internal mixing (dieseis), the fuel ignites spontaneously when it is injected into compressed air heated to a high temperature.

The operating cycle of a four-stroke carburetor internal-combustion engine is carried out in four strokes of the cylinder—that is. in two rotations of the crankshaft. During the first, or intake, stroke the piston moves from top dead center to bottom dead center. At the same time, the intake valve is opened, and the fuel mixture passes from the carburetor to the cylinder. During the second, or compression, stroke, when the cylinder moves from bottom dead center to top dead center, the intake and exhaust valves are closed and the mixture is compressed to a pressure of 0.8–2.0 meganew-tons per sq m (MN/m2), or 8–20 kilograms-force per sq cm (kgf/cm2). The temperature of the mixture at the end of compression is 200°-400° C. At the end of the compression cycle the mixture is ignited by an electric spark, and combustion of the fuel takes place. Combustion takes place when the piston is near top dead center. At the end of combustion the pressure in the cylinder is 3–6 MN/m2 (30–60 kgf/cm2), and the temperature is 1600°-2200° C. The third, or expansion, stroke is called the power stroke. During this stroke the heat from combustion of the fuel is transformed into mechanical work. The fourth, or exhaust, stroke occurs as the piston moves from bottom dead center to top dead center. The spent gases are forced out by the piston.

The operating cycle of a two-stroke carburetor internal-combustion engine takes place during two strokes of the cylinder or one rotation of the crankshaft. The processes of compression, combustion, and expansion are virtually the same as the corresponding processes of a four-stroke internal-combustion engine. All other conditions being equal, the two-stroke engine should be twice as powerful as the four-stroke engine, since the power stroke in a two-stroke engine occurs twice as often, but in practice the power of a two-stroke carburetor internal-combustion engine frequently not only does not exceed that of a four-stroke engine with the same cylinder diameter and piston stroke but is even lower. The reason for this is that the piston completes a considerable part of the stroke (20–35 percent) with the ports open, when the pressure in the cylinder is low and the engine is in effect not performing work. The evacuation of the cylinder requires an expenditure of power to compress the air in the scavenging pump; the clearing of the cylinder of the products of gas combustion and filling it with a new charge is considerably poorer than in a four-stroke engine.

The operating cycle of a carburetor internal-combustion engine can take place at a very high frequency of rotation of the shaft (3,000–7,000 rpm). Motors of racing cars and motorcycles can develop 15,000 rpm and more. A normal fuel mixture consists of approximately 15 parts air (by weight) to 1 part gasoline vapor. An engine can operate on a lean mixture (18 : 1) oran enriched mixture (12 : 1). A mixture that is too rich or too lean causes a great reduction in the speed of combustion and cannot provide normal combustion. The power of a carburetor internal-combustion engine is regulated by changing the amount of mixture delivered to the cylinder (quantity control). A high speed of rotation and a favorable fuel-air ratio in the mixture ensure high power per unit volume of the cylinder of a carburetor engine; therefore, these engines have relatively small dimensions and weight (1–4 kg per kilowatt [kg/kW], or 0.75–3.0 kg/hp). The use of low compression ratios means moderate pressures at the end of combustion, so that parts can be made less massive than, for example, in dieseis. When the cylinder diameter of a carburetor internal-combustion engine is increased, the tendency of the engine to detonate increases; therefore, carburetor internal-combustion engines are not made with large-diameter cylinders (as a rule, not more than 150 mm).

The GAZ-21 Volga is an example of a carburetor internal-combustion engine. It is a four-cylinder, four-stroke engine that develops a power of 55 kW (75 hp) at 4,000 rpm and a 6.7 compression ratio. Specific fuel consumption is 290 g/(kW-hr).

The most powerful four-stroke carburetor internal-combustion engine is rated at 600 kW (800 hp). Two-stroke and four-stroke engines for motorcycles have a power of 3.5–45 kW (5–60 hp). Aviation piston engines with direct gasoline injection and spark ignition develop as much as 1,100 kW (1,500 hp).

Carburetor internal-combustion engines are complicated units that include a number of subassemblies and systems.

The engine framework is a group of stationary parts that are the basis for all other mechanisms and systems. It includes the cylinder block, the cylinder head or heads, the crankshaft bearing covers, the front and rear block covers, the oil pan, and a number of small parts.

The propulsion mechanism is a group of moving parts that receive the pressure of the gases in the cylinders and convert it into torque on the crankshaft. The propulsion mechanism includes the piston assembly (pistons, rods, crankshaft, and flywheel).

The valve-timing gear serves to admit the fuel mixture into the cylinders at the proper time and to exhaust the spent gases. These functions are performed by the camshaft, which is driven by the crankshaft, as well as by the valve lifters, rods, and rocker arms, which open the valves. The valves are closed by valve springs.

The lubrication system is a system of units and channels that provide lubricant to friction surfaces. The oil in the oil pan is brought by a pump to a coarse filter, from which it passes under pressure through the main oil line in the cylinder block to the bearings of the crankshaft and camshaft and to the pistons and parts of the valve-timing gear. The cylinders, valve lifters, and other parts are lubricated by oil vapor formed from the splashing oil that comes from the clearance in the bearings of the rotating parts. Some of the oil is diverted along parallel channels to a fine filter, from which it flows back into the pan.

The cooling system can be of the liquid or air type. A liquid cooling system consists of the cylinder sleeves and heads filled with a fluid (water, antifreeze, and so on), a pump, a radiator in which the fluid is cooled by the flow of air caused by a fan, and devices to regulate the water temperature. Air cooling is effected by blowing air onto the cylinders with a fan, or by the airflow (in motorcycles).

The fuel system prepares the mixture of fuel and air in a proportion corresponding to the operating conditions and depending quantitively on the power of the engine. The system consists of the fuel tank, fuel pump, fuel filter, and fuel lines, and the carburetor, which is the main element of the system.

The ignition system serves to form a spark in the combustion chamber to ignite the fuel mixture. The ignition system includes a source of current (a generator and battery), as well as a contact breaker, which determines the moment of delivery of the spark. The system includes a distributor of high-voltage current to the proper cylinders. A condenser to improve the operation of the contact breaker and an ignition coil from which the high voltage is taken (12–20 kV) are in the same unit as the contact breaker. Before internal-combustion engines had electric ignition, hot bulbs were used for ignition.

The starting system consists of an electric starter, transmission gears from the starter to the flywheel, a current source (battery), and remote-control elements. The system serves to rotate the motor shaft for starting.

The intake and exhaust system consists of pipes, an intake air filter, and a muffler on the exhaust.

Gas internal-combustion engines operate mainly on natural gas and gases obtained in the production of liquid fuel. In addition, the gas generated by the incomplete combustion of solid fuel, metallurgical gas, and sewer gas can be used. Both four-stroke and two-stroke engines are used. Gas engines are classified according to the principle of mixture formation and ignition as engines with external mixture formation and spark ignition, in which the process of operation is analogous to the process in carburetor engines; engines with external mixture formation and ignition by a jet of liquid fuel that ignites by compression; and engines with internal mixture formation and spark ignition. Engines that operate on natural gas are used at stationary electric power plants and compressor gas-pumping installations. Liquefied butane-propane mixtures are used for motor-vehicle transportation.

The economy of operation of internal-combustion engines is characterized by the efficiency, which is the ratio of useful work to the amount of heat emitted to do the work with complete combustion of the fuel. The maximum efficiency of the best internal-combustion engines is about 44 percent.

The main advantage of internal-combustion engines, as well as of other heat engines (such as jet engines), over hydraulic and electric motors is that they do not depend on constant sources of energy (water resources, electric power plants, and so on), which means that installations equipped with internal-combustion engines can be freely moved and located anywhere. This has led to widespread use of internal-combustion engines for transportation (motor vehicles, farm machinery, road-building machinery, and self-propelled military equipment).

The refinement of the internal-combustion engine is directed at increasing its power, reliability, and durability; diminishing its weight and bulk; and providing new types (for example, the Wankel rotary combustion engine). Other trends in the development of the internal-combustion engine include the gradual replacement of carburetor engines by dieseis in motor-vehicle transportation, the use of multifuel engines, and an increase in the speed of rotation.

REFERENCES

Dvigateli vnutrennego sgoraniia,vols. 1–3. Moscow, 1957–62.
Dvigateli vnulrennego sgoraniia.Moscow, 1968.

D. N. VYRUBOV and V. P. ALEKSEEV

internal-combustion engine

a heat engine in which heat is supplied by burning the fuel in the working fluid (usually air)
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