any type of machine that obtains mechanical energy directly from the expenditure of the chemical energy of fuel burned in a combustion chamber that is an integral part of the engine (see COMBUSTION,). Four principal types of internal-combustion engines are in general use: the Otto-cycle, the diesel, and the rotary engine; and the gas turbine. For a discussion of the last-named type, see GAS TURBINE,. For a discussion of internal-combustion engines employing the principle of jet propulsion, see JET PROPULSION,; ROCKET,. The Otto-cycle and the diesel are the most widely used internal-combustion engines; both are piston engines.

PISTON ENGINES

The Otto-cycle engine, invented by the German engineer Nikolaus August Otto (1832–91), is the familiar gasoline engine (although it can be modified to run on a variety of alternative fuels) used in automobiles and airplanes. The diesel engine, invented by the German engineer Rudolf Christian Karl Diesel, operates on a different principle and usually uses oil as a fuel; it is used in electric-generating and marine-power plants, and in trucks, buses, and some automobiles. Otto-cycle and diesel engines are manufactured in two-stroke and four-stroke cycle models, with four-stroke models predominating.

Components of Engines.

The essential parts of Otto-cycle and diesel engines are the same. The combustion chamber consists of a cylinder, usually fixed, which is closed at one end and in which a close-fitting piston slides. The in-and-out motion of the piston varies the volume of the chamber between the inner face of the piston and the closed end of the cylinder. The outer face of the piston is attached to a crankshaft by a connecting rod. The crankshaft transforms the reciprocating motion of the piston into rotary motion. In multicylindered engines the crankshaft has one offset portion, called a crankpin, for each connecting rod, so that the power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshafts have heavy flywheels and counterweights, which by their inertia minimize irregularity in the motion of the shaft and thus reduce harshness and vibration. Engines may have from 1 to as many as 28 cylinders, the most common being 4-, 6-, and 8-cylinder engines.

The fuel supply of an internal-combustion engine consists of a tank, a fuel pump, and a device for vaporizing or atomizing the liquid fuel. This device has historically been a carburetor in Otto-cycle engines and a mechanical fuel-injection system in diesel engines; however, by the 1990s electronic fuel-injection systems were being used in most Otto-cycle and diesel engines, providing more precise control over the fuel-air mixture to improve performance and fuel economy, and to reduce exhaust emissions. The vaporized fuel in most multicylindered engines is conveyed to the cylinders through a branched pipe called the intake manifold; in many engines, a similar exhaust manifold is provided to carry off the gases produced by combustion. The fuel is admitted to each cylinder and the waste gases are exhausted through mechanically operated poppet valves or sleeve valves. The valves are normally held closed by the pressure of springs and are opened at the proper time during the operating cycle by cams on a rotating camshaft that is geared to the crankshaft (see CAM,).

In all engines some means of igniting the fuel in the cylinder must be provided. For example, the ignition system of Otto-cycle engines described below consists of a source of low-voltage, direct-current electricity that is connected to the primary of a TRANSFORMER, (q.v.) called an ignition coil. The current is interrupted many times a second by an automatic switch called the timer. The pulsations of the current in the primary induce a pulsating, high-voltage current in the secondary. The high-voltage current is led to each cylinder in turn by a rotary switch called the distributor. The actual ignition device is the spark plug, an insulated conductor set in the wall or top of each cylinder. At the inner end of the spark plug is a small gap between two wires. The high-voltage current arcs across this gap, yielding the spark that ignites the fuel mixture in the cylinder. By the 1990s, spark plugs had been replaced in most new automotive engines by fuel-injectors, and electronic actuators had replaced traditional distributors. Most automotive engine manufacturers had begun producing multivalve engines, which improve low-end performance (acceleration) without sacrificing fuel economy. These engines typically have four valves per cylinder, twice that of standard engines, to improve engine “breathing” in the intake and exhaust cycles.

Because of the heat of combustion, all engines must be equipped with some type of cooling system. Some aircraft and automobile engines, small stationary engines, and outboard motors for boats are cooled by air. In this system the outside surfaces of the cylinder are shaped in a series of radiating fins with a large area of metal to radiate heat from a cylinder. Other engines are water-cooled and have their cylinders enclosed in an external water jacket. Water is circulated through the jacket by means of a water pump and cooled by passing through the finned coils of a radiator. Some automobile engines are also air-cooled, and in marine engines sea water is used for cooling.

Unlike steam engines and turbines, internal-combustion engines develop no torque when starting, and therefore provision must be made for turning the crankshaft so that the cycle of operation can begin. Automobile engines are normally started by means of an electric motor or starter that is geared to the crankshaft with a clutch that automatically disengages the motor after the engine has started. Small engines are sometimes started manually by turning the crankshaft with a crank or by pulling a rope wound several times around the flywheel. Methods of starting large engines include the inertia starter, which consists of a flywheel that is rotated by hand or by means of an electric motor until its kinetic energy is sufficient to turn the crankshaft, and the explosive starter, which employs the explosion of a blank cartridge to drive a turbine wheel that is coupled to the engine. The inertia and explosive starters are chiefly used to start airplane engines.

Otto-Cycle Engines.

The ordinary Otto-cycle engine is a four-stroke engine; that is, its pistons make four strokes, two toward the head (closed head) of a cylinder and two away from the head, in a complete power cycle. During the first stroke of the cycle, the piston moves away from the cylinder head while simultaneously the intake valve is opened. The motion of the piston during this stroke sucks a quantity of a fuel and air mixture into the combustion chamber. During the next stroke the piston moves toward the cylinder head and compresses the fuel mixture in the combustion chamber. At the moment when the piston reaches the end of this stroke and the volume of the combustion chamber is at a minimum, the fuel mixture is ignited by the spark plug or injector and burns, expanding and exerting a pressure on the piston, which is then driven away from the cylinder head in the third stroke. During the final stroke, the exhaust valve is opened and the piston moves toward the cylinder head, driving the exhaust gases out of the combustion chamber and leaving the cylinder ready to repeat the cycle.

The efficiency of a modern Otto-cycle engine is limited by certain factors, such as losses by cooling and by friction. In general the efficiency of such engines is determined by their compression ratio (the ratio between the maximum and minimum volumes of the combustion chamber), which is usually about 8 to 1 or 10 to 1 in these engines. Higher compression ratios, up to about 12 to 1, with a resulting increase in efficiency, are possible with the use of high-octane antiknock fuels. The efficiencies of good modern Otto-cycle engines range between 20 and 25 percent (in other words, only this percentage of the heat energy of the fuel is transformed into mechanical energy).

Diesel Engines.

Theoretically the diesel cycle differs from the Otto cycle in that combustion takes place at constant volume rather than at constant pressure. Most diesels, like most Otto-cycle engines, are four-stroke engines, but diesels operate differently. The first, or suction, stroke draws air, but no fuel, into the combustion chamber through an intake valve. On the second, or compression, stroke the air is compressed to a fraction of its former volume and is thereby heated to about 440° C (about 820° F). At the end of this stroke vaporized fuel is injected into the combustion chamber and burns instantly due to the high temperature of the air in the chamber. Some diesels have auxiliary electrical ignition systems to ignite the fuel when the engine starts, and until it warms up. This combustion drives the piston back on the third, or power, stroke of the cycle. The fourth stroke, as in the Otto-cycle engine, is an exhaust stroke.

The efficiency of the diesel engine is inherently greater than that of any Otto-cycle engine and in actual engines today is slightly more than 40 percent. Diesels are in general slow-speed engines with crankshaft speeds of 100 to 700 revolutions per minute (rpm) as compared to 2500 to 5000 rpm for typical Otto-cycle engines. Because diesels use compression ratios of 14 or more to 1, they are generally more heavily built, but this disadvantage is counterbalanced by their greater efficiency and the fact that they can be operated on less expensive fuel oils.

Diesel engines are widely used in trucks and buses and gained brief popularity in the U.S. in automobiles after the oil crises of the 1970s; however, high emissions, excess noise, and generally poor performance combined with soft fuel prices and stringent federal emissions standards in the 1980s and ‘90s set back the use of diesels in automobiles. Turbocharged automotive diesels continued to enjoy a small market, however.

Two-Stroke Engines.

Otto-cycle and diesel engines can also be designed to operate as two-stroke, or two-cycle, engines; these engines have a power stroke every other stroke of the piston instead of once every four strokes. The efficiency of such engines is usually less than that of four-stroke engines, and therefore the power of a two-stroke engine is less than half that of a four-stroke engine of comparable size.

The general principle of the two-stroke engine is to shorten the periods in which fuel is introduced to the combustion chamber and in which the spent gases are exhausted to a small fraction of the duration of a stroke instead of allowing each of these operations to occupy a full stroke. In the two-stroke cycle the fuel mixture or air is introduced through the intake port when the piston is fully withdrawn from the cylinder. The compression stroke follows and the charge is ignited at the end of this stroke. The piston then moves outward on the power stroke, uncovering the exhaust port and permitting the gases to escape.

Two-stroke engines are most commonly used in motorcycles, boats, and lawn mowers; however, automakers were developing two-stroke Otto-cycle engines using advanced electronics, with large-scale production expected in the mid- to late 1990s. These engines are about 25 percent less bulky and about 15 percent more fuel efficient than standard four-stroke Otto-cycle engines.

THE ROTARY ENGINE

In the 1950s the German engineer Felix Wankel (1902–88) developed his concept of an internal-combustion engine of a radically new design, in which the piston and cylinder were replaced by a three-cornered rotor turning in a roughly oval chamber. The fuel-air mixture is drawn in through an intake port and trapped between one face of the turning rotor and the wall of the oval chamber. The turning of the rotor compresses the mixture, which is ignited by a spark plug. The exhaust gases are then expelled through an exhaust port by the action of the turning rotor. The cycle takes place alternately at each face of the rotor, giving three power strokes for each turn of the rotor. The engine offers practically vibration-free operation, and its mechanical simplicity provides low manufacturing costs. The rotary usually runs on gasoline but is adaptable to alternative fuels. With the rise in gasoline prices during the 1970s and ‘80s, the rotary’s compact size and lower weight as compared with piston engines increased its value and importance as an automobile engine; its popularity soon waned because of difficulty in meeting exhaust emission standards and low fuel efficiency. By the mid-1990s only one automaker continued to offer rotary-powered cars.

OTHER DEVELOPMENTS

Ongoing concern with fuel efficiency and exhaust emissions of internal combustion engines, particularly in automobiles, has prompted their continued modification. In the 1980s the stratified charge engine, a modification of the conventional Otto-cycle engine, was developed; it not only increased fuel efficiency, but reduced emissions without the need for an exhaust-gas recirculation system or a catalytic converter. In the 1990s, electronic, intelligent engine-management systems, which “talk” to transmissions and other parts of a vehicle for ultimate efficiency, were coming into widespread use.

For further information on this topic, see the Bibliography, section 536. Small engines.