Internal Combustion Engines

Internal Combustion EnginesAn internal-combustion engine is a heat engine that burns fuel and airinside a combustion chamber located within the engine proper. Simply stated, aheat engine is an engine that converts heat energy to mechanical energy. Theinternal- combustion engine should be distinguished from the external-combustion engine, for example, the steam engine and the Stirling engine, whichburns fuel outside the prime mover, that is, the device that actually producesmechanical motion.

Both basic types produce hot, expanding gases, which may thenbe employed to move pistons, turn turbine rotors, or cause locomotion throughthe reaction principle as they escape through the nozzle.Most people are familiar with the internal-combustion reciprocating engine,which is used to power most automobiles, boats, lawn mowers, and home generators.Based on the means of ignition, two types of internal-combustion reciprocatingengines can be distinguished: spark-ignition engines and compression-ignitionengines. In the former, a spark ignites a combustible mixture of air and fuel;in the latter, high compression raises the temperature of the air in the chamberand ignites the injected fuel without a spark. The diesel engine is acompression-ignition engine. This article emphasizes the spark-ignition engine.

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The invention and early development of internal-combustion engines areusually credited to three Germans. Nikolaus Otto patented and built (1876) thefirst such engine; Karl Benz built the first automobile to be powered by such anengine (1885); and Gottlieb Daimler designed the first high-speed internal-combustion engine (1885) and carburetor. Rudolf Diesel invented a successfulcompression-ignition engine (the diesel engine) in 1892.The operation of the internal-combustion reciprocating engine employseither a four-stroke cycle or a two-stroke cycle.

A stroke is one continuousmovement of the piston within the cylinder.In the four-stroke cycle, also known as the Otto cycle, the downwardmovement of a piston located within a cylinder creates a partial vacuum. Valveslocated inside the combustion chamber are controlled by the motion of a camshaftconnected to the crankshaft. The four strokes are called, in order of sequence,intake, compression, power, and exhaust.

On the first stroke the intake valve isopened while the exhaust valve is closed; atmospheric pressure forces a mixtureof gas and air to fill the chamber. On the second stroke the intake and exhaustvalves are both closed as the piston starts upward. The mixture is compressedfrom normal atmospheric pressure (1 kg/sq cm, or 14.7 lb/sq in) to between 4.9and 8.8 kg/sq cm (70 and 125 lb/sq in). During the third stroke the compressedmixture is ignited–either by compression ignition or by spark ignition. Theheat produced by the combustion causes the gases to expand within the cylinder,thus forcing the piston downward.

The piston’s connecting rod transmits thepower from the piston to the crankshaft. This assembly changes reciprocating–inother words, up-and-down or back-and-forth motion–to rotary motion. On thefourth stroke the exhaust valve is opened so that the burned gases can escape asthe piston moves upward; this prepares the cylinder for another cycle.Internal-combustion spark-ignition engines having a two-stroke cycle combineintake and compression in a single first stroke and power and exhaust in asecond stroke.The internal-combustion reciprocating engine contains several subsystems:ignition, fuel, cooling, and exhaust systems.The ignition system of a spark-ignition engine consists of the sparkingdevice (the spark plug); the connecting wire from the plug to the distributor;and the distributor, which distributes the spark to the proper cylinder at theproper time.

The distributor receives a high-energy spark from a coil, ormagneto, that converts low-voltage energy to high-voltage energy. Some ignitionsystems employ transistorized circuitry, which is generally more efficient andless troublesome than the mechanical breaker-point system used in the past. Mostignition systems require an external electrical energy source in the form of abattery or a magneto.

Spark-ignition engines require a means for mixing fuel and air. This may beeither a carburetor or fuel injection. A carburetor atomizes the fuel into theengine’s incoming air supply.

The mixture is then vaporized in the intakemanifold on its way to the combustion chamber. fuel injection sprays acontrolled mist of fuel into the airstream, either in the intake manifold orjust before the intake valve or valves of each cylinder. Both carburetors andfuel injectors maintain the correct fuel- to-air ratio, about one part fuel tofifteen parts air, over a wide range of air temperatures, engine speeds, andloads. Fuel injection can compensate for changes in altitude as well.

Internal-combustion engines require some type of starting system. Smallengines are generally started by pulling a starting rope or kicking a lever.Larger engines may use compressed air or an electric starting system. The latterincludes a starter–a high-torque electric motor–to turn the crankshaft untilthe engine starts. Starting motors are extremely powerful for their size and aredesigned to utilize high currents (200 to 300 amperes).

The large startingcurrents can cause a battery to drain rapidly; for this reason a heavy- dutybattery is usually used. Interrupting this connection is an electrical switchcalled a solenoid, which is activated by the low- voltage starting switch. Inthis way the ignition switch can be located away from the starter and yet stillturn the starter on and off.The cooling system is important because internal-combustion engines operateat high temperatures of combustion–spark- ignition engines at approximately2,760 degrees C (5,000 degrees F) and diesel engines at even higher temperatures.

If it were not for the cooling system, these high temperatures would damage andmelt many parts of the engine. The cooling system essentially dissipates theheat of combustion in metal, water, or air and automatically regulates thetemperature so that the engine can operate at its optimum temperature–about 93degrees C (200 degrees F).Air-cooled engines, popularly used to power small lawn mowers, chain saws,power generators, and motorcycles, as well as small cars and airplanes, oftenrequire no moving parts, and therefore little or no maintenance, for the coolingsystem. The head, or uppermost part, of the cylinder and the cylinder block havefins cast into them; these fins increase the surface exposed to the surroundingair, allowing more heat to be radiated. Usually a cover or shroud channels theair flow over the fins. A fan is sometimes included if the engine is locatedaway from a stream of fast-moving air.

Water-cooled engines have water jackets built into the engine block. Thesejackets surround the cylinders. Usually a centrifugal water pump is used tocirculate the water continuously through the water jackets. In this way the highheat of combustion is drawn off the cylinder wall into the circulating water.The water must then be cooled in a radiator that transfers the heat energy ofthe water to the radiator’s cooler surrounding fluid. The surrounding fluid canbe air or water, depending on the application of the engine.Internal-combustion engines include an exhaust system, which allows the hotexhaust gases to escape efficiently from the engine.

In some small engines theexhaust gases can exit directly into the atmosphere. Larger engines are noisierand require some type of muffler or sound deadener, usually a canister with aninner shell that breaks up the sound waves, dissipating their energy within themuffler before the exhaust gases are permitted to escape.The power capacity of an engine depends on a number of characteristics,including the volume of the combustion chamber. The volume can be increased byincreasing the size of the piston and cylinder and by increasing the number ofcylinders.

The cylinder configuration, or arrangement of cylinders, can bestraight, or in-line (one cylinder located behind the other); radial (cylinderslocated around a circle); in a V (cylinders located in a V configuration); oropposed (cylinders located opposite each other). Another type of internal-combustion engine, the Wankel engine, has no cylinders; instead, it has a rotorthat moves through a combustion chamber.An internal-combustion engine must also have some kind of transmissionsystem to control and direct the mechanical energy where it is needed; forexample, in an automobile the energy must be directed to the driving wheels.Since these engines are not able to start under a load, a transmission systemmust be used to “disengage” the engine from the load during starting and then toapply the load when the engine reaches its operating speed.