Areciprocating engine, more often known as apiston engine, is typically aheat engine that uses one or morereciprocatingpistons to convert high temperature and highpressure into arotating motion.[1] This article describes the common features of all types. The main types are: theinternal combustion engine, used extensively inmotor vehicles; thesteam engine, the mainstay of theIndustrial Revolution; and theStirling engine for niche applications. Internal combustion engines are further classified in two ways: either aspark-ignition (SI) engine, where the spark plug initiates the combustion; or acompression-ignition (CI) engine, where the air within the cylinder is compressed,thus heating it, so that the heated air ignites fuel that is injectedthen orearlier.[2][3][4][5][6]
There may be one or more pistons. Each piston is inside acylinder, into which a gas is introduced, either already under pressure (e.g.steam engine), or heated inside the cylinder either byignition of a fuel air mixture (internal combustion engine) or by contact with a hot heat exchanger in the cylinder (Stirling engine). The hot gases expand, pushing the piston to the bottom of the cylinder. This position is also known as the bottomdead center (BDC), or where the piston forms the largest volume in the cylinder. The piston is returned to the cylinder top (top dead center) (TDC) by aflywheel, the power from other pistons connected to the same shaft or (in adouble acting cylinder) by the same process acting on the other side of the piston. This is where the piston forms the smallest volume in the cylinder. In most types the expanded or "exhausted" gases are removed from the cylinder by thisstroke. The exception is theStirling engine, which repeatedly heats and cools the same sealed quantity of gas. The stroke is simply the distance between the TDC and the BDC, or the greatest distance that the piston can travel in one direction.
In some designs the piston may be powered in both directions in the cylinder, in which case it is said to bedouble-acting.
In most types, the linear movement of the piston is converted to a rotating movement via aconnecting rod and acrankshaft or by aswashplate or other suitable mechanism. Aflywheel is often used to ensure smooth rotation or to store energy to carry the engine through an un-powered part of the cycle. The more cylinders a reciprocating engine has, generally, the more vibration-free (smoothly) it can operate. The power of a reciprocating engine is proportional to the volume of the combined pistons' displacement.
A seal must be made between the slidingpiston and the walls of thecylinder so that the high pressure gas above the piston does not leak past it and reduce the efficiency of the engine. This seal is usually provided by one or morepiston rings. These are rings made of a hard metal, and are sprung into a circular groove in the piston head. The rings fit closely in the groove and press lightly against the cylinder wall to form a seal, and more heavily when higher combustion pressure moves around to their inner surfaces.
It is common to classify such engines by the number and alignment of cylinders and total volume ofdisplacement of gas by the pistons moving in the cylinders usually measured in cubic centimetres (cm3 or cc) orlitres (l) or (L) (US: liter). For example, for internal combustion engines, single and two-cylinder designs are common in smaller vehicles such asmotorcycles, whileautomobiles typically have between four and eight, andlocomotives andships may have a dozen cylinders or more. Cylinder capacities may range from 10 cm3 or less in model engines up to thousands of liters in ships' engines.[7]
Thecompression ratio affects the performance in most types of reciprocating engine. It is the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.
Thebore/stroke ratio is the ratio of the diameter of the piston, or "bore", to the length of travel within the cylinder, or "stroke". If this is around 1 the engine is said to be "square". If it is greater than 1, i.e. the bore is larger than the stroke, it is "oversquare". If it is less than 1, i.e. the stroke is larger than the bore, it is "undersquare".
Cylinders may be alignedin line, in aV configuration,horizontally opposite each other, orradially around the crankshaft.Opposed-piston engines put two pistons working at opposite ends of the same cylinder and this has been extended into triangular arrangements such as theNapier Deltic. Some designs have set the cylinders in motion around the shaft, such as therotary engine.
In some steam engines, the cylinders may be of varying size with the smallest bore cylinder working the highest pressure steam. This is then fed through one or more, increasingly larger bore cylinders successively, to extract power from the steam at increasingly lower pressures. These engines are calledcompound engines.
Aside from looking at the power that the engine can produce, the mean effective pressure (MEP), can also be used in comparing the power output and performance of reciprocating engines of the same size. The mean effective pressure is the fictitious pressure which would produce the same amount of net work that was produced during the power stroke cycle. This is shown by:
where is the total piston area of the engine, is the stroke length of the pistons, and is the total displacement volume of the engine. Therefore:
Whichever engine with the larger value of MEP produces more net work per cycle and performs more efficiently.[2]
In steam engines and internal combustion engines, valves are required to allow the entry and exit of gases at the correct times in the piston's cycle. These are worked by cams, eccentrics or cranks driven by the shaft of the engine. Early designs used theD slide valve but this has been largely superseded bypiston valve orpoppet valve designs. In steam engines the point in the piston cycle at which the steam inlet valve closes is called thecutoff and this can often be controlled to adjust thetorque supplied by the engine and improve efficiency. In some steam engines, the action of the valves can be replaced by anoscillating cylinder.
Internal combustion engines operate through a sequence of strokes that admit and remove gases to and from the cylinder. These operations are repeated cyclically and an engine is said to be2-stroke,4-stroke or6-stroke depending on the number of strokes it takes to complete a cycle.
The most common type is 4-stroke, which has following cycles.
The reciprocating engine developed in Europe during the 18th century, first as theatmospheric engine then later as thesteam engine. These were followed by theStirling engine andinternal combustion engine in the 19th century. Today the most common form of reciprocating engine is the internal combustion engine running on the combustion ofpetrol,diesel,liquefied petroleum gas (LPG) orcompressed natural gas (CNG) and used to powermotor vehicles andengine power plants.
One notable reciprocating engine from the World War II era was the 28-cylinder, 3,500 hp (2,600 kW)Pratt & Whitney R-4360 Wasp Major radial engine. It powered the last generation of large piston-engined planes before jet engines and turboprops took over from 1944 onward. It had a total engine capacity of 71.5 L (4,360 cu in), and a highpower-to-weight ratio.
The largest reciprocating engine in production at present, but not the largest ever built, is theWärtsilä-Sulzer RTA96-C turbocharged two-stroke diesel engine of 2006 built byWärtsilä. It is used to power the largest modern container ships such as theEmma Mærsk. It is five stories high (13.5 m or 44 ft), 27 m (89 ft) long, and weighs over 2,300metric tons (2,535short tons; 2,264long tons) in its largest 14 cylinders version producing more than 84.42 MW (113,209 bhp). Each cylinder has a capacity of 1,820 L (64 cu ft), making a total capacity of 25,480 L (900 cu ft) for the largest versions.
For piston engines, an engine's capacity is theengine displacement, in other words the volume swept by all the pistons of an engine in a single movement. It is generally measured inlitres (l) or cubic inches (c.i.d., cu in,or in3) for larger engines, and cubic centimetres (abbreviated cc) for smaller engines. All else being equal, engines with greater capacities are more powerful and consumption of fuel increases accordingly (although this is not true of every reciprocating engine), although power and fuel consumption are affected by many factors outside of engine displacement.
Reciprocating engines can be characterized by theirspecific power, which is typically given in kilowatts per litre ofengine displacement (in the U.S. alsohorsepower per cubic inch). The result offers an approximation of the peak power output of an engine. This is not to be confused withfuel efficiency, since high efficiency often requires a lean fuel-air ratio, and thus lower power density. A modern high-performance car engine makes in excess of 75 kW/L (1.65 hp/in3).
Reciprocating engines that are powered by compressed air, steam or other hot gases are still used in some applications such as to drive many modern torpedoes or as pollution-free motive power. Most steam-driven applications usesteam turbines, which are more efficient than piston engines.
The French-designed FlowAIR vehicles use compressed air stored in a cylinder to drive a reciprocating engine in a local-pollution-free urban vehicle.[8]
Torpedoes may use a working gas produced byhigh test peroxide orOtto fuel II, which pressurize without combustion. The 230 kg (510 lb)Mark 46 torpedo, for example, can travel 11 km (6.8 mi) underwater at 74 km/h (46 mph) fuelled by Otto fuel withoutoxidant.
Quantum heat engines are devices that generate power from heat that flows from a hot to a cold reservoir.The mechanism of operation of the engine can be described by the laws ofquantum mechanics.Quantum refrigerators are devices that consume power with the purpose to pump heat from a cold to a hot reservoir.
In a reciprocating quantum heat engine, the working medium is a quantum system such as spin systems or a harmonic oscillator.TheCarnot cycle andOtto cycle are the ones most studied.[9]The quantum versions obey the laws ofthermodynamics. In addition, these models can justify the assumptions ofendoreversible thermodynamics.A theoretical study has shown that it is possible and practical to build a reciprocating engine that is composed of a single oscillating atom. This is an area for future research and could have applications innanotechnology.[10]
There are a large number of unusual varieties of piston engines that have various claimed advantages, many of which see little if any current use:
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