The crankshaft, sometimes casually abbreviated to crank, is that part of an engine which translates reciprocating linear piston motion into rotation. It typically connects to a flywheel, to reduce the pulsation characteristic of the four stroke cycle, and sometimes a torsional or vibrational damper at the opposite end, to reduce the torsion vibrations often caused along the length of the crankshaft by the cylinders furthest from the output end acting on the torsional elasticity of the metal.

Contents
1 Design
2 Construction
3 Stress analysis of crankshaft

Design
Large engines are usually multicylinder to reduce pulsations from individual firing strokes, with more than one piston attached to a more complex crankshaft; but many small engines, such as those found in mopeds or garden machinery, are single cylinder and use only a single piston, simplifying crankshaft design. The crankshaft has a linear axis about which it rotates, typically with several bearing journals riding on replaceable bearings held in the engine block, the main bearings. As the crankshaft undergoes a great deal of sideways load from each cylinder in a multicylinder engine, it must be supported by several such bearings, not just one at each end; this was also a factor in the rise of V8 engines with their shorter crankshafts, in preference to straight-8 engines. High performance engines will often have more main bearings than their lower performance cousins, for this reason. In addition, to convert the reciprocating motion into rotation, the crankshaft has "crank throws" or "crank pins", additional bearing surfaces whose axis is offset from that of the crank, to which the "big ends" of the connecting rods from each cylinder attach. The distance of the axis of the crank throws from the axis of the crankshaft determines the piston stroke measurement, and thus engine displacement; a common way to increase the power of an engine is to increase the stroke. This also increases the reciprocating vibration, however, limiting the high RPM capability of the engine; in compensation, it improves the low speed operation of the engine, as the longer intake stroke through smaller valve(s) results in greater turbulence and mixing of the intake charge. For this reason, even such high speed production engines as current Honda engines are classified as long-stroke, in that the stroke is larger than the diameter of the cylinder bore. In production V or flat engines, neighboring connecting rods attach side by side to the same crank throw, simplifying crank design.

The configuration and number of pistons in relation to each other and the crank leads to straight, V or flat engines. The same basic engine block can be used with different crankshafts, however, to alter the firing order; for instance, the 90 degree V6 engine configuration, usually derived by using six cylinders of a V8 engine with what is basically a shortened version of the V8 crankshaft, produces an engine with an inherent pulsation in the power flow due to the "missing" two cylinders, often reduced by use of balance shafts. The same engine, however, can be made to provide evenly spaced power pulses by using a crankshaft with an individual crank throw for each cylinder, spaced so that the pistons are actually phased 60 degrees apart, as in the GM 3800 engine. Similarly, while production V8 engines use 4 crank throws spaced 90 degrees apart, racing engines often use a "flat" crankshaft with throws spaced 180 degrees apart, accounting for the higher pitched, smoother sound of IRL engines compared to NASCAR engines, for example. In engines other than the flat configuration, it is necessary to provide counterweights for the reciprocating mass of each piston and connecting rod; these are typically cast as part of the crankshaft, but occasionally are bolt-on pieces. This adds considerably to the weight of the crankshaft; crankshafts from Volkswagen, Porsche, and Corvair flat engines, lacking counterweights, are easily carried around by hand, compared to crankshafts for inline or V engines, which need to be handled and transported as heavy chunks of metal.

Many early aircraft engines (and a few in other applications) had the crankshaft fixed to the airframe and instead the cylinders rotated, known as a rotary engine design.

In the Wankel engine, the rotors drive the eccentric shaft, which can be considered the equivalent of the crankshaft in a piston engine.

Construction
Crankshafts can be forged or cast from either mild steel or high strength steel, or machined out of a single billet of forged steel. Mild steel is only used for engines in models or other such applications, where the engine runs but does not supply power. Cast crankshafts are usually found in production engines, with forged and billet crankshafts being more expensive but reliable for higher performance. The rough casting or forging is machined to size and shape, the holes are drilled, the main and connecting rod bearing journals are precision ground and case hardened, and the appropriate holes are threaded.

Stress analysis of crankshaft
The crankshaft is subjected to various forces but it needs to be checked in two positions. Firstly, failure may occur at the position of maximum bending. In such a condition the failure is due to bending and the pressure in the cylinder is maximal. Secondly, the crank may fail due to twisting, so the crankpin needs to be checked for shear at the position of maximal twisting. The pressure at this position is not the maximal pressure, but a fraction of maximal pressure.

Invention

Aeolipile
The first piston steam engine, developed by Denis Papin in 1690.The first steam device, the aeolipile, was invented by Heron of Alexandria, a Greek, in the 1st century AD, but used only as a toy. Incidently 700 years earlier in Corinth, Greece, rail tracks were invented; however the Greeks never thought of putting the two together.

In 1690, Denis Papin, a French physicist, built a working model of a steam engine after observing steam escaping from his pressure cooker in about 1679. Sir Samuel Morland also developed ideas for a steam engine during the same period. Early industrial steam engines were designed by Thomas Savery (1698), Thomas Newcomen (1712), and James Watt (patented in 1769), each adding new refinements. Humphrey Gainsborough produced a model condensing steam engine in the 1760s. In 1802 William Symington built the "first practical steamboat", and in 1807 Robert Fulton used the Watt steam engine to power the first commercially successful steamboat.
Early engines worked by the vacuum of condensing steam, whereas later types (such as steam locomotives) used the power of expanding steam.

Use and development

A diagram of Cameron's aero-steam engine, from an 1876 dictionaryThe first industrial applications of the vacuum engines were in the pumping of water from deep mineshafts. The Newcomen engine operated by admitting steam to the operating chamber, closing the valve, and then admitting a spray of cold water. The water vapor condenses to a much smaller volume of water, creating a vacuum in the chamber. Atmospheric pressure, operating on the opposite side of a piston, pushes the piston to the bottom of the chamber. In mineshaft pumps, the piston was connected to an operating rod that descended the shaft to a pump chamber. The oscillations of the operating rod are transferred to a pump piston that moves the water, through check valves, to the top of the shaft.

The first significant improvement, 60 years later, was creation of a separate condensing chamber with a valve between the operating chamber and the condensing chamber. This improvement was invented on Glasgow Green, Scotland by James Watt and subsequently developed by him in Birmingham, England, to produce the Watt steam engine with greatly increased efficiency. The next improvement was the replacement of manually operated valves with valves operated by the engine itself.
Such early vacuum, or condensing, engines are severely limited in their efficiency but are relatively safe since the steam is at very low pressure and structural failure of the engine will be by inward collapse rather than an outward explosion. Their power is limited by the ambient air pressure, the displacement of the working chamber, the combustion and evaporation rates, and the condenser capacity. The maximum theoretical efficiency is limited by the relatively low boiling point of water at near atmospheric pressure (100 °C, 212 °F).

The next big improvement in efficiency came with Richard Trevithick's use of pressurized steam, which used a far greater pressure, but more importantly (from a thermodynamic standpoint) operates at a higher temperature differential. But with this added pressure came much danger and many disasters due to exploding boilers and machinery. The most important refinement at this point was the safety valve, which releases excess pressure. Reliable and safe operation came only with a great deal of experience and codification of construction, operating, and maintenance procedures.

Boilers

Richard Trevithick's No. 14 engine, built by Hazeldine and Co., Bridgnorth, about 1804, and illustrated after being rescued circa 1885; from Scientific American Supplement, Vol. XIX, No. 470, Jan. 3, 1885. Now on display in the National Museum of Science and Industry (The Science Museum), London.Boilers are of two main types:

Fire tube construction is typical of early maritime installations for boats and ships and the boilers of steam locomotives. In a fire tube boiler, the hot gases from the firebox (a combustion chamber) are passed through tubes connecting perforated end plates. The gases then enter a smokebox or smoke chest and pass on to a smokestack. The boiler may be vertical or horizontal. For an example of a vertical boiler of this type observe the boiler in the small riverboat used in the movie The African Queen. This type is also used in some boilers that provide steam for steam heating of a building and was also used in the steam shovel. Locomotives and early ships used a horizontal orientation and early ships would usually require a tall smokestack to provide draft, not having a fan to provide a forced draft. In a steam locomotive the draft is generally augmented at startup by directing the steam exhaust through the smokestack, which provides a partial vacuum.
In a water tube boiler the water is heated in multiple tubes exposed to the hot gases. The tubes are joined to a steam collector chamber at the top. A significant advantage of this type is that there is less chance of catastrophic failure, as there is not a great amount of water in the boiler, nor are there large mechanical elements subject to failure. There may be additional tubes above the collector in the upper portion of the hot gas exhaust - this device, called a superheater, provides additional temperature (and hence pressure) and increases the thermal efficiency of the entire mechanism. Superheaters were also used in some of the later versions of the steam locomotive. There are also rarer variants, for example the drum boiler used in some steam cars.

There is also another division between boilers: natural aspiration, which is nearly all of them, and forced-draft, or "pressure-fired" boilers. This technology, equivalent to supercharging for an internal combustion engine, was developed by the Germans and acquired by the US Navy to be used in some frigates built after the Second World War. In it, a fan is used to increase the rate of burning; the boiler must be constructed to get that extra heat to the water. An engine using this kind of boiler has the greatest acceleration from a standing start of any marine powerplant.

Engines
High pressure steam engines are of various types but most are either reciprocating piston or turbine devices.

Reciprocating

Double-acting
After the development of pressurized steam technology, the next major advance was to the use of double-acting pistons, with pressurized steam admitted alternately to each side while the other side is exhausted to the atmosphere or to a condenser. Most reciprocating engines now use this technology. Power is removed by a sliding rod, sealed against the escape of steam. This rod in turn drives (via a sliding crosshead bearing a connecting rod connected to a crank to convert the reciprocating motion to rotary motion. An additional crank or eccentric is used to drive the valve gear, usually through a reversing mechanism to allow reversal of the rotary motion.

When a pair of double acting pistons is used, their crank phasing is offset by 90 degrees of angle; this is called quartering. This ensures that the engine will always operate, no matter what position the crank is in.
Some ferryboats have used only a single double-acting piston, driving paddlewheels on each side by connection to an overhead rocker arm. When shutting down such an engine it was important that the piston be away from either extreme range of its travel so that it could be readily restarted.

Multiple expansion

Model of a triple expansion engineAnother type uses multiple (typically three) single acting cylinders of progressively increasing diameter and stroke (and hence volume.

High pressure steam from the boiler is used to drive the first and smallest diameter piston downward. On the upward stroke the partially expanded steam is driven into a second cylinder that is beginning its downward stroke. This accomplishes further expansion of the relatively high pressure exhaust from the first chamber. Similarly, the intermediate chamber exhausts to the final chamber, which in turn exhausts to a condenser.The image at the right shows a model of such an engine. The steam travels through the engine from left to right. The valve chest for each of the first two cylinders is to the left of the corresponding cylinder while that of the third is to the right.

One modification of the triple-expansion engine is to use two smaller pistons that sum to the area of the third piston to replace it. This results in the more balanced unit of a total of four pistons arranged in a vee-configuration.
The development of this type of engine was important for its use in steamships, for the condenser would, by taking back a little of the power, turn the steam back to water for its reuse in the boiler. Land-based steam engines could exhaust much of their steam and be refilled from a fresh water tower, but at sea this was not possible. This sort of engine dominated merchant marine applications prior to and during World War II. It even was used in warships before the HMS Dreadnought of 1905.

Uniflow
Another type of reciprocating steam engine is the "uniflow'' type. In this, valves (which act similarly to those used in internal combustion engines) are operated by cams. The inlet valves open to admit steam when minimum expansion volume has been reached a the top of the stroke. For a period of the crank cycle steam is admitted and the poppet inlet are then closed, allowing continued expansion of the steam during the downstroke. Near the bottom of the stroke the piston will expose exhaust ports in the side of the cylindrical chamber. These ports are connected by a manifold and piping to the condenser, lowering the pressure in the chamber to below that of the atmosphere. Continued rotation of the crank moves the piston upward. Engines of this type always have multiple cylinders in an inline arrangement and may be single or double acting. A particular advantage of this type is that the valves may be operated by the effect of multiple camshafts, and by changing the relative phase of these camshafts, the amount of steam admitted may be increased for high torque at low speed and may be decreased at cruising speed for economy of operation, and by changing the absolute phase the engine's direction of rotation may be changed. The uniflow design also maintains a constant temperature gradient through the cylinder, avoiding passing hot and cold steam through the same end of the cylinder. (The uniflow concept is also employed in two stroke supercharged diesel engines used for marine, locomotive, and stationary applications. Such diesels do not need the economizer feature and use a simpler sliding camshaft for reversing.)

Turbine type
Steam turbines for high power applications will use a number of rotating disks containing propeller-like blades at their outer edge. These moving "rotor" disks alternate with stationary "stator" blade rings affixed to the turbine case that serve to redirect the steam flow for the next stage. Owing to the high speed of operation such turbines are usually connected to a reduction gear to drive another mechanism such as a ship's propeller. Steam turbines are more durable, smoother operating, and require far less maintenance than reciprocating engines. A limited number of steam locomotives were manufactured that used turbine technology. While these engines had the typical rods connecting the drive wheels they had no driving rods or cylinders, and no valve links or reversing gear, appearing strangely incomplete to most observers. This locomotive was modeled by Lionel but proved unpopular due to its simple appearance — modelers preferred the complexity and excited motion of the more conventional types.

Rotary type
In theory, it might be possible to use a mechanism based on a pistonless rotary engine such as the Wankel engine in place of the cylinders and valve gear of a conventional reciprocating steam engine. Lack of control of the cutoff is a major problem with such designs, and none has been demonstrated in practice.

Steam powered vehicles

The 1923 Stanley Steam CarNicolas-Joseph Cugnot demonstrated the first functional self-propelled steam vehicle, his "steam wagon", in 1769. Arguably, this was the first automobile. While not generally successful as a transportation device, the self-propelled steam tractor proved very useful as a self mobile power source to drive other farm machinery such as grain threshers or hay balers.Steam engine powered automobiles continued to compete with other motive systems into the early decades of the 20th century. However steam engines are less favored for automobiles, which are generally powered by internal combustion engines, because steam requires at least thirty seconds (in a flash boiler) or so to develop pressure.On February 21, 1804 at the Pen-y-Darren ironworks in Wales, the first self-propelled railway steam engine or steam locomotive built by Richard Trevithick was demonstrated.

Advantages
The strength of the steam engine for modern purposes is in its ability to convert heat from almost any source into mechanical work. Unlike the internal combustion engine, the steam engine is not particular about the source of heat. Most notably, without the use of a steam engine nuclear energy could not be harnessed for useful work, as a nuclear reactor does not directly generate either mechanical work or electrical energy - the reactor itself simply heats water. It is the steam engine which converts the heat energy into useful work. Steam may also be produced without combustion of fuel, through solar concentrators. A demonstration power plant has been built using a central heat collecting tower and a large number of solar tracking mirrors, (called heliostats.Similar advantages are found in a different type of external combustion engine, the Stirling engine, which offers efficient power in a compact engine, but which is difficult to operate over a wide range of operating conditions, difficulties which are readily addressed by the modern hybrid vehicle.

Steam locomotives are especially advantageous at high elevations as they are not especially adversely affected by the lower atmospheric pressure. This was inadvertently discovered when steam engines operated at high altitudes in the mountains of South America were replaced by diesel-electric engines of equivalent sea level power. They were quickly replaced by much more powerful locomotives capable of producing sufficient power at high altitude.In Switzerland (Brienz Rothhorn) and Austria (Schafberg Bahn) new rack steam locomotives have proved very successful. They were designed based on a 1930s design of Swiss Locomotive and Machine Works (SLM) but with all of today's possible improvements like roller bearings, heat insulation, light-oil firing, improved inner streamlining, one-man-driving and so on. These resulted in 60 percent lower fuel consumption per passenger and massively reduced costs for maintenance and handling. Economics now are similar or better than with most advanced diesel or electric systems. Also a steam train with similar speed and capacity is 50 percent lighter than an electric or diesel train, thus, especially on rack railways, significantly reducing wear and tear on the track. Also, a new steam engine for a paddle steam ship on Lake Geneva, the "Montreux" was designed and built, being the world's first ship steam engine with an electronic remote control. The steam group of SLM in 2000 created a wholly-owned company called DLM to design modern steam engines and steam locomotives.

Efficiency
To get the efficiency of an engine, divide the number of joules of mechanical work that the engine produces by the number of joules of energy input to the engine by the burning fuel. In general, the rest of the energy is dumped into the environment as heat. No pure heat engine can be more efficient than the Carnot cycle, in which heat is moved from a high temperature reservoir to one at a low temperature, and the efficiency depends on the temperature difference. Hence, steam engines should ideally be operated at the highest steam temperature possible, and release the waste heat at the lowest temperature possible.

In practice, a steam engine exhausting the steam to atmosphere will have an efficiency (including the boiler) of 5%, but with the addition of a condenser the efficiency is greatly improved to 25% or better. A power station with exhaust reheat, etc. will achieve 30% efficiency. Combined cycle in which the burning material is first used to drive a gas turbine can produce 60% efficiency. It is also possible to capture the waste heat using cogeneration in which the residual steam is used for heating. It is therefore possible to use about 90% of the energy produced by burning fuel - only 10% of the energy produced by the combustion of the fuel goes wasted into the atmosphere.
One source of inefficiency is that the condenser causes losses by being somewhat hotter than the outside world, although this can be mitigated by condensing the steam in a heat exchanger and using the recovered heat, for example to pre-heat the air being used in the burner of an external combustion engine.

The operation of the engine portion alone is not dependent upon steam; any pressurised gas may be used. Compressed air is sometimes used to test or demonstrate small model "steam" engines.

Festivals and museums
Steam Era in Milton, Ontario
Ontario Agricultural Museum in Milton, Ontario
Missouri River Valley Steam Engine Association Back to the Farm Reunion in central Missouri, USA. This is not a steam-only festival, but it has always had a good showing of running steam engines.
Hamilton Museum of Steam and Technology in Hamilton, Ontario. An old municipal pumphouse dating to 1860 with it's original two Woolf Compound Rotative Beam Engines, one of which still operates.

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