A cataract was a speed governing device used for early single-acting beam engines, particularly atmospheric engines and Cornish engines. It was a kind of water clock.
The cataract is distinctly different from the centrifugal governor, in that it does not control the speed of the engine's stroke, but rather the timing between strokes.
The typical installation of a house-built beam engine spanned four floors. The cylinder and the engine driver's usual working position were located in the 'bottom chamber', approximately at ground level. Above this were the 'middle chamber', with the cylinder top cover and 'top nozzle' (the upper valve chest), and above that the 'top chamber' or beam chamber. [3] The cataracts were located in the lowest part of the engine house, in a chamber below the bottom chamber, along with the exhaust pipe. This space was awkward to access and not visited in normal operation.
The valve gear (or 'working gear') of a Newcomen or Cornish engine is based on the plug rod. This is a vertical rod, hung from the beam, and moving in parallel to the piston. Adjustable tappets are attached to this rod. These tappets strike long curved iron levers or 'horns' that are carried on three horizontal shafts or 'arbors'. [lower-roman 1] Each arbor works one of the engine's valves. For the Cornish cycles, these valves are the upper steam inlet to the top of the cylinder, the equilibrium valve that links upper and lower portions of the cylinder, and the lower exhaust and condensing water injection valves, which share an arbor. [4] Unlike most other steam engines, these engines could be run intermittently: making a single stroke before stopping and waiting for the valves to be restarted again. [5] The speed of each power stroke or 'coming indoors' was a feature of the engine and was not easily varied, but there was no need for the engines to run continuously, stroke after stroke. [4] [6] This was a direct contrast to the rotative beam engine, and the rotary nature of almost all other steam engines. With the original Newcomen cycle, the speed of the return stroke varied according to the boiler pressure, although this still did not affect the strength or speed of the power stroke. [7]
Use of a cataract could allow an engine to be operated at only a third of its ungoverned speed. [8] When pumping load was variable, cataracts could also be connected and disconnected as required, allowing the engine to work at full speed for a period and then stopped in between. [9]
The cataract itself resembled a small plunger pump. [2] It was an iron box in a cistern filled with water, with a plunger or piston set in the top and pressed downwards by a weight. The water within the pump could only escape through a small tap or valve. [10] [11] As the plunger gradually fell, its motion was passed upwards by a rocking lever and a rod to the valvegear in the middle chamber. Once the rod had risen sufficiently, this opened the first valve to admit steam into the upper part of the cylinder, beginning a new stroke. [lower-roman 2]
Once the stroke had begun, the cataract's rocking lever was pushed downwards by the engine. This lifted the plunger, which acted as a suction pump within the cataract to refill the plunger box, through a flap valve from its surrounding cistern. [13] The cistern was kept filled with water by the pump that the engine itself was working.
The water outlet valve was controlled by a rod from the bottom chamber. This was used by the engine's driver to control the working speed, according to the work required. [13]
The cataract's actuating rod also had a screw adjuster, which acted to vary the water injection time (Newcomen) or the phasing between the inlet and exhaust valves (Cornish). [13] This could be used to give a longer and more effective condensation time, if the condensing water supply was warm, as in the Summer. This adjustment appears to have been poorly understood though, and little used by the engine drivers. [10]
The cataract first appeared on Newcomen engines in Cornwall, although their inventor is unknown. They were known in Smeaton's time, and they may be another of the developments to Newcomen's engine for which he was responsible. [6] James Watt encountered these on his trip to Cornwall in 1777. [14] They were of a simpler type, these early cataracts or 'jack in the box' were a simple tumbling box: a wooden box on a pivot was filled with water through an adjustable cock. [6] When the box was filled sufficiently to overbalance, the engine's injection valve would be triggered.
Boulton and Watt used the simple tumbling box design of cataract for some years afterwards, to around 1779. [15] [16] After this other designs were used, including a water cataract where the same water was used and recycled continuously and also an air cataract using a circular bellows. An air cataract of this type was supplied for the Ale and Cakes Mine. The plunger pump design of cataract had appeared in Cornwall by 1785, but was not Watt's invention. [15]
The term 'cataract' became a synonym for dashpot, at least where this was associated with steam engines and their governors. They were used as a damping device to avoid over-sensitivity with centrifugal governors. [17]
Cataracts were also used as an over-speed safety device for direct-acting water pumps. [lower-roman 3] A seesaw or 'differential' lever was placed between the pump's piston rod and a cataract adjusted for the pump's normal working speed. If the pump suddenly accelerated, owing to the pump bursting or similar, the piston would overtake the cataract and the action of the differential lever would then close the pump's steam inlet valve and stop the pump, limiting possible damage. [18]
The cataract, like most regulators, is an example of a servomechanism. However unlike the better-known Watt centrifugal governor, this is an open-loop, rather than closed-loop control. The cataract runs at its own speed, but does not measure the resultant speed of the engine. The cataract has also been described as a 'water clock'. [15] This assumes that the relationship between the cataract's operation and the engine's speed is fixed, which is a valid assumption for a beam engine as the cataract controls the timing of the engine's stroke, rather than a variable power or throttle valve. Where a governor controls such a throttle valve, as for the Watt governor, the speed of the engine depends on a complex and unpredictable relation between the engine load, the valve position and the varying efficiency of the engine. Such governors must use a closed-loop control if they are to maintain an effective and precise regulation.
One advantage of the independent and open loop nature of the cataract's control was that two engines could be adjusted to run in synchronisation, but in antiphase. With pumping engines, this gave a more even output to their pumping. [19]
Although the centrifugal governor was already known from its use for water- and windmills, it was not until 1788 when Watt was the first to apply it to a steam engine. [20] This was the 'Lap Engine', an early rotative engine now preserved in the Science Museum, London.
With a rotative engine, it was necessary to control the rate at which an engine moved throughout its stroke, not merely to vary the timing between strokes. This required the use of a throttle valve in the steam supply, controlled by the governor. As the load on mill engines and similar uses could vary, closed-loop control such as the engine speed-based centrifugal governor was also needed. [20] The cataract was thus not used on rotative engines, not even where single-acting Cornish winding engines were still used in Cornwall. [21]
Cornish engines were not amenable to control by a throttle valve, as their operating cycle depended on the condensation time more than a throttled steam supply. Non-rotative beam engines also had no easy means to drive a centrifugal governor. For these reasons the cataract remained in service for as long as the Cornish engine did. [22]
A steam engine is a heat engine that performs mechanical work using steam as its working fluid. The steam engine uses the force produced by steam pressure to push a piston back and forth inside a cylinder. This pushing force can be transformed, by a connecting rod and crank, into rotational force for work. The term "steam engine" is generally applied only to reciprocating engines as just described, not to the steam turbine. Steam engines are external combustion engines, where the working fluid is separated from the combustion products. The ideal thermodynamic cycle used to analyze this process is called the Rankine cycle. In general usage, the term steam engine can refer to either complete steam plants, such as railway steam locomotives and portable engines, or may refer to the piston or turbine machinery alone, as in the beam engine and stationary steam engine.
A centrifugal governor is a specific type of governor with a feedback system that controls the speed of an engine by regulating the flow of fuel or working fluid, so as to maintain a near-constant speed. It uses the principle of proportional control.
The atmospheric engine was invented by Thomas Newcomen in 1712, and is often referred to as the Newcomen fire engine or simply as a Newcomen engine. The engine was operated by condensing steam drawn into the cylinder, thereby creating a partial vacuum which allowed the atmospheric pressure to push the piston into the cylinder. It was historically significant as the first practical device to harness steam to produce mechanical work. Newcomen engines were used throughout Britain and Europe, principally to pump water out of mines. Hundreds were constructed throughout the 18th century.
The Watt steam engine design became synonymous with steam engines, and it was many years before significantly new designs began to replace the basic Watt design.
A governor, or speed limiter or controller, is a device used to measure and regulate the speed of a machine, such as an engine.
Stationary steam engines are fixed steam engines used for pumping or driving mills and factories, and for power generation. They are distinct from locomotive engines used on railways, traction engines for heavy steam haulage on roads, steam cars, agricultural engines used for ploughing or threshing, marine engines, and the steam turbines used as the mechanism of power generation for most nuclear power plants.
Steam power developed slowly over a period of several hundred years, progressing through expensive and fairly limited devices in the early 17th century, to useful pumps for mining in 1700, and then to Watt's improved steam engine designs in the late 18th century. It is these later designs, introduced just when the need for practical power was growing due to the Industrial Revolution, that truly made steam power commonplace.
Main components found on a typical steam locomotive include:
Improvements to the steam engine were some of the most important technologies of the Industrial Revolution, although steam did not replace water power in importance in Britain until after the Industrial Revolution. From Englishman Thomas Newcomen's atmospheric engine, of 1712, through major developments by Scottish inventor and mechanical engineer James Watt, the steam engine began to be used in many industrial settings, not just in mining, where the first engines had been used to pump water from deep workings. Early mills had run successfully with water power, but by using a steam engine a factory could be located anywhere, not just close to a water source. Water power varied with the seasons and was not always available.
A beam engine is a type of steam engine where a pivoted overhead beam is used to apply the force from a vertical piston to a vertical connecting rod. This configuration, with the engine directly driving a pump, was first used by Thomas Newcomen around 1705 to remove water from mines in Cornwall. The efficiency of the engines was improved by engineers including James Watt, who added a separate condenser; Jonathan Hornblower and Arthur Woolf, who compounded the cylinders; and William McNaught, who devised a method of compounding an existing engine. Beam engines were first used to pump water out of mines or into canals but could be used to pump water to supplement the flow for a waterwheel powering a mill.
A Corliss steam engine is a steam engine, fitted with rotary valves and with variable valve timing patented in 1849, invented by and named after the US engineer George Henry Corliss of Providence, Rhode Island.
A Cornish engine is a type of steam engine developed in Cornwall, England, mainly for pumping water from a mine. It is a form of beam engine that uses steam at a higher pressure than the earlier engines designed by James Watt. The engines were also used for powering man engines to assist the underground miners' journeys to and from their working levels, for winching materials into and out of the mine, and for powering on-site ore stamping machinery.
The first recorded rudimentary steam engine was the aeolipile mentioned by Vitruvius between 30 and 15 BC and, described by Heron of Alexandria in 1st-century Roman Egypt. Several steam-powered devices were later experimented with or proposed, such as Taqi al-Din's steam jack, a steam turbine in 16th-century Ottoman Egypt, and Thomas Savery's steam pump in 17th-century England. In 1712, Thomas Newcomen's atmospheric engine became the first commercially successful engine using the principle of the piston and cylinder, which was the fundamental type of steam engine used until the early 20th century. The steam engine was used to pump water out of coal mines.
An expansion valve is a device in steam engine valve gear that improves engine efficiency. It operates by closing off the supply of steam early, before the piston has travelled through its full stroke. This cut-off allows the steam to then expand within the cylinder. This expanding steam is still sufficient to drive the piston, even though its pressure decreases as it expands. As less steam is supplied in the shorter time for which the valve is open, use of the expansion valve reduces the steam consumed and thus the fuel required. The engine may deliver two-thirds of the work, for only one-third of the steam.
A blowing engine is a large stationary steam engine or internal combustion engine directly coupled to air pumping cylinders. They deliver a very large quantity of air at a pressure lower than an air compressor, but greater than a centrifugal fan.
Old Bess is an early beam engine built by the partnership of Boulton and Watt. The engine was constructed in 1777 and worked until 1848.
High-speed steam engines were one of the final developments of the stationary steam engine. They ran at a high speed, of several hundred rpm, which was needed by tasks such as electricity generation.
Resolution was an early beam engine, installed between 1781 and 1782 at Coalbrookdale as a water-returning engine to power the blast furnaces and ironworks there. It was one of the last water-returning engines to be constructed, before the rotative beam engine made this type of engine obsolete.
The Lap Engine is a beam engine designed by James Watt, built by Boulton and Watt in 1788. It is now preserved at the Science Museum, London.
Valve gear opens and closes valves in the correct order. In rotating engines valve timings can be driven by eccentrics or cranks, but in non-rotative beam engines these options are not available. In the Cornish engine valves are driven either manually or through ‘plug rods’ and tappets driven from the beam. This permits the insertions of delays at various points in the cycle, allowing a Cornish Engine to vary from one stroke in ten minutes, to ten or more strokes in one minute, but also leads to some less familiar components when compared with rotative engines.
