Steam engine

A steam engine is a heat engine that makes use of the thermal energy that exists in steam, converting it to mechanical work. Steam engines were used in pumps, locomotive trains, steam ships and steam tractors, and were essential to the Industrial Revolution. They are still used for electrical power generation using steam turbines.

Engines

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

Related Topics:
Reciprocating piston - Turbine

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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.

Related Topics:
Crosshead bearing - Connecting rod - Crank - Eccentric

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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.

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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.

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Multiple expansion

Another type uses multiple (typically three) single-acting cylinders of progressively increasing diameter and stroke (and hence volume).

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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.

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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.

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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.

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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.

Related Topics:
World War II - HMS Dreadnought - 1905

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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.)

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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).

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Rotary type

The Quasiturbine is a recent concept of rotary steam engine, with the mechanical concept also applicable as an internal combustion engine.

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