As I alluded to in a previous post, when originally proposed, the steam engine did not resemble what we are now familiar with. It was invented by Thomas Newcomen in 1712 following earlier work by Thomas Savery. In this engine, more properly called an atmospheric steam engine, the steam is not directly involved in the power stroke. Before the power stroke, the steam, which is at roughly atmospheric pressure, has been drawn into the vertical cylinder by the rising of the piston. After the cylinder is full of steam, a jet of water is sprayed into the now closed chamber, which cools the steam and condenses it, causing a partial vacuum in the chamber. Atmospheric pressure presses down on the top of the piston in the power stroke, lowering the piston and causing a water pump attached to the piston to operate a single stroke. The chamber is reopened to the steam source, and the weight of the beam raises the piston, drawing new steam into the chamber. For an illustration, see technology.niagarac.on.ca/people/mcsele/newcomen.htm
Note in this explanation, there is only one power stroke, in one direction. The piston moves in the other direction by action of the weight of the beam, which has been raised in the power stroke. Because power is exerted on only one side of the piston, such a steam engine is "single action." Another example of a "single action" piston would be any of the pistons in an internal combustion engine in an automobile. The power stroke in the cylinder of an automobile engine always acts on one, and only one side of the piston.
James Watt was able to make many improvements on Newcomen's invention. For instance, rather than spray water into the cylinder, which lowered the temperature of the cylinder with each stroke, Watt condensed the steam using a separate condenser chamber. But his engine initially was also a "single action", atmospheric engine. In the course of further improvements, he realized that rather than using the condensation of steam to cause the atmosphere to power the engine, he could use the expansion of pressurized steam to power the piston directly. He also realized that he could derive a power stroke from both directions of piston movement if the cylinder was designed to do so.
In proposing these improvements, James Watt invented the "double action" steam engine, the modern steam engine. It is "double action" because there are two power strokes, one in each direction.
|QUOTE: From A HISTORY OF THE GROWTH OF THE STEAM-ENGINE.|
by ROBERT H. THURSTON, A. M., C. E.,
The double-acting engine was a modification of the single-acting engine, and was very soon determined upon after the successful working of the latter had become assured.
Watt had covered in the top of his single-acting engine, to prevent cooling the interior of the cylinder by contact with the comparatively cold atmosphere. When this had been done, there was but a single step required to convert the machine into the double-acting engine. This alteration, by which the steam was permitted to act upon the upper and the lower sides of the piston alternately, had been proposed by Watt as early as 1767, and a drawing of the engine was laid before a committee of the House of Commons in 177S'75. By this simple change Watt doubled the power of his engine. Although invented much earlier, the plan was not patented until he was, as he states, driven to take out the patent by the " plagiarists and pirates " who were always ready to profit by his ingenuity. This form of engine is now almost universally used. The single-acting pumping-engine remains in use in Cornwall, and in a few other localities, and now and then an engine is built for other purposes, in which steam acts only on one side of the piston; but these are rare exceptions to the general rule.
Watt also invented the compound steam engine, the principal of which was used by Anatole Mallet to develop the type of articulated locomotive named after him.
Compound steam engines are common in marine engines for example. An excellent illustration of a compound marine engine can be found in the Wikipedia article at http://en.wikipedia.org/wiki/Steam_engine It is an example of a triple expansion compound steam engine with double-acting pistons.
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