II.

History

The first piston engine was developed in 1690 by the French physicist and inventor Denis Papin and was used for pumping water. Papin's engine, which was little more than a curiosity, was a crude machine in which the actual work was done by air rather than steam pressure. It consisted of a single cylinder that also served as a boiler. A small amount of water was placed in the bottom of the cylinder and heated until steam was formed. The pressure of this steam raised a piston fitting in the cylinder, and, after it was raised, the source of heat was removed from the bottom of the cylinder. As the cylinder cooled, the steam condensed and air pressure on the upper side of the piston forced the piston down.

In 1698, the English engineer Thomas Savery built a steam engine that used two copper vessels alternately filled with steam from a boiler. Savery's engine was also used for pumping water, as was the so-called atmospheric engine designed by the English inventor Thomas Newcomen in 1705. This device had a vertical cylinder and a piston that was counterweighted. Steam admitted to the bottom of the cylinder at very low pressure acted with the counterweight to move the piston to the top of the cylinder. When the piston reached this point, a valve opened automatically and sprayed a jet of cold water into the cylinder. The water condensed the steam, and atmospheric pressure forced the piston back to the bottom of the cylinder. A rod attached to the arm of the pivoted beam that connected piston and counterweight moved up and down as the piston moved, actuating a pump. Newcomen's engine was not efficient, but it was sufficiently practical to be used extensively for pumping water from coal mines.

In the course of making improvements to the Newcomen engine, the Scottish engineer and inventor James Watt produced a series of inventions that made possible the modern steam engine. Watt's first important development was the design of an engine that incorporated a separate condensing chamber for the steam. This engine, patented in 1769, greatly increased the economy of the Newcomen machine by avoiding the loss of steam that occurred in alternate heating and cooling of the engine cylinder. In Watt's engine, the cylinder was insulated and remained at steam temperature. The separate condenser chamber, which was water-cooled, was equipped with a pump to maintain a vacuum so that the steam was drawn from the cylinder to the condenser. The pump was also used to remove the water from the condenser chamber.

Another radical departure in the design of the early Watt engines was the use of steam pressure instead of atmospheric pressure to perform the actual work of the engine. Watt also devised a method in which the reciprocating pistons of engines drove a revolving flywheel. He accomplished this first by a system of gearing, and later by means of a crankshaft, as in modern engines. Watt's other improvements and inventions included application of the principle of double action, whereby steam was admitted to each end of the cylinder alternately to drive the piston back and forth. He also equipped his engines with throttle valves to control speed and also with governors in order to maintain automatically a constant speed of operation.

The next important development in the field of steam engines was the introduction of practical noncondensing engines. Although Watt had recognized the principle of the noncondensing engine, he had been unable to perfect machines of this type, probably because he used steam at extremely low pressure. At the beginning of the 19th century the British engineer and inventor Richard Trevithick and the American inventor Oliver Evans devised successful noncondensing engines using the high-pressure steam. Trevithick used this model of steam engine to power the first railway locomotive ever made. Both Trevithick and Evans also built steam-powered carriages for road travel.

At about the same time, the first compound steam engines were built by the British engineer and inventor Arthur Woolf. In the compound engine, steam at high pressure is used in one cylinder and then, after it has expanded and consequently lessened in pressure, is piped to another cylinder, in which it expands still further. Woolf's original engines were of the two-cylinder type, but later types of compound engines used triple and even quadruple expansion. The advantage of compounding two or more cylinders is that less energy is lost in the heating of the cylinder walls; as a result, the engine is more efficient.