In 1788, seventy six years had elapsed since Newcomen's first successful engine was erected. The flying shuttle and spinning jenny were used in factories that were still powered by water. The process of smelting iron with coke rather than with charcoal had been developed. One of the incentives for improved iron-working techniques was the demand for cylinders for Newcomen mine pumping engines. Iron cylinders cost one-tenth as much as the brass cylinders that Newcomen had been forced to use, and by 1775 they could be cast in sizes up to six feet in diameter and ten feet in length, and be finished by machine.
In building his engine, Thomas Newcomen combined a layman's understanding of imperfectly explained scientific theory with an artisan's knowledge of material and fabrication techniques. Many men prospered by erecting Newcomen engines in England and Europe. Some of these men improved the details of the engine. One engineer, John Smeaton, reduced the design of the engine almost to a science, and made so many improvements on it that he increased its efficiency by fifty percent. But until James Watt, nobody successfully altered the basic philosophy of the engine.
James Watt was almost the ideal man for making the many radical changes that he did in the Newcomen Engine. During his early lifetime the philosophical study of heat had become a quantitative science. Much of the work was done at Glasgow University where Watt was a builder and repairer of mathematical instruments. Joseph Black, who developed the concepts of specific heat of materials and latent heat of steam, was a close friend of Watt's, and Watt not only built instruments for him, but also assisted him in his experiments. As a result of his education and trade Watt had an excellent grasp of mathematical, mechanical and geometric principles, and could apply them skillfully to practical problems.
James Watt first became interested in steam power while repairing a model of a Newcomen Engine in 1763. He soon realised that it could be improved, and spent the rest of his active life on the project.
Watt's first improvement on the Newcomen engine, the separate condenser, increased the efficiency of the device by a factor of three and a half. Watt devised the separate condenser after he realised that most of the steam which entered the cylinder of the Newcomen engine was being used to reheat the cylinder. This heat energy was lost each time the spray of cold water cooled the cylinder during the power stroke. Watt reasoned that if the steam could be condensed elsewhere to create the vacuum needed for the power stroke the cylinder would remain hot, and a saving in fuel would result.
Watt's external condenser is shown below and to the left of the cylinder of this engine, in the wooden well. The pump, driven by the great lever overhead, removes the condensate from the condenser to a hot water reservoir which, in turn, feeds the boiler. When the engine is running the well is filled with cold water to hasten the process of condensation. The condenser operates by a spray of cold water which is turned on at the start of each power stroke.
The second stage of Watt's development was to surround the cylinder with a steam jacket to keep it hot, and to apply steam rather than air and water to the atmospheric side of the piston for the same reason. This required that the piston rod pass through a stuffing box at the top of the cylinder to prevent steam leakage.
Since both ends of the cylinder were now scaled it was a logical next step to make both strokes of the piston do work, by alternately connecting both ends of the cylinder to the vacuum. First, however, a rigid connection between the piston and the great lever had to be devised, since the piston now had to push the lever upward as well as pull it downward. Out of his many inventions, Watt was proudest of the parallel motion linkage, shown on this model between the top of the piston rod and the left end of the great lever. This linkage removes all side motion from the end of the piston rod so it can pass freely up and down through the stuffing box at the top of the cylinder.
Prior to Watt's development of rotative motion from a steam engine, steam had already been used to power factories. This was done by supplying reservoirs with water by Newcomen pumps and using the water to run waterwheels. These devices were known as water return engines. Although Watt could not see the reason for improving on this technique, and was content to supervise the erection of pumping engines, he developed the rotative motion shown here on the insistence of his partner, Matthew Boulton.
Watt was undoubtedly a genius but he had a limitation that held up the progress of his inventions many times. Unlike men like Newcomen and Smeaton who would use the best available technique or material, Watt disliked using anything that someone else had thought of. An inventor name Pickard had patented a crank operated steam engine. He offered Watt the use of his invention on liberal terms. Although the crank was a well known device and was used in such homely devices as the spinning wheel, Watt refused to use it on his steam engine, and even claimed that it would not work. He patented five other actions for converting oscillating motion to rotation. Of these, only the sun and planet gear arrangement shown on this engine was practical. All Watt's engines used this action until Pickard's patent ran out. After that Watt quietly converted to the crank. In the meantime, many Newcomen engines had been converted to rotative engines with simple crank arrangements. However, James Wattt was the first man to build a steam engine that could power mills and factories.
A notable exception to Watt's rejection of prior art is the flyball governor. This device was originally used to control the distance between grindstones of wind and water operated mills. Watt used it to adjust the throttle valve of his engine so that the engine would run at constant speed under varying loads. If the engine speed increases beyond the operating level the two rotating weights speed up and are thrown outward by increased centrifugal force. This outward motion is transferred from the governor to the throttle valve by a mechanical linkage, and causes the throttle to close partially, thereby slowing down the engine. Conversely, if the engine slows down, the centrifugal force on the weights is lessened and they move inward. This causes the throttle to open, and the engine speeds up.
Two auxiliary pumps are operated from the right side of the great lever. One of these supplies water to the condenser well. The other transfers water from the hot water reservoir, located between the main supports of the great lever, to the boiler which is not shown in this model.
The operation of the engine is as follows:
With the piston at the bottom of its stroke, steam under slight pressure is admitted to the lower half of the cylinder. Simultaneously the upper half of the cylinder is connected to the condenser by operation of a valve. The steam above the piston rushes into the condenser where it is condensed by a spray of cold water, thus increasing the vacuum. The difference in pressure across the piston drives the piston upward in a power stroke. The power is transferred to the great lever by the parallel motion linkage. From the opposite end of the great lever the power is converted to rotary motion by the sun and planet gears.
At the top of the stroke the valve action is reversed and the piston is driven downward. Because this engine derives two power strokes from each cycle it is classed as a double-acting engine.
James Watt used steam slightly above atmospheric pressure for this engine. This enabled him to close his steam inlet valves before the end of the stroke, allowing the steam to cool and expand as the piston travelled further. This use of the expansive power of steam accounted for great operating economies in the Watt engine, and is a feature of all modern steam engines. Watt refused to raise the pressure of steam beyond the modest pressures used in this engine and fought the concept vigorously. He considered this engine to be the most advanced that would ever be required. It took other men to make the steam engine smaller, more compact, more powerful and more efficient.