Developed between 1763 and 1775, the Watt steam engine was an extension and improvement of the Newcomen engine. The Watt was the first steam engine to drive its piston via pressurized steam and a partial vacuum. James Watt’s design is considered a key step in the evolution of modern mechanical engines.
Improving the Newcomen Engine
Developed by Thomas Newcomen, the Newcomen engine was far superior to previous incarnations of the steam engine, offering significant improvements in efficiency. Older steam engine models consistently lost steam, and therefore power, at the end of each stroke.
The Newcomen engine’s “atmospheric” design used a cylinder with a movable piston connected by a chain to one end of a pivoting beam, which operated a mechanical pump at the opposite end. Steam entered the cylinder below the piston with each stroke, followed by water, which condensed the steam. As the upper end of the steam cylinder was open, this created a partial vacuum that drew the piston down and raised the far end of the beam.
After being tasked with repairing a Newcomen engine at the University of Glasgow in Scotland in 1763, Watt noticed its inefficiencies and saw room for improvement. Based on his observations, Watt made several changes to the Newcomen engine’s design.
A separate condenser outside of the steam cylinder itself condensed the steam without the need for water spray, which cooled the piston and cylinder walls, reducing efficiency. The two chambers were connected, allowing for condensation and power transfer without loss of heat—the condenser remained cool, while the steam cylinder stayed hot.
Watt sealed the top of the steam cylinder and devised a method of injecting low-pressure steam into the area above the piston. This boosted the vacuum and increased the power of the down stroke, which in turn improved the speed and efficiency of the engine.
Finally, adjusting the design to produce rotary motion, instead of the previous oscillating movement of the Newcomen, proved more useful for industrial applications.
The Boulton Partnership
Though his design was completed and he had built a functioning model (ca. 1774), Watt was unconvinced that a marketable version of his engine could be developed. Watt entering into a partnership with Matthew Boulton, a Birmingham entrepreneur. Boulton funded the development of a full-scale test engine, giving Watt access to the facilities, capital, and craftsmen needed to bring his vision to life.
Boulton and Watt went into business selling and installing the steam engines in mines, ironworks, and other industrial locations. The bulk of their profit, however, came from a licensing fee charged to all owners of the steam engines, based on the fuel savings they generated—a Watt engine used only about one-quarter the fuel that a similarly-sized Newcomen engine would. Despite the licensing fees, this exceptional fuel efficiency made the Watt steam engine the far more attractive option for most businessmen.
The first Watt steam engines used hammer iron cylinders, which were out of round and caused leakage in the piston. By 1776, an inventor named John Wilkinson had developed a boring machine that was able to produce cylinders up to 50 inches in diameter with perfect precision. The introduction of bored cylinders further improved the Watt engine’s performance.
With a special arrangement of valves, steam could be admitted to either end of the engine. This allowed the direction of the power stroke to be reversed, creating the world’s first double-acting steam engine. This development also improved the engine’s efficiency and speed, and produced a more regular, stable motion.
Using a unique four-bar linkage, coupled with a pantograph—a development Watt dubbed “parallel motion”—the chain that once connected the piston rod and the engine’s moving beam was replaced. This allowed the piston to both push and pull with equal force.
This, in turn, made it possible for the motion of the beam to turn a wheel. Watt’s first rotary motion solution connected the beam to a wheel by a crank—however, the use of the crank was patented, so another method had to be developed. Using an epicyclic sun and planet gear system, Watt created a unique solution. (Later, after the patent on the crank expired, Watt reverted to this more-effective method.)
To ensure that the turning wheel operated at a consistent speed, a steam regulator valve was attached to a centrifugal governor. Watt based this design on the automatic speed controls used on windmills of his day.
With these improvements, Watt’s steam engine became an effective and reliable replacement for the water wheels and (literal) horsepower that had, until that point, powered British industry. Because it significantly improve efficiency and removed the need for a source of flowing water, the Watt steam engine allowed for vast expansion of industry throughout the country and drove the Industrial Revolution to new heights.