High-pressure, high-volume, small-scale production processes were still nearly two centuries away when Watt developed the first practical steam engine in 1765. His design was an efficient hybrid of the Newcomen and vacuum piston engines that used only 25% as much fuel as a Newcomen engine while also expending 25% less steam. There are many ways to produce power from heat, mechanical energy, or chemical reactions. The most useful for industrial manufacturing is the power cycle: heat → expansion (or pressure) → mechanical output. The most useful process for producing continuous mechanical output is an engine. The following article details the technical principles and specific components of Watt’s steam engine. It explains how his design made it possible to use a single valve to both exhaust and introduce steam into the cylinder a significant improvement over earlier engines and how it simplified other parts so that they could be produced on a large scale with standard dimensions and materials.
The Watt Steam Engine
The Watt steam engine is one of the most important machines in the history of civilization. Watt’s improvements over earlier designs made it possible to use a single valve to both exhaust and introduce steam into the cylinder a significant improvement over earlier engines and simplified other parts so that they could be produced on a large scale with standard dimensions and materials. This made it practical to run an engine 24 hours a day, 7 days a week, which enabled factories to run more products at higher speeds and with more consistent quality. The steam engine’s continuous production cycle of heat → expansion of air → work → cooling was so efficient that it powered the Industrial Revolution. Watt’s engine improved on the vacuum piston design by using steam at a higher pressure, around 100 psi (690 kPa), and a lower volume, around 1–2 gallons (3–8 liters) per minute. The higher pressure and smaller volume enabled him to put a separate valve on the steam inlet, which shut off the steam supply completely and allowed the cylinder to be completely emptied of steam. This control of steam pressure and volume made it possible for the engine to be used for a variety of different tasks, such as powering cranes, pumps, and textile looms.
The Steam Valve
To completely shut off the steam supply, Watt invented a steam valve that replaced Newcomen’s ball-and-pit system. Newcomen’s steam valve was essentially a gate that was opened and closed by a sliding rod attached to a lever. To control the valve, a worker had to stop the engine, lift the rod, and manually turn the valve. Watt replaced this system with a rotating plug inside a cylinder (a valve seat) that was turned by a stationary rod attached to the lever. This allowed the valve to be easily and accurately turned by the lever, with just the right amount of force to fully close the valve. Watt’s valve had two separate ports on the front and back of the plug and two corresponding holes in the valve seat. Steam from the boiler entered the front port and also pushed against the back of the plug, which was opened by a spring. When the lever was pulled, the plug turned and a port on the back of the plug was closed by the valve seat. This completely shut off the steam supply and allowed the engine to be stopped and restarted as needed. The valve seat rotated with the plug and always formed a seal against the rotating plug.
A Flyball Governor
The flyball governor was a centrifugal controller that regulated the speed of the engine. The more steam that was allowed into the engine, the faster it spun, and vice versa. The flyball worked by allowing steam to enter one side of the engine and a ball to enter the other. A weighted flywheel was attached to the ball and spun at a constant rate. When the engine was running too fast, the flywheel pulled the ball towards the other side and opened a valve that allowed more steam into the engine. When the engine was running too slowly, the flywheel pulled the ball back and closed the valve to reduce the amount of steam. Because the flywheel turned at a constant rate, it was necessary to add or remove a ball to change the overall speed of the engine. The number of ball weights needed to run the engine at a certain speed was calculated by careful measurement and recorded on a table. This allowed the flyball governor to run the engine at a consistent speed and was a simple but effective way to maintain consistent power output and speed.
Watt’s condenser: reheating the steam
The steam engine is an efficient but wasteful process that uses fuel to heat water to create a power cycle. This process is like heating a bathtub full of water with a fire and then immediately dumping the water down the drain. The flyball governor only controls the speed of the engine, not its efficiency. To reduce the amount of wasted energy and fuel, Watt added a condenser to his engine. The condenser was a separate tank that contained a set of interconnected pipes with a valve on the top and bottom. Steam from the engine was directed to the top of the condenser, where it cooled and condensed back into the water. The water was collected at the bottom of the tank and then piped back to the engine to be heated and converted into steam again.
Watt’s rotary valve
Watt’s rotary valve was a rotary plug in the steam valve that allowed the cylinder to be filled with steam and then quickly and automatically shut off again when the steam was exhausted. The rotary valve was shaped like a spool and turned freely inside a cylinder that was closed on both ends. Two steam ports in the spool were connected to the boiler and the cylinder. When the engine was producing power, steam flowed into the cylinder and pushed against the spool. The spool turned and pressed against the steam ports, closing them off. The valve was then opened so that steam could flow from the boiler and fill the cylinder. When the steam was exhausted and the spool was pushed back towards the ports, the spool rotated and shut off the steam from the boiler.
Watt’s crank and connecting rod mechanism
When the rotary valve was opened, the steam pressure and volume inside the cylinder were controlled by stopping the engine and closing the valves. When the engine was restarted, the valves had to be opened once again and the engine had to be started by hand. Watt’s solution to this problem was to attach a flywheel to the piston and use a crank and connecting rod mechanism to convert the piston’s rotating motion into a back-and-forth motion. This allowed the piston to be turned with a crank that was turned by a flywheel attached to a rotating shaft. When the engine was stopped, the connecting rod was extended so that the piston was pushed by the steam pressure against the flywheel and turned the crank. When the piston was pushed back, the connecting rod turned and was pulled towards the flywheel by the piston. This allowed the crank to drive the piston back and forth and power the engine.
Watt’s design was an efficient hybrid of the Newcomen and vacuum piston engines that used only 25% as much fuel as a Newcomen engine while also expending 25% less steam. Watt’s efficiency improvements were made possible by using steam at a higher pressure and a lower volume, which enabled a separate valve on the steam inlet, allowing it to be completely shut off. This control of steam pressure and volume made it possible for the engine to be used for a variety of different tasks, such as powering cranes, pumps, and textile looms. Watt’s efficiency improvements were also made possible by using steam at a higher pressure and a lower volume, which enabled a rotary valve to be used to fill and exhaust the cylinder. Watt’s efficiency improvements were also made possible by using steam at a higher pressure and a lower volume, which enabled a crank and connecting rod mechanism to be used to convert the piston’s rotating motion into a back-and-forth motion.