Slide valves controlled steam flow in steam engines, allowing steam to enter and exhaust to exit. They were replaced by piston valves due to their need for more power and lubrication. Modern machines use more efficient valves, such as poppet valves in cars and throttle valves in certain tools.
Slide valves were commonly used in steam engines to control the flow of steam into and out of the engine. The valves were slid from side to side in time with the other moving parts to allow steam to enter the engine and exhaust to exit through ports, or openings, in the top of a single cylinder. They serve the same purpose as the valves in today’s cars, to allow a gas mixture to enter and exit the engine in order to generate power.
When the slider valve is slid to the side, it exposes the intake port and allows vapor to fill the cylinder. This steam pushes the piston, which is inside the cylinder, and the piston in turn pushes a rod that turns the flywheel. The flywheel turns to create motion so that the steam engine can move. When the slide valve slides to the opposite side, it opens the exhaust port and releases the vapor as the piston slides back and the process starts all over again.
Car engines also use valves to allow fuel and gas to enter the cylinders and exhaust to exit. While they perform the same functions, cars generally use poppet valves that move up and down on springs. Slide valves in steam engines move back and forth by the rod without the use of springs. Eventually slide valves were replaced by piston valves. Although their function is similar, piston valves have several advantages over the slide valve.
The pitch in which a slide valve operated had to be quite long, and more power was needed to move the valve back and forth. This move also required a lot of lubrication to ensure the valves didn’t stick or slow down, causing the engine to malfunction. However, piston valves could operate in shorter steps and were much easier to lubricate. This increased the efficiency of the steam engine, and the slide valve became less common.
The use of superheated steam (steam produced at a temperature above the boiling point of water) also led to the decline of slide valves and the rise in popularity of piston valves. These super hot temperatures interfere with the lubrication needed for slide valves. Modern machines have moved from these earlier valve types to more efficient valves. In addition to the poppet valves used in automobiles, throttle valves are still used in certain aircraft and tools, such as riding lawn mowers. The goal when choosing a valve for a machine is to create the highest engine efficiency with the least amount of work to open and close the valves for proper operation.
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