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What is a Slide Valve?

By Jessica Reed
Updated May 23, 2024
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Slide valves were commonly used in steam engines to control the flow of steam in and out of the engine. The valves slid from side to side in time with the other moving parts to allow steam to enter the engine and the exhaust to exit through the ports, or openings, at the top of a single cylinder. They serve the same purpose valves in today’s automobiles do, to let a gaseous mixture enter and exit the engine for the purpose of creating power.

When the slide valve slides to one side, it uncovers the intake port and allows steam to fill the cylinder. This steam pushes on the piston, found inside the cylinder, and the piston in turn pushes on a rod that turns the flywheel. The flywheel turns to create motion so the steam engine can move. When the slide valve slides to the opposite side, it opens the exhaust port and releases the steam while the piston slides back and the process starts over again.

Car engines also use valves to allow fuel and gas into the cylinders and exhaust to exit. While they perform the same functions, cars typically use poppet valves which move up and down on springs. Slide valves on steam engines moved back and forth by rod without the use of springs. Eventually slide valves were replaced by piston valves. Though similar in function, piston valves carried several advantages over the slide valve.

The passage a slide valve operated in had to be fairly long and more energy was needed to move the valve back and forth. This movement also required lots of lubrication to ensure the valves did not stick or slow down, causing a malfunction in the engine. Piston valves, however, could operate in shorter passages and were much easier to lubricate. This increased the efficiency of the steam engine, and the slide valve became less common.

Using superheated steam — steam produced at a temperature higher than water’s boiling point — also led to the fall of slide valves and the rise in popularity of piston valves. These super hot temperatures interfered with lubrication needed for the slide valves. Modern day machines have moved from these earlier valve types to more efficient valves. Aside from the poppet valves used in automobiles, choke valves are still used in certain planes and tools, such as riding lawn mowers. The goal when choosing a valve for a machine is to create the greatest engine efficiency with the least amount of work to open and close the valves for proper functioning.

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