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Here is a diagram of a disk brake. Notice that there is a piston (in red):

Disc Brake

This is a piston, most commonly used inside the internal combustion engine:

Piston

Image 1 Source: http://sjam4uphysics.pbworks.com/w/page/38936885/Regenerative%20Braking Image 2 Source: https://www.emaze.com/@AOTOZWQZ/Piston,-Piston-Ring,-Cylinder

These two types of piston are clearly similar enough to have the same name but different enough not to be interchangeable with one another.

What are the characteristic properties of a piston by definition?

What is the difference between the two types of pistons?

  • Fwiw the origin of the word "piston" itself is from various languages where it means "to pound" (as a verb) or "a tool for pounding". From this I assume the real defining characteristics of any piston is its blunt shape and its linear motion against / in response to changes in pressure against its surface. There is also the modern English definition of the word indicating that over time "piston" has specifically narrowed that down to mean sealed, cylindrical shaped objects acting on a fluid. – Jason C Jul 12 '16 at 6:02
  • It's the thing that makes the bang in the hole - as in suck, squeeze, bang, blow. – Robert S. Barnes Oct 28 '16 at 11:20
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What are the characteristic properties of a piston by definition?

Quite simply put: A piston plugs a hole.

Wikipedia says:

It is the moving component that is contained by a cylinder and is made gas-tight by piston rings.

(Note: I would disagree in the wiki assessment only to say it is sealed tight by sealing rings. Not all pistons have rings attached to them, as I'll explain.)

In your example, the similarity stops there.

What is the difference between the two types of pistons?

Realistically, a piston can be made of any material so long as it does the job at hand. Pistons used in modern internal combustion engines are most likely made of cast hypereutectic (over saturation of silicon content) aluminum. This is due to their light weight and strength to weight ratio. For more heavy duty applications, forged aluminum might be used. Forged aluminum is more resistant to detonation which might occur under high cylinder pressure. Hypereutectic pistons are shatter prone under high stress. Older vehicles may have used cast iron pistons, back when refining aluminum was very expensive.

A brake piston, on the other hand, can be made of plastic, aluminum, or chrome-plated steel. (Most I've seen are the latter.) Again, all it does is plug a hole.

A brake piston is smooth on the outside radius. It is sealed from the inside of the caliper to hold the brake fluid in. The top of the brake caliper piston (the side which brake fluid acts upon) is relatively flat. This is to ensure the brake fluid is acting upon the piston equally across the entire face.

An engine piston has ring groves machined into them. These ring groves provide space for piston rings which seals the cylinder almost completely tight, with two compression rings to provide the sealing (some truck applications use three sealing rings). There is a third ring grove at the bottom of the piston which provides space for an oil control set of rings. This helps scrape oil off of the cylinder walls, forcing it back down into the bottom to the oil pan. The face of the engine piston can be of many different varieties. Some have indentations to allow the valves a place to be (instead of hitting the piston!). There can also be crowns for higher compression and depressions for lower compression.

2

A piston's job is essentially to couple force between mechanisms and fluids. It does this by moving in and out of the (usually cylindrical) fluid chamber.

In the engine during combustion hot gasses apply a force to the piston which in turn applies a force to the connecting rods which in turn applies a force to the crank. During other strokes of the engine the piston operates the other way round using force from the crankshaft to push out the exhaust gasses, draw in new air (and sometimes fuel) and compress the air (and sometimes fuel) ready for combustion.

Similarlly in the brake caliper the hydralic fluid presses on the piston which in turn presses on the brake pads.

The piston in the combustion engine is a high-spec part designed for demanding conditions. It must provide just the right compression ratio, it must move at high speed all the time, reapeatly changing directions, it must maintain a very good seal against the cylinder walls. The couplings between piston, connecting rod and crankshaft must also be able to rotate.

The brake caliper is far less demanding. The piston is only dealing with hydralic oil, not hot gasses and only moves a short distance infrequently.

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