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Grades 3–5 · Machines

How does a bicycle pump push air into a tire?

A bicycle tire may already contain compressed air, yet a hand pump can put more air inside. It must first make the pressure in its own chamber high enough to overcome the pressure at the tire connection. A moving piston and one-way valves make that possible.

T009 visual explanationA piston and valves manage one-way air flow; Intake; Compression; Intake admits air; the check valve prevents return flow; The compression stroke pushes air outward through the outlet.A piston and valves manage one-way air flowIntakeCompressionIntake admits air; the check valve prevents return flowThe compression stroke pushes air outward through the outlet.

In the diagramA piston and valves manage one-way air flowIntakeCompressionIntake admits air; the check valve prevents return flowThe compression stroke pushes air outward through the outlet.

An intake stroke admits air; a compression stroke pushes it through an outlet. A check valve resists the return flow on the next intake.

During intake, the piston increases the chamber's space. Its pressure drops below the surrounding air pressure, and an inlet route opens so outside air enters. A check valve, a valve that permits flow mainly one way, prevents the tire's compressed air from simply flowing back into that growing space.

During compression, the piston makes the chamber smaller. The inlet closes and the trapped air's pressure rises. Once chamber pressure exceeds the pressure on the outlet side by enough to open the valve, air flows through the hose toward the tire. The outlet closes against reverse flow during the next intake.

Repeating the strokes adds air rather than merely squeezing the same air back and forth. The person moving the handle supplies the energy. Higher pressure pushes back more strongly on the piston, explaining why the handle can become harder to push as a tire is filled. Some supplied energy also becomes heat. The valve routes, rather than the piston alone, turn repeated strokes into one-way delivery.

About this explanation

Starting knowledge: Understand that air occupies space and can push harder when compressed.

Sources

  • SKS Germany — Floor pump non-return valve system
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