Why does the cylinder wall heat up after pumping for a period of time?

Because the further down, the more work mechanical energy does and the more internal energy in the gas increases, so the higher the temperature is. When we pump air, we should try to reduce the friction between the piston and the cylinder wall (we often need to refuel), so the friction work will make the cylinder wall heat very little, so the main reason for the heat is that the mechanical energy compresses the gas to do work, not the gas you mentioned, and overcoming the friction work is not the main reason.

Related instructions

The principle of the old rubber bowl pump: there is a gap between the piston of the pump and the cylinder wall, and there is a downward concave rubber bowl on the piston. When the piston is pulled upward, the air below the piston increases and the pressure decreases, and the air above the piston is squeezed downward from around the rubber bowl. When the piston is pressed down, the amount of air below the piston decreases and the pressure increases, so that the rubber bowl abuts against the cylinder wall to prevent air from leaking to the upper part of the piston.

Continue to press the piston downward. When the air pressure is enough to open the tire valve core, compressed air will enter the tire. At the same time, the air outside the cylinder will enter the piston from the gap at the upper end of the cylinder.

The new pump with sliding outer tube is the simplest one-way valve. When the outer tube is lifted, air enters the air pump from the small hole in the middle sleeve at the lower end of the outer tube. Press the handle, the air will enter the inner tube of the air pump through the one-way valve at the top of the inner tube, and enter the valve of the tire along the hose!