Not quite right. It’s the absolute pressure that increases, 8 psi + 14.7 is 22.7 absolute, which increases to 24.2 = 9.5 psi.
In reality, it’s probably a combination of higher pressure, and the softening of the plastic. Let it down a bit if it’s hot!
It takes energy to compress an ideal gas, and that same amount of energy is output as heat. But we also gain the potential energy of the compressed gas. It seems to me that we are doubling up our energy.
Can anybody explain in simple, practical terms (to a humble mech engineer) how this works...