A weird thermal lag engine/ heat pump

Hi folks,

BLADE is here again, because I made a small fast little thermal lag engine
to test my glass piston.
The little engine worked well, it riched the 50 Hz easily with the correct tension of a rubber rope.
If you want to check my video about this engine:

But today I thought about a heat pump with high frequency.
I wanted to see how cool down the end of this little engine.
The engine started to cool down, but not on the closed end
that's weird!

The engine hot side isn't changed as every normal heat engine if we mechanically drive them.
I've tried to drive it 1-50Hz but thte result was same.

The question is, how could I change the hot side to the cold side on my little heat pump?


Thanks any help!
BLADE