Algodoo

Yeah, the solution is not to use FIXATES or GLUE to stick an object that changes radius to another. Just use a hinge with constant 1e-30 (least possible constant) and hinge the other circle to the back of th engine. The circle that changes radius will be able to move, but the other will not fall.
Or, if you think the gearbox will go to more than 50 rad/s, just replace the fixate with 2 hinges (or a braked hinge at the middle)

It works, thanks

There are lots of things you have to improve to make it stronger, but remember that a spring engine will always be stronger than a spawn engine!

First make those V-shapes in the upper part of the engine bigger (like getting to the middle of the cylinder or even more). This will give the balls even more energy.
Make the balls' radius 0.0695 so that they cover all the cylinder all the time (only change density) and make them push the piston until it's ALL THE WAY DOWN. When it's down kill the ball.

Use:

pos := pos;
vel := entity.vel;

Just copy that, erase the pos and vel that are already there, and paste it

Thanks for the tips, I thought spawn engines are as powerful.

And from what I get so far, basicly a spawned circle have to push the piston to the lowest position at spawn, so it won't need to travel much to lose much energy

No, the circle has to push the piston until you can't push it anymore, and then you delete it. if you don't do that, you are making the piston move a lot by inertia