Air Engine Research, Working with some Old & New Idea Designs 4-20-24 

You seem worthy of one of my secrets.😆 If you are willing to sacrifice being able to run it at various powerlevels you can use springs in stead of pistons. It draws some power from the pistons and returns it under different angles. If you for example have 2 pistons, pick the position in which their force translates to work most efficiently. Then you add 2 springs that pull the shaft into that position with about half the force of the desired output. Then it won't run with less than 1/2 but will reasonably work beyond 1. I do this with old rusty bicycles and the effect is spectacular.