Not saying it's impossible, made it slow and weak to show accurately (something that your engines FAIL) and easy-to-see the four strokes.
Anyways, nice work on your engines
Let's see:
For the gearbox, try to make the ratios a little bit bigger, so they can engage properly. Make them of restitution 0 and if it's made of hinges, disengage the previous gear first!
I'd like to see the gearbox, to explore the problem.