The solution would be a steadier and more controlled flow of Fuel (water).
At the current configuration, the water flow is controlled by the gravitational force, and you can't control it yourself, that means that if the cylinder is full with water, water will still get in, and would pour out from the exit valve.
I think you can achieve that by placing the tank under the entering valve, and using rotating gears (just like what used) in order to control the flow, or setting a configuration of controlled valves to allow a bigger, smaller or none flow of water into the cylinder.
Overall, the cylinder is quite good and with further adjustments you can achieve a great engine.
