Safe reaction to maintain 30psi (2 bar) of gas, while that gas is released at random?

Question

I'm going to use it to power a Lego Pneumatic Engine (LPE) (random example, not mine), that will be self-contained, and start and stop a lot using a throttle valve. So the requirements are:

1. Use common materials to run the reaction
   • Might not even need to be a reaction. Vapor pressure would do just as well. See #5.
2. Be completely safe, both for indoor use with no special collection, and for the materials involved:
   • ABS Plastic for valves, cylinder walls, and plumbing hard lines
   • Rubber seals and hoses
   • Steel piston shaft
3. Fit in a small (0.5L) plastic water bottle
   • There's another bottle available, about 1/2 that size, that can be air-displaced into the main one through the standard rubber hoses and ABS hard lines (perhaps to start the process without losing a lot of gas), but I'd rather not use it if I don't have to. It is available though, if needed.
4. Produce a large volume of gas (long runtime of the LPE)
5. Self-regulate to around 30psi (2 bar), so no worries about over-pressure when the LPE is not running
   • (this might be the tough one)
6. Not need special cleaning afterwards

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My first thought was Baking Soda and Vinegar to generate CO2, with a pressure-regulated air pump to displace the vinegar into the baking soda. But there are two problems with it:

• The original reactants can get into the LPE as contaminants, which then need to be cleaned out. (the reason for #6 above)
• It doesn't stop at Lego's maximum pressure of about 30-35psi. (the friction-fit hoses pop off around 37psi, and the reason for #5 above)

I have a design for a "smart" pump controller that accounts for most of the pressure coming from a finite-rate reaction and not the pump itself, but it seems a bit rube-goldberg-y to me, and still can't account very well for the expected run/stop pattern of the engine that uses it.

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So, is there a different reaction that I can use instead, that can satisfy the requirements up top?

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Some pictures may help:

The water bottle sits horizontally, under the black "hold-down bridge", with the bottom against the programmable brick. (big yellow tower on the back)

"Steam" locomotive with Walschaerts valve gear. And yes, it runs. :-) Both directions.

There's also an optional tender car with the other, smaller water bottle and the motorized air pump to displace something out of it, which is also controlled by the programmable brick.

Answer 1

Without seeing the setup, it's hard to judge but what about grinding up citric acid and sodium bicarbonate and with a touch of water (literally some drops) make a pellet. Then can you have a small water feed that allows a drop in now and then? If you can make a hard pellet, it will slow the reaction rate giving off the CO2, but you may have to play with the hardness of the pellet.