The OP's basic proposition: can a charged SCUBA
tank start an engine, has merit.
The advantage, apart from the fact he already has it, is that it will stay charged almost indefinitely.
There is a way to directly exploit the considerable energy potential of a charged high-pressure air tank which would be affordable if you had the knowledge and the time to buy the necessary hardware
at sacrifice prices on eBay: the high pressure air from your tank could be applied to one side of a piston in a 'free piston accumulator' (other types are available but this is the easiest to make). On the other side of the piston would be hydraulic fluid, of which a small portion could be drawn off rapidly at virtually the same pressure as your air, and used to run a hydraulic starter motor
Using the stored air to drive either a pneumatic starter motor, or an air motor to generate electrical
power has a problem: the air is at MUCH too high a pressure to be usable. There is no affordable way I'm aware of to exchange pressure for volume, which is what you need to do here (the air analogy to an transformer for AC)electricity, which drops the voltage while increasing the current
Furthermore, pneumatic motors are notoriously inefficient, especially the sort fitted to inexpensive hand tools.
Thermodynamics, the gas laws (scientific rather than legislative), and the frictional losses using intensification devices in reverse are all against you.
power is the least sensible option, it seems to me. Apart from the time factor and the inevitable conversion losses, it means that you are still entirely dependent on a battery
holding charge, and an electrical starter motor delivering it to cranking.
Someone scammed Tata (Indian corporation which makes motor vehicles among other things) with a pseudo-solution to the problems of direct conversion of high pressure air to mechanical prime mover. They wasted billions of rupees and years of 'research' to no avail trying to make it happen.
It was immediately obvious from the 'inventor's' claims that it had elements of a perpetual motion scam, purporting to be able to extract energy gratis from the environment
, with a bit of hand-waving.
Sure, heat pumps appear to do this, but the energy they 'extract' (actually, move from point A to point B) is low grade (low temperature, high volume) energy, much lower grade than the electrical energy they consume in order to do the moving. Low grade energy has very limited uses in automotive applications, once you've heated the interior
sufficient for comfort...
Returning to viable solutions:
However you do it, it will make a mockery of 'saving money
by using a SCUBA
tank I already have', but it could be fun. To be useful in the long term, you would want to fit your engine room with a dive compressor
, to close the loop and recharge the bottle for next time. And/or a bit of diving
on the side, like when you foul an anchor
or wrap a pot line around a prop ...
generator sets use exactly this technique to start large diesel
engines very reliably after months or years of inactivity, except that they use compressed nitrogen rather than air (better, especially for material longevity, eg seals
, but not crucial AFAIK)