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There are cars powered with around 300 bar compressed air (around 4000 psi), stepped down to around 500 psi, with a range of 50 - 100 miles.

Reading about early torpedos, it was found that heating the air released from the storage cylinder would increase its propelling efficiency by increasing its pressure, or volume at a given pressure. These were called "dry heaters".

Additional expansion was gained by adding water to the heated air to create steam, which also served to control temperature levels in the combustion chamber. These were called "wet heaters".

Increasing volume at a given pressure is ... a jet engine! In this case, the air is already compressed, and the exhaust drives a piston, instead of a turbine (to extract mechanical energy).

According to Gas Law, pressure and volume are proportional to temperature (kelvin). Water to steam is around 1 to 1600 expansion in volume. Heat could be more fully utilized instead of wasted as in an internal combustion engine.

Would a "heater" be a viable improvement$^1$ to a compressed air vehicle?

$^1$ see SAAB Ranotor reference

Robert DiGiovanni
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Modern torpedoes are external combustion engines. You supply hot gases to move pistons. Certainly you could supply compressed air to do this, and of course heating the air would give you more energy.

The question is why bother? Compressed gas has a poor energy density. You'd need energy carried along to heat the gas. Which leads us to use the common energy sources of hydrocarbons or electrical batteries. Both of these have energy densities that make them much more usable. You could theoretically run a car by having a big weight you lift up and capture the energy of it falling, but other than a physics experiment, there isn't much point. We used to run cars (and locomotives) by boiling water, but there are just better ways to do it now.

Tiger Guy
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  • Yes, and a giant flywheel might work too. The goal is to improve the range (50-100 miles) of existing compressed air automobiles. But +1 for realizing this is an external combustion process. Internal combustion tends to produce excess heat in the cylinder, which must be removed. With compressed air, I'm seeing a very small, efficient heating area with a larger volume output, driving the pistons. – Robert DiGiovanni Dec 13 '21 at 09:12
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    "automobiles" is probably overselling the vehicles that can make that type of range on compressed air. – Tiger Guy Dec 13 '21 at 12:38
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Viable? No. The whole point of compressed air is simplicity and you can use a wide variety of energy sources to compress the air beforehand. But when you start adding burning fuels and heating, the pre-compression of the air becomes redundant.

Even in space where you already need to carry propellant with you, instead of compressing a gas you go one step farther and carry solid or liquid instead and react it to turn it into a much higher volume gas. On Earth, it is even less viable because you're surrounded by combustible air and you can just carry fuel and heat the air to pressures far past what you can get through compression.

DKNguyen
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  • DKNguyen thanks for pursuing this question. The goal is to extend the range of a compressed air vehicle. What we must consider is: 1. CO2 savings by compressing air with "green energy". 2. Energy savings by utilizing heat of combustion more efficiently, instead of the required air or liquid cooling systems which simply dissipate heat as waste in standard internal combustion engines. Need to see some numbers to support pro and cons. – Robert DiGiovanni Dec 12 '21 at 19:19
  • @RobertDiGiovanni The problem I see there is you have to draw a line somewhere and to me it's not clear where you draw the line with regards to "extending the range" while still having it remain a compressed air vehicle. Because as you burn more and more fuel you just approach the range of a internal combustion engine. It seems to me that you would almost have to arbitrarily go "I want to achieve this range a compressed air tank of X size" and then add a heating system that can achieve that rather than having a sweet spot to target for. – DKNguyen Dec 12 '21 at 19:25
  • I'm talking just about heating here, not water injection since that's yet another complication and more weight. – DKNguyen Dec 12 '21 at 19:29
  • I would definitely start with the simpler "heat only" approach. And yes, I agree that would have to be weighed against just adding another compressed air tank. Amazing though that these were in use in 1915. – Robert DiGiovanni Dec 12 '21 at 19:31