Compressed Air engines for cars?

I saw a TV show (DC?) about alternative engines for cars. How practical is this compressed air engine? Given the brurst strenght od the tank (where the compressed air is stored0, how much energy can the tank store? Waht is the upper limit on the tank pressure/ Would it make more sense to have a liquid air tank, and boil it off (to provide the compressed air)? anyone actually drive one of these cars? what’s your impression-are they glorified golf carts/

ISTR that this technology, while widely hyped (the inventor had plans to sell factory franchises to local investors across Africa, I believe), has yet to be really verified by third parties.

Compressed air is the most inefficient energy that we use. Granted, we’re manufacturing, but some of that has to carry over into car motility.

In processing any compound into a fuel you’re going to have inefficienies, but compressing air (or any other fluid) is incredibly inefficient. The reason for this is that when you compress air, you add kinetic energy to it in the form of higher pressure. Most of this energy becomes heat, which in any real system has to be dissipated. When you release the compressed air, in the process of doing work (in the case of a piston engine, driving the reciprocating mechanism) it extracts energy from the environment because its temperature drops when it expands. The net result is that you end up throwing away a lot of the work done on the system in the form of heat, rather than storing it in chemical bonds or charge potentials. Kinetic energy, like hyperactive children, just don’t store very well.

Another problem is the pressures that would be required for operation at reasonable ranges and speeds would be in the tens of thousands of bars (1 bar ~ .98 atm ~ 14.5 psi); modern standard high pressure hydraulic fittings and tanks are rated for somewhere between 200-340 bar. There are high strength fiber-reinforced composite tanks used in aerospace applications (for ullage and fuel pressurization systems) that are rated for 300-800 bar but these are intended for a limited number of pressure cycles (often just one), as the composite material will fatigue and lose strength dramatically with use. (See this recent article about concerns of Shuttle ullage tanks as an example.) Practical mobile systems capable of handling pressures in the thousands of bars are the realm of science ficition.

As much as petroleum distillates are a polluting nightmare, they are about the most ideal chemical fuel for a reciprocating engine sufficiently compact and powerful enough to drive an automobile. The temperature and pressure increases in the internal combustion processes are sufficient to get a reasonable thermodynamic efficiency without exceeding the mechanical properties of conventional engine materials. Other alternative combustable fuels–in particular, alcohols and hydrogen–have a number of characteristics (volitility issues at environmental temperatures, corrosive potential, energy density, storage) which make them less than ideal with current engine and materials technology.

If someone can figure out how to make a really efficient pure thermal energy source that can drive a Sterling cycle-based closed-cycle engine, or a solid state electrical/magnetic field generating source (or even a battery technology a couple of orders of magnitude better than what we have today) then perhaps we can dispense with hydrocarbon IC engines in favor of a comperable non-polluting technology, but right now it’s the favored process for powering autonomous transporation systems because of its thermodynamic and overall power efficiency. Short of some kind of tabletop fusion device, it’ll probably remain so for the forseeable future.



And now

And wouldn’t it be fun if you got rear-ended with 10,000 PSI of compressed air in the trunk? Have you ever seen what happens if a SCUBA tank gets ruptured? Multiply that force by however much compressed air you’d have to haul.

And I wouldn’t work at a filling station - you just know that someone’s going to drive in with a tank that’s been damaged or corroded, and it’s going to let go while being pressurized.

The good part is that you’d have cheap air conditioning.