# Small pneumatic engines?

Discussion in 'General Questions' started by jawnn, Feb 15, 2012.

1. ### jawnnMember

I need a small compressed air engine for a bicycle (about one HP or maybe less). If there is one, please give me a link.

If there just is not any, close to one HP, it's about time someone made one. I don't like gasoline nor the batteries of electric motors.

Most people seem to be afraid of exploding air tanks, but that is just not going to happen. If it cracks it will just pop!

Hydrogen with a tiny bit of oxygen is what you should be afraid of. But if no oxygen it will implode.

Last edited by a moderator: Dec 15, 2015

3. ### loquinActive Member

The issue with using compressed air as 'fuel' for any sort of portable power is it's terrible energy density.

At 150 bar (2175 PSI) you have an energy density of only 17 Wh/liter. (Gasoline has an energy density of 9000 Wh/liter, for comparison...)

This means that, if you want to run a 1 HP, 100 percent efficient (obviously, not possible) pneumatic motor for 1 hour, you would need a 44 liter (12 gallon) air tank at 2175 psi. (actually, it would need to be quite a bit larger, as the energy density of compressed air falls as the pressure in the tank drops.

When you factor in both air motor inefficiencies, and energy density drop with pressure drop, you would be doing very well to get 1 HP for 1 hour using a 50 gallon, 2175 PSI tank. Which is the volume of a decent water heater.

Now, despite what you believe, air at over 2000 psi does not just 'pop.' There can be shrapnel; there can be a tank (or regulator) shooting off like a rocket. If you're in the way of shrapnel, tank or regulator, you could easily end up in the morgue. There's some members here who are welders by trade; they could tell you more about the characteristics of metal pipe and/or tanks at scuba-type pressures.

A 50 gallon (185 liter) 2175 PSI tank is also VERY heavy. It would be roughly 20 inches in diameter by 40 inches in length, with a steel wall thickness of at least 1/2 inch. The reasons that the tank would have to be so strong, is 2175 pounds per square inch, spread over roughly 2400 square inches, means that there's 5.3 million pounds of force against the walls of the tank!

(You could get a tank made from exotic materials (spun carbon fiber/epoxy over a thin stainless liner, for instance - it's similar to what NASA uses for Xenon tanks on ion drive rockets - they only weigh about 25 pounds for a 50 gallon tank... but the cost of a tank like that is probably more than a new car...)

Last edited: Feb 16, 2012
4. ### jawnnMember

So how are they making the new compressed air cars work? Run on flat land only? Sound like a hopeless case.

5. ### loquinActive Member

They're not. No one has been able to get a working, practical, pneumatically driven car, and be able to sell it for a profit. Yes, a few folks have built prototypes, and tried to drum up development money, but, from an engineering viewpoint, almost ANY other power storage technique is more efficient... so investors would be pouring their money down a rathole!

There have been a few cases where pneumatic motors have been tried on hybrid systems, (using the pneumatic motor as a compressor to store kinetic energy when braking, then use the stored compressed air to drive the motor again when accelerating.) But even lead acid batteries are ten times more efficient in storing energy, and an electric motor/generator can be made more efficient, and easier to control than the pneumatic equivalent.

The only time that pneumatic energy storage makes sense is in large, fixed installations that are able to use abandoned mines or natural caverns for air storage. (and, where you have wind/solar generation facilities, and the power needs to be stored duing off-peak hours, then released for use during peak periods.) One of the other issues associated with pneumatic storage is the air temperature increase during compression, and the air temp reduction during expansion. The reduction in temps can lead to freezing, so the engineers also have to heat the air up as it comes out of the air motor exhaust. Rather than use part of the stored air pressure to generate the heat to warm up the exhaust, they store the heat generated by compressing the air, then use that stored heat to warm the exhaust air after the pressure is released, to have a system that works. In effect, they end up with two separate energy storage mechanisms...

Last edited: Feb 27, 2012

7. ### MotorBicycleRacingWell-Known Member

Last edited: Mar 1, 2012
8. ### loquinActive Member

Definitely a scam. There is no such thing as perpetual motion. Which is what this device supposedly is. There's certainly no such thing as free energy, with a motor-generator set...

9. ### V 35Member

Does anyone remember the Rankine Cycle type engine ? [ 1970's ]

The theroy was simple, basicly a compressed air engine running on Freon,in a closed loop system. A prototype motorcycle was tested . To ride, you switched on the electric heater, the freon expanded, running the motor, the exhaust went through a heat exchanger, cooled, and was liquified, and returned.

In theroy, the cost of operation would be the electric heater battery charge.

10. ### loquinActive Member

The theoretical efficiency on these was about 40-45%, due to the relatively small temperature differences when using Freon. (which is still much better than with a small internal combustion engine.)

Even with the much higher temperature difference when using water-to-superheated steam as the driving fluid, you're looking at a theoretical efficiency in the 60-65% range.

12. ### marc99Guest

im confused, commercial air motors rated at 1hp consume on average 12 to 15 cfm so roughly 6 litres per second thats 360 a minute and so 21600 an hour not 44 litres an hour, something wrong in my calculationm?