How important is it to keep an air motor warm?

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I want to make a compressed air powered bicycle to get me to and from work, and research has said that a main issue is the cooling caused by expanding gas. How important is it to, and to what extent should I, keep the motor warm?
My plan is to use the compressor at work as a free ride to and from work. I want to install an air tank (a rated one, not an improvised one) onto a bicycle and then run an air motor to turn the wheel (with whatever gears etc needed to make it work). I will probably be using an air tool, such as an air wrench, to run the wheel.

My research has highlighted that the motors will get cold due to the expansion of the gas, and that a heat exchanger is needed to combat this, but I cannot find much explaining why this is an issue? I realise that the oil in a conventional engine needs to be warm to flow correctly, but this can be combatted by using an oil which will operate at low temperatures. I doubt that there would be significant ice build-up inside the motor as it will constantly be moving, but would this be the issue? or will there be a significant loss of power due to the cold environment the air would be expanding to?

In order to prevent the issue, what would be the best option? Will I need an electric heater system to maintain ambient temperature? will the surrounding air be sufficient (I wouldn't be using it in freezing conditions, air temp ~15°C+)? How cold is too cold?

If this prototype works, I will be considering converting a 125cc 4-stroke petrol engine to run on compressed air, fitted to a motorcycle. I imagine that this will need a lot more engineering than my initial "strap a windy motor to a bicycle with an air tank" but hey, start small. I've seen a lot of articles about using existing engines to run on compressed air, and they don't mention heat exchangers, so I'm really confused about how necessary they actually are.

Has anyone here done anything like this?

To pre-empt a potential flaw regarding recharging - I plan to fit an air motor to a compressor on the bike itself, so that the 200psi compressor at work can charge it up to the maximum pressure of the vessel, which I gather can reach 4500psi on some scuba tanks. If it works I may try to create energy harvesting brakes to charge on downhill stretches, but again, I'm starting with the basics.


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The temperature of the motor itself is a lesser point. What will happen is that the tank will cool and that will reduce the pressure. For the system to operate efficiently, you need to allow the tank to be warmed by its surroundings. The higher the rate at which you are using the compressed air, the more of a problem this is. If your compressed air lasts for all of a 60 minute trip, then it has 60 minutes to warm. But if you use the compressed air in only 5 minutes ...


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Decades ago I used to fly small model aircraft powered by compressed CO2. As I recall we had issues with both the tank and the engine. Water vapour would collect and freeze to ice insulating the tank/head reducing power. Think we also got dry ice forming around the exhaust sometimes.


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Decades ago I used to fly small model aircraft powered by compressed CO2. As I recall we had issues with both the tank and the engine. Water vapour would collect and freeze to ice insulating the tank/head reducing power. Think we also got dry ice forming around the exhaust sometimes.
Yeah, although I suspect the OP won't run into quite the same problems, as his parts and chambers and stuff are probably an order of magnitude larger than an RC engine.


Have you done any investigations into an air tool that might fit your application. Basic information would be HP, torque, rpm vs air consumption @ a given input pressure? Also, price, unless you have an unlimited budget?


Science Advisor
Air motors are designed to run from an air supply at about 100 PSI. You will need a regulator to reduce the air pressure because 4500 PSI will instantly destroy a 100 PSI air motor.

Air motors run best with dry air, but real world compressed air has moisture. Air motors live with that moisture every day. Air motors do not come with heaters because they do not need them.

Do not try to make a 4500 PSI air compressor system. Just don't. Search scuba tank explosion for what can happen when things go wrong and some (not all) information about safety.

A rule of thumb for industrial compressed air systems is that it takes a 4 hp air compressor to run a 1 hp air motor. That's actual horsepower, not rated power. The inefficiency is from isothermal vs adiabatic compression and expansion, plus line losses, and mechanical efficiency losses. Compressing air to higher pressure, followed by reducing the pressure through a regulator, decreases system efficiency even more.


I want to install an air tank (a rated one, not an improvised one) onto a bicycle and then run an air motor to turn the wheel
So, I guess I need to address a couple obvious points...
  • An air tank is a lot of added weight. You mention hills in your commute -- will you be trying to add pedal power to mitigate the extra drain on the air tank?
  • I manage to crash my bike every once in a while. Sometimes it is (sort of) my own fault (seams in the road, surprise curbs, deer deciding to jump out in front of me, you get the idea), and sometimes it is someone else's fault (car idiots). I'm reluctant to put a bomb between my legs that can go off in one of these crashes...
Have you looked at electric bike options? There are some great options now, and they are coming down in cost a lot because of their popularity. I'm considering buying an electric MTB for commuting and weekend rides -- I'll still do plenty of pedaling, but having the electric assist for the steepest weekend climbs and the 20 mile daily commute with 1000 feet of elevation would be nice...


Science Advisor
Don't envy the bicycle, envy the rider. . . get inspired ! . :oldwink:

And how long does it take to get a Masters in Bicycle?
Thanks for the replies guys!

Thanks for the advice on the air tank. I will probably aim to get a tank which is capable of as much pressure as I can afford, but then run it lower than that for safety. I'll also be thinking of different places for the tank to sit than between my legs, and of course, crash bars!

@ Scott. So it sounds like the ideal combination is the largest air tank I can fit (to slow down the rate of depressurisation) and maximum surface area to increase the absorption of heat from the surroundings. Perhaps multiple smaller tanks, for the added surface area, and a decreased potential damage from an explosion (it's unlikely that all the tanks would go at once, unless it was on fire, which it shouldn't be).

@ Berkeman, I'm not a fan of battery-powered electric, I think it's a much dirtier energy source than it's made out to be. It's something which I ultimately hope we can move away from, but it's being heralded as the saviour of the planet, as we dig up lithium from vast mines, ship it across the planet, slap it in a plastic case with some volatile chemicals and then recycle what we can and throw away the rest in 5 years when the batteries have died. I know the compressed air has to come from somewhere, but it doesn't come from batteries. It's also possible to make this design almost entirely plastic free (sealing washers and the like excepted).

I have ready access to air tools, so can easily experiment with a few different types. I've considered using a windy tool (rotary sanding tool) connected to a drill gearbox to give the torque I need.


If you want to get some background on air powered bikes and vehicles before starting your project, there a lot of information to be found by Googling "compressed air powered bikes"

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