Compressed air engine

[Chriske]

Hi guys,


I'dd like to start a new project next schoolyear (I'm a teacher)
Purpose is to run an air engine, which will spin an alternator(of some
kind) which will feed an electric motor. These motors will be used to
drive a cart capable transporting one person. I'm not planning to make a
speedy machine, it just has to move at a slow rate, that's all.
It'll be an integrated project. I will do the mechanics together with my
pupils, another colleague with his pupils will do the welding and a thirth
will take care of the electronic parts.

Question is this : will a air engine like this one will generate enough power to keep an alternator in motion...?
http://www.youtube.com/watch?v=vcaFWg-F ... 7384E2920D
Ok, I understand I have to build this thing a bit larger, but how far would I have to go. How big should the engine be..?

Thanks

Chris

[Ian S C]

Hi Chris, for a start your going to need a good bit of power at the crankshaft, because by the time it reaches the wheels via a generator to an electric motor, you might have lost about 50%, could you not use a direct drive, or are you going to run the electric motor off a battery, and use the motor as a charger? Ian S C

[Ferraccio]

This is the forum of Stirling engine, that is a particular closed cycle heat engine.
You speak about a compressed air engine (so is the engine in the video).
Is a completely different matter.
However, my opinion:
I suppose that all (reservoir of compressed air, compressed air motor, alternator or DC generator, electrical motor) are on the car.
The main loss in energy are in compressed air (pneumatic) motor, and mechanic system (gears) to the wheels, expecially if belts, more good are bicycle chains.
The translation of energy electrical (alternator) to electrical (motor) gives very small losses.
Is impossible foreseen the total yeld, not knowing the type and the quality of the of the realization (eg the pressure and volume of the reservoir); but take note that a basic text of mechanic gives a good idea, all other parameter are relevant energy, power, rpm.
Not having exigence in speed and yelds there is not so big problem.
I think that may be you do not have considered a problem: the expansion of gas from the reservoir to the pneumatic engine may happen with a considerable decrease in temperature, which can easily reach the freezing temperature, until the formation of frost or ice; the decrease limits in relation, the expansion work in the engine.
It may be necessary in input to the pneumatic motor to move the air in a heater (eg propane burner).
The presence of a heater can greatly increase the yield.
May be explicated why the temp. drops so drastically, and why the air in temp more high than environmental emphasizes the power.

[Ferraccio]

Of course I'm available to give explications considered useful

[Chriske]

Thanks guys,

...for the comment so far.
I think this project, if correctly designed, should work just fine. Possibly the 50% drop of power, as Ian wrote, is a bit on the high side. let's hope it will not be the case...
On the other hand I think we should indeed consider about the cooling, and even freezing, of the aircontainers during airflow. But I think the owners of the aircontainers (firefighters) will help me out here.

I will scale up this compressed air engine and post some drawings of it in this thread.
It'll take me a few days before I start drawing because we're working and preparing on our Open door in our school this very moment.
I hope I will not be confronted with problems scaling up this engine.

Has someone, reading this forum, already made this engine....!
A few sketches or drawings would be handy

Chris

[Ferraccio]

Chriske,
you may not have realized that this is NOT the forum of pneumatic motors.
Therefore likely to be disappointed waiting for a reply that never comes.
So this means that may not be here to experience this type of engine, except that in our general-mechanics culture, which can be less than yours.
The pneumatic motors are vastly simpler than Stirling engines, (even if the stirling engine is called also "air engine", but can be hydrogen or helium processed), pneumatic engine do NOT have absolutely problems in scaling.
Among other things, the creation of a Stirling engine with ambition to make it (only it) with good mechanical efficiency and adequate power is very demanding, perhaps even beyond your reach, (sorry if I say it), ... given the level of your questions, especially if this is to be done (also if built alone) for next school year.

To do a complete system from a pneumatic motor with a electr power-mechanical power processing on the other hand makes sense, since the air motor is really easy, does not involve heat transfer at high temperature, and thermodynamic problems in closed loop.

If you think to do it (the Stirling engine, alone) welcome in the community.
If you plan to do a pneumatic motor, perhaps we need to take another way.

[Ian S C]

Sorry Chris, I was thinking of Hot Air motors, but the generator is the bit that may loose you some efficiency, small generators are not as good as bigger ones. I was wondering, is this an air turbine motor, or a piston type, the turbine / vane motor puts out a lot of power from a small unit, it does of course use a fair amount of air, don't know if the piston type would use less air, proberbly would. Ian S C

[Ferraccio]

The pistons compressed air engine is simple and is in fact the same of the steam piston engine, may be is scaled without problem.
Not having complications as speed control, (speed may be controlled by a simple valve), ans no inversion device of rotation, are simply piston crankskshaft and valves.

[jimlarsen]

Chriskie,

I am not sure what you are teaching with this project. But if you want to teach efficiency or engineering I suggest you simplify the system. The others who mentioned power loss are correct. Every time you convert your power you will lose a lot of it, especially in a simple hand built system. You could build an air motor and power your car with that. It would skip two of the conversion steps in your energy path and put much more power to the wheels.

[Chriske]

Thanks again for the input and the tips guys,

Chriskie,
I am not sure what you are teaching with this project. But if you want to teach efficiency or engineering I suggest you simplify the system. The others who mentioned power loss are correct. Every time you convert your power you will lose a lot of it, especially in a simple hand built system. You could build an air motor and power your car with that. It would skip two of the conversion steps in your energy path and put much more power to the wheels.

Jim,
The reason why I want to do this kind of project is because it is a possible way of learning pupils working together, to work and think as a team, planning, discussing, drawing, solving problems the moment they occur. I know this project could be made far more efficient, but again I choose to it this way for the reason I mentioned before. One of the goals in the field of education is integrated education, it is in fact mandatory. Thanks anyway pointing this out...;-)
I know this is completely out of topic (sorry administrator..) This is what I (together with the aid of my colleageus and most of all all our pupils( 15 to 18 years old..! ) made last schoolyear. We all had lots of fun working on this hugh project...! (it's all in Dutch, but the images speak for themselves)
http://gti-duffel.be/forum/viewtopic.ph ... a610cd058f

The pistons compressed air engine is simple and is in fact the same of the steam piston engine, may be is scaled without problem.
Not having complications as speed control, (speed may be controlled by a simple valve), ans no inversion device of rotation, are simply piston crankskshaft and valves.

That's exactly the reason why I choose this engine.. It's simplicity...!
And I'm glad you mentioned scaling up is not a problem, thanks..!

Chris

[jimlarsen]

The Engineering 112 class at Stephen F. Austin University in Texas uses a Stirling engine project for similar instructional goals. They build them as a team. The project is used to teach problem solving skills while also teaching about momentum, friction, and physics. They are not machinists, so they build tin can engines. They publish their plans on the web (in case you want to come over into the light). You can find the plans by searching for the SFA Stirling Engine Project.