Air Powered Motors Theory of Design Questions...?

[JAndrew]

Hello,

Several questions relating to AIR powered motors:

1.What is the effective RPM range for air motors? I read somewhere that above a certain rpm air motors don't have enough time to fill the cylinder with air so they need a lower rpm range than IC motors.

2.When would double acting pistons be more efficient than single acting? The two trains of thought are that a double acting has more friction due to packing but may be more efficient in instances where there are less cylinders.

3.How do the bore to stroke ratios of air motors compare to those of IC engines? Should you utilize a longer stroke since your rpm range is lower to gain efficiency? In the IC engine world higher-reving engines like motorcycles use a shorter stroke due to material friction concerns.

4.For the same total displacement, driving the same load, would there be any efficiency gains by using a multi-stage air motor? (ie a small piston, which exhausts to a bigger piston etc.)

5.Any other design characteristics that vary air motors from their IC bretheren?

Thanks in advance! These forums are a wealth of knowledge and invaluable to someone like me, just starting out.

-J.Andrew

 

[MCRIPPPer]

more pressure makes more efficient engine. for the same amount of power as an ic engine, you need lots of pressure. think of how many psi a gas engine has in it just under the compression stroke(100-200 psi) . now how much would it have under combustion(1000+?) ? you need that much to make the same torque, assuming everything else stays the same( bore, stroke.) i have found that the biggest factor in rpm is the freedom of movement of the air through the engine. at lower rpm this does not really affect your power, but it really drags it down at high speed.

 

[Tin Falcon]

first of all interesting set of questions. I admit not sure how to answer most of them. A good basic mechanical designs text book may help you.
I will do my best to help you .

1) for the most part air engines (CO2 engines are a different animal) are build to demonstrate mechanical principles . They are for the most part the same as steam engines except the ideal valving will be different as steam has more expansive properties. The test of how well built an air engine is is slow it runs not how fast.

2)Double acting is about more power output roughly twice that of a single acting engine without adding the weight of a second cylinder piston etc.

3) I will pass on this one.

4) Multi stage engines do not work well on compressed air but do work for steam.

5)Air engines are two stroke. The pressure and in take are one in the same then the exhaust stroke.


6) the formula for a steam engine is (P x L x A x N)/33,000

P pressure
L length of stroke
A area of piston
number of stokes per minute.

Tin


Tin

 

[MachineTom]

To give you an idea of efficient or lack there of: My die grinder is rated at .25 HP at 25,000 RPM, to keep that baby running I use two compressors, a 2HP and a 1HP, and it can only maintain 90psi. Both compressors running all the time the grinder is in use.

They are nice to put a fair amount of power in a small package. Also handy in certain industrial applications, Nuclear power, No spark type environments.

The heat losses when compressing air are huge and that begins the poor performance.

 

[gus]

Air motors have very low efficiencies. A 6" Grinder will require a 10 hp air compressor to drive it. A 7 1/2 hp air compressor cannot keep up.
A 2 hp air compressor will not generate enough air output to a small die grinder.

Major air tool user plants such car assembly plants are our best major air compressor customers.

Gus,32 years with Ingersoll-Rand.

 

[Tin Falcon]

Two perspectives here.

I am talking bout small air powered reciprocating engines. Basically steam type engines. the others are mentioning air tool turbines.
I expect well made air tolls are more efficient.than there Asian cousins.
but I do agree efficiency is not there in any case.

The ME shows I have been to all use large comprssors usualy deisel powered of the type used to power jack hammers. even the small venues have a 5HP unit just to run a bunch of small air engines and few if any of the engines are doining any work.
Tin

 

[John Rus]

As for the bore and stroke question the main reason for choosing one over the is the other is the torque and horse power range is different, long strokes are more torquey and short strokes are more revy.

John.

 

[JAndrew]

All,
Thanks for the replies. They all make sense.

As for the bore:stroke ratio question.

As for the bore and stroke question the main reason for choosing one over the is the other is the torque and horse power range is different, long strokes are more torquey and short strokes are more revy.
John.

John,
What you're saying makes good sense but there's still one thing that nags at me on this one...

Suppose you have two single piston air motors. Motor A has a short stroke and larger bore and motor B has a long stroke and smaller bore. These two motors have the same total displacement and get supplied a constant equal air pressure.

Motor A (short stroke) will rev up and down quicker but have less torque because the crank throw is lower. Motor B's connecting rod applies force on the crank with more leverage thus more torque.

This statement jives with what you said before but the part that troubles me is: What about the larger area of motor A's piston?

Pressure=force/area
Rearrange that equation and you see that a larger bored piston will provide more force for the same air pressure.

So wouldnt a larger bore with a larger force applied prevail? Or maybe motor A would have higher HP and lower torque and B would be higher torque and less HP?

MCRIPPPer,
All very good points. Thank you.

TinFalcon,
Excellent point about well built engines running better at low speeds. I guess the one thing I should have asked is: What IS the effective rpm range of these air motors (reciprocating piston type without variable valve timing)? I'm guessing it would be similar to old steam engines (0-500ish RPM)?

Also the PLAN/33000 formula you provided, I'm assuming that's to estimate Horsepower?

Gus,
I wonder how many of the compressors I've worked with came from you!

Gus and MachineTom,
You're right that die grinders ARE fairly inefficient air motors. But they are simple and cheap to make and have their good applications (+1 nuclear plants).

Thanks all again. Very good info.

-J.Andrew

 

[gus]

In the oil rig fab yards in Singapore,if a welder's assistant ever tried using an Ingersoll-Rand 6"/8" Disc Grinder to do pipe bevelling and weld repair dressing,he is addicted to it forever and will never go back to use electric grinders which are heavy.

Air Grinders have no problem with heavy grinding and does not get overloaded and burnt out. A few drops of Singer Sewing Machine oil in the morning and lunch tine into the inlet will do.

The work output from air grinders is great.So is the air consumption and when the vane motor begins to wear out,the air consumption gets bigger. And we sell a few more Ingersoll-Rand DR 150/Dr 250/ DR365/DR600 Diesel Drive Rotary Vane Compressors.
This was way back in 1970--------1980s before Gus got desk bound.
Oh .We love leaky hose joints too. We also love very long 1" rubber hose and undersized hose which cause pressure drop.

 

[JAndrew]

6) the formula for a steam engine is (P x L x A x N)/33,000

P pressure
L length of stroke
A area of piston
number of stokes per minute.

Tin

Also Tin...

If you have a steam/air engine with multiple cylinders can you just multiply PLAN by the number of cylinders?

I tried running the numbers like this and they seem about right. Maybe the equation should be PLANN instead with one of the N's being "number of cylinders"?

Thanks,
-J.Andrew

 

[Tin Falcon]

N is stokes per minute in a a singe colder sing stoke this would = rpm.

in a 2 cylinder double action there will be 4 strokes per revolution. this is sometimes confused in the formula. The formula does not aply to multistage muti pressure engines.(Compound) each stage needs to be caclutated seperately.


Also remember IIRC dimensions are in feet. for stroke and area.
Tin

 

[gbritnell]

Here's is the link to the video of one of my small double acting steam/air engines. I have never measured the rpm but I'm guessing on 40 lbs. of air it will spin in the neighborhood of 10,000 rpm. As was stated it depends on the accuracy of the build.
[ame]http://youtu.be/tcfrKsOaDNw[/ame]
[ame]http://youtu.be/szfLonCq2ms[/ame]
gbritnell

 

[MachineTom]

JA, In your post you questioned why a larger bore/stroke did not lead to an increase in power as it had a greater area. In a non crosshead engine the connecting rod stroke circle diameter gets smaller as the stroke decreases. This decrease results in a loss of torque as the crank now has a shorter lever to make torque. Or, imagine a 500 lbs rock, a fulcrum at 1 foot from center of rock, a pry bar 2 feet long, you need 500 lbs on the end of the bar to lift the rock, Not gonna happen, so you grab a 3 foot, bar and now need 250 lbs, maybe so you grab a 4 foot bar now you need 166 lbs to lift the rock. But the downside is that to move the rock with the 2" bar, the bar would lift the rock 1inch, and the bar travel is 1 inch, the 3 foot bar needed 2 inch travel to move the rock 1 inch, the 4' bar needs 3 " of travel to move the rock 1 inch.

So as the bore gets bigger the stroke gets shorter, any increase in area or the piston is met by a decrease in torque. Horsepower is RPM * torque /5252= horsepower.

Hope this comes out clear for you.

 

[Entropy455]

1.What is the effective RPM range for air motors?

For air turbines, the upper RPM range is normally limited by centrifugal force on the turbine blades/buckets. Generally speaking, the smaller the air turbine, the faster it can turn without damage.

For piston engines, there are several design limitations. First, the mechanical limitations of the motor cannot be exceeded (connecting rod stress, wrist-pin stress, bearing stress, etc). For this reason (generally speaking) smaller motors can turn faster. There are two other issues with regard to the air itself. Compressed air has appreciably higher density, and inertia losses become significant when the airflow is rapidly started and stopped. Another design limitation is that you cannot have supersonic flow within air supply piping. Airflow into the cylinder is the only place where supersonic flow would be acceptable – assuming your goal is very high rpm operation.

2.When would double acting pistons be more efficient than single acting? The two trains of thought are that a double acting has more friction due to packing but may be more efficient in instances where there are less cylinders.

Unless you are dealing with a very high pressure air supply, anything more than a single stage is a waste. Air has a low heat capacity and becomes very cold when expanded – which lowers pressures, which lowers work extraction. This is why it is advantageous to add heat to the air prior to work extraction (aka the combustion stage of the Otto Cycle, the Brayton Cycle, and the Diesel Cycle).

3.How do the bore to stroke ratios of air motors compare to those of IC engines? Should you utilize a longer stroke since your rpm range is lower to gain efficiency? In the IC engine world higher-reving engines like motorcycles use a shorter stroke due to material friction concerns.

Generally speaking, a square, or slightly over-square cylinder would be ideal for a high rpm air engine. Note that you can alternately apply air pressure to a diaphragm, and make a pretty good “piston” type motor. It’s pretty effective machine, considering it has no cylinder, no rings, no piston, no crankshaft, etc – yet quite capable of doing work.

4.For the same total displacement, driving the same load, would there be any efficiency gains by using a multi-stage air motor? (ie a small piston, which exhausts to a bigger piston etc.)

Your biggest advantage would be to add heat into the air prior to the power stroke. (see answer to question 2)

5.Any other design characteristics that vary air motors from their IC bretheren?

An air motor will have a different camshaft profile. For best efficiency, you’ll want to maximize what would otherwise be the compression ratio.

 

[MCRIPPPer]

remember a motor with more torque does not really have less horsepower. it buggs me when people say " does this have more torque or horsepower?". horsepower is a measure of how fast work can be done ( amount of energy over a period of time). a low hp engine can do the same work as a super powerful one, but it will take longer. torque has nothing to do with energy, but how much force something has. force requires no energy to sustain. torque is pretty much meaningless unless you add an element of time (rpm). you have to add another layer of time to figure out how much energy you are actually expending. for example saying "my car uses 5 gallons of gas when it puts out 100hp", has no real meaning, compared to saying, my car uses 5 gallons per hour at 100hp". this is the same reason electricity is measured in watt hours, not just watts. watts are directly comparable to hp. just like torque and rpm make up hp, the components of wattage are volts and amps. each bare bones component means nothing by it self in most circumstances. watts and hp have a little bit more meaning, but not as much as watt hours, or i guess you could say hp/x time unit.

 

[JAndrew]

Thanks all for the replies. Sorry for the delay in my reply (coming off nightshifts sucks up a lot of time).

MachineTom,
I see it now. I didnt realize the role of torque in an engine's HP. Just like MCRIPPPER said people too often think torque and HP are balanced against each other.

Entropy455,
Brilliant! Just the kind of answers I was looking for! I'm very curious about the camshaft profile differences that you mentioned. I will google this topic. Any examples of these profiles or duration numbers you can offer would be greatly appreciated.

Tin,
Thanks for the clarification. I see my error in thought.

GBritnell,
Thanks for the videos. Incredible work! I guess I assumed that the RPM range would be much lower since I read somewhere that their steam engine bretheren max out at relatively low RPM due to steam taking longer to enter the cylinder (and packing losses).

Thanks all! Almost finished with the Atlas restoration and then I'll attempt to fumble my way through a single cylinder oscillator build like Tubal Cain did in his youtube video series (except smaller scale)
-J.Andrew

 

[Entropy455]

The intake valve should open at (just a tad past) TDC, where compressed air will drive the piston down. The intake valve should then close before BDC. Generally speaking - the earlier the intake valve closes, the less torque the motor will make – however the more ultimate work extraction you’ll get from the air – which means the more efficient the motor will be. On the other hand, the longer you hold the intake valve open, the more torque the motor will produce, thus the more power it will make. However it will also drive compressed air consumption way up, and significantly lower the motor efficiency – which means you’ll suck A LOT of air from your compressor.

The intake valve should be fully closed before BDC, and the exhaust valve should open just after BDC. The exhaust valve should stay open until just prior to TDC, where the exhaust valve closes - then the process repeats.

Unlike a gasoline engine, the intake and exhaust valves should never be open at the same time (aka no valve overlap). It’s similar to a 4 cycle engine operating without a compression and power stroke – thus you'll have a unique valve timing, that’s similar in a way to a 2 stroke.

What boat did you serve on? (I’m the guy on the right)

 

[JAndrew]

Entropy455,

I was on the Alabama (SSBN-731B). It was the best-worst time of my life. I'm sure you know what I mean by that. I have a similar picture to that one (but wearing sound powered phones). I'll have to find it.

Looks like a fast attack? Can't tell where that is. You guys have a Fairbanks Morse ND38 8 1/8 Diesel? I'd love to make a model of one of those someday.

Thanks for the cam info. I'll file your posts away for future projects.

-JAndrew