Designing a Quiet Air Compressor

[Toymaker]

The thinking was twofold:

1. As the piston went to bottom dead center, the oil on the back end of piston would get pushed out of the cylinder and then work its way into other areas.
2. When the pump stopped, the oil would then fall onto the center bearings.

As for the other comments following my test run, they're all good comments. I fully understand that you all are only trying to help. But there's a bigger issue:

The Air Pump Don't Pump Air!

At this stage in the game, I'm so far off from my original intentions, that I really need to evaluate the cross leakage. I'm not even really worried about volume right now. A noisy air pump that actually pumps air is better than a quiet air pump that doesn't do squat.

Thus far I'm thinking of:

• Replacing the possibly ill-fitting barbed fittings
• Making a new crankshaft to incorporate 100 series (.103 thickness) o-rings instead of 000 series (.070 thickness) to have more tolerance for air gaps/mis-alignment.
• Making something akin to springed motor brushes made of either teflon or brass that would press up against the crankshaft for port sealing.

Please understand that I'm not trying to be an ass. Yesterday was quite the disappointment.

...ved.

I hope I'm wrong, but the only place I see your lubricating oil going is a one-way trip out the High Pressure air exhaust port. At 1700 rpm centrifugal force will move all the oil that comes into contact with the cylinder wall past the O-rings, into the head area of the cylinder, and there it will be pushed out of the air exhaust port. I can see some oil being trapped on the bottom side of the piston, but the surface you want most to be continuously lubricated, the cylinder walls, will most likely run dry after a few minutes of operation. I just don't see where oil is being continuously moved onto the cylinder walls. I don't understand how the oil your piston pushes out of the cylinder finds it's way back onto the cylinder walls.

 

[Steamchick]

Ved, you are not being an ass, so don't berate yourself. What you are doing single handed is something a factory would probably spend a year doing, then another testing, refining, making cheaper before going into production.
I have done it many times with many products - and sub-assemblies of products - so it isn't a hobby to me. It was a high stressed job for 40 to 60 hours a week, for up to 4 years on some products. (and 40 plus years in total). So you are just going through the phases that will be repeated many times, with each one being a step closer to success. So stick with it, as you are solving more problems than you are creating, so that is a win every day.
Stage 1: (Many never get past this!): Convert your idea into drawings parts then assemblies.
Stage 2: (Often the killer for a project!) Make that first one run.
Stage 3: Identify where it just isn't adequate, and re-design so it will last more than "zero" life (Zero life is less than 10 runs without anything worn out or broken).
Stage 4: Re-design and build the second one, often using some parts from the original. - I reckon you are now here already.
Stage 5: re-***** what you want from the project, as you will have a sensible running prototype. - Just needs tuning and fettling, or just play with it, knowing its foibles...
Then things get better all the way, as you define the further stages you want from the project.
And don't forget, a Hobby is for enjoyment. When enjoyment stops, take a break and enjoy something different for a while, then come back refreshed.
We appreciate being able to "spectate" on your project. (Spectators are those who tell the referee and players how to play the game!).
Stay happy.
K2

 

[vederstein]

only place I see your lubricating oil going is a one-way trip out the High Pressure air exhaust port

Certainly a possibility.

A quick calculation shows that the oil pressure against the piston o-ring is 77psi @ 1725 rpm. That's easily within the range of a buna-n oring.

But yeah, there's going to be blowby. How much? I dunno. Perhaps a bit of leakage will help lubricate the crankshaft.

Time will tell.

...Ved.

 

[vederstein]

I have done it many times with many products - and sub-assemblies of products - so it isn't a hobby to me. It was a high stressed job for 40 to 60 hours a week, for up to 4 years on some products. (and 40 plus years in total). So you are just going through the phases that will be repeated many times, with each one being a step closer to success.

Me too.

I'm an industrial machine designer. I've designed machines for the trucking industry, automotive, medical, aerospace, and currently I do electronics assembly. I have five patents with my name on them and I've been doing this for over 25 years.

A design, particularly a custom one-off machine never, ever, works perfectly on the first try.

...Ved.

 

[vederstein]

Looking at the design, I'm guessing the cross channel leaks are coming from the boundary layer between the body and the crankshaft. I don't it's actually happening, but possibly there's a bit of seal misalignment with the porting that's causing issues.

Therefore I went ahead and added some spring loaded PTFE tubing somewhat akin to motor brushes on an electic motor. I redesigned the crankshaft to have:

• Smaller Ports to allow the brushes to overlap the ports
• Shorter port length to again increase the boundary layer.
• Larger (.103) O-rings that should be more tolerant to any misalignment

Once I get these part made up I'l test again. If there's little or no improvement, I guess I conceded that my ideas didn't work and I'll have to incorporate some type of check valving system.

...Ved.

 

[Zeb]

Tightening up the ports makes sense.

Just some ideas if that's ok...
-On the audio side, are you getting any evidence of piston slap on the skirts?
-The hose creates a decent sized reservoir and restriction between the piston face all the way to the valve. I'm wondering if the piston is filling up that volume before the flow has a chance to get captured by the crank valve. Similar to the idea of filling up a tractor tire vs a shock strut. A little reed valve at the cylinder top would alleviate but I guess it defeats the idea of running quieter.
-It'd be interesting to see piston pressures and flows for all three in phase.
-I'm guessing the crankcase end is vented to atmosphere, unlike the valve end?

 

[Steamchick]

Hi Ved, just a stupid comment from me. How has sealing improved with running-in?
Running-in is essential a wearing process, so I had an engine where compression increased, peaked, then reduced as parts became worn-out, during what I thought should be just a running-in process driven by an electric motor. Time to make a new piston and cylinder!
Do you have a similar problem?
Ken

 

[vederstein]

sealing improved with running-in

I never got that far. If I was in the 50-70 psi range, I'd agree. But with only 10 psi there's far bigger issues going on.

When I get the brushes made/installed, there will certainly be a break-in period.

I'll probably set it up outside, covered in the box. I don't particularly want to damage my '52 MG (or myself) if something blows apart.

I haven't remade any chips yet. I just didn't have the motivation yesterday when I usually spend time in the garage.

(It's a hobby. If I don't want to work in it, it can sit there.)

...Ved.

 

[Steamchick]

Thanks Ved. I feel less guilty about the "job on the bench" postponed for 6 months...
K2

 

[KellisRJ]

Only six months? That's not even long enough for dust to settle. Some of mine have several layers, from opposite sides of the continent. <rolls eyes at self>

Ron

 

[vederstein]

I made up the port brushes, found some applicable springs in my stash, and reassembled the engine. (No reason for pictures because, externally, it looks the same.)

Performance is worse than before.

Therefore: I'm done. This project is officially a failure. Not that I didn't learn things, but there's a point that one just has to say screw it.

I'll convert the pump into an air motor and leave it at that.

To be honest, this project stopped being enjoyable a couple of months ago. And for a hobby if it ain't fun, don't do it.

Maybe I'll go back to analyzing the Walshearts valve mechanism.

Anybody want to buy a 1-1/2 horsepower motor (1725 rpm, single phase, 56C face, 120/220V)?

...Ved.