Designing a Quiet Air Compressor

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Compressor timing varies with the amount of pressure in the tank

I agree with your assessment when working with check valves. But think of an industrial scroll compressor. That type of compressor has no valves.

In my proposed design, each cylinder inlet stroke takes in .31 cu-in of air which equals .0021 moles. Those .0021 moles are then pushed into the air tank. Prior to opening the cylinder to atmosphere again, the cylinder is isolated from the tank through the radial valving.

I guess I just don't see the difference other than the elimination of the check valves. But I will lose some pumping efficiency due to the inevitable leakage between the high and low pressure air channels. For this project, I'm willing to lose some efficiency for a quiet air compressor.

I need to take another look at my numbers though. I'm not sure I got the massflow correct.

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I recently saw a video, perhaps on this forum, of a Stuart compressor running. The loudest noise it seemed to be making was a sort of plopping, slightly comical, which I put down to the inrush of air when the pistons uncovered the rather small inlet port.
An observation on valve "pressure noise" and air column dynamics:
To "compromise" for the inertia of columns of air, infernal combustion engines use overlap of valve timing - and even sonic wave reflection (as in 2-stroke expansion boxes, reverse megaphone 4 stroke exhausts, etc.) to push gases back into the combustion chamber thus increasing the mass of combustion charge in the cylinder.
Of course, compressors do not need to do that. They work directly with pressure differences across valves. - Hence reed valves are very good - self adjusting for the varying pressure on the receiver side of the valve. Reed valves also have very low mass, short movement, and seal based on the air pressure across them. - Almost ideal for most small compressors.
Pneumatic noise is the biggest cause for concern with compressors, usually intake noise as the "vacuum" in the cylinder opens to air, sucks air in then closes the reed valve rapidly. This rapid closure causes a very short period shock wave (positive pressure wave?) to be generated at the valve just prior to closure of the valve, when the mass of air column moving towards the valve then sees a "closed door" and builds the pressure wave further - which then travels at the speed of sound back up and out of the intake. Not "Valve" mechanical noise but Dynamic gas pressure shock wave noise created by the valve operation. A Helmholtz resonator adjacent to the intake valve can be tuned to the mass and frequency of the noise and make the compressor close to quiet. But it is added cost to an otherwise cheap and simple compressor, so no-one bothers!
Try modelling that on your existing compressor and making it quiet?
I considered doing it on my 90dB compressor until I realised the intake valve was a reed valve on the top of the piston, with air drawn through the piston (to cool it? or just design for "Cheap manufacturing"?).
But using a mechanically timed valve you should be able to tune the timing so the valve closes just when the pressure difference across the valve is zero - and thus reduce the initial "kick" for a valve that is closed by pressure difference. This does NOT eliminate the air-column shock wave when the valve closes though. A Helmholtz resonator does that.
Food for thought...?
An observation on valve "pressure noise" and air column dynamics

Is there some engineering subject that you don't know?!?!?!

Anyways if I read your comments correctly, you're saying the inrush of air when the intake valve opens is the cause of the noise. I can see that because it will take a certain pressure differential to counteract the spring tension on the valve.

As far as my compressor concept goes, I may have half of that issue resolved, being that the valving is based on the rotation of the cylinder body and not on a check valve.

When the piston completes its push stroke, there's a dwell between the cylinder being the cylinder being open to the receiver and open to atmosphere. Once the cylinder is open to atmosphere, there will be an out rush of compressed air from the cylinder to the outside world. If the entire cylinder volume + internal passageways is determined, it's possible to open the cylinder to atmosphere just when the pressure in the cylinder reaches atmosphere thus minimizing the an air out rush.

This would greatly decrease the efficiency of the compressor and may be a step too far.

I'll need to study this a bit more and determine the impact.

Thank you for the thoughts.

very clever design . Our shop has probably the loudes compressor ever made it’s almost like running a top fuel engine in the shop you can clearly , loudly running even when table sawing big wood now on the other hand I have a California air compressor at home that you can barely hear running in the next room it’s actually two cylinder but only runs about 2/2 normal speed the intake is nearly silent using the air blower is far noisier Running air to a couple of my steam turbines creates higher db noise I YHINK it’s the high frequency whine that dominates the noise level I’d suggest some kind of baffled maybe with a foam or paper filter air intake
As i understand it, step-down transformers are not used in the modern TIG welders, it is all done electronically. That means there can be much greater control. I have a transformer type TIG welder in which one has to strike the arc like a match. It was one of the "new" ones made for small businesses back in 1980. I got it for a Lincoln 250 welder. The linconln is very nice, -- infinitely adjustable from 30 to 250 amps. But the TIG welder is COMPLETELY basic. I used it for a bit of practice then put it away as too difficult to use and control unless I absolutely HAD to use it which I never did need.

However, last year I bought a newer model, controlled and run by electronic methods.--that is, the new type of TIG machine. Many years ago, I learned to always check the % duty rating. IMNSHO, one should always try to get 100% duty rating so there need be no down time. If you are working professionally, this is a must. If hobby work, not necessarily a must. Also, this 100% refers to the higherst setting you can weld with. If you are welding at 200 amps, then your machine has a 40% duty cycle. If you are welding at a more likely setting of 40-120 amps, your duty cycle is longer, say something in the order of 60%.. If you are welding at 40 amps, your duty cycle is probably around 100%.

Another thing: since you are in the Land of the Thais, you are in a very hot country--that means the heat will not dissapate as rapidly as in a cooler environment which reduces your duty cycle. Keep a fan on your machine and that will help.

K2--Advertising? Yes, FALSE advertising! The Chinese have been known for centuries to give misrepresentations. However, it really is the same everywhere. In the US, false representation is often a illegal. But a rat can squeeze thru any hole in which it can get it's ratty litltle head thru, so any rat in business will keep chewing till it absolutely breaks the law. Unfortunately for the rats in business, they DO get a reputation, in this case, a "ratputation". -- HA, I just invented a new word, it will now have to be placed in the Oxford Dictionary. Will I be able to patent th word? How about a . . .
Hi Ved.. "Is there some engineering subject that you don't know?!?!?!" - A hell of a lot!!
But I have worked on car engine design (and had a few weeks training in the Japan Design office) - which covers a multitude of sins. The visible sins are discovered by mechanics the world over, especially all those who know better and are not constrained by Legislation, Sales and Manufacturing departments' limitations, customers, accountants, etc. - but those guys usually know a hell of a lot that I don't know. - So I listen to everyone...
e.g. the design engineer will fit Helmholtz resonators, until the accountants (cost experts!), or aftermarket mechanics decide to remove them for some purpose. - After all, what is a bit more noise? - when the customer isn't happy with an "aged" car he'll buy a new one. - Which sounds quieter.. because it has the Helmholtz resonators that keep it quiet... Same with exhaust "silencers". To make after-market parts cheaper, the "luxury" bits of silencing are omitted as they were just there "to get the car through the homologation test" - and not "necessary" to the after-market customer. The fact you may lose 1% of fuel economy in the process or make the Ozone layer dissolve just a tad quicker, isn't seen by those accountants who make the bits cheaper!
And don't ask me about tyres. I am not an expert. But the tyre makers only sell (the best) 20% of their tyres to car makers, because the (Poorer quality) 80% go to after-market sales, where price is paramount... They will make a small loss on the 20% OEM just to get the brand name and size, etc. on the car as new. And the original performance required by the Designer is met by those 20%.... The rest are simply rejects, sold at a whacking profit - because they can. Ever tried a non-original branded tyre on your car and noticed the difference? - Hard to spot except the name. But 2~5% MORE fuel consumption, 3dB MORE noise, 5~10% GREATER stopping distance (You only measure that when you crash), etc. are factors that make those cheaper. (And I stress "More" for "less money").
But that's how the world is going around at the moment.
It is my ambition to become a knowledgeable Engineer in many subjects, but so far succeeding poorly.... Which is why I read many of these threads! - Simply to learn.
Thanks for asking.
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Oh Ved, I leave it to you and other experts to filter out or correct my errors, wrong statements, Junk, Rubbish, simple B.S., etc. as I REALLY don't know what I am talking about half the time... - But I try to learn from some of my mistakes.
I cannot take offence from any corrections from you lot. - I simply enjoy all the expertise available here.
Before I retired work had a 60 hp piston air compressor it was very quiet for a piston type compressor, it compressed air on both sides of the piston, top & bottom. I'm not sure how efficiently is determined but it was very efficient for a piston pump compressor, we only serviced the valves about once every 10 years & ran good for 20 years and was still in good running condition. Then the company was convinced an industrial scroll compressor will be more efficient and same money so the old compressor was replaced but left in place to be a back up compressor. The new industrial scroll compressor was loud like an airplane jet engine. No more piston stroke sounds just a jet engine sound. I think most of the jet engine sound came from the high speed cooling fans. The scroll compressor had to be serviced more often to make sure it has oil and cooling, etc. it also had safeties that turned it off if there is a problem. WE had people that were trained to service the industrial scroll compressor. The industrial scroll compressor self destructed after 1 year and was replaced the 2nd new industrial scroll compressor ran good for 6 years.
In the 1960s I worked (Saturday mornings and holidays) as a school kid at an Engine Machine shop. They also serviced some large Broom Wade compressors for a Paper Mill. 10 inch dia pistons, 2 ft stroke, and the annual service was oil changes, then simply to clean valves and fit new carbon (3- piece) piston rings. They were about 20 years old? Like steam engines.. They were double acting with cross-heads. I machined the cross-head castings on the huge lathe in the workshop. Measured diameters with the large micrometer, checking from above and below as the large micrometer changed size depending on whether you offered it up or down the the job. True size was the mid-point of the 2 measurements. They lasted to the end of the Mill in the late 1980s. But I never saw them running to know the noise....
About 6 months ago, another forum member, dazz, suggested I look into building a Guided Rotor Compressor (GRC). At that time, I had already finished building my wobble plate compressor and therefore had no need to build another, different compressor, which I had never heard of.

Ved, I know you want to do your own design, but since you're still in the design phase, you may want to consider using the GRC as a basis for that design. I've not been able to find an animation showing how the various parts move so I can only guess at how the rotor moves around inside the outer case. I suspect the yellow circles in the drawing below represent roller bearings,...but that's just a guess. I also suspect that there are no moving valves, but rather only intake and outlet ports,....but again, I'm guessing.

Perhaps another forum member can supply more details about how the GRC works. Where are the in and out ports, etc.

This design is very similar in principle to the trochoidal oil pumps on the end of the crankshaft on most modern cars. Intake ports are oestrogen when the chamber expands, and outlet ports open when chambers are reducing volume. Disadvantages are the tight tolerances of clearances required (10 x tighter tolerances than regular home workshops). All the leak paths are difficult to seal for leak rate to LP from HP.
Looks similar to a gerotor. I designed a hydraulic pump once using one of these. I was trying to get 1000psi. Only when I started violating the recommended tolerances did it put out the required pressures. Strangely, I had to put in more clearance than what was recommended by the gerotor manufacturer to go from about 750psi and make the full 1000psi.

Looks similar to a gerotor. I designed a hydraulic pump once using one of these. I was trying to get 1000psi. Only when I started violating the recommended tolerances did it put out the required pressures. Strangely, I had to put in more clearance than what was recommended by the gerotor manufacturer to go from about 750psi and make the full 1000psi.

View attachment 142098

I'd be right curious as to how that pump was actually 'made'.
I designed that pump about 20 years ago and the company went bankrupt since then. I don't have access to any of the design files anymore.

A quick search and the actual gerotor manufacturer was Nichols-Portland which is one of the many divisions of Parker-Hannifin. I remember they published a design guide back then. Perhaps it's still available.

I worked a few more hours on the pump design.

The pistons are fairly simple, but being that the internals will be oiled via splash lubrication, I added some weep holes in the side of the piston so oil the cylinder walls between the piston.


I had to play around with the geometry to keep eliminate some interferences, but it's coming together. I also fixed the inlet and outlet valve porting and I until I realize I screwed up somewhere.

I have the air intake on the radius of the pump. The air outlet is out through the center of the crankshaft.


And now the standard computer rendered video:

Until next post.

With the pump essentially figured out, it was time to mount it. I want to use a 56C frame motor which is an industrial standard for face mounting of motor accessories. I can direct drive the pump off the motor shaft and then support the crankshaft fast with some bracketry:


I went ahead and made the drawings for this design. Usually, I don't publish until I make the first one, but being that I'm sharing the entire design process, I'm posting draft drawing set. I'll re-post the design files after I make the prototype, but the attached .zip file is what it looks like before I build it:


  • Ultra-Quiet Air
    3.1 MB · Views: 14
It's been over month since I've made any posts concerning this project. As you can see in the following photo, I have been making some progress. In the manufacturing stage, I've noticed a couple of design errors and/or manufacturability improvements.

The crankshaft really took some planning prior to making chips because the thread pattern in the end of the shaft must be "clocked" correctly to the radial ports. Thus I created a jig (triangular part in the upper left of the photo) so I could put the threads in the end of the shaft then remove when resetting the mill, keep my orientation correct.

I screwed up one of the three pistons and I'll need to remake that component.

I guess that's it for now.


I have a California ultra quiet air compressor actually the harbor freight knock off . While not high volume itbis so quiet that you can sit right next to it and hardly Lear it run . My son has a larger on that is only slightly more noisy . I YHINK these are two cylinder but short stroke there aren’t any drawings of construction . Someone posted a video on you tube of a disassembled one they really don’t have reed valve. I think they are more automotive engine construction , can driven piper valves . I’m not sure but even a big v 8 car engine doesn’t make a lot of noise even cranking with a starter with plugs removed. I would think a iston valve engine might be very quiet too as an alternate. There is a certain amount of noise on the intake side but that is easily quieted by air cleaner
Th actual CAC model is made in China anyways. I wonder if they're made in the same factoey.

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