Convert wounded GM 6.0 into a compressor?

[Warpspeed]

That is the whole idea Mapper, Strantor wishes to build a very low rpm "stealth" silent compressor.
He should be able to reach the required piston displacement from sheer size and number of cylinders.
Low piston speeds, relatively low internal air velocities, that should drastically reduce the noise.

Just because nobody else does it that way does not mean its not going to work.
Sometimes, way out there fringe engineering can work amazingly well for a very special purpose.

 

[mapper]

That is the whole idea Mapper, Strantor wishes to build a very low rpm "stealth" silent compressor.
He should be able to reach the required piston displacement from sheer size and number of cylinders.
Low piston speeds, relatively low internal air velocities, that should drastically reduce the noise.

Just because nobody else does it that way does not mean its not going to work.
Sometimes, way out there fringe engineering can work amazingly well for a very special purpose.

I was responding to the posts that were suggesting operating on 4 cylinders driving the other four.
My brother in law built a compressor from spare parts some years ago using a rather large 2 cylinder compressor, (don't know if it's one or two stage) and powered it with a 5hp(?) 1800 RPM motor. It runs a lot slower than what it was rated but is very quiet. You can carry on a conversation in a normal voice while standing next to it. Knowing him and his scrounging ability it probably cost him less than $200 and a bit of fabrication to put it together.

 

[strantor]

Well this was really interesting. Ran across it by chance. It's the opposite of my goal but even more radical.

DIY & Crafts - Turning A Car Compressor Into A Working Engine | Facebook

 

[MossRoad]

Well this was really interesting. Ran across it by chance. It's the opposite of my goal but even more radical.

DIY & Crafts - Turning A Car Compressor Into A Working Engine | Facebook

That was pretty cool! Thanks.

 

[strantor]

I知 not sure how you壇 go about calculating the CFM. It痴 not as simple as a liter to CF conversion times rpm divided by compression strokes. The problem is due to compression it痴 not moving 6 liters per compression stroke on every cylinder.

I've been doing some more reading, and learned that you and I were wrong. It really is that simple. Compressor manufacturers present misleading stats. When they say (ex) 17 CFM @ 120PSI, that's essentially a bold faced lie. It is NOT moving 17 CFM of at "AT" 120PSI. It's moving 17CFM AT ambient pressure, and building 120 PSI after the compressor. The CFM figures published by compressor manufacturers represent the volume of air at ambient pressure inhaled by the compressor per minute. The cubic feet of air released at the outlet is less than was inhaled, mostly due to pressure drop-off caused by temperature change and also check valves being <100% efficient, and various other factors. But if you take the displacement of the cylinders multiplied by RPM and add about 1/3, that's a pretty estimate of output CFM as it returns to ambient pressure. But the CFM of compressed air moving through the pressure vessel, that is never discussed or disclosed. It would probably look closer to my math. My math was based on volume of compressed air flowing through the pressure vessel, and when I re-worked the numbers to match how the OEMs are presenting their figures, it turns out I was off by almost a factor of 10. Instead of 980rpm, I could hit the 20CFM goal turning at just over 100 rpm.

 

[texasranger556]

Cheap compressors are rated in SCFM which is as you state, input volume. I do believe industrial grade units rate CFM at a particular output pressure. Marketing eh?

 

[strantor]

Cheap compressors are rated in SCFM which is as you state, input volume. I do believe industrial grade units rate CFM at a particular output pressure. Marketing eh?

CFM, SCFM, ACFM, all apply (according to what I've uncovered) to the air moved at ambient pressure. That goes for commercial and consumer units alike (again, according to what I've read). ACFM is SCFM corrected for ambient temperature, humidity, elevation, etc. And that's the source of the up-to-33% thumbrule I stated previously.

SCFM vs ACFM | Standard CFM vs Actual CFM

I started down this rabbit hole of "CFM of compressed air" vs "CFM of ambient pressure air" while investigating the use of large (10-20 ton) A/C scroll compressors as a source of compressed air. I was unable to find clearly defined CFM values for the vast lot of them, and it seems the reason is because they dont move [gas at ambient pressure] to [gas at a specific higher pressure]; they move [gas at a wildly variable "low" pressure] to [gas at relatively higher pressure]. So I deduced that they are not (typically) rated in CFM like an air compressor because there is no ambient pressure reference. My research seems to confirm that.

Evaluating True Horsepower and CFM Ratings of Air Compressors

When a compressor pumps one "CFM" (cubic foot per minute), that means the intake port inhales one cubic foot of "free air" (air at atmospheric pressure, which is 0 psig) per minute. (Note: A CFM does not mean in any sense the compressed volume.) So the unit really measures the mass of air flowing per minute, not volume per minute, since a cubic foot of free air is a unit of mass. Some people labor under a stubborn misundertanding that these units refer to the flow of compressed volume (as opposed to free air volume), but this is flatly wrong. The confusion arises because the term "cubic foot" sounds like a measure of volume, when in fact this term in this context is an abbreviation for "the mass of a cubic foot of atmospheric air", which is a measure of mass. This nomenclature dates back to the 19th-century era of steam power, which is still quaintly with us.

There usually is some difference between the ratings of anything consumer grade (almost assuredly skewed/inflated) and its industrial cousin. Industrial units are more often specified by Engineers who need factual data to ensure capacities in compliance with their design specifications. Consumers are usually after the bigger number for the smaller price and rarely verify the big numbers or make any kind of weighty stink when they dont add up. In this case, I think the MFGs of consumer grade products are (in contrast to commercial MFGs) guilty, not of failing to specify the capacity in terms of volume of delivery at system pressure (a difference of 1,000%+ from ambient), but of:

1. As you said, by listing their rating in SCFM rather than ACFM (a difference in my estimation somewhere between 5% and 30%)

2. Using the "@"/"at" term to imply that the capacity is of volume of compressed air delivery. Which, in my opinion is an outright lie. I mean, we all know what the word "at" means, right? How is its use in that context to be interpreted as anything other than cubic feet per minute of air at system pressure? But I'm sure that through some muddled abortion of the legal wing of the broken English language (or an outright payoff) they've "proven" to some court that the term is sufficiently ambiguous to preclude the conviction of false advertisement.

 

[texasranger556]

Good info. It seems I went a down a shorter rabbit hole than you have!

From my understanding, stating for example 10 SCFM @90 PSI, they're specifying the input rate of the compressor when the output pressure is 90 PSI. I don't view that is an outright lie, but unfortunately it is not relevant to a consumer trying to determine if said compressor has enough capacity to run a particular tool with a stated CFM @90 PSI.

The SCFM rating at a particular pressure is only relevant for comparing the performance of compressor A with compressor B.

What does annoy me is manufacturers stating SCFM numbers but leaving off the "S". I consider that an outright lie, whether it is intentional or not.

 

[Renze]

The diesel engine is more efficient than a spark ignition engine for two reasons: one is compression ratio, the other is absence of a throttle valve in a Diesel, which creates an intake vacuum at partial load or idle.

The reason gas engines idle whisper quiet is the same: they are controlled by a throttle valve, making them inhale on a vacuum so they get very small effective compression pressure.

The noise level you would get when operating as a compresis about the same as when running at full load.

Furthermore, i think your 5hp motor struggles to overcome the mechanical friction alone, of that 6 liter engine at 1385rpm.

You will need to drive it from the pto of a 150hp tractor to utilise the full capacity.

Also, the compression and ignition strokes will be idle, yet cause friction and piston ring leakage.

Any way i see this working is with a small 1.5 liter Honda engine, the head removed and replaced by an aluminium plate with 8 non-return valves threaded into them: four for intake with air filters fitted, and four joined into each other which will be your pressure output.

And dont use rubber nonreturn valves as these will burn in the heated gases, halving the volume means doubling the temperature because thermal energy vs volume is a constant (adiabatic compression)