Convert wounded GM 6.0 into a compressor?

[Industrial Toys]

I don't know but feel the other poster mentioning the size of motor needed might be correct. Probably any such capacity compressor would use a 480/600volt 3 phase motor, as it has exceeded anything practical for single phase.

From that standpoint, a project using half the motor to run it, for occassional use, would make more sense.

 

[Warpspeed]

The original poster wants to keep it very quiet, so an electric motor and an unusually low compressor rpm might go a long way towards that goal.

I agree with you though, that running half the engine, and using the other half as a compressor makes perfect sense for portable applications. The difficulty with that would be getting sufficient Hp to drive the compressor side. Even the mighty 6.0 liter LS engine might struggle to produce enough power at only 1,200 to 1,500 rpm with a high flow and discharge air pressure. It should be very capable though at much higher engine rpm, but that might not be what is wanted in this case.

A good approximation of just the very basic thermodynamic power required to compress air, neglecting all the various mechanical and flow inefficiencies would be Hp = psi x CFM divided by 229.

I don't know what the original posters flow requirements are, but 50 CFM at 120 psi would be 6,000/229 or 26 Hp. The real shaft drive horsepower required might approach twice that. That is a BIG electric motor.

 

[strantor]

If you had it patched up enough to work you could probably run it on 4 cylinders. Why not just use the 4-4 or in this case 4-3 compression idea. I think an electric driven v8 would be a horribly inefficient. If you go that approach I think you壇 be better off to remove the cam shaft, close all the valves and put a check valve in the spark plug hole. It can suck air in and every stroke is a compression stroke. You could take it a step farther and trade the heads for a flat plate which would increase the compression ratio and increase the air pumped per stroke. You could probably find an old industrial compressor and be off to a lot better start. 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.

The more that I think about it modifying the heads to close the headspace to pretty much zero would be a must do otherwise youæ±*e wasting most of the compressed air.

My math for CFM/PSI is an unsanctioned extension of Boyle's law in reverse, looks something like this:
Goals:
150 PSI
25CFM

25 cubic feet at 150PSI converted to uncompressed air (absolute)(1 atmosphere):
Boyle's law: P1V1 = P2V2
Rearranged: V2 = (P1V1)/P2
P1 = 150PSI
V1 = 25ft^3
P2 = 14.7PSI

V2 = (150*25)/14.7 = 255ft^3

So I'm considering this engine to be equivalent to a blower of uncompressed air, capable of 255CFM.

7/8 of a 6.0 engine is a 5.25L engine. 5.25L = 0.1854 cubic feet (of uncompressed air, pumped in a single rev)
255ft^3 / 0.1854ft^3 = 1375 revolutions (per minute)

I'm not sure how many laws of physics or internal combustion engine principles that violates, but it's what I'm operating under. Feel free to correct any glaring false assumptions.

I did all that math many times, different versions of it, more complicated versions, versions involving pumping air from two cylinders into one for a 2-stage (or 3 stage, perhaps optimal use of 7 cylinders), versions factoring in volume of the chambers in the heads, and I reached the same conclusion about eliminating head space. I agree that a blocked-off head is the way to go. A flat plate with a couple of check valves is what I have in mind.

 

[strantor]

Sell the GMC as-is and use the proceeds to buy a compressor.
If you want something to do, fix the GMC and then sell it or use it.

boring...
(probably sound advice though)

 

[strantor]

I don't know but feel the other poster mentioning the size of motor needed might be correct. Probably any such capacity compressor would use a 480/600volt 3 phase motor, as it has exceeded anything practical for single phase.

From that standpoint, a project using half the motor to run it, for occasional use, would make more sense.

Available compressors delivering 25CFM/150PSI are using 7.5 - 10HP motors (example - 7.5HP). I assume mine will be much less efficient; requiring perhaps double the motor, as was mentioned earlier by others. I'm tentatively planning on needing a 15-20HP motor as I said previously. If the stars align I might be able to get away with 10HP? I don't know... but you're right, I'll need a 3 phase motor, and that's the other part of this story I didn't get into. I plan to put it on a variable frequency drive running a PID loop so that it maintains an exact pressure without cycling on/off and causing rolling brown-outs for my neighbors. If I just blast a little air or use a die grinder for a few seconds and system pressure drops by a few PSI, the compressor will slowly chug-a-chug at a few RPM for a minute to get it back up to setpoint. If I open a valve and just start blasting my air investment across the shop, the motor quickly accelerate from chug-a-chug to full chooch, trying to catch up.

I don't want to use half the engine for combustion power, because... I could list reasons but you or others would probably try picking them apart and I'd eventually just have to throw up my hands and say "look, just because. I just want to do it this way because that's what I want, and I'm stubborn." So I'll just get that out of the way now. I just want it to be electric, and as quiet as possible.

 

[strantor]

The original poster wants to keep it very quiet, so an electric motor and an unusually low compressor rpm might go a long way towards that goal.

I agree with you though, that running half the engine, and using the other half as a compressor makes perfect sense for portable applications. The difficulty with that would be getting sufficient Hp to drive the compressor side. Even the mighty 6.0 liter LS engine might struggle to produce enough power at only 1,200 to 1,500 rpm with a high flow and discharge air pressure. It should be very capable though at much higher engine rpm, but that might not be what is wanted in this case.

A good approximation of just the very basic thermodynamic power required to compress air, neglecting all the various mechanical and flow inefficiencies would be Hp = psi x CFM divided by 229.

I don't know what the original posters flow requirements are, but 50 CFM at 120 psi would be 6,000/229 or 26 Hp. The real shaft drive horsepower required might approach twice that. That is a BIG electric motor.

Your calculation in my case yields (25cfm * 150PSI)/229 = 16.4HP; way higher than the motors put on marketed compressors (7.5-10HP) - see example I just posted previously. That discrepancy is exactly why I suspect your formula is accurate. I know compressor manufacturers are full of shrap, worse than vacuum manufacturers (or does my "5.5HP" 120V shopvac defy physics running on a 15A breaker? LOL). I guess what I'm after isn't strictly 150 PSI at 25CFM; what I'm after is a compressor comparable to the commercial units advertised to deliver 150PSI/25CFM. Maybe that means 150PSI max and 25CFM @ 90PSI? Or worse? Probably.

 

[4570Man]

I’m thinking a v twin lawnmower engine converted to pump every stroke would be a lot better candidate for this. The v8 has huge parasitic losses and and weighs like 800 pounds. I don’t think a commercial built air compressor that pumps the volume you want has anything close to 6 liter displacement.

 

[Warpspeed]

The Hp figure I quoted was from an authoritative book on sizing and driving superchargers which has proven reliable in most of its other claims.

Most common shop air compressors fall typically into the 100psi to 120psi class, and intermittent duty cycle. So a 25CFM compressor might require about 10.9 to 13.1 Hp on that basis. But the motor would not be expected to run continuously for that type of service, so an actual 7.5Hp to 10Hp motor rating sounds quite reasonable sizing for a competitive commercial product. Electric motors are rated for continuous service, so they can be pushed a bit harder for intermittent service.

Now how much air your compressor will actually flow is determined by how much NEW air can be inhaled each cycle (volumetric efficiency).
There are two aspects to that. Flow restriction through the intake valves, and how much compressed air is left over in any clearance volume in the cylinder head, that expands back as the piston descends. No new air is going to enter the cylinder until the cylinder pressure falls below atmospheric.

A converted gas or diesel engine is going to have a huge combustion chamber space that is totally detrimental to operation as an air compressor.
That is where a valve plate design really pays off, with absolutely minimal dead volume at top dead centre.

 

[4570Man]

A converted gas or diesel engine is going to have a huge combustion chamber space that is totally detrimental to operation as an air compressor.
That is where a valve plate design really pays off, with absolutely minimal dead volume at top dead centre.

Which is why I suggested removing the head and replacing it with a flat piece of steel. It wouldn’t be that hard.

 

[Warpspeed]

I’m thinking a v twin lawnmower engine converted to pump every stroke would be a lot better candidate for this. The v8 has huge parasitic losses and and weighs like 800 pounds. I don’t think a commercial built air compressor that pumps the volume you want has anything close to 6 liter displacement.

Many other candidates such as motorcycle or small air cooled stationary engines. Belt driven refrigeration compressors in the larger sizes should not be overlooked either. These already have a valve plate, their only disadvantage is that some of them don't have oil scraper rings and can blow a lot of oil fog into the air stream. That might be an advantage for air tools, but not for spray painting !