Air compressor calculations for engine super charger

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I am an engineer but have a rather dumb question regarding superchargers for engines and probably compressors in general.

Most specifications are give as XXcfm at XX psi. I am trying to understand if this truly means we are moving that amount of air at the rated psi or if that means that volume of air at ambient pressure, compressed to XXpsi??

I guess my misunderstanding regarding SCs for engines is the actual volume of gas going through the engine does not change assuming all engine parameters stay the same. The only thing that changes is the pressure or density of the gas. So by cranking up a compressor, we really not not going to be using more CFM, we will be using the same CFM at a higher pressure. Am I off track with this? It is late so....
 
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  • #2
brewnog
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It's the volume at the quoted pressure.

The volume flow rate obviously changes depending on the point in the air system you're considering it. Deal with mass flow rate instead; it's a lot easier since it remains constant through the whole engine (except where you add fuel or lose blowby).
 
  • #3
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I think what I am trying to grasp is what people are trying to say when they indicate 1.5cfm/hp for a normally aspirated engine. A 2.0 pressure ratio would be 14.7 gauge pressure to the inlet. Some people have indicated this means now twice the volume of air at 2.0 PR. uh, cfm of what???? At what density???

I guess I am trying to get all the hot air out of all these calcs and better understand

maybe we can just look at an example.

Engine, 1L engine, 50hp, 1.5cfm/hp = 75HP, no boosting
Want to add a blower to double HP. Inlet would need to see twice the pressure or 14.7psi at the same density (temperature) as before the blower.

What is the specification of pump is needed? What I experienced with other air pumps is they specify a "free air" volume, say 100cfm, then a max pressure, say 10spi. Then there is a performance graph presented that relates flow to pressure but nothing really relating mass to this. Obviously we have temp increases in the compression of a gas so there is no way the actual density is going to stay the same so I might be moving XXX CFM but need to know the the delta T across the pump to calculate density loss.
 
  • #4
brewnog
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Not sure what the issue is. If you're pressure charging, you're increasing your mass flow rate. I don't know what a particular manufacturer is getting at, but if you look at how much mass flow you need, then volume flow rate is academic at best, and at worst just a case of applying the universal gas law. The mass flow rate is the same at the air filter, at the supercharger inlet, in the inlet manifold...
 
  • #5
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I guess what is confusing me is I am not thinking some of these calculations are actually looking at mass flow, but rather volume flow. cfm, not scfm.

is this all just clouding the waters? should I just be looking at inlet flow where things will be much more standardized? IE, double inlet flow, double HP? obviously more complicated but for eval, lets exclude eff losses, etc.
 
  • #6
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The essence is that the engine only knows how many oxygen molecules have been inducted and for the most part doesn't care at what temperature or pressure they're at. Using STP simplifies that process. The more oxygen that can be ingested, the more fuel can be combusted and the more power can be made.

Let's look at the commonly used method of sizing a blower is, using commonly available parts; a 350 cid engine and a 144 cid supercharger. Both are considered positive displacement, assuming 100% volumetric efficiency at STP. However the engine's displacement is rated per combustion cycle, which requires two crank revolutions to complete. The effective displacement of the engine per revolution is 350/2 = 175 cid. The supercharger's rating is 144 cid per revolution.

To double the flow through the engine, the supercharger needs to ingest double the amount of air needed by the engine. That gets us back to 350 cid "worth" of air (175 cid per revolution, doubled). Divide what the engine wants by what the supercharger can deliver (350/144 = 2.43) and you'll find that the supercharger needs to be driven 2.43X faster than the engine to double the flow. Note that this is a ratio and doesn't bring time (cfm or rpm) into the calculation.

Any boost level for a specific engine and supercharger can be quickly calculated by using this. It assumes ideal conditions of course; actual mileage may vary!
 
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  • #7
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I guess I am a touch confused by that. a 350ci engine does displace 350ci but only ingests air every other stroke. That gets you down to 175ci/rev. 4 poke takes 4 strokes or 2 crank revs to complete its work so just cid/2. We are probably on the same page but have different ways of saying it.

Also, was the 144ci charger just used as example? I will admit that is a small pump for a 350 engine. From your estimates, you are basing those just on inlet flow right? IE, no real need to get too technical with gas laws if you are just looking at the inlet flow and temp since the pressures would be normalized?
 
  • #8
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I guess I am a touch confused by that. a 350ci engine does displace 350ci but only ingests air every other stroke. That gets you down to 175ci/rev. 4 poke takes 4 strokes or 2 crank revs to complete its work so just cid/2. We are probably on the same page but have different ways of saying it.

Also, was the 144ci charger just used as example? I will admit that is a small pump for a 350 engine. From your estimates, you are basing those just on inlet flow right? IE, no real need to get too technical with gas laws if you are just looking at the inlet flow and temp since the pressures would be normalized?
Yep, same thing.

144 is a popular and typical supercharger size:
http://www.aa1car.com/library/supercharge.htm

This is for general sizing; you do have to consider the temperature rise from compressing the air in a real system to avoid other issues but not for component selection.
 
  • #9
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Has intercooling of the air charge just proven more efficient than chilling of the compressor itself? Would seem the intercooler would largely affect the flow rate and decrease overall compression eff but I guess we have to look at this as a system whole with the engine and a cold, dense air charge wins every time. Just really surprised there is not an integrated system for these.
 
  • #10
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It's easier to use an intercooler than to try cooling the air as it passes through the compressor. The intercooler does cause a pressure drop due to internal flow resistance but it uses passive cooling - usually mounted in front of the radiator - whereas cooling a compressor would likely involve at least a water pump and more weight/complexity, the need to isolate/insulate the compressor case, etc. Interesting thought, though.
 
  • #11
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So what are the more efficient pump designs out there for SC applications? I have difficulty understanding how centrifugal designs are super efficient, but on paper, it looks that way. Do you have more or less pumping loss with internal compression pos displacement? It looks many other older designs have been pushed aside like the scroll, vane, etc. There is even some stuff out today that is mono screw for HVAC stuff but have not seen that for SC apps and not sure why.

It would almost seem to only function to efficiency here is to reduce pumping loss and frictional drag.
 
  • #12
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It's also about heating the air. The old Roots style supercharger apparently was only about 50% efficient, which increased the parasitic power draw and increased the air temp to the engine. That increased the octane requirement and reduced the net power output.

New designs are being developed to close the gap between positive displacement and centrifugal supercharger efficiency. Here's a link:
http://www.kennebell.net/superchargers/BigBore/BigBore.html [Broken]

As far as the efficiency of the two methods, one might liken them to a paddlewheel and a propeller.
 
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  • #13
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So you are agreeing that centrifugal is ruling the roost in terms of eff at this point? It would sure seem that a high helix or more "gentle" compression would be ideal like a really high helix twin screw?

Could a mono screw be a better design due to the elimination of the "paddling" effects of the roots style rotors?
 
  • #14
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Centrifugal has the highest efficiency for IC supercharging applications that I know of.

High helix twin screws are more efficient than Roots because of the reduction in pulsing (I'm assuming) but you're getting beyond my range of experience. I think that centrifugal compression works best at (relatively) low pressure/high flow and other methods will be better as the pressure ratio goes up. Could be that multi-staged centrifugal compression is the most efficient there too!
 
  • #15
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Please again excuse my ignorance regarding pump design but for example, on a pos displacement pump, would the actual flow of the pump open air simply be expressed as swept volume x operating rpm x volumetric eff? Ie, is the swept, uncompressed volume an accurate way to estimate pumping ability in air?

For some reason, I thought this might only apply to fluids but it has been a day or two...
 
  • #16
563
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Please again excuse my ignorance regarding pump design but for example, on a pos displacement pump, would the actual flow of the pump open air simply be expressed as swept volume x operating rpm x volumetric eff? Ie, is the swept, uncompressed volume an accurate way to estimate pumping ability in air?

For some reason, I thought this might only apply to fluids but it has been a day or two...
Yes; that's how all engines are rated - which is a positive displacement air pump.
 

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