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Car engineering: PSI and CFM

  1. Oct 23, 2007 #1
    hello physics experts! i have a simple question about PSI and CFM and would like your input.

    context of the question:

    a car's turbocharger is essentially an air compressor that forces positive "boost" into the intake manifold. now a great debate among car enthusiasts is why a small turbocharger will produce less power than a big turbocharger at the same psi, same car. 2 answers that are often given: turbocharger efficiency (more efficient = cooler and denser charge) and different CFMs per turbo under the same psi. the underlined section is the topic in question.

    question:

    If a car's turbocharger is set for 19psi, isn't there a corresponding CFM associated with 19psi? Isn't the pressure that the manifold sees (in this case, 19psi), directly related to how much airflow (measured in cfm) the manifold is getting hit with? How can the same car, same conditions, experience 19psi by 2 different flow rates?

    2 different levels of cfm cannot achieve the same level of pressure (psi) in the intake manifold, correct? Some people are arguing that different turbos, although set to the same psi, will produce different CFM. If the flow rate is different, then I don't see how the manifold can experience 19psi under both conditions. Need some clarification here by some physics gurus :smile:.
     
    Last edited: Oct 23, 2007
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  3. Oct 24, 2007 #2

    FredGarvin

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    To me it seems that, all things being equal, the same pressure differential, i.e. turbo output-engine demand, will produce the same flow. It has to. What this issue appears to be is more along the lines of a performance matching issue.

    If you have a larger compressor, it should have to turn at a lower speed than the smaller. The same engine producing the same exhaust flow can't turn the two the same speed (or can they? I'm not a car guy). It would seem that one of the compressors will not be operating at it's best efficiency point or any other good spot on its map.
     
  4. Oct 24, 2007 #3

    rcgldr

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    With the engine running at different rpm, or with a partial throttle opening.

    If the rated CFM is not large enough, the pressure at the manifold will be less at full throttle and rpms above a threshold.
     
    Last edited: Oct 24, 2007
  5. Oct 24, 2007 #4

    chemisttree

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    You can have both turbos give you the same psi and have different maximum CFM values. The waste gate will open and attenuate the exhaust flow across the turbine in the larger turbo when the engine doesn't need it and it will close the waste gate to deliver the higher CFM demanded at WOT. A larger turbo should give you more air at lower rpm which will positively affect performance during spool up. Big turbo... big wastegate actuator and spring... faster turbo response.

    http://www.bankspower.com/Tech_howturbowastegateworks.cfm
     
  6. Oct 24, 2007 #5

    FredGarvin

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    Nice bit of information Chemistree. So there is definitely not an apples to apples comparison between the two units. All things are not equal.
     
  7. Oct 24, 2007 #6
    First I would like to thank all of you for taking a shot at the question.

    FredGarvin you must be too smart for me cause I don't understand what you're saying. I understand and like your signature though :smile:.

    Jeff Reid's statement about partial throttle and different RPMs definately makes sense. I should have stated, however, that both hypothetical turbo equipped cars are under same conditions (including same rpm, same throttle).

    I think I understand part of Chemisttree's statement. So a big turbo does pump out more cfm than a smaller turbo when both are dialed in for a certain X amount of psi of pressure in the manifold. it's just that a lot of the air that is compressed and on its way to the motor leaks out through the wastegate to achieve the given psi in the manifold--is this what you're saying Chemisttree?

    So the first question about different cfms achieving a given psi has been solved. Now a follow up question if you guys don't mind :wink:.

    In light of what was mentioned in your responses, is this statement true or false:

    Bigger turbos make more power than smaller turbos (at the same psi, same car, same conditions) because bigger turbos flow more air/cfm even though both turbos are dialed for the same amount of psi.

    Because surplus airflow exits through the wastegate, extra air/cfm cannot contribute to the gain in power observed by a big turbo. It must be due to turbocharger efficiency, which allows for cooler compression of air, making the air charge more dense with oxygen, correct? I'd greatly appreciate your thoughts.
     
    Last edited: Oct 24, 2007
  8. Oct 24, 2007 #7

    FredGarvin

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    I definitely wouldn't say that. A turbocharger is different enough from a regular compressor system to mess me up, so I am learning here too. I was muttering out loud in my post. I hope I didn't ramble too much.


    That is how I see it. The wastegate acts as the main pressure relief to the system. That allows the larger compressor to put out more air but only have the required amount actually get into the manifold.

    In light of what Chemistree said, I would believe that statement to be true.
     
  9. Oct 24, 2007 #8

    stewartcs

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    Larger turbos have a higher lag so they typically take longer to spin up and provide boost. At higher rpms they will provide greater boost over a smaller turbo. However, the smaller turbo has a smaller lag so it generally provides more boost at lower rpms because it accelerates quicker.

    The lag is related to the rotational inertia of the turbo.

    If you do use a smaller turbo, then you'll need a wastegate to keep the turbine from spinning too fast. If the turbine starts to spin too fast and you slam the throttle plate shut (let off the gas), the air in the intake manifold will have no where to go but back through the compressor. When that happens you get a compressor surge (a blow off valve will prevent this from happening).

    It sounds like Chemisttree has answered your question though (along with Fred). The wastegate will keep the boost pressure at a maximum preset level so even if the volumetric flow rate in the intake manifold varies the pressure shouldn't exceed that preset value.

    It seems to me that the mass flow rate would be the dominating factor between the large and small turbos. The greater the mass flow rate into the engine the more power you get due to the increased oxygen content in the cylinders. This would increase the efficiency of the turbo since it doesn't have to work as hard to get the same boost level.

    Of course this is just my understanding of turbos so take it with a grain of salt!
     
    Last edited: Oct 24, 2007
  10. Oct 25, 2007 #9

    chemisttree

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    The wastegate is on the exhaust side of the turbo. The exhaust gas is routed around the turbine when the wastegate is open and the high pressure gas is directed across the turbine when it is closed. A bigger turbo will usually do this more efficiently since the actuator and spring are beefier.

    A bigger turbo won't have to spin as fast to reach the pressure preset and the CFM associated with it.
     
    Last edited: Oct 25, 2007
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