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How air goes thru your engine.....(basic)

Discussion in 'Technical Chat' started by chris4x4, Nov 30, 2008.

  1. Nov 30, 2008 at 2:16 PM
    #1
    chris4x4

    chris4x4 [OP] With sufficient thrust, pigs fly just fine. Staff Member

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    Have you ever noticed that if you make a quick "puff" at a candle from a short distance, that the air, even though you are not blowing anymore, still makes it to the candle and blows it out? That's because air has "mass", and when in motion, it must be acted upon to stop it. Have you ever noticed that as a kid when blowing bubbles, if you try to blow a bubble to keep it aloft, it seemed that sometimes the bubble would be passed by the air, only to wiggle around, and then "follow" the air you just blew? These little things also apply in an engine.

    As the air is going through the intake, and through the throttle body, there is a bit of turbulence created by the throttle plate. As it continues on to the intake runners, the air is straightened out into a nice laminar flow, as the intake valves open, and the piston is rapidly moveing downward, the air rushes in to fill the combustion chamber, flowing down the sides of the cylinder wall, hitting the top of the piston and moving up the other side and thru the middle, and swirling. as the intake valves close very fast, its like a door slamming shut. The air that was following in to the combustion chamber is now suddenly stopped. This sends a "shock wave" backwards through the intake runners, and if timed properly, this "shockwave" or "Pulse" is now helping to "push" more air into another cylinder.

    Meanwhile, as the air in the first cylinder is being ignited, and producing power on the downward stroke, the piston now begins its upward stroke. The exhaust stroke.This is where scavenging comes into play. Scavenging is when the exhaust valves open and the exhaust exits the cylinder leaving a vacuum behind it. At that point the piston moves the exhaust out rapidly in a "puff". This puff, once again has mass. There is a high pressure area leading the way in front of the exhaust pulse, and when the exhaust valve closes, it happens abruptly, creating a low pressure area behind the exhaust pulse. This pulse travels down the exhaust, and past another cylinder. The low pressure of the pulse "pulls" the exhaust gasses from the next cylinder, when doing this, it creates a vacuum in the combustion chamber helping to "pull" more air into the cylinder as the intake valve is starting to open while the exhaust valve is still open, this is called overlap.

    All these pulses need to keep up their velocity to remain efficient. They do this by staying hot, and by the size of the exhaust pipes. Newer vehicles with variable valve timing (VVT) keep these pulses timed throughout the rpm range. There is a "Goldie Locks" effect taking place. Too large of a pipe, and the pulses slow, exhaust cools too fast, and the engine looses efficiency. Too small of a pipe, and the exhaust is sped up beyond how the timming of the valves was designed, the exhaust temps rise, and the engine looses efficiency. Any change in the system, be it a larger exhaust, or whatever, has an effect on it. Sometimes positive, as the auto makers have to abide by emissions standards, but many times negative, as some aftermarket manufactures just want it to "sound cool".

    When installing a Turbo, or Supercharger, the scavenging effect is not as important, as the higher pressure "pushes" everything through the system, in which case, a larger free flowing exhaust would be the most beneficial.

    I have oversimplified this just a bit, as I don't have time to go into EVERY thing, or break down the physics involved, but thought it would be a good read for those thinking about changeing their exhaust system and wondering how the new exhaust may affect performance. :)


    Special thanks to 007Tacoma for adding the hyperlinks, and editing for me. :)

    Found on a different site about "Back Pressure":

     
  2. Nov 30, 2008 at 2:23 PM
    #2
    HerNameIsLucy

    HerNameIsLucy I miss Lucy. :-(

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    Nice writeup Chris. Thanks!
     
  3. Nov 30, 2008 at 2:45 PM
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    007Tacoma

    007Tacoma I dub thee malicious!

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    awesome job.
     
  4. Nov 30, 2008 at 3:43 PM
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    Packman73

    Packman73 ^^^^ 3%er ^^^^

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    AWESOME write-up! :cool:
     
  5. Nov 30, 2008 at 3:44 PM
    #5
    tacomaman06

    tacomaman06 Carolina Alliance: Lead, follow, or get the hell o Staff Member

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    great writeup chris!!!:D
     
  6. Nov 30, 2008 at 4:49 PM
    #6
    Brunes

    Brunes abides. Staff Member

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    Bored a bit chris??


    Nicely done...
     
  7. Nov 30, 2008 at 4:54 PM
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    chris4x4

    chris4x4 [OP] With sufficient thrust, pigs fly just fine. Staff Member

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    LOL. I wrote one up a while ago, but it was too long, and the system told me to shorten it. In the process, I became pretty frustrated so I just said F it. I had some time this morning, so I thought I would throw it out. Seems alot of people think that you can do whatever you want to an exhaust and get 4000 more h.p., so I wanted to give a little insight into how things work.
     
  8. Nov 30, 2008 at 5:23 PM
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    Ridgerunner

    Ridgerunner Well-Known Member

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    Yup Chris explained this process to me on the phone like 3 times. Read his post till you get it and you will know way more than most everyone how an engine runs and what NOT to do to your intake and exhaust until you know how it will affect the balance of that process. Unless youre just doin it for sound or bling:D great knowledge as usual Chris:)
     
  9. Nov 30, 2008 at 5:32 PM
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    Ridgerunner

    Ridgerunner Well-Known Member

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    I move we make this a sticky especially because it has the most comprehensive explanation of the combustion process from nose to tail and in fine detail too.
     
  10. Nov 30, 2008 at 6:12 PM
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    luk8272

    luk8272 Poodoo

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    Good explanation chris. Nice and easy. Thanks.
     
  11. Dec 17, 2008 at 2:05 PM
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    chris4x4

    chris4x4 [OP] With sufficient thrust, pigs fly just fine. Staff Member

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    Bumpin it up for the new folks. :)
     
  12. Dec 17, 2008 at 2:07 PM
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    Packman73

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    Should be a sticky...:cool:
     
  13. Dec 17, 2008 at 2:14 PM
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    crawfishsoul

    crawfishsoul Look at my member

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  14. Dec 17, 2008 at 2:17 PM
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    Fluffymonkey

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    Nice writeup!
     
  15. Mar 3, 2009 at 6:24 AM
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    chris4x4

    chris4x4 [OP] With sufficient thrust, pigs fly just fine. Staff Member

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    Bumpin for new members. :)
     
  16. Mar 3, 2009 at 6:47 AM
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    TacticalBacon13

    TacticalBacon13 Sorry for party rockin'

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    nice write up chris i would give you +1 but i cant because i gave it to you like three times this week. haha
     
  17. Mar 3, 2009 at 7:11 AM
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    lsocoee

    lsocoee My hair is all natural Staff Member

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    Great writeup Chris, but I'd like to add a few things here

    You won't see laminar flow going into the engine. There are too many surface variations that create a turbulent flow. It's not really important whether the flow is turbulent or laminar for fuel injected engines, I just don't want people to get confused over this issue.

    I understand you are keeping it simple, but it might be beneficial to point out that the turbocharger is the ultimate scavenger. Also, I would venture to say that the turbocharger creates some back pressure due to that scavenging. The supercharger does not create any back pressure.
     
  18. Mar 3, 2009 at 7:59 AM
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    98tacoma27

    98tacoma27 :POOPCORN: Staff Member

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    Whoa, thats what she said!
     
  19. Mar 3, 2009 at 8:01 AM
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    chris4x4

    chris4x4 [OP] With sufficient thrust, pigs fly just fine. Staff Member

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    It is important if the air is turbulent or not. The intake runners are designed to straigten the air flow as much as possible before going into the combustion chamber. A laminar flow will more efficiently fill the combustion chamber than a turbulent flow. In the old days of carburation, turbulence was a goodthing, as it helped to atomize the fuel. Now, with Fuel Injection, a laminar flow is best, and is what the manufatures strive for.


    Yes...the turbo IS, in fact, the unltimate "scavenger". And I have pointed out that a turbo does have a parasitic drag on the engine in the form of an exhaust blockage. However, Scavenging is what I described in the first post.
     
  20. Mar 3, 2009 at 8:01 AM
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    98tacoma27

    98tacoma27 :POOPCORN: Staff Member

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    I still think this has some validity

    [​IMG]
     
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