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Automotive Accelerator pedal and rpm

  1. Jul 5, 2014 #1
    Hello everyone,

    this is my first contribution in this forum,

    I'm aware that this question had been asked many times either in this forum or elsewhere and I have indeed read at least one thread with a similar topic, however, unfortunately, I have yet to find specific answers that nail the core fundamentals in a solid means, that's why I preferred to post this thread separately rather than bumping up an older thread in hope it will be more focused on the subject this time.

    How does the action of the accelerator pedal changes the rpm of the internal combustion engine ?

    And which quantities are or should be* constant and which are variable through the whole rpm range (e.g from idle to red line) as the engine accelerates when pressing the pedal ?
    1- Compression ratio
    2- Air to fuel ratio
    3- Ignition rate
    4- Air flow rate through intake manifold


    Please note that :
    - This subject is beyond the basic fundamentals that describe the primary functions and terminology of internal combustion engine and their theoretical cycles.
    - *: In general I'm not referring to an absolute constant quantity, so if there would be any variation that is not controllable or not meant to regulate the engine speed (rpm) then this should not be considered in the explanation, such variations are the resultant of thermal deficiencies or mechanical friction.
     
    Last edited: Jul 5, 2014
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  3. Jul 5, 2014 #2

    PhysicoRaj

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    Hi and welcome to the forum. What do you think the answer is? Which of them is constant and which are variable according to you?
     
  4. Jul 5, 2014 #3

    PhysicoRaj

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    If you have a good idea of what those four things are (and how they work), it is easy to figure this out.
     
  5. Jul 5, 2014 #4

    Baluncore

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    Fascinating. I have read carefully through the post and have even examined the title.
    I cannot find a question mark anywhere. If there is no question, then we have no destination.

    We don't know where we are going, so why must we determine anything ?
     
  6. Jul 5, 2014 #5

    PhysicoRaj

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    :tongue::rofl:
     
  7. Jul 5, 2014 #6

    PhysicoRaj

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    Atleast the OP used a 'wh' word..
    OP can be forgiven for that :tongue:
     
  8. Jul 5, 2014 #7

    Ranger Mike

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    Welcome sailor,

    A very thought out question!
    Made me stop and think about it..

    Compression ratio- of all 4 items listed, the compression ratio is going to be that which will change the least. More specially the CR is as the engine is built. There are small variances in dynamic compression ratio when things start spinning up to 7000 RPM. But Dynamic CR will always be lower than static CR and Dynamic CR will not change over the operation of the engine rpm.

    every other item listed will vary. If you have fixed fuel air mixture at idle and no provision to make it richer as RPM increases , the mixture will lean out. Ignition rate may appear to be fixed but this too will vary as rpm increase and the initiation of the ignition must occur earlier as rpm increase so we have a mechanism to advance the spark. As a matter of physics as the rpm increases so does the air flow through the intake manifold.
     
  9. Jul 5, 2014 #8
    Hi PhysicoRaj and thanks for the welcoming.

    I would say that both the compression ratio and the air to fuel ratio are (should be) constant, while the firing rate and the air flow rate are variable, therefor, theoretically it's these two that should control the engine speed.
    And more specifically, since the the accelerator pedal is linked to the butterfly which in essence control the air flow through the manifold, it should be the primary influential factor in the process, because the other variable which is the firing rate is dependent on the resultant rpm through either the distributor in old vehicles or the ECU input and crankcase positioning sensor in modern ones.

    That's what I think is the case, theoretically at least, so what are your thoughts about it.
     
  10. Jul 5, 2014 #9
    :rofl:

    sure if you are not interested in the subject, then you mustn't do anything about it

    at least you could make an objection and try to define the relation between the accelerator pedal and the change in rpm in a better way

    duh..., again, in case you're interested in the first place :biggrin:
    It isn't a new question that's why I supposed the relation in the title is self explanatory

    and further more I hinted about the lack of solid answers to it :wink:
     
  11. Jul 5, 2014 #10

    PhysicoRaj

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    You are correct about the compression ratio. But regarding air fuel ratio, you are wrong.
    Ranger Mike has already provided an answer, a detailed explanation too. I hope you find that post informative enough for you. If you still find any difficulties, ask it over!
     
  12. Jul 5, 2014 #11
    Hi Ranger Mike, thank you for welcoming and the elaborated input,

    I agree that the CR should almost held constant theoretically or at least through some rpm range, but what if we already had rich air-fuel mixture at idle ? then we can assume it is almost constant ...

    I have assumed the ignition rate to be variable, and it actually happens as a resultant from the incrementation of the rpm and not beforehand, this is especially true in the case of the distributor and the timing belt,
     
  13. Jul 5, 2014 #12

    Baluncore

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    Ideally, they are all variable.
    I have a governor that adjusts the fuel to maintain RPM at that directly indicated by the actual position of the accelerator pedal. My air/fuel ratio is highly variable, from lean as hell when idling to the edge of blowing black smoke when working hard. The compression ratio can also be arbitrarily increased through the use of water injection.
     
  14. Jul 5, 2014 #13
    So my question here is what happens after the idle rpm, say from 2000 to 4000 rpm ?
    shouldn't the air to fuel be at the range of 14.7 theoretically ? could there be such miniature incrementation in the air to fuel ratio that leads to the upshift in rpm ?

    And what happens if we had a constant 14.7 ration from idle with a semi-closed butterfly ?
     
    Last edited: Jul 5, 2014
  15. Jul 5, 2014 #14

    PhysicoRaj

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    Agreed. But this isn't used in domestic automotive engines. I know about jet engines and gas turbines in powerplants using water injection, or even alcohol injection sometimes.
     
  16. Jul 5, 2014 #15
    Do they vary due to friction and out of control deficiencies or they are meant to be variable ?
    and what would happens if we made the first two constant ?
     
  17. Jul 5, 2014 #16

    PhysicoRaj

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    Isn't air-fuel ratio the major factor in increasing rev's? The ignition rate automatically increases if it's a magneto, or, if it's a digitized ignition, it is made to increase. It's a different case in diesel engines, where direct fuel injection and auto ignition takes place.

    As for the air flow through the intake manifold: Higher rpm→increased strokes of piston per second→increased amount of air sucked in per second→→increased air flow through intake manifold.
     
  18. Jul 5, 2014 #17

    Baluncore

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    They are deliberately designed to be continuously adjusted to improve fuel economy.
    If you make the first two constant then you reduce the fuel economy by about 50%.
     
  19. Jul 5, 2014 #18

    PhysicoRaj

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    Friction b/n what?
    Constant compression ratio will never affect change in rpm. Actually in all domestic and general use automotive engines, CR is constant. An engine with variable CR will be (with the present technology) bulky, complicated and expensive.
    With a constant fuel-air ratio.. I don't think you can even make the slightest deflection in your tachometer needle! (with compression ratio constant).
     
  20. Jul 5, 2014 #19

    Baluncore

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    With the dominant present technology, yes. But with much older, now largely forgotten technologies, no.
     
  21. Jul 5, 2014 #20

    PhysicoRaj

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    Sorry, but I didn't get what you were referring to..? Which tech. ?
     
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