Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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
  2. jcsd
  3. Jul 5, 2014 #2

    PhysicoRaj

    User Avatar
    Gold Member

    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

    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor

    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

    User Avatar
    Gold Member

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

    PhysicoRaj

    User Avatar
    Gold Member

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

    Ranger Mike

    User Avatar
    Science Advisor
    Gold Member

    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

    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor

    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

    User Avatar
    Gold Member


    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

    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor

    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

    User Avatar
    Gold Member

    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

    User Avatar
    Science Advisor

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

    PhysicoRaj

    User Avatar
    Gold Member

    Sorry, but I didn't get what you were referring to..? Which tech. ?
     
  22. Jul 5, 2014 #21
    That's what I've read in other forums but, I don't think it makes any sens at all, because there is an optimum air to fuel ratio that should be reached all the time, I remember I've once read that at idle it's something like 12:1 but that could be for an economic reason rather than to lower rpm

    the increase in ignition rate happens as a resultant of the increased speed, each time the piston approachs TDC the ECU in modern cars will send a pulse to the spark plugs to ignite and any alteration in the timing of the spark ignition would serve other tasks than controlling the absolute rpm of the engine, some of these tasks are to control detonation

    The air flow will always be restricted partially by the butterfly, your assumption indicates an unrestricted manifold (e.g wide open throttle)
     
  23. Jul 5, 2014 #22
    that's true, but fuel economy has nothing to do with the incrementation in rpm
     
  24. Jul 5, 2014 #23
    The friction could be either between solid parts or simply of the boundary between the sucked air and the manifold housing/butterfly, all that can affects the amount of air the inters the cylinder

    If the rate of filling the cylinder is variable, then injecting the consequent amount of fuel in it should have no influence at the rpm. It is a fact that all engine makers tries to hold the optimum stoichiometric ratio of 14.7 through all the rpm range
     
    Last edited: Jul 5, 2014
  25. Jul 5, 2014 #24

    Baluncore

    User Avatar
    Science Advisor

    What is “the incrementation in rpm”?
    What has it got to do with CR or air/fuel ratio, the first two in your list?
     
  26. Jul 5, 2014 #25

    PhysicoRaj

    User Avatar
    Gold Member

    That's what.. all of the four can be used to change rpm. If you talk about a butterfly throttle, then you will have observe some modification in the air-fuel ratio. Coordination of these two is most important, especially at huge rpm's.

    Your question in the first post seemed that you were asking "which of these four remains constant/variable on rpm increment", rather than "which of these must remain constant/variable in order to increase rpm."
    For the first question, another question as answer: by what means is the rpm increased?(Any of the four? other?)
    For second question: All of them can change the rpm, provided you design the engine that way.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook