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Automotive How does the throttle affect the RPM of an engine?

  1. Sep 11, 2015 #1
    Hello, I'm working on a car simulation and I'm attempting to implement a manual transmission simulation into the game.

    My question is, how do you find the rate of change of the engine's RPM when the gas pedal is pressed in the car?
  2. jcsd
  3. Sep 11, 2015 #2


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    Hi envel.

    There is no one answer to your inquiry.
    How representative of real life does your simulation need to be?

    You can use a mathematical equation to represent the throttle / rpm relationship, or use physical equations to determine the same.
    Both can be very simple or become very complicated.

    The simplest mathematical is just using a direct ratio equation, ie RPM = THROTTLE_MAX times X, where X is the fraction of the throttle opening rangine from 0 to 1.
    In which case, the rate of change of the engine's rpm becomes instantaneous.

    The next simplest mathematical is using a time loop on a linear equation, basically y = mx + b, and one justs counts up the slope from the known Starting RPM and Starting Throttle Position to the known Final RPM and Final Throttle Postion by increments of time t, comparing whether or not you have arrived at the Final RPM.

    Becoming more complicated, you can modify the linear equation to a power function, exponential, quadratic, sine, etc. or parts therof and/or combinations to get what you want to look like is real, such as when you really step on it, or going up or down a hill, or where you are at the throttle position.

    Is the time loop the only way to do it - not sure at the moment.

    Physical equations involve torque, mass, load, in which case you need to know something about your car and its performance. The "something" can be as simple or as complicated as you wish.

    That is of course, just a pre-amble answer, and no doubt there are better and best ways to proceed, so as to minimize computing power, that people have found out.

    Even so, you might like to experiment to see just how different mathematical equations in a time loop affect you car, if you ar the curious type.
  4. Sep 11, 2015 #3
    Thank you for replying! I'm most likely going to have to use that direct ratio equation that you mentioned, and weave the weight of the flywheel into it. RPM = THROTTLE_MAX / currentThrottlePosition / flywheelWeight.
  5. Sep 11, 2015 #4


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    The simple answer is, the throttle alone determines nothing about the engine operation except how much fuel goes in. It is the combustion of the fuel that creates torque which causes the force to propel the car forward. Just enough fuel to generate the torque required to overcome friction of all sorts at the current speed gives you a steady speed. Extra fuel, properly burned, give you excess torque, causing a driving force greater than friction and the vehicle accelerates. The converse is also true.

    Your proposed model makes little sense at all. Look at the dimensions of the factors.
  6. Sep 11, 2015 #5
    Well yes I quickly realized that it should just be currentThrottlePosition / flywheelWeight because else the rpm would increase by a smaller amount as the throttle was pushed in more.

    Also, I'm not making the car simulation as detailed as what you said about fuel. Thanks for the reply!
  7. Sep 12, 2015 #6


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    As the throttle is opened more air is permitted to enter the engine through the carburettor. The mass of air entering is proportional to the RPM * engine capacity * density of the air leaving the carburettor for the inlet manifold. As the virtual throttle moves from 0 to 100% the intake manifold pressure moves from zero to close to 100% of atmospheric pressure. There is a stop to set idle at about 1%.

    The passage of the air through the carburettor causes just the right mass of fuel to be added to the air to make a stoichiometric mix, usually about 14.5 kg of air to 1 kg of fuel. That mix is burned to generate pressure on the piston that translates to torque in the drive train. The potential energy released is proportional to the mass of fuel being burnt. The mass of the entire vehicle accumulates kinetic energy from the burned fuel's potential energy.

    Energy conversion rate is; RPM * capacity * throttle_setting.
    KE = 0.5 * vehicle_mass * velocity2.

    The mass of the flywheel is only important when changing gear.
  8. Sep 12, 2015 #7


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    Envel post #5, you propose to use a model that says
    rpm is proportional to throttle position/flywheel weight.

    Imagine that you are driving at steady speed on a level roadway. The throttle position is constant. Now, with no change in throttle position you start up a hill. Does engine rpm remain the same? Not in my truck!

    Loosely put, the throttle position is a positive input to the rpm, but this must be balanced against the negative inputs, that is, those factors that load the engine down (friction, weight when climbing, etc). If you don't take load into account, you do not have even the slightest hint of a realistic model.
  9. Sep 12, 2015 #8
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