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Can a car's reaction to gas pedal be said to be stable ?

  1. Jul 27, 2015 #1
    As per control theory, if a bounded input produces a bounded output then a system can be said to be stable. So assuming that I press my cars gas pedal such that it reaches a fixed position, then the reaction of my car would be to reach a corresponding velocity, and assuming the road to be even sloped and no other disturbances acting on my car, my car would maintain certain fixed velocity. So in this case can i say that my system ( car) is stable ?
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  3. Jul 27, 2015 #2


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    Setting gas pedal to a specific position will cause car to accelerate until a desired or limiting condition of speed is reached .

    Generally gas pedal is then adjusted to a position where car runs steady state at speed achieved after acceleration .

    There is human feedback in the system .

    Without human feedback this type of control system does not work at all .

    Stability depends on skills of driver . Overall response may be stable or unstable .

    With modern car controls and average driving skills system is usually reasonably stable .

    Older members though will remember the antics of cars starting off from traffic lights with poorly trained drivers and cruder car controls in the 1960's and earlier !
    Last edited: Jul 27, 2015
  4. Jul 27, 2015 #3


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    Things are more complex than you might think. The “gas pedal” is usually an opening “throttle”. It allows air to flow through the carburettor which draws fuel in with the air to the engine. As the car runs faster, the engine also runs faster so the throttled airflow will also change.
    The air to fuel ratio is set by the carburettor to be stoichiometric which gives full combustion.
    On a petrol engine, cruise control is an electronic system that regulates road speed by continuously adjusting throttle opening. It has a difficult job as the road rises and falls with crests and dips.

    A diesel injection pump on the other hand usually has an internal governor. It adjusts the fuel volume injected between minimum and maximum settings determined by the difference between the position set by the drivers foot and the actual engine RPM.
    The diesel engine always takes in as much air as possible. The volume of fuel injected is adjusted by the injection pump. Too much fuel for the engine conditions, cold engine or blocked air filter, will produce black smoke.
    Cruise control on a diesel can be a simple hand control that sets the required speed against the injection pump speed governor position at that speed.

    The speed of a petrol engine includes the driver in the speed control circuit feedback.

    The speed of a diesel engine replaces the driver with an injection pump governor in the speed control circuit.
  5. Jul 27, 2015 #4


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    If the pedal is fixed, there is no feedback mechanism at work, so I think you have an open loop control system which is in a stable condition. If the power input increases a little the speed will also go up a little. But it does not run away. If it were a closed loop control system, for instance if cruise control was fitted, then the change in speed would be zero after a few seconds.
    When a car "stalls", the open loop has become unstable, so that an increased load has brought the engine to a lower speed where the torque curve is falling. Then feedback is required from the driver, making a closed loop, to prevent the engine from stopping.
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