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Why does a car continue to run when the engine is turned off?

  1. Apr 7, 2010 #1
    Hello guys,

    I really don't know much about cars at all. So here is my question

    1. If I keep driving and turn off the key the car continues to run?
    Is this because momentum. Would the car eventually stop due to friction? I mean if spark plug stops working how can the car continue to run?
    2. How does push start make the engine run?
    After push start has started a car does it still use the battery to work the spark plug?

    I might not be knowing what I'm talking about so please help. Thanks :smile:
  2. jcsd
  3. Apr 7, 2010 #2
    It does indeed keep rolling becuase of momentum (from a physics point of view). Also it's becuase wheels can keep rotating even if not being driven (out of gear). If left in gear pushing/rolling the car acutally turns the engine.

    A push (bump) start, still uses the battery/altenator system to fire the injectors and spark plug, but what pushing does is remove the need for a starter motor to turn the engine so it can compress the first load of mixture.

    After a push start a car works as though you started it as though you turned the key.

    If that didnt make sense, or something needs explaining in more detail ask away.
  4. Apr 7, 2010 #3

    Filip Larsen

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    Yes, if you put the gear in neutral (or step on the clutch with a manual gear shift), the engine (running or not) would no longer has its crankshaft connected to the wheels and would therefore no longer be able to "push" the car forward. If the car was already rolling on a flat piece of road, it would continue to roll slower and slower and eventually stop. You are correct that this slowing is due to friction. Actually, the reason a car can keep its speed fairly constant during a normal drive is because the engine delivers the power necessary to counter this friction. At slow speeds where the friction is low, the engine has plenty of excess power over friction so it can accelerate fast. At very high speeds the engine may only just be able to counter friction without overheating. Note, that the precise performance of the engine in this regard also depends on the gearing ratio between engine and wheels.

    In the old days, the spark plugs got activated mechanically when the engine shaft was turning. At the right position of the shaft rotation for each cylinder, the plug would spark due to some electrical components activated by the shaft, so it was possible to push the car into speed (in neutral or with clutch held) and then once it got rolling kick it into a low gear to connect the rotation of the wheels to the crankshaft thus move the cylinders and spark the plugs.

    I'm not exactly sure how todays cars with electronic (computer) injection do it. I would think, that you would need at least a little bit of juice on the main battery so that the computer can run, but I would guess that the electrical energy for the spark itself still comes from the rotation of the crankshaft and that only the timing is controlled by the injection computer. I have had at least one car with electronic injection that, once jump started, was able to run without a working battery.
  5. Apr 7, 2010 #4
    1. The ignition turns off when you turn the key, so the engine doesn't run. The wheels turn the transmission and the engine, so the engine does keep rotating and the tachometer will still show the RPM. It won't respond to throttle though, the car will just come to a stop.

    2. The starter motor simply rotates the engine, pushing the car while it's in gear accomplishes the same thing. If the ignition is on and the engine is rotating fast enough, the car will start. You need a battery to start it even when push starting, but once started, at least on older cars, the engine will keep running without a battery due to the alternator spinning.
  6. Apr 7, 2010 #5


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    Staff: Mentor

    There may be a language barrier here: the word "run" implies the engine is still powered, which it certainly is not if the key is turned off.
  7. Apr 7, 2010 #6


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    A bit confusing here, since older cars with carburetors could go into a diesel mode. The fuel pump was mechanical, the carburetor continued to work, the altenator continued to work, only the spark was cut off, but detonation closer to top dead center could occur with sufficient throttle input because of the heat from compression and the still hot spark plug element.
  8. Apr 7, 2010 #7


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    The question might refer to the fact that the car's fan will sometimes run for a while after turning off the engine, in order to cool the engine. If you don't know much about cars, you might think the fan noise is the sound of the engine still running.

    Or "run" could just mean that the car remains in motion, if it was moving at the time the engine was turned off. Then it's a Newton's First Law question.
  9. Apr 7, 2010 #8
  10. Apr 7, 2010 #9
    The classic reason for an engine to keep running after the ignition is switched off is coking of the plugs or cylinder heads, the carbon glows red for long enough to ignite the fuel mixture and the heat of the burning fuel heats up the carbon enough to propogate the next cycle, I owned an old Vauxhall Viva that I had to stall to stop the engine running.
  11. Apr 7, 2010 #10
    I had an Olds 88 (Rocket 455) that would do that. Even putting it in gear (automatic transmission) would not stop it.
  12. Apr 7, 2010 #11


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    You probably have enough power in the battery to run the computer and the spark plugs, it only takes mA.
    The reason you needed to push start is that the starter motor takes 100A and a half dead (or even not quite perfect) battery can't source enough current
  13. Jun 27, 2010 #12
    Yes, it's called run on, it happens when the gasoline used has a low octane number or the spark plugs are too hot for that engine.

    My mother had a alfa romeo 2000 gtv that had that problem. Just push the acelerator a couple of times to flood the engine with gasoline, cooling it off and ending the run on.
  14. Jun 27, 2010 #13
    electric power comes from the battery amd when rotating fast enough, the alternator. At higher engine RPM the alternator takes over and charges the battery and supplies the modest power for spark and electronic control of engine functions. While an alternator can supply power without a battery, such an arrangement will instantly burn out most alternators unless a special protective device is utilized...ZAP STOP might be one such brand name....Never run an alternator without a battery.
  15. Jun 27, 2010 #14
    Interesting. I did not know that. Thanks!
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