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When you are driving and stop at a light, do the pistons still move up and down due to combustion? If not how will the engine know when to combust the fuel to keep it moving?

  • 8
    what do you think makes that noise ? – njzk2 Apr 28 '16 at 14:49
  • If the engine is running, the pistons are moving. If not, the driver, or the car, must start the engine. Stanley Steamers are an arguable exception and they are not IC. – mckenzm Apr 28 '16 at 19:56
23

The answer to your question is both yes and no.

Yes in majority of the conventional vehicles, the pistons keeps moving even when the vehicles is at a stop light. The idle RPM, which is usually between 600RPM to 1000RPM, signifies the speed of the crankshaft. The fuel is calculated by the ECU (or ECM or PCM) depending upon the load, which is a calculated based on readings from Mass Air Flow sensor, Manifold Absolute Pressure sensor, Intake Air Temperature sensor and Throttle Position Sensor.

Nowadays there is start-stop technology implemented in vehicles (commonly found in hybrid technology) and I have also heard of conventional vehicles having this feature. Basically engine shuts off when the vehicle is not moving to save fuel and as soon as the driver touches the throttle pedal, depending upon the load demand (aka throttle position etc.) fuel and spark timing is calculated by the computer and combustion begins to keep the vehicle transition smooth.

  • In "most newer cars" even standard engines have this "economy mode", I find it annoying, but it does turn the engine fully off. It is quite noticeable though. – coteyr Apr 28 '16 at 17:27
  • You can of course do it manually with the key in older cars, but whether it's worthwhile depends on how efficiently the starter can restart the engine. I suspect newer cars with this feature have the necessary sensors and variable-output starter needed to turn the engine with the minimal necessary electrical power so that you don't lose all your savings from the alternator recharging the battery. – R.. Apr 28 '16 at 19:48
  • Nice Answer..... +1 – DucatiKiller Apr 29 '16 at 15:50
  • R.. I sell starters all day here in NZ, and I have never heard of a variable output starter. Do you have any information available on those? – Bevan May 4 '16 at 7:40
12

In most cars, yes the engine continues to rotate and the pistons go up and down in the cylinder bore due to combustion and the engine continuing to run. Some vehicles have an "auto stop" feature which kills the engine when it's not needed, but that's usually after several seconds of sitting still, as long as other parameters are met as well.

Most vehicles today have a tachometer. This is usually one of the two large gauges in the dash. The other tells vehicle speed. If you look at it when stopped, you'll notice the needle will most likely be pointing at ~600 rpm, meaning the engine is still rotating ~600 rpm even when the vehicle is stopped.

The vehicle you ride in must be one smooth running vehicle, as most vehicles have at least enough vibration to easily tell the engine is running. Good on yah.

  • 3
    Now that I think of it pistons stop motion twice with each full revolution, at the top and the bottom of travel, no kidding. – Moab Apr 28 '16 at 0:33
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    @Moab - Actually, you are right, but realistically in answering this question I think that understanding would confuse things a little. Thanks for the add, though! PS: I already stated about the "Some cars" comment (see: "auto-stop" above). – Pᴀᴜʟsᴛᴇʀ2 Apr 28 '16 at 0:54
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    Auto stop (also knows as StopStart) on manual cars in Europe kicks in as soon as you are in neutral and release the clutch. The car realises its stopped and not in gear and cuts the engine (assuming 1. the battery is can cope with the restart cycle, 2. the engine is up to the minimum temperature for the stopstart function, 3. the ambient temperature is within stopstart operating ranges). – Mauro Apr 28 '16 at 7:44
  • @Moab Depends on what you what parts you define as making up the piston. I personally always considered the connecting rod to be part of the piston. In which case the piston head does stop moving at top and bottom, but the connecting rod is moving sideways at those points, so in that light, the piston really does not stop moving. That however does not make my definition of the piston correct 8)...and yes some mor info to muddy the waters. – Forward Ed Apr 28 '16 at 12:09
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    By your logic the drive wheel is part of the piston?, they are all connected and part of an assembly. – Moab Apr 28 '16 at 13:04
5

To add to the other answers, in a manual vehicle, the road wheels are usually disconnected from the engine when at a stop either by putting the transmission in Neutral, or by depressing the clutch pedal.

In an automatic vehicle, a torque converter essentially does the same thing as a clutch, but does not require manual intervention.

These mechanisms prevent the engine from stalling when the vehicle comes to a stop.

See this video by Engineering Explained to see how a torque converter works, and this one to see the differences between an Automatic and Manual transmission.

3

I don't have enough reputation to comment, but the small economy cars (K or Kei cars) in Japan also have this StopStart feature, as Mauro commented (for European cars).

I think this feature is now common, say the last 3-5 years, across all K-cars (multiple manufacturers).

On these cars, this StopStart feature is also available on automatic cars, so the implementation is slightly different than explained by Mauro.

As an aside, the StopStart feature increases fuel economy.

  • +1 for virgin status on this forum when answering this question. Yeah Jake! – zipzit Apr 28 '16 at 17:56

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