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When a manual transmission car is stopped on level ground, it is possible (for some cars) to make it start moving without using the gas pedal by putting it in gear and bringing the clutch to the friction point. If the clutch pedal is fully released, this acceleration will cease, causing the car to stall if it has not gathered enough momentum.

Why does this happen? What causes the car to accelerate when the clutch is in the friction zone but not when it is fully in gear?

Note that I already asked this, but that question was closed because people didn't understand it.

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  • Fully in gear? How do you manage to let out the clutch without the shifter being fully seated in a gear or in neutral without grinding everything to dust? What make, model, and year of vehicle are you abusing?
    – cory
    May 25, 2017 at 13:13
  • Fully in gear as in the clutch pedal is all the way out. Sorry, I thought that was pretty clear from the context of the rest of the question.
    – intuited
    May 25, 2017 at 15:05
  • The answers to the question marked as a duplicate, while very informative with respect to the functionality of a clutch, don't seem to answer my question. There's no mention of the idle control system, for example.
    – intuited
    May 25, 2017 at 15:25
  • You don't seem to understand what a clutch does, what a gear shift does, what it means for a transmission to be in gear, etc. Do some research about those things. Then, when you are ready to maintain or repair your vehicle, ask a specific question about a specific vehicle.
    – cory
    May 25, 2017 at 15:30
  • I think I have a pretty good understanding of it now but continue to be surprised by how frequently site users focus on verbiage and ignore the actual point of the question. But so I guess you would say that the car is in gear even if the clutch pedal is pushed down? That seems less than obvious from a non-technical standpoint.
    – intuited
    May 26, 2017 at 1:09

2 Answers 2

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A car engine has a maximum torque that it can produce under different conditions: idle, maximum RPM, etc. When the torque of the wheels exceed that produced by the engine, the engine speed will go down.

At idle, the car's idle control system attempts to maintain a constant idle, but can only do so with a finite amount of torque. This is because it is designed for only varying idling loads, like a/c compressors, power steering pumps, and alternator. The idle control may have the ability to increase air through a bypass around the throttle, and adjust fuel amounts by controlling injector duty cycle. The amount of air flow has a maximum at a given RPM due to air movement being governed by a pressure difference, and a corresponding maximum of fuel can be added before combustion gives no increase in power.

When you exceed the torque able to be produced by the engine by releasing the clutch too fast the engine will slow related to the difference in torque applied versus produced and the angular momentum of the system. Eventually the engine will slow until it cannot produce enough vacuum to pull air into the cylinders and will stop.

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  • Note that especially for old cars, there is idle air control valve that can let only a small amount of air through it. More recent cars use the throttle valve instead through electronic control, and therefore, can at least in theory provide greater torque at idle. So I would say that accelerating at standstill with no gas is probably going to work better on more recent cars. Of course, the ECU software may still have a limitation for throttle opening at idle, so all of this depends on the ECU software.
    – juhist
    May 25, 2017 at 12:26
  • Okay, so you're saying that the idle control system provides a low rate of fuel supply to the engine, but only while the clutch pedal is pressed down or the transmission is in neutral? And while the clutch is in the friction zone the idle control system is still providing fuel to the engine? I feel like you've led me to water but not made me drink.
    – intuited
    May 25, 2017 at 15:30
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    This is not a duplicated question. Considering an electronic throttle system, here's what happen: the engine in idle speed is controlled by an ECU which reads its speed and actuates the throttle to maintain idle. Whenever you start releasing the clutch the idle controller increases the engine torque to overcome the load induced by the friction. It does it by opening the throttle and increasing fuel flow. But there's a limit, and this limit is reached when you have enough load caused by the friction that the ECU is fully opening the throttle (WOT) and injecting the maximum amount of fuel... tbc
    – PDuarte
    May 25, 2017 at 16:03
  • ...any further friction will be impossible to overcome to maintain idle speed. At this point, the engine speed reduces as clutch is further released until the engine cannot sustain rotation due to the low speed, and it stalls.
    – PDuarte
    May 25, 2017 at 16:05
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The clutch is slipping, so the stationary load is accelerated. If the clutch pedal is released too quickly, the stationary load stalls the engine as it does not accelerate quickly enough.

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  • Wait so how is the engine able to accelerate the stationary load while the clutch is slipping when it can't even do it with the clutch properly connected? That's kinda confusing. I could see there being some momentum imparted to the clutch plate before the clutch starts to slip, but then since the clutch pedal is staying stationary (in the middle), how would the clutch ever connect again, since it's then moving faster than it was when it slipped, and the wheels are slowing down?
    – intuited
    May 26, 2017 at 18:41
  • When some are learning to drive they release the clutch too quickly ie kangaroo mode ... you can slip the clutch and still provide power torque to the wheels so the csr moves but you are slipping hge clutch - usually ends in smelly blue smoke and dventuslly a nice large repair bill . I suggest that you can find out if you have a manual by seeing how long it takes to generate blue smoke by slipping zhe clutch - at, of course, your own expense...
    – Solar Mike
    May 26, 2017 at 19:09

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