Why do cars accelerate when the clutch is in the friction zone?

Question

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.

Answer 1

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.

Answer 2

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.