Join the Car Talk Community!

Discussion Rules

Welcome to the Car Talk Community!

Want to ask a question or join the discussion? Great! Join now.

Sign In Register

What does it mean exactly to stall a manual car?

edited November -1 in General Discussion
I never quite understood the concept of stalling a manual car. According to my google searches, a stall can be caused "by not timing the release of the clutch and pressing the gas pedal correctly. You need to give it just enough gas not to stall it while slowly releasing on the clutch so you take off smooth and without damaging your car". What I don't understand is why timing the release of the clutch matters to the engine. Isn't the rotational motion of the pistons inside the engine enough the sustain the constant motion to keep the engine running? Why does the timing of the release of the clutch matter? Also, what does pressing the gas pedal have to do with this process. Thanks guys.
Tagged:
«1

Comments

  • edited July 2010
    Engines have very little power and torque at idle speed. A good way to learn to drive manual transmission is to stop on a flat road. Take your right foot off the gas and slowly let out the clutch without stalling the car. This is a good way for beginners to learn how to start off in first gear. If you let the clutch out too fast the engine will stall. One you get the feel of the clutch, you add gas while letting out the clutch for a smooth start.

    Twotone
  • edited July 2010
    When you engage the clutch (remove your foot from it), the friction of the two plates reduces the RPMs. The stall occurs because the RPMs dropped low enough for the gearing to match the engine speed to the drive wheel speed. The bounding condition is when the car is stopped. If the clutch is engaged and the drive wheels are stopped, the engine has to stop to match the drive wheel speed.
  • edited July 2010
    what do you mean by "bounding condition"? Can you elaborate on that please. Thanks.
  • edited July 2010
    It's the condition of not moving that he is talking about. There isn't a condition of "bounding."

    Stalling the engine happens when you try to get the car moving and the clutch is engaged too quickly and the engine quits. That was easy.
  • edited July 2010
    I understand what stalling is but why does it happen? Why does it matter whether or not the clutch is engaged slowly or quickly? How does this cause the engine to stall? Thanks.
  • edited July 2010
    Why does it matter whether or not the clutch is engaged slowly or quickly? How does this cause the engine to stall?

    If you let the clutch out slowly, then as the clutch engages, it slips some and starts moving the car a little without stopping the engine. Doing it too fast would require the engine to provide more energy in a shorter time than it can do so it would stall.

    This is a practice thing and may take some time. I suggest you have an experienced standard shift driver help you out a few times.

    Note: it seems to be less of a problem with most modern cars, but it is possible to damage the clutch practicing this. An experienced coach and help reduce the practice time.

    While my father was great, generally family members may not be the best coach.

    Next step is to learn how to do it on a up grade.
  • edited July 2010
    What is your experience with a manual shift car? Have you ever driven one? Ridden in one and watched the driver's feet closely?

    IF the car had a huge engine, like 10 liters and 500 hp in a Civic body you could simply release the clutch as fast as you want and the car won't stall. It would lurch ahead and perhaps snap you neck; but it won't stall. Such a motor would be too big, too heavy, and too expensive for a normal car.

    The purpose of the transmission is to allow a small motor to move a big vehicle. Using lower gears to get it rolling and then higher gears to add momentum and eventually reach cruising speed. At the same time changing gears keeps the motor in a "sweet spot" of rpm's for producing power and torque.

    The clutch is released slowly in order to keep the car from lurching, bucking, and snapping the driver and occupants around in their seats. Gas is applied in the process because a motor at idle produces about 1/100th of its peak power and torque. You need to have the motor spinning faster to produce enough power to move the car. Gentle starts on level pavement require only a little press on the gas. Fast starts, rapid acceleration, or starting on a steep incline require a much bigger press on the gas petal, and even smoother release of the clutch to accomplish without whipping the occupants around or producing a lot of buring rubber from the drive wheels spinning out.
  • edited July 2010
    Let's try a different approach to explaining an engine stalling. Say you have a regular lawnmower. A rotary blade with a small gas motor. The motor has a shaft the spins and on this shaft the blade is attached. There is no transmission, no belts, the blade is directly attached to the motor.

    You come home from a month vacation and instead of the normal 2 or 3 inches of grass you need to cut, it is over a foot high. You fire up the lawn mower and move it into the high grass and it stalls, the motor stops running. You pull it out and start it and try again, same result. Next time you start the motor, push it in the grass and you hear the motor straining and just before it quits you pull the mower back out a bit and the motor keeps running and gets back up to speed. You find out if you go real slow and work the mower back and forth you can keep it running. It takes a long time but eventually you mow your hay field of a lawn and head inside for a beer.

    Your actions in pushing the mower into and out of the high grass to keep the motor running is essentially what you are doing with a clutch. A gas motor that is overwhelmed by the load on it will stall. You are using the clutch to control the "load" on the motor to keep the motor from stalling and get the car moving.
  • edited July 2010
    Engines have very little power and torque at idle speed. A good way to learn to drive manual transmission is to stop on a flat road. Take your right foot off the gas and slowly let out the clutch without stalling the car. This is a good way for beginners to learn how to start off in first gear. If you let the clutch out too fast the engine will stall. One you get the feel of the clutch, you add gas while letting out the clutch for a smooth start.

    Twotone


    Excellent...I used this technique teaching my wife and two kids, who both still prefer driving standards. That, along with learning low speed maneuvers driving through the local cemetery seemed to help them in Boston traffic.
  • edited July 2010
    Internal combustion engines produce power in short duration pulses. The rest of their cycle is devoted to preparing for the next short duration power pulse.

    There are two ways to increase their instantaneous load capability; produce power pulses at a faster rate such that they are more "continuous" (e.g. more cylinders and/or higher rpm) or store the energy pulses in a large rotating mass such as a flywheel. The latter is not well suited to a modern automobile application.

    Contrary to your thinking, there is not a lot of energy being stored in the rotating mass of the automobile engine. Responsiveness to power application or removal is highly desired in a car engine. It's designed to have just enough rotating mass to keep it running smoothly at idle with a minimial load applied to it.

    By gradually releasing the clutch you reduce the instantaneous load on the engine and by pressing the gas pedal, you increase the engine rpm and therefore produce more power at closer intervals. Both of which help to overcome the load being applied to the engine. There is an optimum range where the clutch can be released with minimal slippage and the engine rpm is capable of handling the instantaneous load of beginning to move the vehicle.

    Feathering of the clutch and gas pedal is an acquired skill. The amount of feathering required is dependant on the vehicle weight, engine power, clutch design etc.
This discussion has been closed.