Eli5: how can you roll start a manual vehicle but not an automatic vehicle?
In: Other
A manual transmission is like a bicycle, you move the wheels by moving the pedals, but if you are riding downhill then the pedals will rotate by themselves because the wheels move the chain whether you want it to or not. When you hit the bottom of the hill you can put your feet on the pedals and your motion is already “started”. Engines need to be running at a certain speed to keep themselves going, and if you can get it moving fast enough to reach that speed then the car will start.
A manual has essentially a direct connection between the wheels and the engine when it’s in gear so turning the wheels turns the engine.
An automatic has a bit in between the engine and the wheels called a torque converter. It’s like a fan submerged in fluid and at low engine speed it makes it hard for the car to accelerate from a stop. Turning the wheels would only turn the outside of the torque converter and not the fluid enough to spin the fan and, in turn, the engine.
Manual transmissions directly connect the rotation of the engine, through the clutch, through the transmission, through the driveshaft, through the differential, through the axles, to the wheels. One end cannot spin without the other end spinning. When the wheels spin it transfers that rotation all the way back to the engine. When the engine spins, it has a timing belt/chain that synchronizes all the bits that keep an engine running, so the fuel starts pumping, the spark plugs start firing, etc. and the car starts. This is essentially the same process as your starter motor manually rotating the engine over, it’s just using the transmission to turn over the engine instead of the starter motor.
Automatic transmissions do not directly connect the engine’s rotation to the transmission. This allows the wheels to be stopped while in gear, but have the engine still rotating. This magic happens between the transmission and the engine, instead of a clutch that joins the two together, you have a torque converter. You can think of a torque converter like two fans that are facing each other. One fan is connected to the engine, and one fan is connected to the transmission, but the fans don’t physically touch each other. The fans are inside a case that is filled with liquid, when the engine spins, it spins the fan, and it makes all the liquid slosh around in a circle. That rotating liquid makes the other fan spin, and that spinning is transferred to your transmission and ultimately to your wheels. This allows you to apply the brakes and stop the car, which is forcing one of those fans to stop spinning, but since it’s not physically connected to the engine side, the engine’s fan is still allowed to spin around and around, not stalling the engine. In practical applications, the engine spins while the wheels do not, not the other way around. Knowing that, the designers designed the fan blades to be more efficient at transferring the rotation from the engine’s fan to the transmission’s fan. This efficiency made the reverse direction very inefficient, spinning the transmission fan does not transfer enough fluid rotation to spin the engine’s fan to overcome the friction and compression that is needed to spin the engine over. In theory you could get an automatic transmission car to roll start if you were going fast enough, but in most circumstances a car that isn’t started isn’t going to be getting up to any significant speed. There is a such of a thing as a torque converter lockup, which will use a clutch to link the two sides together, but that is driven by your car’s computer, which activates a solenoid at highway speeds to ‘lock’ the engine rotation to the transmission. The lockup technology was primarily used for fuel efficiency, as you can imagine it’s much more efficient to transfer power when the two are physically touching, rather than relying on rotating fluid to transfer the spinning energy. In theory you could macgyver some way to engage the lockup temporarily to start the car, but that’s far more effort than just replacing the starter or battery.
At a more basic level, an engine when it is turning is doing several things at once. The turning motion pushes and pulls pistons in and out of cylinders. When a piston is being pulled, air and fuel gets sucked into the cylinders, and a spark is generated. That spark ignites the air and fuel, pushing the piston down. That action keeps is what keeps the engine turning. So it’s all a self-sustaining process you just have to get it going. Its the same thing on airplanes when someone in an old movie spins the propeller by hand to start the engine. Modern airplanes work the same way, there is now just a little electronic motor to provide the initial spin so no one gets their hand chopped off. Then the engine just does it’s thing.
As others point out, automatic cars have extra layers between the engine and the wheels turning so pushing the car doesn’t turn the engine the same way.
Well, I have definitely helped roll started an automatic. Early 90s, was in France with friends and the car I was in was in an accident. The owner was fully covered and he got a Fiat Tipo for the duration of the holiday. But couldn’t get it going when we stayed at a farmhouse, so we (ignorantly it seems) tried to bump start. A long farmhouse drive, slight incline in our favour and we got it going. Only discovered later that we shouldn’t have been able to do it.
Would love to know HOW we succeeded if anyone has any ideas, it’s been a mystery ever since… thx
An automobile clutch, clutch being the general term we use for a device that can allow and disallow twisting, is a powerful spring that pushes a friction plates against each other that allow the engine and the transmission to connect to each other while retaining the ability to disconnect them by using a strong lever – like a clutch pedal.
In typical driving the engine is generating power by harnessing enough combustion energy to both push the piston back down, push a corresponding piston up, AND provide enough torque to twist something connected to wheels – in a car that would be a transmission since a car can only develop enough torque to effectively power a vehicle in a narrow band of RPM.
If the car is moving but the engine isn’t, the condition you find yourself in when you depress the clutch and release the brake and are either going down a hill or being pushed, if you release the clutch you send the kinetic energy from the drivetrain into the engine – the opposite of normal driving. If you do this with the ignition on (so the battery is OK-ish) you mimic the behavior of a starter motor. The engine spins until you get a successful inhale/compression/sparl-combustion/exhaust.
A manual has a clutch, which is a rigid coupling. The motion of the wheels is transmitted even at low speeds, there is no slip.
An automatic uses a torque converter, which is a fluid coupling. There has to be significant motion for it to transmit enough energy. It slips constantly.
Imagine you have a fan (the engine) that you want to spin. A manual would be equivalent to you rotating the blade with your hands. An automatic would be like you trying to rotate the blade by gently blowing on it. Since the fan has a lot of resistance (engine compression and friction), gently blowing on it won’t cause it to spin. You’d have to use a leaf blower.
Manuals have a clutch, automatics have a torque converter, the clutch locks the engine to the wheels without slipping (if if in all the way) whereas a torque converter allows the engine to slip at lower RPMS, so you can’t easily force the engine to turn over.
The tired of the vehicle are directly connected to the clutch disc by way of the transmission.
The flywheel and pressure plate (the things that the clutch are compressed between) are directly connected to the engine.
In a traditional automatic transmission the connection between the engine and transmission is a fluid coupler (torque converter) that works off of centripetal force, and you can’t make the force of the engine isn’t running. And in a traditional automatic transmission the transmission itself is hydraulic. But you the engine to run to produce that hydraulic pressure.
So when the engine isn’t running in a traditional automatic transmission there’s no direct connection between the engine and the tires. That’s why there’s a “park” in an automatic transmission. It’s a little lever that gets pushed into a space that stops the wheels from moving. If you just left an automatic car in gear when you turned off the engine. You would be able to push it.
A manual transmission uses a clutch to physically connect and disconnect the engine to the transmission. If the clutch is engaged and the transmission is in gear, when the engine turns the wheels will turn. Also, if the wheels turn the engine turns. That’s how you do a roll/push start. You use the rolling wheels (with the weight of the vehicle) to turn over the engine, performing the same function that the starter does when you start the engine normally.
An automatic transmission uses a torque converter, which is basically a fluid pump, to push the automatic transmission fluid through the transmission to deliver power. Effectively the automatic transmission is a one way connection, the engine can deliver power to the wheels, but the wheels can’t deliver power to the engine. This means the engine and the wheels can turn independent of each other, even when the transmission is in gear.
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