Eli5: how can you roll start a manual vehicle but not an automatic vehicle?
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Roll starting a manual vehicle engages the transmission by using momentum from rolling downhill. Automatic vehicles require the engine to be running for the transmission to engage.
basically, the automatic gearbox is designed in such a way that it doesn’t create a physical connection to the wheels *unless* the engine is running, even if the gearstick is in “drive”, so you can’t therefore use the movment of the wheels to get the cylinders turning and get the engine to start.
In cars with manual transmissions, the engine and transmission are coupled through the clutch. When the clutch is engaged, turning the wheels will turn the engine. In automatic cars, the two are connected through a torque converter. This device allows the engine to keep idling but the wheels to not turn while the car is in gear. In a car with a manual transmission, you must either put it in neutral or depress the clutch. The torque converter is essentially a “fan” on side connected to the engine and a “turbine” on the side connected to the transmission. As the fan spins, transmission fluid flows and pushes against the turbine blades. When enough pressure hits the blades the transmission will begin to turn. You won’t be able to turn the blades fast enough to generate sufficient pressure to turn the engine.
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.
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.
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.
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
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