Eli5: how can you roll start a manual vehicle but not an automatic vehicle?
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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.
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.
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