Eli5: When a car stops randomly or won’t start up, why does pushing the car get it started?
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it only works for manual transmission cars. and only if the car starter is the problem.
the starter for the car is the component responsible for getting the engine spinning. if that part is bad, then you can manually get the engine going by engaging the transmission, pushing the car (which spins the transmission, which spins the engine).
When a car is turned on and you’re driving, the engine is spinning, and the spinning engine corresponds to the wheels spinning because the engine is connected to the axles via the driveshaft.
When you first turn on the car, a small electric motor called the starter is used to get the engine spinning. The engine is too heavy to just start spinning on its own, so when you turn the key the starter gets the engine spinning, and once it has started to spin, it’ll keep spinning under its own power.
In a car with a manual transmission, you can push start the car by putting the car in neutral and pushing it. Once the car is going fast enough, you step on the clutch and turn the key. The reason this works is because when you step on the clutch, it connects the driveshaft to the engine. Since the wheels are turning, the driveshaft is spinning, and if the driveshaft is spinning fast enough, it’ll provide enough force to get the engine spinning – basically replicating what the starter does.
Nooooo do not push an automatic! It causes the gears of a manual transmission to turn if you release the clutch, but it will destroy an automatic transmission. Do not do it!
Imagine you are on the swings at recess. Once you are swinging, it is not that difficult to keep yourself swinging at the same height. But when you are just starting, it can be very hard to get the swing going. It helps if someone gives you a push instead. Likewise, a car engine is good at keeping itself going once it is started, but it is terrible at getting started. A separate motor called a _starter motor_ is used to give the engine a push when it is completely stopped. But if the starter motor is broken, it can’t give the engine a push to get it started.
When driving a car normally, the engine is connected to the transmission, which is then connected to the wheels to make them turn. When you shift the car into neutral, it essentially disconnects the wheels from the transmission. If shift into neutral and get the car rolling a bit (by pushing it down a hill or something), you can suddenly connect the wheels back to the transmission, which will connect to the engine and start turning it. If you can get the engine turning just a little bit, it can create just enough power to keep itself turning and start the car.
This is not how the car is meant to be started, so it is not recommended unless there is a serious problem with the starter motor (or things the starter needs, like the battery). This is also basically impossible on an automatic transmission car because it connects to the wheels differently than a manual. Somebody in the comments is going to claim to have done it, but it’s almost impossible to do in a manual.
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