How does pushing a car that won’t start help start it?
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The car must be a manual transmission for this to work.
An engine starts by the starter motor forcing the engine to turn until it “catches” and then runs on its own.
When you roll start a car, you build up enough speed that when you put it into gear and then release the clutch, the wheels and transmission now force the engine to turn until it catches and then runs on its own.
Basically instead of the engine moving the wheels, you can use the wheels to move the engine. It’s easier in 2nd or even 3rd gear.
Usually to start a car, a little electric motor spins the engine until it has enough strength to spin by itself. This needs a lot of energy from the battery, so a dead or low battery cannot start the car.
However, if the engine is connected to the wheels (referred to as being ‘in gear’), you can spin the wheels to spin the engine, instead. Pushing the car then spins the wheels which spins the engine which can then start and spin by itself.
if the car isn’t starting because the battery is low, what’s missing is actually the initial input from the starter motor. In a manual transmission car, engine and drive shaft are fully connected when the clutch is engaged, so you can start the engine turning by pushing the car with the clutch disengaged (which turns the wheels and thus the shaft) and then engaging the clutch which transfers that rotational energy to the engine. Once the engine is turning over then the power is coming from combustion rather than the battery.
It turns the engine over by powering it backwards through the wheels. Sort of like the pull start on a lawnmower.
This will help if the battery or starter motor is dead, but generally won’t help if there’s more wrong that’s keeping the engine from starting(you need fuel, air, spark, and compression). Also requires a manual transmission, so you can disconnect the engine from the wheels to get the car moving, then reconnect it to start the engine.
To add to what others have said, you also need to make sure that the ignition is on, otherwise the engine doesn’t know to deliver a spark. It’s also best to use 2nd gear as it creates less engine braking. Also remember to dip the clutch if/when it fires up to stop the car from flying off (or stalling, if they hit the brakes)
Y’know how some motorcycle have a kick start? The way those work is that they simply push the engine so that it goes through a couple of rotations. With the electricity on, this causes all the same stages as when a car is fully running, leading up to spark and the engine firing.
Once the engine fires it should continue to drive itself through more rotations.
Pushing a car in neutral lets it build momentum that, similarly, should push the engine through its cycle once you put it in gear while it’s moving.
This works when the battery is low (but not dead) because it bypasses the starter motor, which draws a lot of electricity.
A lot of “it makes the engine run itself” explanations here, without explaining how it does that.
The battery in your car is just an extra power supply for when the engine is off, once the engine is running, it turns something called an alternator which generates electricity. This alternator creates more power than the car uses, so it keeps the car running while also recharging the battery.
A car that won’t start due to a flat battery has no power to start. So instead, you push it and engage the gears, causing the engine to spin. This then turns the alternator, which provides a spark to the engine and allows it to run. As it runs, it turns the alternator more, which provides more sparks, which keeps the engine running and repeat.
If the battery isn’t completely dead, it’ll then be charged while the car runs.
Like others have replied, the vehicle has to have a manual transmission in order to attempt a push to start. Automatic transmission has the gear locked, so you can’t build momentum to pop the clutch into gear.
Before your mower had an electric start you had to pull a rope to spin the engine to get it to fire. Old cars and tractors had hand cranks for yrs. Rolling the standard transmission car allows a start without the electric starter.
It only works if the thing that is preventing it from starting is a dead starter motor. The rest of the electrics need to be in order. The following is for a manual car.
When you turn the key/ press the start button, an electric motor cranks the engine and initiates the firing sequence of the engine. Once that takes hold, the motor disengages and the engine keeps itself alive via the sequential piston motion. This motion is transferred to the wheels via the transmission, which is linked to the engine via the clutch.
When you push a dead car, and the clutch is engaged, the opposite happens. The wheels can turn the engine. So what is happening is you are in effect doing what the starter motor does to initiate the piston firing. Only a fully engaged clutch at standstill is incredibly resistant to motion thanks to the gearing. This is why when doing a roll start, you would have the gear in neutral (wheels and engine disconnected), push the car without the resistance of the engine, then pop the clutch once you’ve built up enough momentum to connect the engine and wheels, causing the wheels to turn the engine enough to start the firing sequence.
I don’t think it works on automatics, but I may be wrong about that.
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