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A few days ago the battery of my car went (almost) dead. As it is a manual car, my father once told me that the way to get it going without jumper cables was to push it or let it roll down a hill, sink the clutch, shift to 2nd gear and then let go the clutch.

After the joy of being able to 'revive' the car, I got to wonder of the reason for why this works. All I could think of was Electromagnetic Induction, however I couldn't find anything on the web to support this. I'm sure that there might be all sorts of engineering details, so I'm only looking for the physical principles involved and the basic process that make this work.

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    $\begingroup$ I don't think it has anything to do with the starter or induction at all. The starter just turns the car's crank shaft. Putting the car in gear down hill also turns the crankshaft. You're using gravity to do the same job as the starter. $\endgroup$ – Brandon Enright Jan 21 '15 at 22:28
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The purpose of the starter motor (using the battery) is to get the engine moving so that the combustion cycle can be initiated. The combustion cycle is self-sustaining, but it relies on the pistons moving to compress the air/fuel mixture, so you have to get them moving in the first place somehow. By putting the car into gear and rolling it down the hill, the motion of the wheels will transfer to the engine and start the pistons moving, in the same way as the starter motor would.

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    $\begingroup$ It's self-sustaining if there's enough power to fire the spark plugs. $\endgroup$ – Nathan Tuggy Jan 22 '15 at 3:41
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    $\begingroup$ Yes, but if the engine is turning at a speed fast enough for the thermo cycle to work, the alternator will provide that power, not the battery. You could (likely) completely remove the battery and the car would still run if started this way. $\endgroup$ – Godric Seer Jan 22 '15 at 4:50
  • $\begingroup$ You'd a decent hill, not just manual pushing on level or near-level ground, to get power from the alternator. $\endgroup$ – R.. GitHub STOP HELPING ICE Jan 22 '15 at 5:20
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    $\begingroup$ By the way, provided you do have enough power for the spark plugs (and possibly fuel injectors), you can do this with no hill and even without any space to move the car (think: parallel parked) if you jack up one wheel and turn it by hand (or with a ratcheting breaker bar on the axle nut). $\endgroup$ – R.. GitHub STOP HELPING ICE Jan 22 '15 at 5:24
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    $\begingroup$ Question stated the battery was (almost) dead. Popping the clutch only works if there is enough power for the spark plugs and engine computer. If the battery is completely drained this will not work, because the alternator will not supply any power during this process. With the clutch pedal pressed in, the crankshaft will not move. Once you engage the clutch, the alternator will be spinning very slowly, probably not enough to produce measurable power. I actually tried this once with a battery with dead cells. Popping the clutch wouldn't work. $\endgroup$ – user57109 Jan 22 '15 at 5:56
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I'm sure that there might be all sorts of engineering details,

Not really. An electric starter motor requires a large amount of power to turn the engine over. If the battery is nearly exhausted, it cannot provide the power to rotate the electric starter motor.

However, as long as there enough power from the battery to run the car's electronic engine controller and ignition system, the engine can be turned over by non-electric means and started.

For older designs without electronic ignition and controllers, no battery is required at all.

I was recently reading about the Coffman starter which used what looked like shotgun shells (cartridges) to start aircraft piston engines. If you've ever watched the movie "Flight of the Phoenix", there is a dramatic scene involving a Coffman starter.

In early automobiles, the engines were often started with a hand crank.

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    $\begingroup$ I have started a car (Morris Minor) with a crank. Make sure you put your thumb over the handle and not around it. $\endgroup$ – Rob Jeffries Jan 21 '15 at 23:24
  • $\begingroup$ I forgot the rear reading lamp on in the car while I was cleaning it and it was turned on a day or two. Then when I inserted the key a dim lights appeared on the dashboard but they faded away very soon - so I knew that the battery was completely empty. Anyway we tried to push start it several times on the street but it did not started. I suppose there were not any electricity for the car computer or spark plug to ignite any cylinder. We wasted our time. It is useful to have jump start cables or/and battery charger if your battery is old or the car is sitting for longer periods of time parked. $\endgroup$ – Combine Oct 17 '19 at 20:31
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Actually, you can push/roll start a manual car in this fashion with no battery at all. The momentum of the car is used to turn over the engine when the clutch is released, hence why the car slows noticeably at this point.

Second gear is used to get the engine turning fast enough that the alternator provides sufficient power to run the engine and other electronics, and the pistons move fast enough for the engine to start.

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    $\begingroup$ I think many, but not all cars could be push started without a battery. Some modern electronic or computerized ignitions might not be happy about this. It may also be necessary to connect the two battery leads to complete the circuit. I hope this isn't nitpicky, I am trying to think through the problem and wondering where the nearest hill is to test it! $\endgroup$ – Freiheit Jan 22 '15 at 1:39
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    $\begingroup$ @Freiheit It's very difficult to push start an automatic. The torque converter may give just enough so that the engine does not turn over. $\endgroup$ – LDC3 Jan 22 '15 at 3:52
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    $\begingroup$ I've seen manual button-start cars without batteries started in this fashion. You can't generally push-start automatics. $\endgroup$ – Monty Wild Jan 22 '15 at 4:08
  • $\begingroup$ If your goal is to get the engine spinning faster, wouldn't you use 1st gear instead of 2nd gear -- the higher the gear, the slower the engine spins for a given speed. I thought the alternators used in modern cars needed the battery to energize the field coils, without that initial voltage from the battery, the alternator wouldn't produce any power? $\endgroup$ – Johnny Jan 22 '15 at 7:19
  • $\begingroup$ @Johnny: Maybe you get too much resistance in first and the car might not roll as easily? $\endgroup$ – Time4Tea Jan 22 '15 at 12:38
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In the days before Electric Rear Window Defrosters, cars still had generators, you could push start the car with a completely dead battery. Generators make electricity, whereas today's cars are equiped with alternators.

Alternators take electricity to make more electricity. They have a much higher output for today's cars but are useless completely flat. Perhaps if the battery has voltage but not enough to turn over the starter you could push start the car...

My first car had a hand crank and a generator. I do not miss generators, carburators, or points.

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    $\begingroup$ Alternators are generators: they produce electricity from the rotational motion supplied by the engine. An alternator works without any input from the battery, and in fact charges the car battery while the motor is running. $\endgroup$ – Asher Jan 22 '15 at 2:51
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    $\begingroup$ True, for an indepth lesson on this go to rowand.net/Shop/Tech/AlternatorGeneratorTheory.htm $\endgroup$ – SgtWallace Jan 22 '15 at 3:37

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