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Energy required to start a car

approximately how much energy is required to start a car? I know it depends upon engine size, temperature, etc… I’m just looking for a range of values — min to max.


A Couple Cups Of Relatively Strong Coffee Does It For Me !

I’d say current draw 50 amps (small, warm 4 cylinder) to 300 amps (cold V8) at 12V. That works out to 600 to 3600 watts. Then, it might take 0.5 second to start a warm, well tuned motor and 5 seconds to start a cold, finicky motor.

So my answer is 300 to 18000 watt-seconds (joules). More for a diesel and/or very large motor

The current value will vary with a given engine, more so with a four than an 8. When a piston comes up on compression, the starter motor will work harder drawing more amps. The battery voltage will rise and drop too as current varies. The old rule of thumb used to be around 1 amp per cubic inch of engine displacement but with some starters having permanent magnet fields and some not, the range can vary.

Starter motor run time will also have an effect on the total energy use. I can’t even guess how anyone would have accurate information on all of this unless they worked on starter design.

It would be easy to get this information with proper equipment such as a Hall Effect current transducer and a dual trace integrating oscilloscope to follow amps and volts but who has that? A dc clamp-on ammeter, a voltmeter and a stopwatch would give a very rough idea but dc clamp-on ammeters are very uncommon and you can’t just run out to Harbor Freight to buy a cheap one as they don’t have them; I looked.

Thanks for your helpful response.

Thanks for you helpful response. You and responder #2 have given me what I wanted – I think.

Add to the current draw the energy from the gasoline…without combustion in the cylinders you could crank all day without success!

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