Other Question #3867

Gloria, a 20 year old female from Toronto asks on May 14, 2007,

On average, how much petrol is burned per minute of idling?

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The answer

John Jones answered on May 14, 2007

On average, about 30ml of petrol is burned per minute of idling. An engine runs at between 600 and 1000 rpm (Revolutions Per Minute) when idling. Engine cylinder volumes range from about 400 cc (cubic centimeters) to 625 cc resulting in engine sizes of about 1.6 litres for a 4 cylinder compact, up to an SUV with a 5 litre 8 cylinder power plant.

Now, the engine can't actually draw in a complete cylinder full of air, since the flow losses in the intake valve reduce the cylinder pressure to below atmospheric. The actual mass of gas drawn in will be 80-90% of a full volume at atmospheric pressure. (We call this the *volumetric efficiency* of the engine.)

Also, each cylinder fills with air once every four strokes, so the total volume drawn in is:
4 small cylinders at 600 rpm: 1600 * 0.8 * 600 / 4 = 192 litres/minute
8 large cylinders at 1,000 rpm: 5000 * 0.9 * 1000 / 4 = 1,125 litres/minute

1000 litres of air weigh about a kilogram, so that's 0.192 kg air/minute for small engines and 1.125 kg air/minute for big engines. At idle, the ratio of air to fuel is approximately 15 to 1 by weight, that is, we take in 15 grams of air for every gram of fuel burned. So we need to multiply these air values by 1/15 to get the fuel weight.

Converting to liquid measure, the volume of fuel consumed per minute is in the range 15 ml/min to 75 ml/min. To put it another way, if gasoline costs $1/litre, it would cost 7.5 cents/minute to idle in a big SUV. NOTE: the higher value is probably too high -- a big 8-cylinder engine would probably idle more slowly than 1,000 rpm.

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