Theoretical efficiency.

[Dr Jekyll]

Suppose I had a perfectly efficient engine which I wanted to run for an hour on one gallon of petrol. How many horsepower could I sustain for that hour? And how much different would it be for diesel?

[robbieduncan]

Petrol contains 46.4 MJ/kg of energy (diesel is 48) (https://en.wikipedia.org/wiki/Energy_density).

1 hp = 735.5 W (https://energyeducation.ca/encyclopedia/Horsepower)

46.4 MJ = 12 888.89 W hours (https://www.traditionaloven.com/tutorials/energy/convert-mega-joule-mj-to-watt-hour-h.html)

So this is 17.52 hp for 1 hour (unless any of these websites or my maths or basic understanding is wrong)

Edit: oops that's for 1kg. A gallon of petrol weighs 3.27Kg (4.54609 litres at 720g per litre)

So your gallon of petrol would only produce 5.36 hp for an hour

Edited by robbieduncan on Thursday 25th October 14:25

Edited by robbieduncan on Thursday 25th October 14:25

[mikeveal]

http://bfy.tw/KXTU
http://bfy.tw/KXTZ

[Dr Jekyll]

It was getting from stored energy to HP that was causing me problems.

5.6 HP does sound very low though. It implies a real engine would produce < 2HP for an hour, since I can drive at 50+ for an hour on one gallon I was sure it would be several times that.

[kambites]

robbieduncan said:

Edit: oops that's for 1kg. A gallon of petrol weighs 3.27Kg (4.54609 litres at 720g per litre)

So your gallon of petrol would only produce 5.36 hp for an hour

You need to multiple not divide! So 57.3 hp.

Edited by kambites on Thursday 25th October 14:57

[jet_noise]

I get:

kWh in a gallon 36.6
W in a hp 735 (as above)

So using up that whole gallon in an hour
hp= 36600/735= 49.8

[robbieduncan]

kambites said:

You need to multiple not divide! So 57.3 hp.

Edited by kambites on Thursday 25th October 14:57

Oops indeed! You are correct. This probably addresses the concern the OP had with the very low result too

[ATG]

If by "perfect efficiency" you mean the maximum theoretical efficiency and you fancy exercising your brain, have a look at the Carnot cycle.

[mikeveal]

robbieduncan said:

Petrol contains 46.4 MJ/kg of energy (diesel is 48) (https://en.wikipedia.org/wiki/Energy_density).

1 hp = 735.5 W (https://energyeducation.ca/encyclopedia/Horsepower)

46.4 MJ = 12 888.89 W hours (https://www.traditionaloven.com/tutorials/energy/convert-mega-joule-mj-to-watt-hour-h.html)

So this is 17.52 hp for 1 hour (unless any of these websites or my maths or basic understanding is wrong)

Edit: oops that's for 1kg. A gallon of petrol weighs 3.27Kg (4.54609 litres at 720g per litre)

So your gallon of petrol would only produce 5.36 hp for an hour

Edited by robbieduncan on Thursday 25th October 14:25

Edited by robbieduncan on Thursday 25th October 14:25

Joules = MJ/Kg * Kg = 46.4 * 3.27 = 151.728MJ

Knowing that
1J = 1W * 1s

W * seconds per hour = Watt Hours = Joules / seconds per hour.

1hp is 735.5W

Number of hp hours = Joules / (seconds per hour * Watts per hp)


So Number of hp hours = 151.728*10^6 / (3600 * 735.5) = 57.3