- #1

mastermechanic

- 108

- 15

I was looking at the technical specs of Porsche 911. It has 427 kW power and 7.5 liters/100 km fuel consumption on highway. I made a quick calculation but I got strange result. Please tell me why ;

First I assumed that the car has 90 km/h constant speed on highway so it will take 1.11 hours to travel 100 km.

$$ 1.11 x 3600sec= 4000 sec$$

$$ 427kW.4.10^3 = 1708 MJ$$

$$ Q= m.H$$ where the m is mass of fuel and H is heating value

Gasoline's heating value = 45 MJ/kg

$$1708 MJ = m. 45\frac{MJ}{kg}$$

$$ m= 38 kg$$

and density of gasoline= $$680\frac{kg}{m^3}$$ =>$$\frac{38}{680} = 0.055 m^3$$

$$ 0.055 m^3 . (\frac{1000L}{1 m^3})= 55 L$$ and it's far away from 7.5 L

Edit: I guess we should take into account 25% efficiency but it implies that tech. specs don't show the real power of the car, it shows theoretical power? My mind has blown :D

First I assumed that the car has 90 km/h constant speed on highway so it will take 1.11 hours to travel 100 km.

$$ 1.11 x 3600sec= 4000 sec$$

$$ 427kW.4.10^3 = 1708 MJ$$

$$ Q= m.H$$ where the m is mass of fuel and H is heating value

Gasoline's heating value = 45 MJ/kg

$$1708 MJ = m. 45\frac{MJ}{kg}$$

$$ m= 38 kg$$

and density of gasoline= $$680\frac{kg}{m^3}$$ =>$$\frac{38}{680} = 0.055 m^3$$

$$ 0.055 m^3 . (\frac{1000L}{1 m^3})= 55 L$$ and it's far away from 7.5 L

Edit: I guess we should take into account 25% efficiency but it implies that tech. specs don't show the real power of the car, it shows theoretical power? My mind has blown :D

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