- #1

- 5

- 0

I once measured my cars efficiency (an old Renault 5).

I drove 100 km/h (28m/s) on a flat and level freeway, with no wind, and set the gears in neutral. It took the car about 30 seconds to slow down to 90 km/h (25m/s). The car weighs about 900kg.

So we have E

_{0}=0.5*m*v

^{2}= 353kJ and E

_{1}=281kJ. The car lost 72kJ in 30 seconds or 2.4kW

So it takes just

**2.4kW**to keep a small car cruising at 100kph on a freeway. The stated gasoline consumption of that car is about 1 liter/18 km at 90 kph so 1.3 ml/s of gasoline. Gasoline has ca 32MJ/l energy content, so 1.3ml/s is equivalent to 44kW.

The system efficiency of a car cruising on a flat freeway is therefore about 5%; I define system efficiency as the power that gets applied to the wheels divided by the gasoline energy content used per second.

Another way to calculate this:

I lost 3 m/s in 30 seconds, so my deceleration is 0.1m/s^2. The car weighs 900kg, so F= 90N. If I had applied 90N, the car would have stayed at speed. 90N * 28m/s = 2.5kW. This gives the same figure.

*Am I making some big mistake in my reasoning?*