# How to convert car power into energy (MJ) used per km?

I am currently attempting to do an energy audit regarding how much energy my car uses per kilometer. I am not sure whether my approach to this is right, and it feels like I have made too many simplifications along the way.

I first started my calculation by using a power torque curve for my Hyundai Elantra. I then took the horsepower that corresponded to my estimated average rpm for the trip, in order to find the average power (kW).

Using an average rpm of $$n = 2500$$ for a 37.6 km trip that takes 50 minutes, I determined the energy used accordingly:

\begin {align*} P(n) &= P(2500) \approx 55 \phantom{x}hp \\ &= 40.45 \phantom{x} kW \end{align*}

From there, I took the duration of the trip (in seconds) and multiplied it to obtain the energy consumed during the trip. After doing this for several destinations, I then took the mean to get the average energy consumption per km.

\begin {align*} \frac{E}{\Delta d} &= \frac{P\Delta t}{\Delta d} \\ &= \frac{(40.45)(3000)}{37} \\ &= 4.9 \phantom{x} \frac{MJ}{km} \end {align*}

• The curves you are using only apply during wide open throttle. Sep 22 at 22:43

Tip: 'KW' is a kelvin-watt (which I don't think is useful for anything). Use 'kW' for kilowatt. (Capitals matter!)

You could really simplify the problem by using the energy density of the fuel. If you look it up you should find that it's about 10 kWh/kg of petrol or diesel. 10 kWh/kg × 3600 s/h = 36000 kWs/kg = 36000 kJ/kg = 36 MJ/kg.

My diesel VW does about 5.5 L/100 km which is 0.055 L/km. Assuming diesel has a density of 1 kg/L (it doesn't) we can use 0.055 kg/km. That gives you 36 MJ/kg × 0.055 kg/km = 1.98 MJ/km.

I think this will lead to an overestimation because you are assuming the load is 100% during the whole time (I would expect it to be somewhere 40 and 50% on average).

Assuming 35 MJ/L, your calculation as it is would yield about 15 L per 100 km, which is too much.

Given the average ratio of load, it would yield something between 7.5 and 6 lit per 100 km which -IMHO - is far more reasonable.

However your approach needs to assume an average for the rpm and - IMHO - the average load, which are difficult to estimate, and also very easy to change.

• this, although I expect that general throttle position for a car is probably closer to 10-15%. Sep 22 at 22:42

Can you not simply take the miles per gallon value times the Heating Value of the fuel?

• that would give the average for certain. Sep 22 at 22:43