A Simple Metric for Vehicle Efficiency

Vehicle driven by electric or fossil fuel energy to reach its range.

Simple Engineering Units

For BEVs the energy storage capacity is given in kWh and their range in miles. The first step is to express both together as a ratio, kWh/mile, i.e. how much energy is needed per mile travelled, or alternatively as its inverse, how many miles can be travelled with a given amount of energy, miles/kWh. For instance, a Tesla Model S advertised with both a 57.5kWh battery and a range of 235miles would yield 4.09 miles/kWh or 0.24 kWh/mile.

Further Simplification

Looking at the term kWh/mile tells me that there is still scope for simplification, in particular since the unit mile is not universal. Since one mile is equivalent to 1,609.340m and one kWh is equivalent to 3,600 kWs, i.e. 3,600 kJ, the above metric of kWh/mile can be converted such that 1 kWh/mile = 3,600kJ/1,609.340m = 2.237 kJ/m. The metric or unit chosen, kJ/m, is simply the energy spent in kJ for moving a vehicle by one meter. Conversely, one could conceive the inverse, m/kJ, or the distance in meters the energy expenditure of 1kJ would move the vehicle. Along that line, 1 mile/kWh would translate into 0.447m/kJ.

Energy Efficiency Examples

Having downloaded a couple dozen samples from the Electric Vehicle Database shows the order of magnitude of the specific motion energy.

Comparison of the specific motion energy for various EVs
Comparison of specific motion energy for various ICE vehicles

Ceterum censeo

The energetic gain obtained with the electrification of individually owned cars is laudable, but the electrification on its own is not as far reaching as a transformation of society would be. Ideally, society would not rely on individual car ownership, at least in urban areas. Rather, human-centred urban planning allows people to walk, cycle and use public means of transport. Fewer cars is the call, and smaller cars, together with their electrification, and possibly autonomous vehicles for shared transport.

Notes

  1. Anglo-Saxon countries express fuel efficiency in a very pragmatic manner: given an amount of fuel at hand (or better in a container), how far can the vehicle get with that amount of fuel? It is a very tangible metric and one only needs to be able to multiply in order to work out the distance for a given amount, e.g. 4 gallons in the tank on a 40 MPG car results in a 160 miles range. In contrast, European countries use litres-per-100km which is rather an Engineering approach: what is the amount of fuel needed to travel a known and well defined distance.
  2. The energy expenditure is proportional to the square of the speed, the vehicle mass, drag coefficient, rolling resistance, drive train losses as well as many other factors. The range will depend primarily on the speed the vehicle travelled when the test was carried out. However, the fuel efficiency will plateau at some speed, usually around 55–60 mph, and as long as the same speed profiles are used for both ICE based vehicles and BEVs, the absolute speed would not matter in a comparison.

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Peter Wurmsdobler

Peter Wurmsdobler

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Works on the technological foundations of autonomous vehicles at Five, UK. Interested in personal mobility, renewable energy and regenerative agriculture.