Stephan Paukner :: syslog

I recently read^[1] that 10% of the energy an automobile consumes during its lifetime already occur during production, i.e., only 90% of energy are burnt fuel. This made me wonder, looking at the option of trading an old car for a new one with a more efficient engine that burns less fuel per distance: When does the saved fuel pay off the production of the second vehicle? (Note that we’re not talking about when the saved fuel pays off financially.)

Let’s make some assumptions for a first guess: The old car burns 0.07 ℓ/km (7 ℓ/100 km) of whatever fuel type. Assuming a lifetime of 8 years with 20,000 km/y, we drive 160,000 km and thus burn 11,200 ℓ of fuel. These are our 90%. Thus, 1,244 ℓ fuel-equivalents of energy are used for production; let’s take pessimistic 1,300 ℓ, because we don’t know whether the production of the new car is more or less energy-expensive than the old one anyway.

The two sides of the equation reflect the two possibilities: Either produce only one car and spend more fuel, or produce two cars, spend some fuel for the first and then less fuel for the second. When should we switch, and for how long should we drive the second car? It turns out that the “when should we switch” is irrelevant, we only have to consider that producing two cars is more expensive than just one, and we have to recover from that difference by saving fuel.

So, we spend 1,300 ℓ production plus 20,000 km/y × years used × 0.07 ℓ/km fuel for the old car. (^km⁄_y × y × ^ℓ⁄_km = ℓ.) Assume further that the new car only needs 0.06 ℓ/km, what yields 2 × 1300 + 20000 y × 0.06 correspondingly. The equation thus turns into

20000 y (0.07−0.06) = 1300 or y = ¹³⁰⁰⁄₂₀₀ = 6.5,

meaning that switching cars only pays off towards the end of the lifetime of the second car if we only gain 0.01 ℓ/km. If the new car saves twice as much, 0.02 ℓ/km, it takes only half that time, but still 3–4 years. And it’s only until after that time that our economy environment gains some benefit—but still only the saved fuel, not the total, of course.

As a conclusion, it does pay off, but only hardly. The best option is still not to drive (and fly!) that much. And don’t think that electric vehicles are that much better: They still need energy for production and for charging the battery, and the corresponding electric infrastructure has to be scaled up and supplied, demanding even more energy from... where? In addition, and in contrast to fuel, the battery is just an energy carrier, not an energy source, so something is always lost during conversion.

If you have the impression that our industrial society as it is today is doomed, you’re right, sadly. If you don’t have that impression, I recommend you to read this book, even if it’s already ten years old:

1. Richard Heinberg: “The Party’s Over: Oil, War and the Fate of Industrial Societies”