3 Cars

For our first chapter on consumption, let’s study that icon of modern civilization: the car with a lone person in it.

How much power does a regular car-user consume? Once we know the conversion rates, it’s simple arithmetic:

[\begin{matrix} {\text{energy\ used\ per\ day} = \frac{\text{distance\ travelled\ per\ day}}{\text{distance\ per\ unit\ of\ fuel}}} \ {\times \text{energy\ per\ unit\ of\ fuel}} \ \end{matrix}]

For the distance travelled per day, let’s use 50 km (30 miles). ¹

For the distance per unit of fuel, also known as the economy of the car, let’s use 33 miles per UK gallon ² (taken from an advertisement for a family car):

[\text{33\ miles\ per\ imperial\ gallon} \simeq \text{12\ km\ per\ litre.}]

(The symbol ≃ means “is approximately equal to.“)

What about the energy per unit of fuel (also called the calorific value or energy density)? Instead of looking it up, it’s fun to estimate this sort of quantity by a bit of lateral thinking. Automobile fuels (whether diesel or petrol) are all hydrocarbons; and hydrocarbons can also be found on our breakfast table, with the calorific value conveniently written on the side: roughly 8 kWh per kg (figure 3.2). Since we’ve estimated the economy of the car in miles per unit volume of fuel, we need to express the calorific value as an energy per unit volume. To turn our fuel’s “8 kWh per kg” (an energy per unit mass) into an energy per unit volume, we need to know the density of the fuel. What’s the density of butter? Well, butter just floats on water, as do fuel-spills, so its density must be a little less than water’s, which is 1 kg per litre. If we guess a density of 0.8 kg per litre ³, we obtain a calorific value of:

[\text{8\ kWh\ per\ kg\ ×\ 0.8\ kg\ per\ litre} \simeq \text{7\ kWh\ per\ litre.}]

Rather than willfully perpetuate an inaccurate estimate, let’s switch to the actual value, for petrol, of 10 kWh per litre. ⁴

[\begin{matrix} \ {= \frac{\text{distance\ travelled\ per\ day}}{\text{distance\ per\ unit\ of\ fuel}} \times \text{energy\ per\ unit\ of\ fuel}} \ \left. = \frac{\left. \text{50\ km}/\text{day} \right.}{\left. \text{12\ km}/\text{day} \right.} \times \text{10\ kWh}/\text{litre}\phantom{\text{AAA,,}} \right. \ {\simeq \left. \text{40\ kWh}/\text{day}\phantom{\text{AAAAAAAAAAAA}} \right.} \ \end{matrix}]

Congratulations! We’ve made our first estimate of consumption. I’ve displayed this estimate in the left-hand stack in figure 3.3. The red box’s height represents 40 kWh per day per person.

This is the estimate for a typical car-driver driving a typical car today. Later chapters will discuss the average consumption of all the people in Britain, taking into account the fact that not everyone drives. We’ll also discuss in Part II what the consumption could be, with the help of other technologies such as electric cars.

Why does the car deliver 33 miles per gallon? Where’s that energy going? Could we manufacture cars that do 3300 miles per gallon? If we are interested in trying to reduce cars’ consumption, we need to understand the physics behind cars’ consumption. These questions are answered in the accompanying technical chapter A, which provides a cartoon theory of cars’ consumption. I encourage you to read the technical chapters if formulae like (\frac{1}{2}mv^{2}) don’t give you medical problems.

Chapter 3’s conclusion: a typical car-driver uses about 40 kWh per day. Next we need to get the sustainable-production stack going, so we have something to compare this estimate with.

Queries

What about the energy-cost of producing the car’s fuel?

Good point. When I estimate the energy consumed by a particular activity, I tend to choose a fairly tight “boundary” around the activity. This choice makes the estimation easier, but I agree that it’s a good idea to try to estimate the full energy impact of an activity. It’s been estimated that making each unit of petrol requires an input of 1.4 units of oil and other primary fuels (Treloar et al., 2004).

What about the energy-cost of manufacturing the car?

Yes, that cost fell outside the boundary of this calculation too. We’ll talk about car-making in Chapter 15.

Notes and further reading

calorific values

petrol

10 kWh per litre

diesel

11 kWh per litre