http://www.ck12.org Chapter 2. Numbers, Not Adjectives
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 Stuff.
Figure 3.4:How British people travel to work, according to the 2001 census.
Notes and further reading
For the distance travelled per day, let’s use 50 km.This corresponds to 18000 km (11000 miles) per year. Roughly
half of the British population drive to work. The total amount of car travel in the UK is 686 billion passenger-km
per year, which corresponds to an “average distance travelled by car per British person” of 30 km per day. Source:
Department for Transport [5647rh]. As I said, I aim to estimate the consumption of a “typical moderately-affluent
person” – the consumption that many people aspire to. Some people don’t drive much. In this chapter, I want to
estimate the energy consumed by someone who chooses to drive, rather than depersonalize the answer by reporting
the UK average, which mixes together the drivers and non-drivers. If I said “the average use of energy for car driving
in the UK is 24 kWh/d per person,” I bet some people would misunderstand and say: “I’m a car driver so I guess I
use 24 kWh/d.”
...let’s use 33 miles per UK gallon.In the European language, this is 8.6 litres per 100 km. 33 miles per gallon was
the average for UK cars in 2005 [27jdc5]. Petrol cars have an average fuel consumption of 31 mpg; diesel cars, 39
mpg; new petrol cars (less than two years old), 32 mpg (Dept. for Transport, 2007). Honda, “the most fuel-efficient
auto company in America,” records that its fleet of new cars sold in 2005 has an average top-level fuel economy of
35 miles per UK gallon [28abpm].
Let’s guess a density of 0.8 kg per litre.Petrol’s density is 0.737. Diesel’s is 0.820–0.950 [nmn4l].
TABLE2.1:
calorific values
petrol 10 kWh per litre
diesel 11 kWh per litre...the actual value of 10 kWh per litre.ORNL [2hcgdh] provide the following calorific values: diesel: 10.7 kWh/l;