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.”E
29... 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 [ ].
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 [ ].
29Let’s guess a density of 0.8 kg per litre. Petrol’s density is 0.737. Diesel’s is
0.820–0.950 [ ].
–... the actual value of 10 kWh per litre. ORNL [ ] provide the following
calorific values: diesel: 10.7 kWh/l; jet fuel: 10.4 kWh/l; petrol: 9.7 kWh/l.
When looking up calorific values, you’ll find “gross calorific value” and
“net calorific value” listed (also known as “high heat value” and “low heat
value”). These differ by only 6% for motor fuels, so it’s not crucial to distinguish
them here, but let me explain anyway. The gross calorific value is the
actual chemical energy released when the fuel is burned. One of the products
of combustion is water, and in most engines and power stations, part
of the energy goes into vaporizing this water. The net calorific value measures
how much energy is left over assuming this energy of vaporization is
discarded and wasted.
When we ask “how much energy does my lifestyle consume?” the gross
calorific value is the right quantity to use. The net calorific value, on the
other hand, is of interest to a power station engineer, who needs to decide
which fuel to burn in his power station. Throughout this book I’ve tried to
use gross calorific values.
A final note for party-pooping pedants who say “butter is not a hydrocarbon”:
OK, butter is not a pure hydrocarbon; but it’s a good approximation to
say that the main component of butter is long hydrocarbon chains, just like
petrol. The proof of the pudding is, this approximation got us within 30%
of the correct answer. Welcome to guerrilla physics.