Sunday, November 23, 2008

Petrol, diesel, miles per gallon, litres per 100 km, energy, and emissions

Too many units! Too many things to measure! In Europe they talk about "the one litre car" (using 1 litre of fuel per 100 km). In Britain, drivers of the Prius are happy to do "more than 50 miles per gallon". In the USA, gallons are different. Then there's emissions (does it emit less than 100 grams of CO2 per km?) and finally there's energy measures (for example, the average British car consumes 80 kWh per 100 km).

And, while we're dealing with all these different units, the most annoying detail of all is that petrol is different from diesel. Diesel has bigger energy per litre (roughly 10% more), and it has bigger carbon emissions per litre too.

I've put together a graph that makes it possible to compare and convert some of these measures of vehicle performance.

Some memorable anchors on this diagram:
  1. A 90 mpg petrol vehicle is roughly equivalent (in energy and emissions) to a 100 mpg diesel car. Both use an energy of about 30 kWh per 100 km and have emissions of about 75 g per km. People have sometimes lampooned the Prius for consuming more fuel than a BMW. If the Prius is using petrol and the BMW is using diesel, then it's not fair to compare the numbers of litres used.
  2. A 'one litre car' delivers 282 mpg, and uses about 10 kWh per 100 km. This is the energy consumption, incidentally, of quite a few prototype electric cars (measured at the socket).
  3. My 'average UK car today' uses 80 kWh per 100km and emits 200 g per km. Europeans would call it an 8-litre car.

For more about energy consumption of eletric vehicles and hydrogen vehicles, see Sustainable Energy - without the hot air.
Small print: 'mpg' means miles per imperial gallon. 'g' means grams of carbon dioxide.
Energy contents (high heat values) and emissions were assumed to be:
Petrol: 34.7 MJ per litre; 2344 g per litre.
Diesel: 37.9 MJ per litre; 2682 g per litre.