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.


Geoff said...

Would be neat to say how many kwh are in a litre/gallon of petrol & diesel and compare that to standard lead acid batteries/

Heather said...

Just to clarify - imperial gallons are British gallons? And how do they compare in volume to US gallons?

David MacKay FRS said...

"Would be neat to say how many kwh are in a litre/gallon of petrol & diesel and compare that to standard lead acid batteries"
- Yes, that is one of the many neat facts that's in my neat book. Please help yourself!

Yes, imperial gallons are British gallons, which are 1.2 of the New Empire's (US) gallons. US gallons are 5/6 of real gallons.

Unknown said...

Buying the car with the lowest official CO2 emission might not necessarily give you the lowest CO2 emission in practice.

My driving may not be typical, but it is almost exclusively motorway driving. My Diesel Clio (officially 115g/km) manages 56mpg at 70mph, whereas my wife's Diesel Golf (145g/km)manages 62mpg over the same journey and speed.

Andy in Hawick said...

@Geoff 9kWh/litre for petrol & 10 for diesel (roughly!)

Good old 'standard' lead-acid cells can only store about a tenth. A standard car battery of 80Ah at 12V only stores 1kWh and is typically 5-10 litres in size. The modern rechargeable technologies do significantly better but can't match the energy density of fossil fuels.

Martin Joinson said...

I've just come back from 2 weeks in france with my wife and 3 kids in our Nissan Leaf electric car. O fuel cost :) Haven't had a fossil car for over a year (just two electrics). Look at this and tell me you don't want one...
300 miles/charge under 6 secs 0-60 7 seats....

Joe Public said...

"Diesel has bigger energy per litre (roughly 10% more), and it has bigger carbon emissions per litre too."



Unknown said...

How does this analysis look when also considering the embodied energy & carbon for:
a) petrol vs diesel fuel (which requires less refining); and
b) petrol vs diesel engines(which requires more steel and less other stuff like spark plugs)