TapThought experiment

In the discussions in the EU today, a limit of 130 g/km carbon dioxide emission is being suggested for cars. I was wondering exactly what that meant.

All carbon emitted by combustion must have gone in via the fuel tank, the oxygen came from the atmosphere. The other major emission, water, is the same—the hydrogen from the fuel and the oxygen from the air—but it is not regarded as a pollutant. Apart from some impurities there is no other input or output from an internal combustion engine.

30 MPG is a good yardstick for a petrol engined car’s performance. It is not a standard as some do very much better and others do a lot worse but it is a value that people understand. Converting to metric this comes out at 12.75km/l, or inverted as 78.4ml/km.

Petrol is made up of a mixture of hydrocarbons with a specific gravity between 0.6 and 0.8—let us say 0.7 for convenience. So we are using 55g of fuel per kilometre.

In the next part I am not so sure of the science but the atomic weight of carbon is 6 times that of hydrogen and hydrocarbons are composed of roughly 2 hydrogen atoms for each carbon. That suggests to me that by weight, petrol is 3/4 carbon and 1/4 hydrogen. Hence we are consuming about 41g of carbon per kilometre.

Oxygen has an atomic weight of 8 and they are 2:1 for carbon in CO2 so the total output is a nice round 150g of carbon dioxide per kilometre so it looks like we need to get nearer 35 MPG to meet the new standard. The calculation would be different for diesel partly due to the different composition and density of the fuel but also because some of the output is in the form of solid (particulate) carbon. I leave that as an exercise for the reader.

The important thing to realise is that size or car, size of engine, speed and fuzzy dice have nothing to do with it. The only thing important is fuel consumption. If your people tractor can do 150mph and still turn in 40MPG then fine … for the moment.

Comments are closed.

^ Top