Vehicle emissions - Pt. 1

A few years ago, I bought a new car that told me the average mpg I was achieving on the latest tankful of petrol and I started to wonder about what gCO2/km this meant I was achieving at a specific fuel economy. A little bit of research revealed a fairly linear relationship between mpg and CO2/km that was true for almost any car for a given fuel type, and a bit more thinking made me realise that complete combustion of a fixed quantity of any given fuel would give off a fixed quantity of CO2 irrespective of the engine it was burnt in. (Obvious when you think about it, but it took me a while!) And that this was directly related to the weight of carbon in the fuel, which could be determined by looking at the molecular weight of its chemical composition and its density.

Which led me to develop a "magic number" for petrol which, divided by the mpg I was achieving gave me a good idea of my emissions on the latest tankful - "6600" for "petrol". So, if I achieved 40 mpg, my emissions were roughly 165 gCO2/km, 50 gave 132 gCO2/km and 60 gave 110 gCO2/km. So, if I wanted to break the magic 100 gCO2/km, I needed to achieve 66mpg (which I have done on occasion!). And the same is true for any petrol engined car / 95 octane fuel brand within a small margin of a few grammes / km. (Blended additives in commercial fuels and the temperature of the fuel when its volume was measured affects the calcs a little). And the equivalent magic figure for "diesel" is "7575", and for "LPG" is "3250", so average 76 mpg in a diesel engined car or 33mpg in a LPG car, and you are achieving better than 100 gCO2/km, and you can hold your head up whenever you pass a "hybrid".

Which brings me to the reason why I've started to ponder on this again. I was watching BBC Countryfile last night and there was a segment on biofuels and the EU Directive on blending fossil fuels with biofuels to reduce overall emissions and I started to wonder about what the magic number might be for "Biodiesel" and "Bioethanol" - just the "combustion carbon" ignoring any offsetting from its production methods, so that I could quickly calculate a compound "magic number" for any given blend.

Which also made me wonder if engines run better on different types of fuel - not just more economically, but actually better? For example, assuming that "petrol" and "bioethanol" have exactly the same magic number, ie their theoretical "combustion carbon" content is exactly the same, would an engine tuned for petrol give exactly the same CO2 emissions per power output as the same engine tuned for bioethanol, or would one make the engine more efficient overall than the other? (Not as daft as it sounds - the combustion characteristics of a fuel affect its power output - hence the interest in laser igniters as an alternative to traditional spark plugs - faster more uniform combustion = more power for the same carbon)

So I guess its back to scratching my head, prowling around for info, and punching buttons on my calculator (and hence Part 1 on the title) Watch out for Part 2 when I have some answers!