it is actually carbon content purely in all automotive fuels that = power. So the more carbon, the more power and the more that ends up as CO2 in the atmosphere (aka greenhouse gas), but that is why diesel gets better mileage (more carbon in the fuel than gasoline) combined with the slightly more efficient nature of diesels.
The rest is mostly contaminents and pollution. CO2 is not much of a pollutant (a global warming fanatic would chop my head off with that statement as they sell it as a "pollutant"). All it is the same stuff we are breathing out of our bodies. It does lead to real issues related to global warming though but that is its only real negative, or might have some inversion aspects but in an open air environment (like a normal city) it would have little negative issues. CO2 affects atmospheric issues not local so it does not produce smog in non-valley areas, that I am aware of.
There is cetane and octane, a lot of peope think this equals power but it does not, it is the speed at which the fuel burns. Higher cetane (or octane for gasoline vehicles) burns faster.
Most natural fuels have much higher cetane but lower overall carbon content than petroleum fuels. So this is why 100% biodiesel is about 7% less power than diesel even though biodiesel has much higher cetane...