A higher Cetane value makes the diesel fuel combust more readily when mixed with air and compressed. You are likely to get a more complete burn with a high Cetane value fuel because ignitions starts earlier in the compression cycle. There is always the risk of the molecules at the higher end of the chain length range not completely combusting if ignition starts later in the compression cycle due to a low Cetane rating, so it makes sense to up the Cetane level – to a point. You don’t want the fuel so volatile that it’s like petrol. Cetane rating of 56-58 is considered optimum.
Normal Diesel is a mixture of Hydrocarbons varying from 8 to 21 carbon atoms in length. I suspect that V-power (with a proportion made from gas to liquid process for higher purity) ends up with a lower average hydrocarbon chain length because creation of longer carbon chains than the starting material is precise, hence the lower density compared to regular diesel.
In effect, V-power has less energy per litre to liberate under combustion (as does Petrol – it contains about 12% less chemical energy than Diesel), but the relative purity of the stuff as well as the added lubricity components is supposed to make the most of what is there under combustion and so level the playing field. If my car ends up with more mpg when I switch to normal Shell then I would definitely say that is not the case.
Difference between branded fuels and supermarket fuels isn’t the fuel itself, but the additive packs – the stuff like the added detergents which help keep your engine clean, some other additives (mainly alkyl nitrates) which lower the temperature and pressure conditions required for combustion (by increasing Cetane number). You tend to get a better set of additives in the branded forecourts.
