Re: is the calorific value of a fuel always a const.

Dear User,

If under the "calorific value" you understand "specific heat of combustion"
then yes. With one remark - "fuel" is not a chemical substance. Its
composition varies quite significantly depending on the manufacturer , 
process of purification and other factors. Hence combustion heats of fuel
will
not be always the same, but would generally have a steady average over
many batches.

For references, i suggest searching the American Bureau of Standards.

Hope it helps.

A.G.E.