Dont forget most diesel engines are DI - direct injection so the only place where they 'mix' together with air is in the cylnder itself.
As RedPhil says, the colder the air, the more dense, so the mass drawn into the cylinders, is higher on cold days.
Hotter days less air mass = less power.
More throttle input to compensate= more fuel burnt.
Last edited by E_S_P; 16th July 2013 at 13:38.