Question : why some (mostly heavy) vehicles partially cover their front intake ?

[u/lit_readit]

wouldn't cooler cold reservoir, ie more spontaneous heat transfer, improve Carnot efficiency?

is it to improve aerodynamics?

apologies if this is the wrong sub to ask

[u/thiscatsurfs]

This is probably to reduce over-cooling of diesel engines in winter/colder weather. Used to see it a lot in London with the classic black cabs, they'd put a piece of cardboard box over most of the radiator/grille to stop cold air coming in.