I am currently adding an engine to Revell's 1/72-scale M1A2 Abrams. Viewed from the rear of the tank, the left-hand cooling exhaust duct has a triangular item fitted on top, with a large hose attached to the front. Does anyone know what this particular item is? For some reason, it is not repeated on the right-hand side.

That is for the air intake.
It has a metal prefilter (that absorbs all the sticks, rocks etc) over the top of 3 metal air filters (just like in your auto). Wouldn't want a bunch of stuff getting into the turbine. That would be messy!!

It is actually called the pre-cleaner. It works as joepanzer mentioned but actually catches the large particles such as dust particles, grains of sand, and similar items to prevent early clogging of the V-Packs (air cleaners), which lie under the pre-cleaner. In an extremely dusty environment, the pre-cleaner and v-packs will have to be removed and cleaned often, pre-cleaner by banging it gently against the hull to remove contaminates and the v-packs by being blown out with high pressure air via a cleaning "wand" and hose that is connected to another tank's engine with a special fitting.

John