BOTH Supply & Exhaust
is best, but if only 1 fan, Exhaust
is essential. Install the Exhaust fan High, and the Supply air duct Low.
Air is required in engine
compartments, for two separate purposes:
- engine combustion
- engine room cooling
- Engine compartment ventilation is NOT intended to vent noxious products of combustion, which should be exhausted through the engine exhaust.
An ideal system would provide fully mechanical ventilation - both supply and exhaust air fans.
As implied by Brett, the supply fan might be sized about 10% lower than the exhaust fan (less combustion air), to provide a slight negative pressure in the engine room, thereby preventing fumes entering the occupied spaces.
If engine compartment ventilation is provided by a single fan - it should be an exhaust (extractor) fa,, located in the upper 1/3 of the compartment.
For every 10° above 77° F, an engine may lose 1% to 1.5% of its power output. So the engine room must be supplied with cool air, supplied as low down as possible (lower 1/3 of compartment), and exhausted out from the upper (1/3) part of the engine compartment.
Formulas for engine room air required for engine combustion, and for cooling
can be found in most engine installation manuals
. Hot climates may require as much as twice the ventilating (cooling) capacity as do temperate climates.