DIESEL WORLD MAGAZINE
BY JIM BIGLEY
COWL INDUCTION HOODS
Can you explain how a hood’s cowl induction, not directly connected to air intake, can affect the air pressure inside engine bay, which according to what I’ve read could affect engine cooling? I’ve also heard that cowl induction could slow the flow of air entering through the grille area. I would think that if the air enters through the grille and exits through the hood and/or through the open bottom of the engine bay it would not cause heating issues, compared to just exiting through the bottom of the engine bay without cowl induction?
James North Via Email
Good question. Here’s a theory. Airflow through the cooling stack (ATF cooler, A/C condenser, intercooler, radiator) is a result of a pressure differential—higher pressure ahead of the stack and lower pressure behind, either due to road speed or fan operation. Also, at speed, there’s a high-pressure area located at the lower edge of the windshield due the horizontal airflow across the hood stacking up against the windshield. This is why cabin ventilation inlet ducting exists at that location— it’s forced-air ventilation. This area of higher pressure will force air into the engine compartment via a cowl induction hood unless the cowl is sealed and is used to supply air only to the engine’s intake system.
So, any forced air entering the engine compartment, other than the cooling stack, can and will change the pressure differential and could likely reduce the amount of airflow through the airflow-resistant cooling stack. Because of the large areas surrounding the engine that allow for hot under-hood air to exit, no hot underhood air exits through the cowl hood because of the pressure differential that exists when the vehicle is moving at normal road speeds. Without the cooling stack (again, ATF cooler, A/C condenser, intercooler, radiator), airflow through the grille would overwhelm the cowl induction hood in terms of air forced into the engine compartment, but the stack makes it a lot harder for air to flow through even at highway speeds… whereas open-cowl induction is pretty much a straight shot into the engine bay.
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