Why Do Some Cars Have Vents Behind The Rear Bumper?

Almost every car has a feature that you may not even know is there. Called air extractors — or cabin exhausters, pressure relief valves, or cabin pressure relief vents — these flaps are usually located under your car's bumper cover, under the trunk, or behind a rear panel, which is not the most visible placement. Despite being so elusive, these air extractors are actually a very important component of your car.

Air extractors are elastomer flaps inside a plastic frame that open when the pressure inside your cabin is higher than that outside the vehicle. This pressure often happens when a door is closed or when the heater or air conditioning is in use. By providing more air circulation through and out the rear of the car, drivers and their passengers can feel comfortable inside, even with the windows up (which is among the tips recommended for fuel efficiency). The release of air pressure can also protect your car's windows, which can sometimes feel strain at the top edges and start to push outward if there's too much pressure. 

The primary task of air extractors is to reduce cabin pressure when a door is closed or the HVAC system is running. By allowing air to flow out the back, the air extractors keep passengers comfortable and reduce the force required to close the doors. The pressure that builds up when closing a door is referred to as "air bind," which can account for 40% to 60% of the effort required to close doors. Too much air pressure, or a pressure spike, produces torque on the door, which slows down the door's movement when passengers attempt to close it (but that doesn't mean you should slam your car door). The issue at hand for carmakers is determining how to reduce this air pressure, which is increased due to door seals that don't allow noise and weather to enter the cabin. 

According to a study by Ford, the Institute for Dynamic Systems and Controls, and Lawrence Technological University, "air extractors are necessary because vehicle cabins today are practically sealed airtight to prevent exterior noises from entering the passenger cabin and lowering the perceived quality of the vehicle." It's quite interesting that something most drivers didn't even know was there has such a significant impact on cabin pressure.