Firewall (engine)

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Firewall in a car illustrated by a red line
Cutaway drawing of the 1934 Citroen Traction Avant, showing the firewall
Firewall of an aircraft's engine nacelle

In automotive engineering, the firewall (American English) or bulkhead (British English) is the part of the automobile body (unibody or body-on-frame) that separates the engine compartment from the passenger compartment (driver and passengers). It is most commonly a separate component of the body or, in monocoque construction, a separate steel pressing, but may be continuous with the floorpan, or its edges may form part of the door pillars. The name originates from steam-powered vehicles, where the firewall separated the driver from the fire heating the boiler.

In aviation, a firewall on an aircraft isolates the engine(s) from other parts of the airframe. In single-engine aircraft, it is the part of the fuselage that separates the engine compartment from the cockpit. In most multi-engine propeller aircraft, the firewall typically divides the nacelle from the wing of the aircraft, or divides the nacelle into two zones.