bonnet
The hinged cover over the engine of a motor car.
bonnet: the hinged engine cover that lets you get at the works
The bonnet is the hinged panel that covers the engine bay of a motor car, providing access to the engine, radiator, battery, and ancillary components. On most modern vehicles, it opens forwards or upwards from the windscreen end of the vehicle. Its primary function is to protect the engine from weather and debris while allowing technicians and owners to service, repair, and inspect the powerplant without removing the entire front fascia.
Bonnets are typically fabricated from steel, aluminium, or composite materials depending on the vehicle's design philosophy and weight targets. Steel bonnets remain common in volume production because of their cost and repairability; damage can be hammered out or the panel replaced affordably. Aluminium bonnets save weight but require different tooling and repair techniques. High-performance or lightweight vehicles may use carbon fibre or fibreglass composites, though these are expensive and harder to repair in the field.
Design and fitment challenges
The bonnet must seal adequately against weather ingress while remaining light enough to open and close without excessive effort. Most bonnets incorporate a rubber gasket or foam seal around the perimeter to exclude water and dust. The hinge mechanism, typically two-point or four-point, must support the full weight without drift, and the strut or prop-rod that holds the bonnet open must be robust enough to prevent collapse if the vehicle is parked on a slope. Misalignment of the bonnet relative to the wings and grille is a common complaint after minor frontal damage or poor repair work.
The term 'bonnet' is primarily British and Commonwealth usage; American English uses 'hood'. The etymology is obscure but likely derives from the bonnet as a covering garment, with the automotive sense emerging early in the twentieth century. Some sources suggest it was first applied to the soft fabric covers fitted to early petrol engines before rigid metal panels became standard.
In service, bonnets are vulnerable to corrosion from road salt, impacts from debris on motorways, and hail damage. They are among the first panels to suffer stone chips and rust creep around the edges and hinge points. Replacement or respray is routine bodywork; hinge wear or latch failure is usually addressed as part of preventive maintenance on higher-mileage vehicles.