Drivability Practice Exam

The key idea is how the injector seals fuel flow. A disc valve injector uses a small, flat circular disc that sits directly on a matching flat valve seat to shut off flow. This flat, round plate provides a simple, uniform seal, and its low mass gives quick opening and closing, which helps with precise timing and high-pressure operation. When the injector is energized, the disc is lifted away from the seat to allow fuel to spray, then the spring or return mechanism pushes it back to seal. Top feed and bottom feed describe how fuel is supplied to the injector, not the shape of the sealing element, so they don’t define the valve’s sealing method. A poppet nozzle injector, on the other hand, uses a poppet valve (a small valve with a stem) that seals against its seat, which is a different geometry from a flat disc.