From the Workshop: Diesel Fuel Injectors
A correct-functioning mechanical injector should atomise fuel with the correct spray pattern, open at the correct pressure, and not dribble fuel. The operation of typical injector is probably simpler than you imagine. High pressure fuel from the injection pump enters the inlet, and travels down a drilling in the body of the injector to the needle valve, which is held in the closed position by the valve spring. As the pressure rises in the void around the needle valve, force is exerted on the tapered shoulder of the needle. When sufficient pressure has developed to counteract the spring, the needle lifts upwards, opening the nozzle orifice and ejecting an atomised spray of fuel into the combustion chamber.
When the injection pump ceases pumping, the pressure falls and the needle valve shuts against its seat in the nozzle. A small amount of fuel leaks up past the needle valve, lubricating the needle and cooling the injector. This ‘leak-off’ fuel leaves the injector through a low-pressure leakoff line, where it is either returned to the fuel tank or the injection pump.
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