These nozzle inject the fuel directly into the cylinder, with a combution chamber formed in the top of the piston. The injection pump begins its pumping stroke, and fuel enters the injector via an inlet connection, and travels down the fuel passages to the fuel gallery which surrounds the needle valve. Fuel pressure rises forcing the needle valve to open at the pre-set pressure.
Fuel then passes through a small space called the sac to the injection hole or holes.
The fuel comes out of these holes in the form of a fine spray. As pumping pressure falls, the valve spring closes the needle valve and fuel injection is ended The valve then forms a seal against combustion pressure to prevent exhaust gas being forced back through the injector and into the fuel line. It is relatively simple design, and incorporates no ball or roller bearing, gears or highly stressed springs. The number of working parts remains the same irrespective of the number of engine cylinders the pump is required to serve. The pump is flange mounted to the engine.