- The two main basic parts are its nozzle and nozzle holder or body.
- The high-pressure fuel enters and travels down a passage in the body and then into a passage in the nozzle, ending finally in a chamber surrounding the needle valve.
- The needle valve is held closed on a mitred seat by an intermediate spindle and a spring in the injector body.
- The spring pressure, and hence the injector opening pressure, can be set by a compression nut which acts on the spring.
- The nozzle and injector body are manufactured as a matching pair and are accurately ground to give a good oil seal.
- The two are joined by a nozzle nut.
- The needle valve will open when the fuel pressure acting on the needle valve tapered face exerts a sufficient force to overcome the spring compression.
- The fuel then flows into a lower chamber and is forced out through a series of tiny holes.
- The small holes are sized and arranged to atomise, or break into tiny drops, all of the fuel oil, which will then readily burn.
- Once the injector pump or timing valve cuts off the high pressure fuel supply the needle valve will shut quickly under the spring compression force.
- All slow-speed two-stroke engines and many medium-speed four-stroke engines are now operated almost continuously on heavy fuel.
- A fuel circulating system is therefore necessary and this is usually arranged within the fuel injector. During injection the high-pressure fuel will open the circulation valve for injection to take place.
- When the engine is stopped the fuel booster pump supplies fuel which the circulation valve directs around the injector body.
- Older engine designs may have fuel injectors which are circulated with cooling water.
A simple Diagram of Fuel Injector
ref: introduction to marine engineering by D A Talyor