Page 41 - Introduction to Marine Engineering

28 Diesel engines

Fuel injection Fuel circulation

Spring

Nozzle Needle
valve

Figure 2.16 Fuel injector

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.