The Two Stroke
The fuel is delivered by the fuel pumps to the
injectors or fuel valves. For the fuel to burn completely at the correct time it
must be broken up into tiny droplets in a process known as
atomisation. These tiny droplets should penetrate far enough
into the combustion space so that they mix with the oxygen.
The temperature of the droplets rise rapidly as they absorb the
heat energy from the hot air in the cylinder, and they
ignite and burn before they can hit the relatively cold surface of
the liner and piston.
Fuel injectors achieve this by making use of a
spring loaded needle valve. The fuel under pressure from the fuel
pump is fed down the injector body to a chamber in the nozzle just
above where the needle valve is held hard against its seat by a
strong spring. As the fuel pump plunger rises in the barrel, pressure builds up
in the chamber, acting on the underside of the needle as shown.
When this force overcomes the downward force exerted by the
spring, the needle valve starts to open. The fuel now acts on the
seating area of the valve, and increases the lift.
As this happens fuel flows into the space under the needle and
is forced through the small holes in the nozzle where it emerges
as an "atomised spray".
At the end of delivery, the pressure drops sharply and the
spring closes the needle valve smartly.
Older loop scavenged engines may have a single injector mounted
centrally in the cylinder head. Because the exhaust valve is in the centre
of the cylinder head on modern uniflow scavenged engines the fuel valves
(2 or 3) are arranged around the periphery of the head.
The pressure at which the injector operates can be adjusted by
adjusting the loading on the spring. The pressure at which the injectors
operate vary depending on the engine, but can be as high as 540bar.
Some injectors have internal cooling passages in them extending into
the nozzle through which cooling water is circulated. This is to prevent
overheating and burning of the nozzle tip.
Injectors on modern
2 stroke crosshead engines do not have internal water cooling passages. They
are cooled by a combination of the intensive bore cooling in the cylinder
head being close to the valve pockets and by the fuel which is recirculated through the injector when the follower is on the base of the
cam or when the engine is stopped.
As well as cooling the injector, recirculating the
fuel when the engine is stopped keeps the fuel at the correct
viscosity for injection by preventing it from cooling down.
The animation opposite shows the principle on which one system
Fuel injectors must be kept in good condition to maintain optimum
efficiency, and to prevent conditions arising which could lead to damage
within the cylinder. Injectors should be changed in line with
manufacturers recommendations, overhauled and tested. Springs can weaken
with repeated operation leading to the injector opening at a lower
pressure than designed. The needle valve and seat can wear which together
with worn nozzle holes will lead to incorrect atomisation and dribbling.