Page 36 - Introduction to Marine Engineering

Diesel engines 23

valve in the injector will lift at a pre-set pressure which ensures that the
fuel will atomise once it enters the cylinder.
There are two particular types of fuel pump in use, the valve-
controlled discharge type and the helix or helical edge pump.
Valve-controlled pumps are used on slow-speed two-stroke engines and
the helix type for all medium- and high-speed four-stroke engines.

Helix-type injector pump
The injector pump is operated by a cam which drives the plunger up
and down. The timing of the injection can be altered by raising or
lowering the pump plunger in relation to the cam. The pump has a
constant stroke and the amount of fuel delivered is regulated by rotating
the pump plunger which has a specially arranged helical groove cut into
it.
The fuel is supplied to the pump through ports or openings at B
(Figure 2.12). As the plunger moves down, fuel enters the cylinder. As
the plunger moves up, the ports at B are closed and the fuel is
pressurised and delivered to the injector nozzle at very high pressure.
When the edge of the helix at C uncovers the spill port D pressure is lost
and fuel delivery to the injector stops. A non-return valve on the
delivery side of the pump closes to stop fuel oil returning from the
injector. Fuel will again be drawn in on the plunger downstroke and the
process will be repeated.
The plunger may be rotated in the cylinder by a rack and pinion
arrangement on a sleeve which is keyed to the plunger. This will move
the edge C up or down to reduce or increase the amount of fuel pumped
into the cylinder. The rack is connected to the throttle control or
governor of the engine.
This type of pump, with minor variations, is used on many four-stroke
diesel engines.

Valve-controlled pump

In the variable injection timing (VIT) pump used in MAN B&W engines
the governor output shaft is the controlling parameter. Two linkages are
actuated by the regulating shaft of the governor.
The upper control linkage changes the injection timing by raising or
lowering the plunger in relation to the cam. The lower linkage rotates
the pump plunger and thus the helix in order to vary the pump output
(Figure 2.13).
In the Sulzer variable injection timing system the governor output is
connected to a suction valve and a spill valve. The closing of the pump
suction valve determines the beginning of injection. Operation of the