fuel injection system & Diagram in marine diesel engine

The fuel injection system is one of the most important parts of a marine diesel engine. A fuel injection system does the work of providing the right amount of fuel to the engine cylinder at the right moment. It is also extremely important that the fuel injected inside the engine enters the cylinder at the right combustion situation for the highest combustion efficiency. It is for this reason that there is a need of a measured fuel supply system which times and monitors the delivery of the fuel and oil in the combustion chamber. This timing device helps to have a perfect atomization of the fuel. The device is known as fuel injector.

Fuel injection is done with the help of cams and camshaft. The speed of the cam shaft is same as the engine speed in a two stroke engine and half the engine speed in a four stroke engine. The adjacent fuel injection system diagram gives a broad view to the reader regarding the fuel injection system. The faded sketch shows the engine in the background whilst the dark coloured schematic represents the fuel system. This helps the reader to understand the concept in conjunction with the given theory.

Types of Injection :
  • Jerk Pump
  • Common Rail
Jerk Injection : The most common system used on modern diesel engines. The fuel pressure is built up at a fuel pump in a few degrees of rotation of the cam operating the plunger. Fuel is directly delivered to spring loaded injectors which are hydraulically opened when the fuel pump has generated sufficient pressure.


Jerk fuel injection, marine disel engine
Jerk fuel injection
The pump consists of a cam operated single acting plunger of a fixed stroke. Helical springs are fitted to return the plunger on its down stroke and to maintain contact of follower on the cam.

A helix or scroll is machined on the plunger which is closely fitted inside a barrel.
As the plunger moves down the suction and spill ports on the barrel open and fuel flows into the barrel. As the plunger moves upwards a pressure is created immediately when the suction and spill ports are covered. This is the pressure at which the injector is set to open.
Injection continues until the point the helical groove on the plunger uncovers the spill port. The high pressure in the barrel is immediately connected to low pressure of fuel suction. The injector will close once the pressure falls below injector opening pressure.

The regulation of quantity of fuel is effected by groove on the plunger. The plunger is free to rotate in the barrel and rotation is achieved by a rack and pinion arrangement. Rack is fitted to the pump to engage with a pinion machined on the outside of the sleeve. As the plunger rotates the position of the helix relative to the port in the barrel will change thus controlling the amount of fuel delivered.

In some pumps a non return spring loaded discharge valve is fitted to ensure positive seating of fuel injector needle and reducing cavitation within pump.

Timing : Adjustment of injection timing is carried out by varying the relative height of plunger and suction/spill ports in the barrel. Lowering the plunger has the effect of retarding the injection. Raising the plunger advances the injection.
Fuel pump Timing Control, marine fuel injection system
Fuel pump Timing Control

Timing can be adjusted by moving the cam with respect to the shaft. Rotating the cam on camshaft in its ahead direction of rotation will cause advancing while rotating opposite with respect to ahead direction of rotation will cause retarding of injection.
Alternatively the fuel pump casing itself may be lowered or raised on its mounting to give the corresponding effect.

Advancing will cause early injection with result of
  • rise in maximum pressure.
  • improvement in specific fuel consumption.
  • exhaust temp will be less
  • improvement in power.
  • high thermal efficiency.

Retarding the cam gives late injection with
  • low peak pressure.
  • high exhaust temp
  • low thermal efficiency
  • possibility of afterburning.

Fuel pump Timing Control Diagram, marine fuel injection system

A-Pump spill closes (approx.-8o)
B-Fuel injector opens (approx. -4o)  Pressure approx 300 -350 bar.
C-Spill opens (approx. 12o)  Max pressure approx 600 bar
D-Fuel injector closes (approx. 16o)
G- Injection period (approx. 20o)

    Author marineGuru

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