Fuel supply to the engine
The fuel oil from the storage tank is transferred and supplied to the engine by use of supply and booster pumps. During the transfer process, the fuel is treated (as explained above) before it reaches the main engine fuel pump and later, the injectors.
The fuel oil supply goes through a three-way change-over valve, coarse filter, an automatic fine filter, flow meter, mixing tank, booster pump, heat exchanger, viscosity control system, and other specially designed components.
The fuel oil in the service tank will flow into the coarse and booster pump through the three-way change-over valve. After filtering by the automatic fine filter, the oil flows into the mixing tank and is then heated by the heat exchanger. After that it goes to the viscosity controller which controls the steam inlet for heating the oil in the heat exchanger, to meet the required viscosity standard.
The fuel oil then reaches the final stage where it will be pumped into the diesel engine injectors via engine fuel injection pumps at high pressure.
The pressure of the fuel supplied to the injector should be sufficient to lift the fuel valve. The fuel injection pressure varies between 200 to 400 bar depending on the type of engine.
A cam activates the fuel injection pump and the quantity of the fuel supplied varies depending on the engine load. Suction and spill valves or ports control the metering of fuel.
In marine large two-stroke diesel engines, two types of fuel injection pumps are used. They are valve-controlled and port-controlled fuel pumps.
The fuel pump comprises a suction valve, spill valve, pump plunger, pushrods, and levers. The plunger stroke is constant and is raised and lowered by the follower. The follower is pressed on the cam by the plunger spring. The pushrods operate the suction and spill valves. These rods are operated from the plunger drive-through levers.
A set of control eccentric acts as a pivot point for these levers. When the plunger is raised upwards, the spill valve is closed. But, the push rod holds its suction valve off its seat. As the plunger continues to rise, the suction valve returns to its seat as its pushrod moves down.
Once the suction valve is seated compression begins as the plunger continues to move upwards. Fuel is pushed into the cylinder, through the high-pressure fuel delivery pipe and fuel injector. Whilst the plunger is moving upwards and injecting fuel, the spill valve push rod also moves upward.
Injection stops as soon as the spill valve is lifted off its seat by its pushrod. On the downward stroke of the plunger, fuel is drawn from the suction space. The suction valve is lifted by the suction effect of the plunger and the fuel transfer pressure in the suction space. In the lowest part of the plunger’s downward stroke, the suction valve is lifted by its pushrod.
The actuating levers of the suction and spill valves’ pushrods are pivoted at an eccentric shaft. The fuel injection time is varied with the rotation of these eccentric shafts. Under normal operation, the suction valve timing and the start of injection are fixed.
The spill valve timing is varied to control the quantity of fuel delivered. Varying the suction valve timing controls the beginning of fuel injection to compensate for the ignition quality of fuel without adjusting the fuel pump cam.
Fuel enters the pump from the supply system through the inlet connection and floods the fuel chamber surrounding the barrel. When the plunger is at the bottom of its stroke, the fuel flows through the barrel ports. It fills the space above the plunger, the vertical slot cut in the plunger, and the cutaway area below the plunger helix (scroll).
As the plunger moves upward, the barrel ports are covered and compression begins. As the plunger continues to move upward, fuel is discharged through the lifted delivery valve into the high-pressure pipe. Fuel delivery stops when the plunger helix uncovers the spill port. The delivery valve returns to its seat and closes.
The quantity of fuel delivered is regulated by the vertical length of the helix which is in line with the suction port. This setting is altered as required by rotating the plunger. A rack is fitted to the pump to engage with a pinion machined on the outside of a sleeve.
The sleeve fits over the plunger and has slots engaging with keys. In this way, the movements of the rack rotate the plunger. The relative angular position of the cam peak to the crankshaft controls the injection timing. It is adjusted by radially moving the cam with respect to the crankshaft. Adjustment is also done by raising or lowering the barrel with respect to the plunger.
The function of the fuel injection system is to inject the correct quantity of fuel into the combustion chamber at the right time within the required pressure to attain the desired atomization.
This measured fuel supply and delivery timing are attained by specialized fuel supply pumps which are usually a jerk type pump or common rail system and they are controlled by the governor and VIT to control the variations that arise on engine load.
The delivery of fuel from the supply pump is controlled by the movement of the cam in the camshaft which rotates at engine speed for a two-stroke engine and at half engine speed for a four-stroke.
In a common rail system, all the cylinders or units are connected to the rail and the fuel pressure is accumulated at the same. The supplied fuel pressure is thus provided through the rail. A similar type of common rail system is also there for the servo oil system for opening the exhaust valves.
A device that receives pressurized fuel as a liquid and sprays it into an engine cylinder as a fine mist. It consists of a nozzle and nozzle holder or a body. The nozzle has a series of small holes around its tip. The fuel is sprayed into the engine cylinder through it.
A typical fuel injector is shown in the above Figure. 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 that acts on the spring. The nozzle and injector body is manufactured as a matching pair and is accurately ground to give a good oil seal. The two are joined by a nozzle nut.