Sketch and describe Heavy fuel oil system of a large bore internal combustion engine and give temperature and pressure at important points. Discuss atomization. penetration and swirl and their inter relationship.
Structured for DG Shipping MEO Class II examination scoring criteria.
Sketch and describe the heavy fuel oil system of a large-bore internal combustion engine and give temperature and pressure at important points. Discuss atomization, penetration and swirl and their inter-relationship.
[Sketch notes: The HFO system consists of: (1) the fuel service tank; (2) a settling tank; (3) the centrifugal separators (purifier/clarifier); (4) a fuel supply pump; (5) a fuel heater; (6) a viscosity controller; (7) a fuel filter; (8) the fuel injection pump; (9) the fuel injector; (10) a fuel return line; (11) a circulating pump.]
The heavy fuel oil system: The HFO is stored in the settling tank, where it is heated and settled to remove water and sludge. It is then purified by the centrifugal separators (purifier and clarifier) to remove water and solid contaminants. The purified fuel is supplied by the fuel supply pump to the fuel heater, where it is heated to the correct temperature (about 135-155 deg C for 380 cSt fuel) to reduce the viscosity to the injection value (about 12-14 cSt at the injector). The viscosity is controlled by a viscosity controller. The fuel passes through a filter to the fuel injection pump, which delivers it at high pressure (e.g. 600-900 bar) to the fuel injector, which injects it into the cylinder. The excess fuel returns to the system via the return line.
Atomization, penetration and swirl and their inter-relationship:
Atomization is the breaking up of the fuel jet into fine droplets by the injector nozzle. Good atomization (fine, uniform droplets) gives a large surface area for mixing with the air, promoting rapid and complete combustion. Atomization depends on the injection pressure, the nozzle design, and the fuel viscosity.
Penetration is the distance the fuel spray travels into the combustion chamber. Adequate penetration is needed to distribute the fuel throughout the air charge, but excessive penetration can cause the fuel to impinge on the liner or the piston crown (wall wetting), causing deposits and poor combustion.
Swirl is the rotary motion of the air charge in the cylinder, which carries the fuel droplets and mixes them with the air.
Inter-relationship: The three are inter-related. Good atomization gives fine droplets that are easily carried by the swirl and mixed with the air. The penetration must be matched to the combustion chamber size and the swirl: with strong swirl, the fuel is carried around the chamber, so less penetration is needed; with weak swirl, more penetration is needed to distribute the fuel. The atomization, penetration, and swirl together determine the quality of the air-fuel mixing and hence the completeness and efficiency of the combustion. They are optimised by the nozzle design, the injection pressure, and the combustion chamber/swirl design.