Describe Main Engine Fuel supply system functions and operation.
21 Jun '17, 06:55
For correct combustion to occur in the cylinder, the fuel must be atomized correctly so that the droplets are at the correct size, so they can penetrate the combustion space, yet heat up and burn completely before they come into contact with the piston crown or liner surface. If the viscosity is too low, the droplets will be too small and will begin to burn too quickly, effectively advancing the combustion timing. However combustion may not be complete, because each droplet of fuel may not have enough air surrounding it to supply the oxygen required. If viscosity is too high, then the atomised droplets will be too large, combustion will be delayed and the burning droplets will hit the piston crown and liner walls, cooling down and not burning completely.
Modern residual fuels vary in their viscosity (usually quoted in centistokes at 50 or 100°C) This is because they are blended from low grade bitumens with lighter fractions added to give the correct grade. Because this will vary from bunker port to bunker port, heating the fuel to one set temperature will not ensure that it is always at the correct viscosity. Fuel oil may be bunkered "on top". The fuels may blend together giving a fuel at a different viscosity from the two original specifications, or they may even separate into layers, each with a different viscosity.
Fuel oil supply and return system incorporating a heater and viscotherm. Both the heater and the viscotherm would be fitted with a by pass, and the heater steam supply could be controlled by hand if required. There will be more than one heater to allow for redundancy, and there would be two supply pumps and two booster pumps.
Fuel leaves the settling tank via a quick closing valve and passes through the change over valve and suction filter before entering the mixing column, where it mixes with fuel from the return line before being pumped through the heater by the booster pumps. After the heater the fuel viscosity is monitored by the viscotherm. An amplified output signal from the viscotherm operates the heater control valve. The fuel then enters the engine fuel rail via a set of backflushing filters. The fuel is continuously recirculated back down a return line to a mixing column, from where it enters the system once again; this ensures that the fuel in the fuel rail is kept at the correct temperature to maintain the desired viscosity. The system thermally insulated to prevent unnecessary heat loss, and normally fitted with trace heating.
21 Jun '17, 06:59