Fuel Oil And Fuel Injection Page 11-13 Flashcards
How is high viscosity fuel oil treated between the storage tank and main engine?
The fuel is stored and heated in the settling tank, allowing gravity to separate sediment and water, which is drained off as sludge every watch. It is then drawn to a centrifugal separator and filtered in duplexed filters, which are changed and cleaned every watch to remove further particulate. Within the purifier, the fuel is heated to around 90°C to aid separation of particulate, and water and sediment are removed as the oil passes through the unit. After purification, the fuel is stored in the service tank at a high temperature, allowing further settling and removal of water and particulate as sludge. The fuel is drawn to the booster unit via fine hot filters, circulated through heaters and a viscometer to maintain correct viscosity for optimum injection and combustion, and finally pressurized to around 7bar into the main fuel rail, which is trace heated with steam to maintain viscosity.
What is the purpose of treating high viscosity fuel oil?
To ensure a clean and reliable fuel supply, to prevent damage, and maintain efficiency of combustion.
How is high viscosity fuel oil treated prior to entering the main engine?
Fuel is heated to the correct temperature to reduce viscosity. It is purified to remove any remaining water or small particles, then filtered to remove solids or finer particles. The fuel passes to the fuel booster unit through fine hot filters, where it is heated in banks of heaters controlled by a viscometer set to the suitable viscosity for optimum injection, then onto the fuel booster pumps where the pressure is increased to around 7bar before the main fuel rail.
What changes are required in fuel preparation when switching to fuel of lower density and viscosity?
Purifiers require a smaller gravity disc to maintain the water-oil interface, finer filters must be fitted for effective particle separation, and the visco-therm controller must be adjusted to prevent overheating the fuel. Additionally, the engine fuel injection timing must be retarded to compensate for the less viscous fuel.
Describe the test which is carried out on a fuel injector after overhaul.
- Mount the fuel injector into the test rig and connect to the test oil supply. Prime the injector until oil returns down the leak line.
- Do not allow hands in the test compartment during testing. Use the test pump to deliver several quick strokes; the injector should open with a high-pitch chatter. Check that the pressure does not fall quickly after opening and that the opening pressure is within manufacturer tolerance.
- Allow pressure to return to zero, then slowly build pressure just below the opening pressure and check for no dripping or leaking from the injector nozzle.
- Place a clean piece of paper underneath the injector nozzle and observe the spray pattern.
Sketch a section through the nozzle of a fuel injector.
Components include: Pressure pin, Nozzle body, Nozzle needle, Inlet passage, Pressure chamber, Spray hole, Blind hole.
State why the simultaneous injection of fuel oil and starting air into a main engine cylinder is undesirable.
To stop explosion at air start manifold.
Explain how simultaneous injection of fuel oil and starting air is prevented.
The fuel supply index is shut off via an interlocking valve while air is admitted. When manually starting, an air solenoid valve blocks the fuel line until the air start button is released.
What are the effects of early injection in a two-stroke diesel engine?
- Severe stress loading of piston rod, con-rod, cross head bearing, and bottom end bearings.
- Very high unit temperatures, cooling water, and exhaust gas.
- Vibration and irregular engine speed.
What are the effects of late injection in a two-stroke diesel engine?
- Uneven thermal distribution throughout piston liner, causing micro-seizure and liner cracking.
- Heavy carbon deposits on piston and liner due to improper combustion.
- Fuel oil wash down due to incomplete combustion.
Sketch an ‘out of phase’ for a two-stroke diesel engine and indicate the point of fuel injection.
Sketch should illustrate the out of phase timing with the point of fuel injection clearly marked.
What is the function of the interlocking valve in the fuel supply index?
The fuel supply index is shut off via an interlocking valve while air is admitted.
What happens when manually starting an air solenoid valve?
It blocks the fuel line until the air start button is released.
What are the effects of early injection in a two-stroke diesel engine?
Severe stress loading of piston rod, con-rod, cross head bearing, and bottom end bearings; very high unit temperatures, cooling water, and exhaust gas; vibration and irregular engine speed.
What are the effects of late injection in a two-stroke diesel engine?
Uneven thermal distribution throughout piston liner, causing micro-seizure and liner cracking; heavy carbon deposits on piston and liner due to improper combustion; fuel oil wash down due to incomplete combustion.
What is the ignition point of fuel in a two-stroke diesel engine?
Ignition of fuel occurs at TDC (Top Dead Center).
What is the procedure to change the fuel supply from high viscosity fuel to low viscosity fuel?
- Check the status of the diesel tank (quantity and quality).
- Gradually reduce the heat supply to the HFO heater.
- Slowly close the HFO tank valve and open the diesel oil tank valve, leaving the return line to the HFO tank open.
- Monitor the flow situation with respect to temperature and pressure.
- Monitor the visco-therm if fitted.
- After a suitable period for circulation of diesel oil, remove all trace heating.
- Change over the recirculating line from the HFO tank to the diesel oil tank.
- Monitor the condition of the engine (temps, pressures, sound, etc.).
