Specific Fuel Oil Consumption (SFOC) is the amount of fuel consumed by the engine to produce one unit of power for one hour. It is usually expressed in grams per kilowatt-hour (g/kWh) and is an important measure of engine efficiency.

To calculate the SFOC on board a ship, the following procedure is followed:

1. Determine the Brake Power of the Engine

The first step is to calculate the Brake Power (BP) of the main engine.

• Indicator cards are taken from the engine cylinders to determine the Indicated Horse Power (IHP).
• The brake power is then obtained by multiplying the indicated power by the mechanical efficiency of the engine.

The mechanical efficiency is generally taken as approximately 0.95.

The relationship is:

$$BP=Indicated\:Horse\:Power\times Mechanical\:Efficiency\operatorname{}$$

2. Measure Fuel Consumption

The fuel oil consumption rate is determined by measuring the amount of fuel consumed over a fixed period, usually 2 to 3 hours, during steady operating conditions and around the same time the indicator cards are taken.

Fuel consumption is measured in:

• Kilograms per hour (kg/h)

3. Calculate SFOC

The Specific Fuel Oil Consumption is calculated by dividing the hourly fuel consumption by the brake power produced by the engine.

The formula is:

$$SFOC=\frac{Fuel\:Consumption\:per\:hour}{Brake\:Power}$$

The result is normally converted into:

• grams/kWh

4. Correction for Fuel Calorific Value

To compare the calculated SFOC with the manufacturer’s shop trial figures, correction may be required for differences in fuel quality.

• The calculated SFOC is:
  • Divided by the Lower Calorific Value (LCV) of the fuel used during shop trials
  • Multiplied by the LCV of the actual fuel being used on board
  Part (b)

  Factors Affecting SFOC and Its Impact on Efficiency and Operating Cost

  SFOC is a direct indication of the thermodynamic efficiency of the engine. A lower SFOC means the engine produces the required power while consuming less fuel.

  Factors Influencing SFOC

  1. Engine Condition

  Poor maintenance can increase SFOC due to:

  • Worn piston rings
  • Fuel injector problems
  • Incorrect valve timing
  • Fouled turbochargers
  • Poor compression

  Proper maintenance helps maintain low fuel consumption.

  2. Engine Load

  Diesel engines generally operate most efficiently near their designed load range.

  • Very low load operation increases SFOC.
  • Overloading may also reduce efficiency and increase fuel consumption.

  3. Fuel Quality

  Fuel with:

  • Low calorific value
  • High viscosity
  • Poor ignition quality

  can result in incomplete combustion and higher SFOC.

  4. Injection and Combustion Efficiency

  Correct fuel injection timing and proper atomization are essential for efficient combustion.

  Poor combustion leads to:

  • Increased fuel consumption
  • Higher exhaust temperatures
  • Smoke formation

  5. Air Supply and Turbocharger Performance

  Restricted air supply or inefficient turbocharger operation reduces combustion efficiency, thereby increasing SFOC.

  6. Hull and Propeller Condition

  A fouled hull or damaged propeller increases resistance, requiring higher engine power and fuel consumption for the same ship speed.

  Impact of SFOC on Efficiency and Operating Cost

  • Lower SFOC = Higher Efficiency
    • A low SFOC indicates better conversion of fuel energy into useful mechanical power.
  • Fuel Economy
    • Lower fuel consumption reduces operating expenses significantly, especially on long voyages.
  • Operational Cost Reduction
    • Since fuel cost is one of the largest expenses in ship operation, reducing SFOC improves the vessel’s profitability.
  • Environmental Benefits
    • Lower fuel consumption also reduces:
      • CO₂ emissions
      • NOₓ emissions
      • Overall environmental impact

    Importance in Shipbuilding Contracts

    The SFOC of the main engine is normally specified and guaranteed in shipbuilding contracts.

    • During shop trials, the actual SFOC is measured and compared with the guaranteed value.
    • If the measured SFOC exceeds the contractual value beyond the allowable limit (commonly about 3%), the engine manufacturer or shipbuilder may be required to compensate the ship owner.