What is specific fuel consumption?

Short Answer:

Specific fuel consumption (SFC) is the measure of how efficiently an engine or power plant uses fuel to produce useful output power. It is defined as the amount of fuel consumed per unit of power output per hour. The unit of SFC is kg/kWh for engines or kg/hp·h for smaller machines.

A lower specific fuel consumption indicates better fuel efficiency because less fuel is used to produce the same amount of power. It is a key performance parameter used to compare the economy and efficiency of different engines and turbines in mechanical and thermal systems.

Detailed Explanation :

Specific Fuel Consumption

Specific fuel consumption (SFC) is one of the most important indicators of the performance and efficiency of engines, turbines, and power plants. It measures how much fuel is required to produce a certain amount of useful power or energy. In simple terms, it shows the fuel efficiency of a power-producing system.

SFC helps engineers assess how effectively a system converts the chemical energy of fuel into mechanical or electrical energy. It is widely used in the study of internal combustion engines, steam turbines, gas turbines, and aircraft engines.

The general formula for specific fuel consumption is:

The unit is kg/kWh (kilogram of fuel per kilowatt-hour of energy produced).

For engines rated in horsepower (hp), the unit can also be kg/hp·h.

A lower SFC value means that the machine consumes less fuel for the same power output, indicating higher efficiency.

Concept of Specific Fuel Consumption

Every power-producing system — whether it is an internal combustion engine, a steam turbine, or a gas turbine — converts the energy stored in fuel into useful mechanical or electrical energy. During this process, part of the energy is lost due to heat dissipation, friction, and incomplete combustion.

The specific fuel consumption gives a direct indication of how efficiently the system uses the available fuel energy. It compares fuel input to power output and is therefore an important factor in the operational cost and environmental performance of power plants and engines.

If the specific fuel consumption is low, the system is economical, as less fuel is required for the same energy output. Conversely, a high SFC means poor efficiency and higher fuel costs.

Types of Specific Fuel Consumption

There are mainly two types of specific fuel consumption used in engineering analysis, depending on whether the system produces mechanical or electrical power:

  1. Brake Specific Fuel Consumption (BSFC):
  • It is used for engines where the useful power is measured at the output shaft (brake power).
  • It indicates the fuel efficiency of an engine in converting fuel into mechanical power at the crankshaft.
  • Formula:
  • Unit: kg/kWh
  • Lower BSFC values indicate better engine performance.

For example, a modern diesel engine may have a BSFC of 0.2 to 0.25 kg/kWh, whereas a petrol engine may have 0.3 to 0.4 kg/kWh.

  1. Thermal Specific Fuel Consumption (TSFC):
  • Commonly used in aircraft and gas turbines, it measures the fuel flow rate relative to thrust produced.
  • Formula:
  • Unit: kg/N·s
  • It shows how much fuel is required to produce one unit of thrust per second.

Relation Between SFC and Efficiency

Specific fuel consumption is inversely proportional to thermal efficiency. When the efficiency of an engine or turbine increases, its specific fuel consumption decreases.

This relationship can be expressed as:

That means a highly efficient power plant or engine consumes less fuel to generate the same power, resulting in a lower SFC value.

Example:
If a diesel engine consumes 0.25 kg of fuel per kWh, and a similar petrol engine consumes 0.35 kg/kWh, the diesel engine is more efficient since it uses less fuel for the same power output.

Factors Affecting Specific Fuel Consumption

  1. Type of Fuel Used:
    • Fuels with higher calorific value produce more energy per unit mass, resulting in lower SFC.
    • For example, natural gas provides better efficiency compared to coal or biomass.
  2. Engine or Turbine Design:
    • Advanced designs with optimized combustion chambers and aerodynamics achieve lower SFC.
  3. Operating Speed and Load:
    • Engines are most efficient at their rated speed and load. At partial or fluctuating loads, SFC increases.
  4. Combustion Efficiency:
    • Complete and uniform combustion of fuel ensures higher efficiency and lower SFC.
  5. Mechanical and Frictional Losses:
    • Reduced friction in bearings and moving parts helps minimize energy loss, thus lowering SFC.
  6. Air-Fuel Ratio:
    • The correct air-fuel mixture ensures efficient combustion. Too much or too little air increases fuel consumption.
  7. Maintenance and Condition of Equipment:
    • Dirty injectors, fouled filters, or worn-out components can increase SFC by causing incomplete combustion.
  8. Cooling and Lubrication:
    • Efficient cooling prevents overheating, while good lubrication reduces friction losses. Both contribute to better fuel economy.
  9. Ambient Conditions:
    • Temperature, humidity, and altitude affect engine performance, particularly for air-breathing engines like gas turbines.

Methods to Improve Specific Fuel Consumption

  1. Use of High-Quality Fuel:
    • Fuels with high calorific value and clean-burning properties improve efficiency.
  2. Optimization of Air-Fuel Ratio:
    • Electronic fuel injection and automatic control systems maintain the correct mixture for combustion.
  3. Regular Maintenance:
    • Cleaning and replacing filters, lubricating parts, and tuning the engine keep SFC low.
  4. Turbocharging and Supercharging:
    • These increase the intake air pressure, improving combustion efficiency.
  5. Waste Heat Recovery:
    • Using waste heat to preheat air or fuel increases overall energy utilization.
  6. Advanced Engine Design:
    • Modern engines with precision components, reduced friction, and better thermal management have lower SFC.
  7. Efficient Load Management:
    • Operating engines and plants near their rated load ensures optimal fuel consumption.

Significance of Specific Fuel Consumption

  • Performance Indicator:
    SFC helps in evaluating and comparing the fuel efficiency of different engines and turbines.
  • Economic Importance:
    Lower SFC reduces fuel costs, improving profitability in industries and transport systems.
  • Environmental Impact:
    Efficient engines with lower SFC emit fewer greenhouse gases and pollutants.
  • Design Improvement:
    It provides valuable data for engineers to design more efficient systems.
  • Operational Control:
    Continuous monitoring of SFC helps detect performance deterioration and the need for maintenance.
Conclusion :

Specific fuel consumption is an essential performance measure that indicates the amount of fuel required to generate a unit of power output. It directly reflects the efficiency of an engine or power plant — the lower the SFC, the higher the efficiency.

Reducing SFC through improved combustion, advanced materials, better maintenance, and waste heat recovery not only saves fuel costs but also minimizes environmental impact. Hence, optimizing specific fuel consumption is one of the key objectives in modern mechanical and thermal engineering systems.