What does specific fuel consumption (SFC) measure in an internal combustion engine?

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RRB JE ME 22 Apr 2025 Shift 2 CBT 2 Official Paper

The efficiency of the engine’s exhaust system
The total mass of the fuel used during engine operation
The fuel efficiency expressed as the amount of fuel consumed per unit of power produced
The ratio of air intake to fuel delivered per combustion cycle

Answer 1

Explanation:

Specific Fuel Consumption (SFC)

Specific Fuel Consumption (SFC) is a critical parameter in evaluating the performance of internal combustion engines. It measures the fuel efficiency of an engine by calculating the amount of fuel consumed per unit of power produced over a specific time period. This parameter is particularly useful in comparing the performance of engines and determining their operational efficiency. SFC is typically expressed in units such as grams per kilowatt-hour (g/kWh) or pounds per horsepower-hour (lb/hp-hr), depending on the unit system being used.

Formula for SFC:

The specific fuel consumption is calculated using the formula:

SFC = Fuel consumption rate / Power output

Where:

Fuel consumption rate: The amount of fuel consumed by the engine per unit of time (e.g., kg/hr or lb/hr).
Power output: The power produced by the engine during the same time period (e.g., kW or hp).

Significance of SFC:

Fuel Efficiency: SFC serves as a direct indicator of an engine’s fuel efficiency. Lower values of SFC suggest that the engine consumes less fuel to generate the same power output, which is desirable in most applications.
Cost Savings: Improved fuel efficiency, as indicated by a lower SFC, leads to reduced operating costs for vehicles, machinery, and equipment.
Environmental Impact: Engines with lower SFC values produce fewer emissions for the same power output, contributing to reduced environmental pollution.
Design Optimization: By analyzing SFC, engineers can identify areas for improvement in engine design and optimize performance for specific applications.

Factors Affecting SFC:

Engine Design: The design and configuration of the engine, such as the number of cylinders, compression ratio, and turbocharging, significantly influence SFC.
Operating Conditions: SFC varies with engine load, speed, and temperature. Engines tend to be most efficient within a specific range of operating conditions.
Fuel Quality: The type and quality of fuel used, including its calorific value and combustion characteristics, impact SFC.
Maintenance: Regular maintenance and proper tuning of the engine can enhance fuel efficiency and lower SFC.