Brake vs Indicated-Specific Fuel Consumption

Specific Fuel Consumption (SFC) is a measure of fuel efficiency in engines, indicating how much fuel is consumed to produce a unit of power. It is commonly expressed in two ways:

1. Brake-Specific Fuel Consumption (BSFC) – Based on brake power (BP).

2. Indicated-Specific Fuel Consumption (ISFC) – Based on indicated power (IP).

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1. Brake-Specific Fuel Consumption (BSFC)

Definition:
The fuel consumption rate per unit of brake power (BP). It represents the actual fuel efficiency of the engine under load.

Formula:

$$����=\frac{\text{Fuel Consumption Rate (}{\dot{�}}_{�}\text{)}}{\text{Brake Power (BP)}}$$

Units:

• Common: $\text{g/kWh}$ (grams per kilowatt-hour)

• SI: $\text{kg/J}$ (but rarely used)

Typical Values:

Engine Type	BSFC Range (g/kWh)
Diesel Engines	180–220
Petrol (Gasoline) Engines	250–350
High-Efficiency Turbocharged Engines	160–200

Example Calculation:

• Fuel flow rate = 5 kg/h

• Brake Power (BP) = 100 kW

$$����=\frac{5000\text{g/h}}{100\text{kW}}=50\text{g/kWh}$$

(Note: If BP is in HP, convert to kW by multiplying by 0.7457.)

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2. Indicated-Specific Fuel Consumption (ISFC)

Definition:
The fuel consumption rate per unit of indicated power (IP). It represents the engine's theoretical fuel efficiency before mechanical losses.

Formula:

$$����=\frac{\text{Fuel Consumption Rate (}{\dot{�}}_{�}\text{)}}{\text{Indicated Power (IP)}}$$

Units: Same as BSFC ($\text{g/kWh}$).

Relationship with BSFC:
Since $��=��-��$, and ${�}_{�}=\frac{��}{��}$,

$$����=\frac{����}{{�}_{�}}$$

(Mechanical efficiency ${�}_{�}$ reduces BSFC compared to ISFC.)

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3. Factors Affecting SFC

• Engine Load – SFC is lowest near optimal load (~75-85%).

• Engine Speed – Higher RPM increases frictional losses, worsening SFC.

• Combustion Efficiency – Poor combustion (e.g., incorrect air-fuel ratio) increases SFC.

• Friction & Heat Losses – Better lubrication and cooling improve SFC.

• Turbocharging – Increases efficiency, reducing SFC.

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4. Practical Applications

• Engine Tuning: Engineers optimize SFC for better fuel economy.

• Aircraft Engines: BSFC is critical for fuel weight and range calculations.

• Hybrid Vehicles: Combines engine SFC with electric efficiency.

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Example Problem

Given:

• A diesel engine produces 150 kW BP at 2000 RPM.

• Fuel consumption = 30 kg/h.

• Mechanical efficiency = 85%.

Find:

1. BSFC

2. ISFC

Solution:

1. BSFC Calculation:

$$����=\frac{30,000\text{g/h}}{150\text{kW}}=200\text{g/kWh}$$

2. ISFC Calculation:

$$��=\frac{��}{{�}_{�}}=\frac{150}{0.85}\approx 176.47\text{kW}$$$$����=\frac{30,000}{176.47}\approx 170\text{g/kWh}$$

(Note: ISFC < BSFC because it excludes friction losses.)

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Key Takeaways

• BSFC = Real-world fuel efficiency (measured at the crankshaft).

• ISFC = Theoretical efficiency (inside the cylinder).

• Lower SFC = Better fuel economy.

• Turbocharged & high-compression engines typically have lower SFC