Fuel Efficiency by Mass & BSFC Calculator

Propulsion Thermodynamics, BSFC & Mass Fuel Efficiency Metrology

Brake Specific Fuel Consumption (BSFC) measures fuel mass consumed per unit of mechanical energy produced:

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• BSFC Unit Conversion Formulas (SAE Standards)**:

BSFC_lb/(hp·h) = BSFC_g/(kW·h) × 0.001643987

BSFC_g/(kW·h) = BSFC_lb/(hp·h)0.001643987 = BSFC_lb/(hp·h) × 608.277

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• Thermal Brake Efficiency Formula**:

η_th = (3,600) / (BSFC)_g/(kW·h) × LHV_MJ/kg × 100%

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• Gravimetric Distance Conversion**:

1 km/kg = 0.28198 miles/lb 1 mile/lb = 3.5463 km/kg

Mass Consumption = (100) / (km/kg)

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• Standard Propulsion Benchmarks**:

• High-Efficiency Marine Diesel: 200 g/(kW·h) 0.329 lb/(hp·h) η_th = 42.25%

• Automotive Gasoline Engine: 250 g/(kW·h) 0.411 lb/(hp·h) η_th = 33.18%

• CNG Natural Gas Car: 18.0 km/kg 5.08 miles/lb 5.56 kg/100km

• Hydrogen Fuel Cell Car: 100.0 km/kg H_2 28.20 miles/lb 1.00 kg/100km

How to Use This Calculator

Enter your Fuel Efficiency / BSFC Magnitude (e.g. 200 in val), From Metric / Efficiency Unit — choose from Brake Specific Fuel Consumption (BSFC - g/(kW·h)), BSFC in Pounds per Horsepower-Hour (lb/(hp·h)), Distance per Mass (km/kg), To Metric / Efficiency Unit — choose from Brake Specific Fuel Consumption (g/(kW·h)), BSFC in Pounds per Horsepower-Hour (lb/(hp·h)), Distance per Mass (km/kg), Fuel Lower Heating Value (LHV in MJ/kg) (e.g. 42.6 in MJ/kg), and Engine & Propulsion Benchmark — choose from Custom Mass Efficiency, High-Efficiency Marine Diesel (200 g/(kW·h), LHV=42.6) → 0.329 lb/(hp·h) | 42.25% Thermal Efficiency, Automotive Gasoline Engine (250 g/(kW·h), LHV=43.4) → 0.411 lb/(hp·h) | 33.18% Thermal Efficiency. Results update instantly as you type — adjust any value to compare scenarios.

The Formula

Thermal Efficiency: eta = 3600 / (BSFC_g_kWh * LHV_MJ_kg)

Worked Example

A marine propulsion engineer measuring a diesel engine operating at 200.0 g/(kW·h) with fuel LHV of 42.6 MJ/kg determines the engine achieves 42.25% thermal efficiency and consumes 0.329 pounds of fuel per horsepower-hour.

Understanding Your Result

US EPA fuel economy benchmarks (2024): Compact cars 28–35 MPG combined; midsize SUVs 22–28 MPG; pickup trucks 16–22 MPG; hybrid vehicles 40–55 MPG; EVs equivalent 90–120 MPGe. At 3.50/gallon, every 1 MPG improvement saves approximately 35–50/year over 10,000–15,000 annual miles. Real-world fuel economy typically runs 15–20% below EPA ratings in stop-and-go traffic, cold weather (reduces efficiency 15–25%), or with heavy loads.