Fuel Efficiency - Mass Converter

Joule per Kilogram (J/kg)

The SI unit of specific energy, representing the energy content per unit mass. 1 J/kg = 1 kJ/kg ÷ 1,000. Standard for fuel energy content and combustion calculations in modern energy systems.

Common reference: Natural gas: ~50 MJ/kg; Coal: ~25-30 MJ/kg; Gasoline: ~46 MJ/kg; Diesel: ~45 MJ/kg.

Kilojoule per Kilogram (kJ/kg)

One thousandth of a joule per kilogram. 1 kJ/kg = 1,000 J/kg. Convenient for fuels with energy content in thousands of joules per kilogram.

Practical use: Common for coal, biomass, and industrial fuels where energy content is in the MJ/kg range.

Calorie (IT) per Gram (cal/g)

International Table calorie per gram. 1 cal/g = 4.1868 J/g = 4,186.8 J/kg. Common in chemistry and food science for energy content measurements.

Historical and practical use: Food nutrition labels, chemical reaction calculations, and scientific literature.

British Thermal Unit per Pound (Btu/lb)

North American unit of specific energy. 1 Btu/lb = 1.8 cal/g = 7,536 J/kg. Standard in US coal and fuel specifications.

Common use: Coal grading, industrial fuel specifications, and US energy calculations.

Understanding Specific Energy (Energy per Mass)

Specific energy is the energy content per unit mass of fuel or material. It determines:

• Fuel efficiency: Higher specific energy = more energy per unit weight
• Energy density comparison: Allows fair comparison of different fuels
• Calorific value: Gross (includes latent heat of water vapor) vs Net (excludes it)
• System design: Determines tank size, range, and power output requirements

Typical Fuel Energy Content Values

• Gasoline: ~46.5 MJ/kg (11,000-12,000 Btu/lb)
• Diesel: ~45.3 MJ/kg (10,700-11,100 Btu/lb)
• Natural gas (LNG): ~50 MJ/kg (21,000-24,000 Btu/lb)
• Coal (anthracite): ~30 MJ/kg (7,000-8,000 Btu/lb)
• Coal (bituminous): ~25 MJ/kg (6,000-7,000 Btu/lb)
• Wood (dry): ~15-16 MJ/kg (6,500-7,000 Btu/lb)
• Hydrogen gas: ~142 MJ/kg (61,000 Btu/lb) - highest energy density
• Nuclear fission: ~82 TJ/kg (86 million Btu/lb) - nuclear fuel
• Charcoal: ~30 MJ/kg (13,000 Btu/lb)
• Methane: ~55 MJ/kg (23,500 Btu/lb)

Fuel Comparison: Specific Energy Matters

Specific energy determines fuel choice for different applications:

• Liquid fuels (gasoline, diesel): High specific energy, compact, practical for vehicles
• Coal: Lower specific energy, abundant, used for power plants and heating
• Natural gas: Very high specific energy, clean burning, preferred for heating
• Hydrogen: Highest specific energy, but requires special handling
• Nuclear: Incomparably highest energy density, small amount powers cities
• Renewables: Energy per mass varies; solar/wind are intermittent energy flows, not fuels

Gross vs Net Calorific Value - IMPORTANT

Gross (Higher) Calorific Value: Includes energy from condensation of water vapor in exhaust

Net (Lower) Calorific Value: Excludes latent heat of water vapor formation

Difference: For hydrocarbon fuels, net is typically 5-10% lower than gross because water vapor escapes and doesn't condense in normal combustion.

Important: Always specify which value is being used. Modern condensing boilers can recover latent heat and approach gross calorific value efficiency.

Applications of Specific Energy

• Vehicle range calculation: Range = (Tank size × Fuel energy) / (Power consumption rate)
• Aircraft design: Fuel energy density critical for takeoff weight and range
• Power plant efficiency: Fuel energy input vs electrical output determines efficiency
• Heating systems: Determines boiler size and fuel tank requirements
• Industrial processes: Heat requirements based on fuel energy content
• Comparative analysis: Different fuels compared on energy per kg basis
• Energy policy: Governments use energy density for renewable targets and comparisons

Key Conversion Factors

• 1 cal/g = 4.1868 J/g = 4,186.8 J/kg
• 1 J/kg = 0.000238846 cal/g (approximately)
• 1 Btu/lb = 2,326 J/kg ≈ 2.326 MJ/kg
• 1 MJ/kg = 239.006 cal/g ≈ 430 Btu/lb
• Gross vs Net Calorific Value: Net ≈ Gross × 0.90-0.95 for hydrocarbons
• Energy content does not equal efficiency: Fuel energy × engine efficiency = usable energy
• Inverse units (kg/J): Measure fuel consumption per unit energy (fuel economy metric)