Fuel Efficiency by Mass Converter

Understanding Mass-Based Fuel Efficiency

Mass-based fuel efficiency measures distance traveled per unit mass of fuel consumed. This metric matters for applications where fuel mass, not volume, determines performance. Rocket propulsion, hydrogen fuel cell vehicles, and compressed natural gas (CNG) vehicles use mass-based efficiency because fuel density varies with pressure and temperature.

Common units include kilometers per kilogram (km/kg) and miles per pound (mi/lb). Electric vehicles use an analogous measure: kilometers per kilowatt-hour (km/kWh), which relates energy consumption to distance.

Fuel Efficiency Formula

Calculate mass-based fuel efficiency from distance and fuel mass:

Example: A hydrogen fuel cell vehicle travels 100 km while consuming 1.2 kg of hydrogen:

Convert to miles per pound using 1 km/kg = 0.4536 mi/lb:

Specific Impulse for Rocket Propulsion

Rocket engineers use specific impulse (Isp) to measure propulsion efficiency. Specific impulse equals thrust produced per unit weight flow rate of propellant:

Higher specific impulse means more efficient propellant use. Specific impulse in seconds converts to effective exhaust velocity:

Example: A rocket engine with Isp = 300 s has exhaust velocity:

Fuel Efficiency by Mass for Various Propulsion Systems

Note: Rocket specific impulse values shown in seconds. Higher Isp indicates more efficient propellant use but typically lower thrust.

Hydrogen Vehicle Range Calculation

Calculate driving range from fuel tank capacity and efficiency:

Example: A hydrogen vehicle with 5 kg tank capacity and 90 km/kg efficiency:

The Toyota Mirai and Hyundai Nexo achieve approximately 80 to 100 km/kg under real-world driving conditions, giving 500 to 650 km range from their 5 to 6 kg hydrogen tanks.

Unit Conversions

Convert between mass-based fuel efficiency units:

• 1 km/kg = 0.4536 mi/lb
• 1 mi/lb = 2.2046 km/kg
• 1 km/kg = 2.825 mi/US gal (assuming gasoline density 0.75 kg/L)
• 1 mi/lb = 0.354 km/L (assuming gasoline density 0.75 kg/L)

Converting between mass-based and volume-based efficiency requires fuel density. Gasoline density ranges from 0.71 to 0.77 kg/L depending on formulation and temperature.

Applications of Mass-Based Fuel Efficiency

• Hydrogen fuel cell vehicle range estimation
• Compressed natural gas (CNG) fleet management
• Rocket propulsion system design and comparison
• Aircraft fuel planning where mass affects performance
• Alternative fuel vehicle certification and labeling
• Spacecraft mission delta-v calculations
• Fuel cell system optimization

Frequently Asked Questions