What is Brake-Specific Fuel Consumption (BSFC)?

BSFC is the mass rate of fuel burned divided by the mechanical power delivered at the engine crankshaft. In SI units it is expressed in grams per kilowatt-hour (g/kWh); in SAE units it is pounds per horsepower-hour (lb/hp·h).

How does BSFC relate to Brake Thermal Efficiency (BTE)?

Brake thermal efficiency is the ratio of mechanical work output to chemical energy supplied: η_bth = 3600 / (BSFC [g/kWh] × LHV [MJ/kg]). Because it depends directly on fuel energy content, engines burning different fuels (e.g. diesel, gasoline, ethanol) can only be equitably compared via thermal efficiency rather than mass-based BSFC alone.

Why does BSFC vary across engine speed and torque?

BSFC changes continuously over an engine operating map due to variations in volumetric efficiency, in-cylinder heat losses, combustion phasing, throttling/pumping work, and mechanical friction. A single steady-state measurement represents only that individual operating point, not a full cycle average.

What is the conversion factor between g/kWh and lb/(hp·h)?

1 lb/(hp·h) equals approximately 608.277 g/kWh, and 1 g/kWh equals approximately 0.001644 lb/(hp·h), based on the international avoirdupois pound (453.59237 g) and mechanical horsepower (745.69987 W).

Why is fuel density required for volumetric flow?

BSFC is strictly a mass-based metric. Liquid fuel density changes significantly with temperature and blend formulation (e.g., standard gasoline ~0.72–0.78 kg/L, neat ethanol ~0.79 kg/L). Converting volumetric flow (L/h, GPH, cc/min) to mass flow without measured fuel density introduces unacceptable measurement error.

Brake-Specific Fuel Consumption Calculator

Calculate BSFC (g/kWh, lb/hp·h), fuel mass flow rate, or power output. Includes optional brake thermal efficiency (BTE) from fuel lower heating value (LHV).

Engine Parameters

Steady-State Analysis

Calculation Mode

Fuel Mass Flow Rate (ṁ_f)

Brake Power (P_b)

Must be measured shaft/crankshaft brake power output.

Optional Operating Context & Thermal Efficiency

Fuel Lower Heating Value (LHV, MJ/kg) Gasoline ~43–44, Diesel ~42.5–43, E85 ~29.2

Engine Speed (RPM) Optional test point speed

Brake Torque (optional operating point reference)

Calculated Results

Primary SI & SAE equivalents

BSFC (SI / Metric)

— g/kWh

BSFC (SAE / Imperial)

— lb/(hp·h)

Normalized Parameters

Fuel Flow (kg/h): —

Fuel Flow (lb/h): —

Brake Power (kW): —

Brake Power (hp): —

Brake Thermal Efficiency (η_bth)

— %

Requires fuel Lower Heating Value (LHV).

Factual Unit Definitions & Governing Identities

BSFC Definition	BSFC = ṁ_fuel / P_brake
SI to SAE BSFC	1 g/kWh = 0.00164399 lb/(hp·h)
SAE to SI BSFC	1 lb/(hp·h) = 608.2774 g/kWh
Mechanical Horsepower	1 hp = 745.69987 W (≈ 0.7457 kW)
Metric Horsepower	1 PS = 75 kgf·m/s = 735.49875 W
Avoirdupois Mass	1 lb = 453.59237 g = 0.45359237 kg
Brake Thermal Efficiency	η_bth = 3600 / (BSFC_kg/kWh × LHV_kJ/kg)

Engineering Constraints & Required Limitations

Operating-Point Specificity: BSFC is strictly valid only for the measured engine speed, load, and ambient test cell conditions.
Not a Drive-Cycle Representative: A single steady-state measurement point is not a drive-cycle, transient, or whole-engine average.
Power Measurement Baseline: Use measured crankshaft brake power, not indicated (in-cylinder) or nominal rated power, unless indicated specific fuel consumption (ISFC) is deliberately intended.
Mass Flow vs. Volumetric Flow: Never convert volumetric fuel meter readings (L/h, GPH) without adjusting for instantaneous fuel temperature and fluid density.
Lower Heating Value Requirement: Thermal efficiency calculations must use the fuel's Lower Heating Value (LHV, water vapor uncondensed), not Higher Heating Value (HHV), to reflect thermodynamic engine work limits.

Summary

Calculate BSFC (g/kWh, lb/hp·h), fuel mass flow rate, or power output. Includes optional brake thermal efficiency (BTE) from fuel lower heating value (LHV).

How it works

Select the target parameter to solve: BSFC, Fuel Mass Flow, or Brake Power.
Enter known parameters using preferred engineering units (g/s, g/h, kg/h, lb/h for flow; kW, hp, PS for power; g/kWh, lb/hp·h for BSFC).
Optionally provide fuel Lower Heating Value (LHV) in MJ/kg to compute Brake Thermal Efficiency (η_bth).
Optionally record test operating point conditions (engine speed in RPM and shaft torque).
Results convert transparently between metric SI (g/kWh) and imperial SAE (lb/hp·h) units.

Use cases

Calculate steady-state BSFC from dynamometer fuel mass meter and absorber load readings.
Size fuel system mass flow capacity given target brake horsepower and expected BSFC.
Determine shaft power output during fuel-limited steady-state dyno mapping.
Compute brake thermal efficiency (BTE) from fuel lower heating value (LHV).
Convert between metric g/kWh and imperial lb/(hp·h) engineering specifications.

Frequently Asked Questions

Last updated: 2026-09-19 · Reviewed by Nham Vu