What is brake-specific fuel consumption (BSFC)?

BSFC is the fuel mass flow divided by the brake (crankshaft) power: BSFC = ṁf ÷ Pb. In SI units it is g/kWh; in SAE units lb/(hp·h). Lower is more efficient. Example: 18 kg/h at 75 kW is 18,000 ÷ 75 = 240 g/kWh, or 0.3946 lb/(hp·h).

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

1 lb/(hp·h) = 608.277 g/kWh and 1 g/kWh = 0.0016440 lb/(hp·h), using the avoirdupois pound (453.59237 g) and mechanical horsepower (745.69987 W).

At their best operating point, large two-stroke marine diesels reach about 160–175 g/kWh, truck diesels 180–200 g/kWh, car diesels about 195–215 g/kWh and modern gasoline engines about 205–250 g/kWh. At light load BSFC rises sharply, often above 300–400 g/kWh.

How much fuel does an engine need per horsepower?

Fuel flow = power × BSFC. A 400 hp gasoline engine at 0.50 lb/(hp·h) needs 200 lb/h of fuel at full power, which is about 122 L/h or 32 US gal/h at 0.745 kg/L. Injector-sizing rules of thumb use about 0.45–0.50 lb/(hp·h) for naturally aspirated gasoline and 0.55–0.65 for turbocharged or supercharged gasoline.

How do I size fuel injectors from BSFC?

Injector flow = horsepower × BSFC ÷ (number of injectors × maximum duty cycle). For 400 hp at 0.50 lb/(hp·h) on 8 injectors at 80% duty: 400 × 0.50 ÷ (8 × 0.8) = 31.25 lb/h per injector, or about 317 cc/min with gasoline at 0.745 g/cm³.

How does BSFC relate to brake thermal efficiency?

η = 3600 ÷ (BSFC [g/kWh] × LHV [MJ/kg]). A diesel at 200 g/kWh with LHV 42.8 MJ/kg has η = 3600 ÷ (200 × 42.8) = 42.1%. Because fuels differ in heating value, compare engines on different fuels by efficiency, not by mass BSFC.

Why is fuel density needed for volume flow?

BSFC is mass-based. Converting to L/h or gal/h divides by density, which varies with fuel blend and temperature (gasoline about 0.72–0.78 kg/L, diesel about 0.82–0.85 kg/L). Use measured density for test work.

Brake-Specific Fuel Consumption Calculator

BSFC = fuel mass flow ÷ brake power. Solve for BSFC, fuel flow or power in g/kWh or lb/(hp·h); get power from torque × rpm; convert fuel flow to L/h and gal/h with fuel density; size injectors; and get brake thermal efficiency = 3600 ÷ (BSFC × LHV).

Engine Parameters

Steady-State Analysis

Calculation Mode

Fuel Mass Flow Rate (ṁ_f)

Brake Power (P_b)

Crankshaft (brake) power. Leave blank to use torque × rpm below.

Fuel, operating point & injectors (optional)

Fuel

LHV (MJ/kg)

Density (kg/L)

Engine Speed (rpm)

Brake Torque

Number of injectors

Max duty cycle (%)

Calculated Results

SI & SAE equivalents

BSFC (SI / Metric)

—g/kWh

BSFC (SAE / Imperial)

—lb/(hp·h)

Normalized Parameters

Fuel flow (kg/h):—

Fuel flow (lb/h):—

Fuel flow (L/h):—

Fuel flow (US gal/h):—

Brake power (kW):—

Brake power (hp):—

Brake Thermal Efficiency (η_bth)

—%

Requires fuel lower heating value (LHV).

Injector size (per injector)

Enter the number of injectors.

Typical BSFC by engine type

Representative ranges. Efficiency = 3600 ÷ (BSFC × LHV) with the fuel LHV shown. Full-load gasoline and E85 rows are the rules of thumb used for injector sizing.

Engine / condition	g/kWh	lb/(hp·h)	Efficiency
Large two-stroke marine diesel, best point	160–175	0.26–0.29	48–53%
Heavy-duty truck diesel, best point	180–200	0.30–0.33	42–47%
Passenger-car diesel, best point	195–215	0.32–0.35	39–43%
Modern gasoline engine, best point	205–250	0.34–0.41	33–40%
Gasoline engine, light load	300–500	0.49–0.82	17–28%
Naturally aspirated gasoline, full load (injector sizing)	274–304	0.45–0.50	27–30%
Turbo / supercharged gasoline, full load (injector sizing)	335–395	0.55–0.65	21–25%
E85, full load (injector sizing)	395–487	0.65–0.80	25–31%

Unit definitions & identities

BSFC	BSFC = ṁ_fuel ÷ P_brake
Power from torque	P [kW] = 2π × N [rpm] × T [N·m] ÷ 60,000
SI → SAE	1 g/kWh = 0.00164399 lb/(hp·h)
SAE → SI	1 lb/(hp·h) = 608.2774 g/kWh
Horsepower	1 hp = 745.69987 W; 1 PS = 735.49875 W
Thermal efficiency	η = 3600 ÷ (BSFC [g/kWh] × LHV [MJ/kg])
Injector flow	lb/h = hp × BSFC ÷ (n × duty)
lb/h → cc/min	cc/min = lb/h × 453.59237 ÷ 60 ÷ ρ [g/cm³]

Worked example & limits

A diesel on the dyno makes 238.7 N·m at 3,000 rpm and burns 18 kg/h. Power = 2π × 3000 × 238.7 ÷ 60,000 = 74.99 kW, so BSFC = 18,000 ÷ 74.99 = 240.0 g/kWh (0.3946 lb/(hp·h)). With LHV 42.8 MJ/kg, η = 3600 ÷ (240.0 × 42.8) = 35.0%; at 0.835 kg/L the flow is 21.56 L/h (5.69 US gal/h).

BSFC is valid only for the measured speed, load and ambient conditions; it is not a drive-cycle average.
Use measured crankshaft (brake) power, not indicated or catalog power.
Use LHV, not HHV, for thermal efficiency.
Jet engines? Use the thrust-specific fuel consumption (TSFC) calculator.

Summary

Brake-specific fuel consumption (BSFC) is the fuel mass an engine burns per unit of work delivered at the crankshaft, in g/kWh or lb/(hp·h). This calculator solves BSFC from measured fuel flow and brake power (or torque and speed), the fuel flow needed for a target power, or the power a fuel flow supports. With fuel density it converts mass flow to L/h and US gal/h and sizes fuel injectors; with the fuel lower heating value it gives brake thermal efficiency. For jet engines use thrust-specific fuel consumption (TSFC) instead.

How it works

Select what to solve: BSFC, fuel mass flow, or brake power.
Enter the known values in your units (g/s, g/h, kg/h, lb/h; kW, hp, PS; g/kWh, lb/(hp·h)). If you only have dyno torque and rpm, leave power blank and enter them below: P = 2π × N × T ÷ 60.
Pick the fuel to fill its typical lower heating value (LHV) and density, or type measured values.
Read BSFC in SI and SAE units, fuel flow in kg/h, lb/h, L/h and US gal/h, and brake thermal efficiency.
Optionally enter the number of injectors and the maximum duty cycle to get the required injector flow in lb/h and cc/min.

Use cases

Calculate steady-state BSFC from dynamometer fuel-meter and torque readings.
Find the fuel flow (and fuel pump capacity) a target horsepower needs.
Size fuel injectors for a gasoline, E85 or diesel build from horsepower and BSFC.
Compute brake thermal efficiency from BSFC and fuel LHV.
Convert between g/kWh and lb/(hp·h) specifications, and between mass and volume fuel flow.

Frequently Asked Questions

Last updated: 2026-10-09 · Reviewed by Nham Vu