What Is Brake Specific Fuel Consumption (BSFC)?
Every engine is a deal: you feed it fuel, and it hands back useful turning force at the shaft. Brake specific fuel consumption (BSFC) puts a hard number on how good that deal is — it is the mass of fuel the engine burns for each unit of mechanical energy it delivers.
The word "brake" refers to the dynamometer (historically a friction brake) used to measure the power coming out of the crankshaft, and "specific" means per unit of that power. A lower BSFC is always better: it means more work from every gram of fuel, which translates directly into better fuel economy and lower running costs.
The BSFC Formula
BSFC is a single division — fuel flow divided by power output:
[
\text{BSFC} = \frac{\text{Fuel Flow}}{\text{Power Output}}
]
Where:
- Fuel Flow is how fast the engine consumes fuel, e.g. in grams per second (g/s).
- Power Output is the useful shaft power, e.g. in watts (W).
With those units the answer comes out in grams per joule (g/J). Because a joule is tiny compared with the fuel an engine drinks, engineers usually rescale to grams per kilowatt-hour (g/kWh) or pounds per horsepower-hour (lb/hp·h). Since 1 kWh = 3,600,000 J, converting is just a multiplication:
[
\text{BSFC (g/kWh)} = \frac{\text{Fuel Flow (g/s)} \times 3600}{\text{Power (kW)}}
]
Worked Example: A 2-Liter Engine Under Load
Suppose a 2.0 L gasoline engine is on a dyno producing 100 kW while a fuel-flow meter reads 7 g/s. First convert the power: 100 kW = 100,000 W.
[
\text{BSFC} = \frac{7 \text{ g/s}}{100{,}000 \text{ W}} = 0.00007 \text{ g/J}
]
Rescale to practical units by multiplying by 3,600,000 J/kWh:
[
\text{BSFC} = 0.00007 \times 3{,}600{,}000 = 252 \text{ g/kWh}
]
That equals about 0.41 lb/hp·h — a completely healthy figure for a gasoline engine working near full load. Try it in the calculator above: enter 7 g/s and 100 kW and you should get exactly 252.0 g/kWh.
What Does Your BSFC Number Mean?
| BSFC (g/kWh) | Verdict | Interpretation |
|---|---|---|
| ≤ 210 | Excellent | At or near the engine's best-efficiency point — typical of large diesels at full load |
| 211 – 300 | Typical | Normal territory for a gasoline engine working under decent load |
| > 300 | High | Lots of fuel per unit of work — the engine may be lightly loaded, worn or badly tuned |
One trap to avoid: BSFC depends strongly on where on its map the engine is running, not just what engine it is. The same engine can have brilliant BSFC at 80% load and terrible BSFC at idle, when most of the fuel goes to overcoming its own friction.
Typical BSFC Values by Engine Type
| Engine Type | Best-Point BSFC (g/kWh) | Best-Point BSFC (lb/hp·h) |
|---|---|---|
| Large marine / stationary diesel | 170 – 190 | 0.28 – 0.31 |
| Modern automotive diesel | 190 – 240 | 0.31 – 0.39 |
| Modern gasoline (port injection) | 230 – 290 | 0.38 – 0.48 |
| Racing engine at full throttle | 270 – 330 | 0.44 – 0.54 |
| Small engines (mowers, generators) | 300 – 450 | 0.49 – 0.74 |
Notice the pattern: bigger, slower, more highly stressed engines are generally more efficient. A ship diesel turns huge peak pressures into work with remarkably little waste heat, which is why cargo ships still burn heavy fuel oil rather than gasoline.
Unit Conversions Worth Memorizing
| From | To | Multiply by |
|---|---|---|
| g/J | g/kWh | 3,600,000 |
| g/kWh | lb/hp·h | 0.00164 |
| lb/hp·h | g/kWh | 608.3 |
| g/s | g/kWh basis | × 3600, then divide by power in kW |
Quick Recap
- BSFC = fuel flow ÷ shaft power — grams of fuel burned per joule (or kWh, or hp·h) delivered.
- Lower is better: it means more useful work from every gram of fuel.
- Gasoline engines typically land around 230–290 g/kWh at their best point; diesels do better still.
- Always compare figures taken at similar operating points — part-load BSFC is naturally much worse.
If you want to pair this with the other side of the equation, run your engine's specs through the brake horsepower calculator to see how shaft power itself is determined.