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Brake-specific fuel consumption calculator

An engine burning 10 kg of fuel per hour while delivering 50 kW at the shaft has a BSFC of 200 g/kWh. Enter the fuel mass flow and brake power in the units of your dynamometer log and get grams per kilowatt-hour. Free in your browser.

  • Free
  • Runs in your browser
  • No sign-up
  • Your data stays on your device
Run — free How it works & API
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Run — free

Worked example

If you enter
Fuel flow
10
Power
50
You get
Fuel flow unit
Kg h
Fuel flow g h
10,000
Power unit
Kw
Power kw
50

Brake-specific fuel consumption, written BSFC, is the mass of fuel an internal-combustion engine burns for each unit of shaft power it delivers at the flywheel or dynamometer. This calculator takes a fuel mass-flow rate and the matching engine brake power, converts those readings into consistent working units, and reports consumption in grams per kilowatt-hour — the number printed on engine maps, type-approval sheets, and dynamometer logs. A modern automotive diesel at its sweet spot often sits near two hundred grams per kilowatt-hour; a spark-ignition gasoline engine is usually thirstier at the same load. The identity is deterministic: no network, no clock, and no random sampling.

What brake-specific fuel consumption means and why the unit is g/kWh

Brake-specific fuel consumption is a ratio, not a raw burn rate. Two engines can gulp the same kilograms of fuel per hour and still have very different BSFC if one delivers more shaft power at that point. The word brake is historical: it names the power measured at the output shaft against a brake dynamometer, after friction, pumping, and accessory losses have already been paid, rather than the indicated power inferred from cylinder pressure.

Dividing mass flow by that shaft power therefore answers a fair comparison question: how many grams of fuel did this engine need for each kilowatt-hour of useful work. Grams per kilowatt-hour is the SI-friendly unit used on European maps and in most textbooks; the older imperial companion is pounds per horsepower-hour, which this page does not emit because the contract is grams per kilowatt-hour.

Typical best-point values sit near one hundred sixty to two hundred grams per kilowatt-hour for large two-stroke marine diesels, near two hundred for a well-tuned automotive diesel, and two hundred fifty to three hundred fifty for many gasoline spark-ignition engines. A single number never describes a whole engine: BSFC is a function of speed and load, and the map is usually a family of islands around the most efficient region. This calculator evaluates one operating point.

It does not invent a map, apply a heating value, or convert indicated to brake power. If you only have volumetric flow in litres per hour, convert to mass flow with a density first; BSFC is a mass-specific figure and a volume reading without density is a different quantity.

Where a BSFC calculator earns its keep in labs, fleets, and software

Engineers meet this number when they reduce a dynamometer log to a map, when they compare a candidate diesel against a gasoline baseline at the same load, or when a homework problem asks for grams per kilowatt-hour from a measured burn and a measured brake reading. Calibration teams use a fast, offline identity to check that a logged point was not inverted (power in the fuel column, flow in the power column) before it is painted on a contour plot.

Fleet and generator operators use it to see whether a genset is sitting on a thirsty island or near its best specific consumption. Students and instructors need a deterministic oracle that rejects a parked engine with zero brake power instead of returning infinity or a silent empty success. Contest and lab practice often needs a source of quotients that does not depend on a spreadsheet macro under time pressure.

Pair it with fuel-economy converters when the question is vehicle miles per gallon, and keep this page for the engine-lab search: how many grams per kilowatt-hour did this brake power actually cost.

What you can do with it

Reduce a dynamometer point to g/kWh

Convert a logged fuel mass-flow and the matching brake power into BSFC so the point can be painted on an engine map without a hand calculator.

Compare two engines at the same load

Put each engine's burn rate and shaft power through the same identity to see which operating point is thirstier in grams per kilowatt-hour.

FAQ

How do I calculate brake-specific fuel consumption?

Send a positive fuel mass-flow rate and a positive engine brake power.

What happens if engine brake power is not positive?

BSFC is undefined when shaft power is zero or negative, and the failure is not billed.

Which units can I enter for fuel flow and power?

Power accepts kw (default), w, hp (mechanical), and ps (metric horsepower).

Can I enter volumetric fuel flow in litres per hour?

No. Convert volume to mass with a fuel density first. BSFC is a mass-specific figure; a litre-per-hour reading without density is a different quantity.

What is a good BSFC value?

A well-tuned automotive diesel at its best point sits near 200 g/kWh, large two-stroke marine diesels can go lower, and many gasoline engines are clearly thirstier. BSFC changes with speed and load, so one figure describes one operating point, not the whole engine.

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For developers — API access

Everything on this page is available programmatically. This section is for teams who want to wire it into their own systems; everyone else can just use the tool above.

API endpoint

POSThttps://api.kit.forhosting.com/life/brake-specific-fuel-consumption

Prefer to automate it? One authenticated POST creates the task; the result comes back by webhook or a signed link. The same capability also runs here on the web, by email and from Telegram — and soon from our app too.

Call it from your stack

curl -X POST https://api.kit.forhosting.com/life/brake-specific-fuel-consumption \
  -H "Authorization: Bearer $KIT_KEY" \
  -H "Content-Type: application/json" \
  -d '{"fuel_flow":10,"power":50}'

Example request

{
  "fuel_flow": 10,
  "power": 50
}

Example response

{
  "task_id": "tsk_a1b2c3d4e5f6a1b2c3d4e5f6",
  "type": "life.brake_specific_fuel_consumption",
  "status": "queued",
  "_links": {
    "result": "/tasks/tsk_…/result"
  }
}

The API is asynchronous: the call returns a task_id immediately and the result arrives by webhook. Polling is capped at 1 req/s per task.

Pricing

Per request$0.002

Published price — no tokens, no invented credits. A failed task is never charged.

Limits

max_abs_value1000000000000000

Errors

HTTPCodeMeaning
401unauthorizedMissing or invalid API key.
402insufficient_balanceYour balance doesn't cover the task price.
404unknown_typeThat task type doesn't exist.
429rate_limitedToo many requests. Use the webhook instead of polling.

Technical notes

Use the free browser form to check a single operating point, or call the API at $0.002 per successful request. If brake power is zero or negative the task fails with invalid_input, because BSFC is undefined when the engine is not producing positive shaft work.

Input contract, unit conversion, and how the quotient is formed

Mechanical horsepower uses the exact 550 foot-pound-force per second definition; metric horsepower uses 735.49875 watts. The pound is the avoirdupois pound of 453.59237 grams. Fuel flow must be strictly positive. Power must be strictly positive: zero or negative brake power is rejected because dividing a burn rate by a non-positive shaft output is undefined, and those failures are not billed. POST a JSON body with two required finite numbers: fuel_flow, the fuel mass-flow rate, and power, the engine brake power at the same instant. Optional fuel_flow_unit selects g_h, g_s, kg_h, kg_s, lb_h, or lb_s and defaults to kilograms per hour, the unit most dyno logs already print. Optional power_unit selects kw, w, hp, or ps and defaults to kilowatts. Aliases such as mdot, fuel_rate, brake_power, and P are accepted so spreadsheets can reuse column names. Optional precision, with aliases decimals and dp, chooses how many decimal places to expose, defaulting to ten and capped at fifteen so JSON goldens stay stable. The engine converts fuel flow to grams per hour and brake power to kilowatts, then forms bsfc_g_per_kwh as that quotient. The response echoes the inputs, the normalized fuel_flow_g_h and power_kw, the method name, and the formula string. Missing fields, non-finite values, magnitudes above the documented ceiling, and unknown units fail with invalid_input naming the field.

Where a BSFC calculator earns its keep in labs, fleets, and software

Developers call the same contract from scripts to seed test vectors for engine-dashboard widgets, to normalize mixed-unit logs before a report, or to satisfy a product field that must be named bsfc_g_per_kwh rather than a hand-waved miles-per-gallon figure. On this page the work runs entirely in your browser; through the API each successful lookup costs $0.002, failed validation is not charged, and nothing is stored after the response returns. That split — free interactive exploration and a priced, identical API — lets you move from a single-point check to an automated pipeline without changing the mathematical result.

Integration use cases

Lab homework and CI fixtures

Give students or tests a deterministic oracle that returns 200 g/kWh from 10 kg/h at 50 kW and rejects non-positive brake power.

Technical questions

What does the brake-specific fuel consumption calculator cost?

The browser tool is free. API requests use prepaid KIT balance at $0.002 per successful call; invalid input is not billed.

Read the full KIT documentation →