Our BSFC calculator is undoubtedly helpful in a world where reducing consumption, particularly fossil fuel consumption, is of prime importance. In a few simple steps, you will be able to calculate the brake-specific fuel consumption for an engine and even find its efficiency for a given fuel.

In this short article, you will find:

  • What is the brake-specific fuel consumption?

  • How do I calculate the BSFC?

  • What are the uses and applications of the brake-specific fuel consumption formula?

  • How to calculate the efficiency of an engine from the BSFC.

What is Brake specific fuel consumption (BFSC)?

If an engine produces rotational power, we can calculate its brake-specific fuel consumption. This quantity measures the efficiency of an internal combustion engine by calculating the ratio between the fuel consumption and the delivered power. Fuel consumption alone is not enough information to assess an engine's performance: it only tells us how often we need to refill our tank, but no information about the engine's output.

There's more to unpack about this concept, but we need to define the brake-specific fuel consumption formula.

How do I calculate BSFC?

To calculate brake-specific fuel consumption, you need to know two quantities: the output power of the engine and its fuel consumption. The formula for the brake-specific fuel consumption becomes, then:

BSFC=rP,\mathrm{BSFC} = \frac{r}{P},

where:

  • BSFC\mathrm{BSFC}Brake specific fuel consumption in grams per kilowatt-hour (g/kWh\mathrm{g/kWh});
  • rrFuel consumption in grams per second (g/s\mathrm{g/s}); and
  • PPPower measured in watts (W\mathrm{W}).

Since we are dealing with the rotational power, we can swiftly substitute the last quantity with the even more immediate expression we met in the torque to hp calculator:

P=τ×ω,P = \tau \times \omega,

where:

  • PPRotational power;
  • τ\tauTorque; and
  • ω\omegaRotational speed (in rad/s\mathrm{rad/s}).

Take a look at the formula to calculate BSFC: you can clearly see that a lower BSFC is desirable: it corresponds either to a lower fuel consumption or a higher power output. Either way, when checking the values of brake-specific fuel consumption in engines over the years, you will see a downward trend.

How to use the BSFC to calculate the efficiency

The BSFC is not a direct measure of the efficiency of the engine. To find this quantity, you need to consider the energy density of the fuel you feed to it. Once you know this value, multiply it by the BSFC.

The energy density (measured in kWh/g\mathrm{kWh/g}) indicates the amount of available energy in a unit mass of fuel. In contrast, the BSFC indicates the mass of fuel required to produce a given amount of energy. Multiplying these quantities is a measure of the fraction of energy in input that gets transformed into available energy from combustion.

To calculate the efficiency of an engine, refer to the values of energy density for the most commonly used fuels:

  • Methane: 0.0139 kWh/g0.0139\ \mathrm{kWh/g};

  • LPG: 0.01264 kWh/g0.01264\ \mathrm{kWh/g};

  • Gasoline: 0.01206 kWh/g0.01206\ \mathrm{kWh/g};

  • Diesel fuel: 0.01183 kWh/g0.01183\ \mathrm{kWh/g}; and

  • Kerosene: 0.01194 kWh/g0.01194\ \mathrm{kWh/g}.

Finally, to find the engine efficiency, apply the following formula:

e=1denergy×BSFC×100%,e =\frac{1}{d_{\mathrm{energy}}\times \mathrm{BSFC}}\times100\%,

where:

  • eeEngine efficiency calculated as a percentage; and
  • ded_{\mathrm{e}}Energy density.

Use the brake specific fuel consumption to calculate the fuel consumption of an engine

An engine has, of course, different regimes of operations, each of them characterized by a different brake-specific fuel consumption. To calculate the fuel consumption of an engine using the BSFC, you usually refer to an averaged value over the possible regimes. That's also the tabulated value you can look for on operators' manuals and online.

To find the fuel consumption, multiply the brake-specific fuel efficiency by the power delivered by the engine:

r=BSFC×Pr = \mathrm{BSFC}\times P

The impossibility of mapping all the possible regimes at once limits the result of the previous calculations: don't plan your trip according to this result!

How to use our BSFC calculator

Our BSFC calculator implements the formula for brake-specific fuel consumption and the calculations for the efficiency of an engine. The units in these calculations are a bit on the messy side, so we separated the metric calculations and their imperial counterparts. Select your desired measurement system at the top of the tool!

🙋 You may need some conversion done before using our BSFC calculator. We suggest the weight converter (for the fuel consumption rate) and the energy conversion calculator (to help you with the conversion between joules and kilowatt-hour).

You can also calculate the engine's power from the torque and the angular velocity (RPM of the engine): click on advanced mode to show the two variables. Moreover, we inserted a selection of fuel to help you with the efficiency calculations. Do you need a specific fuel that's missing there? Let us know!

🙋 You can use our BSFC calculator in reverse too! Calculate the BSFC for the desired efficiency or the power corresponding to a given BSFC for a fixed fuel consumption.

This tool is very well complemented by other automotive-related tools: visit the fuel consumption calculator for a less theoretical way to calculate your consumptions or the fuel cost calculator for an economic take on the matter!

FAQ

What is the brake-specific fuel consumption?

The brake-specific fuel consumption (BSFC) measures an engine's performance (efficiency). With the BSFC, you have a piece of ready information about the amount of fuel consumption required to deliver a specific power. Engines with a low BSFW have a higher efficiency (they can deliver the same power for lower consumption of fuel).

How do I calculate the brake specific fuel consumption?

To calculate the brake-specific fuel consumption, you need to follow some simple steps:

  1. Find the fuel consumption of the engine r.

  2. Measure the power associated with that specific fuel consumption P.

    • You can also calculate the power with the formula:

      P = τ × ω,

      where:

      • τ — Torque; and
      • ω — Angular speed (in RPM or similar units).
  1. Calculate the BSFC as the ratio between the two quantities above:

    BSFC = r/P

What is the efficiency of a 250 kW gasoline engine consuming 14.78 g/s?

38.9%. To find this result, follow a few simple steps:

  1. Calculate the brake-specific fuel consumption of the engine:
    BSFC = r/P = 14.78 g/s/250 kW =
    0.05912 × 3600 g/kWh = 212.83 g/kWh

  2. Multiply the BSFC by the energy density of gasoline (de = 0.01206 kWh/g):

    BSFC × de = 212.83 g/kWh ×0.01206 kWh/g = 2.57

  3. Calculate the inverse of the result and find the corresponding percentage:

    e = (1/2.57) × 100% = 38.96%

This is not a bad result for an internal combustion engine, but it's still pretty low!

Is a lower BSFC good?

Yes! The lower the brake-specific fuel consumption (BSFC), the higher the power delivered for a given fuel consumption (in g/s), or, vice-versa, the engine requires a lower fuel consumption to achieve the same power.

Davide Borchia
Units
Imperial
Fuel consumption
lb/s
Power
hp(l)
BSFC (brake specific fuel consumption)
lb/hp·h
Fuel type
Gasoline
Efficiency
%
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