BSFC Calculator

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.

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.

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:

where:

• $\mathrm{BSFC}$ — Brake specific fuel consumption in grams per kilowatt-hour ($\mathrm{g/kWh}$);
• $r$ — Fuel consumption in grams per second ($\mathrm{g/s}$); and
• $P$ — Power measured in watts ($\mathrm{W}$).

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:

where:

• $P$ — Rotational power;
• $\tau$ — Torque; and
• $\omega$ — Rotational speed (in $\mathrm{rad/s}$).

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.

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 $\mathrm{kWh/g}$) 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: $0.0139\ \mathrm{kWh/g}$;

• LPG: $0.01264\ \mathrm{kWh/g}$;

• Gasoline: $0.01206\ \mathrm{kWh/g}$;

• Diesel fuel: $0.01183\ \mathrm{kWh/g}$; and

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

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

where:

• $e$ — Engine efficiency calculated as a percentage; and
• $d_{\mathrm{e}}$ — Energy density.

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:

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!

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 can also calculate the engine's power from the torque and the angular velocity (RPM of the engine): click engine power 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!

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!