BSFC is a measure that is used to express the consumption of the engine regardless of the size of the engine, so that the data can be used to compare the consumption of engines of different powers. In the international system, the BSFC is expressed as the mass fuel consumption (gr.) divided by the mechanical energy supplied in kW / h. In this way, an engine that consumes for example 300gr. / kW / h means that it consumes 300gr. of fuel for each kW / h it generates. For a motor that has a power of 2kW for example, this means that it generates 2kW / h of mechanical energy in 1 hour, and therefore the motor will consume 600gr. / hour.
The BSFC for a given engine should be expressed on a table that covers all the engine duty points. Or at least some graphs that show some of the operating points like the above graph.
In practice, manufacturers express the BSFC as a single data without detailing the RPM point and throttle position.
In the case of UAV engines, the right way would be to depict the consumption in the cruise of the aircraft, which can be around 50 to 65% of the accelerator. Correct BSFC data allows the drone operator to predict fuel consumption for a given mission.
To measure the BSFC, the power measurement needs to be arranged as precise as possible, for example in a stationary power bank that allows the motor to be kept at an exact RPM while the motor torque is measured. Engine consumption can be easily measured using a simple test tube and a stopwatch.
For example, if we have measured a consumption of 20ml for 30 seconds, it can be said that the consumption is 3600 seconds / 30 seconds * 20ml = 2.4L / hr, if converted to gr. multiplying by the density of gasoline (0.76kg / hr) a consumed mass of 1824 gr. per hour comes out. If the motor is developing 5kW during the test, it means that in 1 hour it would give a mechanical power of 5kW / h. therefore 1824 gr / hr divided 5kW / hr is 364gr / kW / hr.