Brake specific fuel consumption

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Brake specific fuel consumption (BSFC) is a measure of the efficiency of an internal combustion engine. The term "specific fuel consumption" refers to the amount of fuel used normalized to the amount of power generated, which gives you an efficiency at at certain operating point of the engine. The term "brake" refers to how the power is measured: a brake is attached to the output of the engine for this measurement.

Understanding what operating conditions are necessary to minimize BSFC will improve fuel efficiency when designing systems to use internal combustion engines, or operating vehicles and equipment. Typically this is most critical when driving an automobile.

Here are key takeaways to be aware of:

  • Maximum efficiency, for gasoline engines, is when the throttle is completely open (engine is under 100% load). The reason is that the engine must suck air past the throttle plate, in which case it is acting like a vacuum pump.
  • Following the point above, to achieve maximum mpg when driving, it pretty much boils down to "be in the highest gear possible at the lowest speed possible". Being in a higher gear causes the throttle to be open more generally. At high speeds engine friction and air drag reduce efficiency.

Brake specific fuel consumption at different engine loads (throttle levels). The throttle being fully open is more efficient than it being even half way open, regardless of rpm.
This contour plot shows the "sweet spot" operating condition for fuel efficiency. The smallest closed contour represents the lowest specific fuel consumption. Note that if you have the throttle completely open (100%), then you are operating at the top of the graph over all rpms. At lower throttle levels, you are operating at the bottom of the graph.
This shows what fuel efficiency you get when operating in 5th gear only. At lower speeds you must have the throttle mostly open because you are driving in a much higher gear than normal and you have very little torque. The low torque at low speeds requires the throttle to be open. At higher speeds, the throttle is closed partially because 5th gear has more power at those speeds.
Fuel efficiency of a 1986 Volkswagon Golf GTI at different speeds. This is what is normally seen because it shows common gears being used at those speeds.