The Use of BSFC Numbers As Operating Values and As A Cycle Average Statistic
Any engine will have different BSFC values at different speeds and loads. For example, a reciprocating engine achieves maximum efficiency when the intake air is unthrottled and the engine is running near its torque peak. However, the numbers often reported for a particular engine are a fuel economy cycle average statistic. For example, the cycle average value of BSFC for a gasoline engine is 322 g/(kW·h), translating to an efficiency of 25% (math calculation: 1/(322*0.0122225)=0.2540 ). However, efficiency for that engine can be lower or higher than this average statistic depending on the operating condition. In the case of a production gasoline engine, the most efficient BSFC is approximately 225 g/(kW·h), which is equivalent to a thermodynamic efficiency of 36%.
An iso-BSFC map (AKA fuel island plot) of a diesel engine is shown. The sweet spot at 206 BSFC has 40.6% efficiency. X axis is rpm, y axis is BMEP in bar (bmep is proportional to torque)
Read more about this topic: Brake Specific Fuel Consumption
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