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Specific fuel consumption, abbreviated SFC, compares the ratio of the fuel used by an engine to a certain force such as the amount of power the engine produces. It allows engines of all different sizes to be compared to see which is the most fuel efficient. It allows manufacturers to see which engine will use the least fuel while still producing a high amount of power.
There are different types of SFC: TSFC, thrust specific fuel consumption, and BSFC, brake specific fuel consumption, are two of the most common. TSFC looks at the fuel consumption of an engine with respect to the thrust output, or power, of the engine. Airplane engines, for example, can be compared to see which will produce the most thrust while using the least amount of fuel.
TSFC is expressed in the amount of fuel needed to provide a certain thrust over a period of time. This formula is written as pounds of fuel per hour of thrust. There are disadvantages to this formula, however. The most fuel efficient engine may not always be the best choice. A more lightweight engine may cut down on the need for more fuel to power it, and thus be a better choice even if a heavier engine has a lower TSFC.
BSFC is used to calculate and compare how fuel efficient a reciprocating engine is. The reciprocating engine is a type of engine that uses pistons to create the motion that powers the engine. The most common type is an internal combustion engine, found in most vehicles today.
The formula for measuring BSFC is the fuel rate over power. The fuel rate is expressed as the fuel consumption of the engine in grams per second and power is expressed as the amount of power the engine produces written in watts. The final answer for calculating BSFC is typically expressed in grams per kilowatt-hour.
While specific fuel consumption has its advantages, it has its disadvantages as well. While it allows engines of all sizes to be compared, resulting in a chart that shows the most efficient engine, it can also leave out other important factors. The engine design, what it will be used for, and where it will be used all affect the engine's performance and specific fuel consumption can only make an educated guess at which engine will perform the best.
I would really like to see this formula used to compare modern engines to those dating back 60 years or so. It seems that we're getting a lot more power out of engines (the V-6 power plants used in Ford Mustangs, Chevrolet Camaros and Dodge Challengers, for example) while using a lot less gas than ever before.
Another interesting comparison would be to see how the adoption of emissions controls impacted power and fuel consumption on engines from the 1970s. You could have a 400 cubic inch engine putting out less than 200 horsepower in the last half of the 1970s, so how did that compare against the same sized engine in muscle cars in the 1960s? My guess is the 1970s engines fared a lot worse due to reduced power and perhaps only slightly better fuel economy.