Whether you're an engine builder or engaged in testing them on an engine dynamometer, a clear and working understanding of brake-specific fuel consumption (BSFC) can be of value. The broader category includes parts designers and those interested in evaluating power level changes involving parts or modifications. In one way or another, any changes in power (positive or negative) can be linked to combustion efficiency. And, simply stated, BSFC is keyed to this as well. Despite previous discussions about the subject, we'll expand on it a bit in this presentation.
Now, although "Enginology" is not intended to include an array of mathematical calculations in support of the information provided, it's worth noting how to compute BSFC because that will help in understanding the importance of its numerical relationships. In the English system of units, the computation involves fuel flow in pounds per hour (pph) and "observed" horsepower (uncorrected for barometric pressure and inlet air temperature). Arithmetically, if we divide fuel flow by observed horsepower, the units of measure will be pounds/horsepower-hour. That's the academic perspective.
As a practical matter, BSFC is a measure of how efficiently a given amount of fuel is being converted into a specific amount of horsepower. More broadly stated, it could also be considered a measure of combustion efficiency, and that's key to our discussion, but first we need to include some thoughts about a related subject.
Regardless of the type of fuel being used, it has a specific energy content for a given volume. That means if we were to burn all the fuel and capture all the heat delivered during any particular combustion cycle, we would have extracted the maximum amount of potential horsepower. Unfortunately, however, the internal combustion engine is not an efficient one. And while you can expect certain percentages of energy content will be lost to the exhaust and cooling systems, they can run in the range of a 20-25 percent loss to each system, in the best of cases.
It's not uncommon for these percentages to be higher. So the objective in building, modifying, or tuning a racing engine is to minimize these unavoidable losses. For example, thermal coatings intended to reduce heat losses to the cooling system are attempts to increase the amount of energy focused in the combustion space. The same applies to coating major exhaust system components, like headers. Makes sense.
Stated another way, we're talking about improving an engine's "thermal efficiency" by minimizing heat losses, particularly to the cooling and exhaust systems. As this is accomplished, power stands to increase, and we need a way to evaluate what's going on in the combustion space. This brings us full circle and back to using BSFC as the yardstick. Short of conducting in-cylinder pressure analysis tests that are comparatively more expensive and complex than considering BSFC data, how do we do this?
First, let's consider a practical example. Suppose we're evaluating a gasoline-fueled racing powerplant on an engine dynamometer. At wide-open throttle, full load, and constant rpm (using race gas), the "chemically correct" baseline BSFC was some time ago considered to be 0.500 pounds of fuel flow/horsepower-hour.
As engine builders and modifiers refined ways to improve both thermal and combustion efficiency by methods that included combustion chamber shapes, piston crown designs, exhaust system efficiency, and related areas, the original "standard" for gasoline decreased to somewhere only slightly north of 0.400. This meant that improved combustion was allowing the same amount of fuel to produce an increase in power—e.g., combustion efficiency improved. As a result, BSFC was reduced.