Bobby Clark looks over the...
Bobby Clark looks over the shoulder of engine builder George Pils as they evaluate the results of a dyno run. Photo by Rob Fisher
Over time, I discovered some additional benefits to "BSFC testing" that may be of value to weekly engine builders and testers. In particular, even though engine dyno testing equipment and data acquisition are far more sophisticated than in previous years, there are some fundamental relationships that can be helpful when dyno testing.
First of all, you will find that an engine's BSFC and volumetric efficiency (VE) curves should mirror each other (see figure 1). In a previous Series segment, it was mentioned that VE curves and torque curves (except for the influence of pumping losses) can be essentially overlaid. In addition, unless an engine is experiencing a significant combustion efficiency problem (enrichment dilution from excessive exhaust residue, improper air/fuel mixture ratios, isolated wet-out in the combustion space, or related abnormalities), minimum BSFC should occur at or near peak torque. In fact, it is at this point in an engine's speed range that you can run repeated "spot checks" for fuel calibration adjustments, until further reduction of BSFC nets a power loss. It's then that you've determined minimum BSFC for a given set of engine components and conditions and, correspondingly, the best attending combustion efficiency which in simple terms is the conversion of fuel into power.
Next, this process will also set a bench-mark BSFC that you can use as a template for values below and above the engine's torque peak(s). However, at engine speeds below this rpm point, there isn't sufficient piston speed to generate higher volumetric efficiency, and above peak torque there's not enough time to maintain the VE achieved at peak. As a result, you can expect BSFC numbers to be numerically higher in these two ranges, even though your ultimate goal is to minimize these values.
Figure 1 - As pointed out...
Figure 1 - As pointed out in the text, minimum BSFC. and maximum brake torque should occur at or near the same crankshaft rpm. For a given set of engine components and state of tune, this value of BSFC (once optimized) can be used as a "target" value for best combustion efficiency. Photo by Jim Mcfarland
Finally, remember that BSFC can be used as a measure of combustion efficiency, although the data can be flavored by other conditions that will cause an increase in the actual values. For example, suppose a particular condition exists that's preventing adequate evacuation of combustion residue from the combustion space (e.g., improper valve timing, inefficient exhaust system, etc.) In this case, relatively inert exhaust gases dilute fresh air/fuel charges, upsetting proper enrichment and tending to reduce overall combustion temperature...and combustion efficiency. Consequently, BSFC values will increase.
In another instance, you may note inordinately high BSFC values in engine speed ranges beyond peak torque. If, again for whatever reason, mechanical separation of air and fuel may be taking place (either along the inlet path or in the combustion space), and BSFC values will increase. They will also rise when mixtures are overly rich.
The trick is to compare exhaust gas temperatures with BSFC values (if EGT data is available) and correlate the data accordingly. As an example, undesirably high BSFC values, combined with lower-than-normal EGTs can often be linked with excessive enrichment, even beyond what you can read on spark plugs. Just remember that high BSFC values in the upper rpm ranges can be a blend of the two previously-mentioned conditions, so you'll need to keep an eye of EGTs to pinpoint your analysis.
On The Exhaust Side - Periodically in this Series, we've touched on ways you can influence torque curve "shape." This practice makes more sense if you'll consider some of the major factors that affect where in a given engine speed range torque is produced. What's convenient about this approach is that you'll quickly discover some physical/dimensional relationships between induction and exhaust systems that are common to both. Here's one example of how this can work.