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Old 02-03-2021, 10:43 AM   #21 (permalink)
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mechanical eff. and BSFC

Originally Posted by AeroMcAeroFace View Post
So "mechanical efficiency of the driveline components are a function of transmitted power" a linear one? It can certainly be approximated to linear over the 5% speed increase, so it can be incorporated into "rolling resistance"

How much does BSFC actually vary by? It is a tiny amount that is negligible over the small speed increase, and so can be ignored altogether.
1) Under the circumstances of constant-throttle, and a variable load, according to Sovran, mechanical efficiency would be non-linear. SAE may have papers dedicated to this specific topic.
2) Personally, I'm reluctant to combine variables, choosing the 'long-hand' approach, just for book-keeping purposes. There may be a particular vehicle road test which incompasses all data, from which one could run the numbers each way in order to detect whether or not something 'funny' was introduced into the quanta.
3) At Bonneville, I experienced over a 10% change in speed, over redline rpm. Technically, engine efficiency would be falling. Without a dyno, it's problematic assigning numerical values of a certain precision. We would have no 'a priori' knowledge of a velocity increase with a given modification.
4) The BSFC question is the most important of all, and again, it's an unknown quantity. An engine map from the manufacturer would be of some help. Perhaps.
5) We know for a fact that, any conclusions about a drag reduction from any particular modification could be off by 40% without gear-matching. That's the sticking point for the whole exercise. Any presumptions about quanta cannot have any degree of confidence above 60%.
Any thought exercise I come up with that might help requires a test track, away from public traffic.

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