Quote:
Originally Posted by JCL
You don't actually measure octane when you test a fuel, you measure knock and compare it to the knock from a reference fuel in a test engine under controlled conditions. BMW could easily determine how much the timing was being retarded in response to measured knock, but that wouldn't measure octane, because additional variables of altitude, ambient temperature, carbon buildup, etc, would all impact knock.
|
Correct about the carbon buildup; however, much of the other stuff is readily available to the computer, e.g., ambient temperature, altitude (air density/mass/pressure), etc.
Regarding the diesel, I am not sure how they would derive Cetane; however, if they could they would have a nice Catch-22:
(1) Sorry, the data shows you were not using the recommended 51 Cetane fuel, so you violated the warranty, or
(2) Sorry, the data shows you were using fuel with a Cetane rating of 51. Since such fuel is not in common use in the U.S., you must have been using a Cetane enhancing additive which is prohibited so you violated the warranty.
P.S. A Google search revealed this interesting link:
http://www.spiritpfc.com/shatox_octanemeterbrochure.pdf
The accuracy, if true, is rather surprising, at least to me.