Determining BSFC map from logged OBD2 data

[monroe74]

Originally Posted by fumesucker

as long as the engine is closed loop I think the fuel useage measurements from OBD2 data are probably pretty close

I'm really not sure. I think it depends a lot on the circumstances. For example, I suspect the SG/OBD2 approach also has a problem with cars that have a wideband O2 sensor (e.g., most modern VWs, I think). I think the SG calculations assume AFR that's stoich. With a wideband sensor, real AFR could differ quite a bit from stoich.

I also suspect the problem is insidious, in that the accuracy issues might tend to equalize over the course of a fill. Tracking fills is the traditional way to calibrate a SG. Therefore certain kinds of accuracy problems might be concealed. An instantaneous reading, or a trip-based reading, might be wrong, even though we see good accuracy on a fill-to-fill basis. Therefore I suspect some people might feel confident that they are looking at accurate readings, even though the confidence is misplaced.

But I don't want to overstate the point, since it's far better than what most people have (nothing), and the accuracy problems are probably relatively insignificant for most people, most of the time. And being aware of open-loop vs closed-loop will probably help a lot.

Anyway, I see various smart people (like you folks) trying to tackle these issues in new ways, so I'm hopeful that a lot of the mysteries will start getting solved.