Determining BSFC map from logged OBD2 data
 

I went ahead and bit the bullet and ordered an Elm327 interface for the OBD2 port.. I imagine it should be here in a week or so.

What I'm interested in (besides the obvious mpg data) is logging data that will allow me to derive a BSFC map, I think such a thing would be very helpful for those wanting to keep their engine running in the most efficient portion of its operating range.

It seems to me that BSFC could be determined from speed, injector width, throttle position and rpm signals with a little math. I'm still mulling over what would need to be done with the data but I think I have a handle on it.

Determine horsepower from acceleration and weight, then the BSFC for various rpm and throttle settings could be found by measuring injector pulse width to get fuel flow figures for each set of operating conditions.

Any thoughts?

You'd need something to accurately measure load AND fuel flow to create a BSFC map. Ideally, you'd want to make a 3-dimensional graph with BSFC plotted against RPM and hp. In a perfect world, you'd have access to a hysteresis dyno that would allow you to fix the RPM and vary the throttle at each RPM point to determine the fuel flow at each TPS/hp setting. Absent that, I'd invest in some type of accelerometer.

Acceleration is simply difference in speed over time, that will be logged with OBD2 data.

I'm looking at creating a spreadsheet that can take maybe ten miles worth of OBD2 driving log data over a known course and come up with a rough BSFC map, something to allow the driver to know where the fuel efficiency "sweet spot" is in the operating range of his engine..

I'm not looking for ultimate accuracy, just reasonable qualitative guesstimates .

I'm thinking your best bet would be to do several runs, at 10% throttle, 15%, 20%, etc. Run from as low a speed as you can up to max rpm, choosing a gear that allows you to max out at a reasonable speed, so aero drag isn't a big issue.

I had planned on collecting data on aero drag and rolling resistance through some coast down runs and then incorporating that data in the spreadsheet. Once you characterize the coast down you can then add that to the acceleration data and largely compensate for drag and rolling resistance. My biggest problem is that I live in a relatively hilly area and finding a flat straight road that is free enough of traffic to do coast down tests is hard.

I wrote a performance calculator program in Visual Basic for electric powered model airplanes about a dozen years ago after collecting equations from about thirty years worth of old magazine articles among other sources.. By today's standards my program was pretty primitive but it gave surprisingly accurate results for things like climb rate, motor run time, stall speed and so forth.

I'd like to take the same kind of simulation techniques and apply them to helping people save energy in their driving.

Now that I think about it, air drag shouldn't affect BSFC measurements. You're just measuring the engine output against some kind of drag, but it doesn't matter what it is. The loading will change the acceleration that results, but not the engine output.

You have to know what the power output is in the first place before you can determine BSFC so you will have to have a handle on aero drag..

Even a non level road can be fairly easily compensated for by running tests in both directions and averaging the results.. I haven't been able to find a truly level road anywhere around where I live, the one flat straight stretch I have found is significantly tilted.

Do you need to know the output? Sure, to have a true BSFC chart you do.

I think what's more useful to us, though, is a throttle-based chart. TPS vs rpm, with rings of fuel usage. Throttle position is something we can directly control, vs horsepower output.

I can't directly control throttle position in my DBW car. :)

Anyway, the throttle-based chart is workable. If OBDII could report measured fuel rate it would be very easy. Less easily, it can be done using the fuel injector duty cycle (fuel rate) meter (thanks, monroe74!) combined with the TPS and RPM readings logs. It would be necessary to get video of the meter, along with something for reference -- tachometer maybe. Then you frame through the video and record the meter reading alongside the logged TPS and RPM readings.

I've been considering doing something like that.

Use OBD2 to log speed, tps and all the other stuff and a channel on your audio card to log the injector pulse train if OBD2 does not have that data.

I'm pretty sure everything else you might need can be calculated from that.

You could get a lot of nice data to massage if you did that..

Update rate will be critical in getting good data.

A vacuum gauge will give you engine load..

Essentially the engine will be most efficient at a given vacuum setting and rpm.

Vacuum doesn't necessarily equate strictly with tps.

I should hope not. Except in DBW cars, TPS equates strictly with your right foot.

I was trying to be nice. :)

Sorry I've been away for a while. Thank you for acknowledging me.

I think these various efforts to create BSFC measurements sound very promising. I hope to hear more about what kind of results people come up with.

I got my ELM327 in the mail today.. After a little experimentation this evening I think I may have found the software that will calculate a BSFC map from the OBD2 data..

Take a look and tell me what you think..

https://www.edgeanalysis.com/obd2.asp

That's an interesting product. Thanks for pointing it out.

I see that they are displaying and logging lots of OBD2 data. But I don't see where they are attempting to do any power calculations, like BSFC. So I guess you could export the raw data and apply your own procedure for deriving BSFC, but I think most of that work is going to happen outside this program. They're giving you raw speed data, but you have to translate that into acceleration, and then translate that into power, and then relate that to fuel use.

The power calculation includes tricky factors like whether or not you're traversing a grade, and the effect of aero drag at different speeds. These calculations are much simpler if you live on a planet with no hills and no atmosphere. Anyway, I don't see where that program attempts to tackle these issues. But maybe it's in there and I didn't notice!

And there's a separate issue, which I think you already know about. OBD (at least for most makers) doesn't read the injectors. So any fuel-use calculation is some kind of calculated approximation, based on things like air flow. This is how the Scangauge works, and I think under certain conditions it creates problems with accuracy.

Either way, I'm interested in hearing more about what you're doing, because I think you're on the right track and you're inevitably going to make helpful discoveries.

I haven't had time to do any more testing, but I got the impression that there was the option to do custom math in the program..

One problem I see with the program on the old laptop I have is that that it uses Microshaft's dot net framework and the update rate is considerably slower than a couple of other similar but less capable programs I have tried. The newer high level programming languages can be glacially slow on older machines, it took nearly half an hour to install the dot net framework on my laptop.

I know that there are some potentially tricky calculations to do and a few different factors to take into account.. Anywhere near my home there is no such thing as a flat, straight and actually level road so I plan on taking multiple measurements over the course of probably ten or twenty miles of driving and averaging the results.

I think one of the reasons that Scangauge loses accuracy sometimes is because many cars go into DFCO sometimes and Scangauge does not allow for that. As you say, OBD2 does not seem to directly access injector pulse width so all fuel consumption figures are inferred from other data.

However, as long as the engine is closed loop I think the fuel useage measurements from OBD2 data are probably pretty close, for this reason I intend to monitor the open loop/closed loop status and only process data that is taken during closed loop running.

One piece of data that may help you: OBDII reports Open Loop during DFCO, at least on my VW.

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.

Yup, my 2008 VW is almost certainly wideband O2 equipped.

Such concerns are probably the major thing holding me back from spending on a SG now. I'd like to fine tune my driving more accurately than any OBD device can due to being unable to read actual measured fuel rate. I may get one anyway to use for live data from available sensors.

I guess I'm guilty of assuming that anyone who really needs to save on gas lacks the financial wherewithal to purchase one of the newer vehicles that have wideband sensors.. That's why I didn't address wideband sensor equipped cars in my last post.

Other than some of the lean burn Hondas I'm not sure all that many cars which the financially embarrassed community to which I belong have wideband sensors. Frankly, when you don't have a lot of money it's hard to put yourself in the shoes of someone who does.. Thanks to astronomical medical bills my wife and I have no serious hope of ever getting out of debt and I'm guessing that most people who are really serious about saving gas are also part of the forgotten lower middle class in America.

Mea maxima culpa.. It simply didn't occur to me to include wideband sensor equipped cars in my thoughts.

The ranks of the financially embarrassed might be bigger than you think. And the reality of the national debt means we should all feel financially embarrassed (I realize some members of this forum are outside the US; obviously I don't mean you). But I realize that's not much consolation. Anyway, I send you my best wishes for health and prosperity. Hopefully better times are ahead.

I appreciate your kind words.. Thank you.

Our physical health is actually pretty good for our ages right now but several emergency surgeries that somehow ended up not being covered by insurance have left us in poor financial health.. A tip, read your insurance policies carefully and with an extremely cynical eye. In one case whether or not a surgery was covered was determined by a single "and" in a long and complex document.

I'm well aware that we have a lot of company here in the poor house, just about everyone I know is struggling right now.

In the short term I foresee worse times, probably medium term too.. The long term has so many imponderable variables that it really is impossible to make even a SWAG about what might happen.

SG has a load (LOD) instantaneous reading which may be useful in this.

I suspect all of you are way ahead of me on this, but I didn't see it mentioned.

I am trying to figure out how to create a BSFC chart on a motorcycle, where no OBDII interface is present. I use an SG in one car, and really like it. I'm upgrading to the SGII currently.

First post. There is some good info on this forum.