Flex Fuel Tables interpolation/extrapolation

[koracing]

I would really like in the case of doing a flex fuel tune with E85 to be able to either set the ethanol content values that tables 1 and 2 were tuned at and have the ecu calculate what the values above and below those content values should be, or allow the multi-fuel ratio to go above 100, and below 0 so I can set the slope the way I would like.  More often than not I'm tuning pump gas (about E10 here) and then we finish that tune and put in enough ethanol to get it above 75% and doe the second tune.  It then becomes a bit of an exercise to calculate what the 100% ethanol value would be for fuel table two and what the 0% ethanol value would be for table 1 to truly allow blending any and all ethanol content values from 0-100.  In the case of ignition advance tables, i don't really mind keeping the same timing from 85% to 100% as most of my customers aren't running more than 85% anyway, but mathematically I wish there was a way to make the blending past the tuned tables a bit easier.  

[Adamw]

If you set the multifuel properties for 85% ethanol and set up the blend table so that 100% table 2 is 85% ethanol but tune table 2 with only say 70% ethanol in the tank then that will work how you propose.  The only thing that maynot work properly quick tune, but I have never tried.

[koracing]

I don't think that will work right if the 70% blend is actuallly using some portion of the values from table 1 and perhaps your table 1 values aren't perfected in the region you're trying to tune table 2 set up properly.  Also would this then allow the ecu to extrapolate beyond table 2 if there was 90% ethanol in the tank?

[Adamw]

9 hours ago, koracing said:

don't think that will work right if the 70% blend is actuallly using some portion of the values from table 1 and perhaps your table 1 values aren't perfected in the region you're trying to tune table 2 set up properly.

Of course table 1 will need to be correct in the first place before tuning table 2.  The same applies to your suggestion of extrapolating.  Regardless of whether you are extrapolating or interpolating the two reference points that you are linearising from must be correct.  

 

9 hours ago, koracing said:

Also would this then allow the ecu to extrapolate beyond table 2 if there was 90% ethanol in the tank?

It would go all the way to 100% if you enter the fuel properties for 100% ethanol.  I just used 85% as an example since you originally said "my customers aren't running more than 85% anyway".  The further you interpolate or extrapolate from the tuned point, the greater any errors will be multiplied - so you are best to choose a max value that is relistic and tune with a fuel as close as possible to that.  

[koracing]

While I understand how this work currently, I wish it was a little simpler to set up the points I'm tuning at to be the table 1 and 2 and have the values extrapolated beyond.  I feel like you're missing the point or stating how unnecesary my wish here is by simply pointing out how it works now.  Most commonly I have customer only run up to 85% but sometimes they run higher.  I guess I could tell them "you better not run more than 85% because you're on a link and that's how we had to set up your tune".  What I'm asking for is something other ecus already do, I'm simply putting the idea out there to ask link to consider doing it as well in the future.

[dx4picco]

I mean, going by what Adam is saying, as long as one of your 2 tuning point is the 0 (or the 100), the second one can be anything above (or below), what you mean by extrapolating will work.

0-> E5 tuned on pump gas for table 1.

100-> e98 but only tuning on e80, then changing the table 2 will still be acurate when you would go to more.

same apply if you tune on e85=100% (table 2) and can only achieve e20 when switching back to pump gas

 

[koracing]

The fundamental problem here is that all of the above listed scenarios assume a perfected tune on one table or the other prior to tuning the second table.  Unfortunatley the reality is there is only so much time that can be alloted to tuning one then the other in most cases.  Usually we are E10 on pump here, but there are places you can buy E0 and customers like trying new things ("I didn't know I couldn't do that").  Often times we have a limited tuning window to try and perfect one of the tables before needing to move on to the second table.  Some people come in for WOT dyno tuning only and want to have the tune dialed in in a couple hours.  Some customers only want to run e85 from a petrol/gas station which varies in content. 

When adding 5 gallons of E85 (from a petrol station here which means it can be 70-85%) to a tank of fuel that is on "E" in a car often the ethanol content barely reaches 60%.  Adding 5 more gallons we can get to 70-75%.  I know if they drained the tank completely, which is rarely feasible, we could get much closer to 85%.  Also if they continue putting E85 in the tank they will eventually get closer to 85%.  I have been keeping a barrel of E98 in my shop just because adding e98 to some amount of left over pump E10 is much easier to get the content up to 80-85%, but that isn't something I always have on hand.  I have had customers on occasion want to run E90-E95 as a test to see if there is more power to be had.  None of these scenarios lend themselves to easy tuning and later refining tables 1 and 2 if they aren't perfect to begin with.  In one ecu I've used a lot in the past the table 2 was entirely an overlay table from table 1 and that worked quite well.  You only had to input in the settings what percent ethanol table 1 was tuned with, and what percent ethanol table 2 was tuned with and it would do the remainder of the extrapolation for other content levels.  It felt quicker to tune to me than the way the link is set up.

It may be my own pickiness, but I feel like if a tune is flex fuel it should be flex fuel capable all the way from 0 to 100% ethanol.  Tuning table 2 with let's say 75-77% ethanol in the tank when table 2 is the 100% ethanol table seems a bit of a hassle.  I've tried tuning table 1 at 10% and then setting table 2 at whatever other ethanol content and then using those two points to calculate the 0% and 100% ethanol tables, but with the difference in fuel density the results aren't always as accurate as I would like.  This is the part of the process I would like to see improved.

I guess I will try tuning at 80-85% with the blend ratio set to 100% at 100% ethanol.  I just don't know what my corrections while tuning will end up being (i.e. subtracting 5% from the VE when the lambda is 5% rich probably won't yield a 5% change), and my lambda target changes and everything else will be quite a ways off of what I'm tuning versus what the tables are targetting.    

I guess I must be alone here in feeling this frustration.