EFI bikes - Correction of fuel density

[Meinolf]

Hi,

some years ago I stumbled across the implausible values used in the trim tables for air mass changes following air temperature and ambient pressure. I first noticed it in the 15M. My initial assumption was that Marelli, not quite trusting the reliability of the air/pressure sensors at that time, choose values which wouldn't lead to a much to lean mixture in case of sensor failure or degradation.

Consequently I recalculated the respective trim tables and applied them to the 15M/RC and 5AM BINs of the bikes I own and on which I logged AFR with resulting changes to the fuel and other tables.

However, I found the same deviations from the general gas equation in much later ECUs and all OEM BINs I looked at - all CARC models, Ducati, Morini, Aprilia, BMW, ...

Some time ago the thought popped up that the interpretation of the trim tables was that they applied to air mass changes. But, AFR also includes fuel mass. And there's no trim table for fuel mass changes following the fuel temperature. Compression plays no part, fluid density doesn't change significantly over the ambient pressure delta.

So, the existing trim tables for for air temperature/pressure were enhanced. The coefficient of fuel mass change over temperature is 0,001 ¹/_K. Doesn't seem to be much, but it means that fuel mass changes 2% over a temperature change of 20°C (or °K). Which is in plausible range experienced when fuel is pumped. A reasonable assumption is that fuel temperature follows air temperature, not or much less engine temperature, which I both logged in parallel several years ago.

So, I added the fuel mass change to the existing (corrected) air temperature/pressure) tables of the 5AM BIN. Following the recalculated table for temperature&pressure,  which I've been using so far.





And here's the same table with a fuel mass correction added.





Unfortunately this still doesn't explain the difference to the OEM trim tables values.

I don't expect the change to be really noticeable, it's just one more step in blue-printing and getting a stable AFR in the temperature range I would expect that we drive our bikes in - say -10°C (yeah, I've driven at that temperature) to +40°C. Which we might experience more often even in our areas due to the climate change.

Your thoughts on this topic are appreciated.

Cheers
Meinolf

[berniebee]

So, um, that works out to how many turns in for the mixture screw?   

[Meinolf]

Hi,

So, um, that works out to how many turns in for the mixture screw?

The bypass screws influence the air part, not the fuel part of AFR. Hence, no connection.

Cheers
Meinolf

[berniebee]

Hi,

The bypass screws influence the air part, not the fuel part of AFR. Hence, no connection.

Cheers
Meinolf

I was actually making a joke about how this in depth article related to the carburettors on my old Guzzi, forgetting that there can be adjustment screws on an EFI system. Apologies.

[Meinolf]

Hi,

Will this be a change that we can retrofit to our existing maps, or does the entire map need to be rewritten. (I have a 15M.)

an excellent question.

The jury's still out on the procedure for the 15M/RC, I haven't gotten my head around it yet. I have to check if the engine temp trim table has breakpoint where the factor is 1. If there's one, then the % changes can be applied to the rest of the trim table without having to change any fuel (or other) value in the BIN. If not, a multiplication with 1.02 of all fuel values would then take care of the required fuel value changes.

Concerning the 5AM, the situation's a bit different. The air pressure/air temp table is 2D and the OEM table had a spot in which the correction factor was 1 (it's the green'ed cell in the picture). The previous correction of air pressure/air temp values was based on this cell, hence there was no need to change anything anywhere. And the same applies to the additional fuel density correction. It's also based on the cell with the value 1.

Cheers
Meinolf

[Meinolf]

Hi,

I was actually making a joke about how this in depth article related to the carburettors on my old Guzzi, forgetting that there can be adjustment screws on an EFI system. Apologies.

no apologies needed, in fact it's an astute observation. Carburetors, by design and because they follow the physics, mostly mitigate such changes. I've never done the calculation, but air mass and fuel mass density coefficients change in the same direction, even if the coefficients are different.

Cheers
Meinolf

[moto-uno]

  Would any of this be of some consequence to the maps for a 2018 Eldorado ?  Peter

[Meinolf]

Hi,

  Would any of this be of some consequence to the maps for a 2018 Eldorado ?  Peter

I've never checked the air pressure/air temperature correction values of a Eldorado BIN against the general gas equation. But, as the discrepancy was found in all previous Marelli BIN I did look at, it's likely to be present also.

Cheers
Meinolf