Commanded AFR in Open Loop
#1
Commanded AFR in Open Loop
I was wondering what controls commanded AFR in open loop other than the tables "Loop Open % Change AFR vs MAP" and "Loop Open % Change AFR vs Temp"
The target AFR that I am logging seems to be off from what I would expect the tables to command.
Example @ 50KPa and 92C and forced open loop.
14.7/(1+.0391+.0977)=12.93
Data logging shows it to be commanding close to 14.7.
Maybe I am not understanding the tables correctly.
The target AFR that I am logging seems to be off from what I would expect the tables to command.
Example @ 50KPa and 92C and forced open loop.
14.7/(1+.0391+.0977)=12.93
Data logging shows it to be commanding close to 14.7.
Maybe I am not understanding the tables correctly.
#5
Re: Commanded AFR in Open Loop
The power enrich tables work the way I would expect.
Example @ 4000RPM and 80C.
14.7/(1+.2266-.0430)=12.42
The target AFR from the data log is right in line with this.
I don't quite understand what you mean by Fuel to Air Ratio.
Does the computer calculate the target AFR differently in open loop than it does in power enrich?
Example @ 4000RPM and 80C.
14.7/(1+.2266-.0430)=12.42
The target AFR from the data log is right in line with this.
I don't quite understand what you mean by Fuel to Air Ratio.
Does the computer calculate the target AFR differently in open loop than it does in power enrich?
#6
Moderator
iTrader: (1)
Join Date: Mar 2002
Location: Chasing Electrons
Posts: 18,401
Likes: 0
Received 215 Likes
on
201 Posts
Car: check
Engine: check
Transmission: check
Re: Commanded AFR in Open Loop
Check this link:
https://www.thirdgen.org/techbb2/sho...hreadid=214156
This shows the math as the ECM does it for PE mode. Which is basically the same for open loop AFR.
The modifier tables are a percent change to the FAR value. For 14.7:1 the ECM uses 445. The conversion to AFR is: AFR = 6553.6 / FAR (445)
Note that the FAR isn't the true FAR, but one that has been scaled for use by the ECM.
RBob.
https://www.thirdgen.org/techbb2/sho...hreadid=214156
This shows the math as the ECM does it for PE mode. Which is basically the same for open loop AFR.
The modifier tables are a percent change to the FAR value. For 14.7:1 the ECM uses 445. The conversion to AFR is: AFR = 6553.6 / FAR (445)
Note that the FAR isn't the true FAR, but one that has been scaled for use by the ECM.
RBob.
#7
Moderator
iTrader: (1)
Join Date: Mar 2002
Location: Chasing Electrons
Posts: 18,401
Likes: 0
Received 215 Likes
on
201 Posts
Car: check
Engine: check
Transmission: check
Re: Commanded AFR in Open Loop
Lately there has been a number of requests about how $8D calculates open loop AFR. So... put this together:
The code in the $8D mask works with a fuel to air ratio (FAR) value. This makes the math easier for the ECM to run. This FAR can be converted to AFR by dividing 6553.6 by the ECM's FAR value.
This why stoich is 445 in the ECM: 6553.6 / 445 = 14.7 AFR. Also note how the math is done using binary increments. Such as: 6553.6 * 10 = 65536 in decimal, which is $10000 (hex).
In open loop the ECM uses six parameters to create the FAR. Note that this is standard mode, not in PE mode.
There is:
Stoich FAR
% change versus in idle
% change versus MAP
% change versus CTS
% change versus after-start enrichment
scalar for % change versus CTS
The scaling of the % change values is: % of change = value / 2.56
With the value being the raw BIN value. So a value of 36 is a 14% change to the FAR. And an increase at that, as that is a lower (richer) AFR.
All of the % change values are added together (it is a 16-bit value). The stoich FAR is then multiplied by the total % change and then divided by 256. The result is the commanded FAR.
So:
FAR = (Stoich_FAR * (%_idle + %_MAP + %_CTS + %_AfterStart + CTS_Scalar) ) / 256
Using values from AXCN, at 56* C (25 for 9.8%) and 90 KPa (36 for 14%), afterstart already decayed out. Note that the % change for idle is 0 in this calibration.
FAR = (445 * (0 + 36 + 25 + 0 + 221) ) / 256
FAR = (445 * 282) / 256
FAR = (125490) / 256
FAR = 490
The value of 490 is used by the ECM in the injector PW calculation.
For the AFR:
AFR = 6553.6 / 490
AFR = 13.4
RBob.
The code in the $8D mask works with a fuel to air ratio (FAR) value. This makes the math easier for the ECM to run. This FAR can be converted to AFR by dividing 6553.6 by the ECM's FAR value.
This why stoich is 445 in the ECM: 6553.6 / 445 = 14.7 AFR. Also note how the math is done using binary increments. Such as: 6553.6 * 10 = 65536 in decimal, which is $10000 (hex).
In open loop the ECM uses six parameters to create the FAR. Note that this is standard mode, not in PE mode.
There is:
Stoich FAR
% change versus in idle
% change versus MAP
% change versus CTS
% change versus after-start enrichment
scalar for % change versus CTS
The scaling of the % change values is: % of change = value / 2.56
With the value being the raw BIN value. So a value of 36 is a 14% change to the FAR. And an increase at that, as that is a lower (richer) AFR.
All of the % change values are added together (it is a 16-bit value). The stoich FAR is then multiplied by the total % change and then divided by 256. The result is the commanded FAR.
So:
FAR = (Stoich_FAR * (%_idle + %_MAP + %_CTS + %_AfterStart + CTS_Scalar) ) / 256
Using values from AXCN, at 56* C (25 for 9.8%) and 90 KPa (36 for 14%), afterstart already decayed out. Note that the % change for idle is 0 in this calibration.
FAR = (445 * (0 + 36 + 25 + 0 + 221) ) / 256
FAR = (445 * 282) / 256
FAR = (125490) / 256
FAR = 490
The value of 490 is used by the ECM in the injector PW calculation.
For the AFR:
AFR = 6553.6 / 490
AFR = 13.4
RBob.
Last edited by RBob; 08-25-2011 at 03:17 PM.
Trending Topics
#9
Re: Commanded AFR in Open Loop
Thanks RBob.
This clears things up a lot.
The only thing I cant locate in the tuning software is "scalar for % change versus CTS".
Is this some sort of constant that cant be changed?
Thanks again.
This clears things up a lot.
The only thing I cant locate in the tuning software is "scalar for % change versus CTS".
Is this some sort of constant that cant be changed?
Thanks again.
#10
Moderator
iTrader: (1)
Join Date: Mar 2002
Location: Chasing Electrons
Posts: 18,401
Likes: 0
Received 215 Likes
on
201 Posts
Car: check
Engine: check
Transmission: check
Re: Commanded AFR in Open Loop
Code:
;======================================= ; OPEN LOOP AIR/FUEL PCT CHANGE vs COOLANT ; ; ; TBL = (PCT CHG * 2.56) * SCALAR ;======================================= L845D: FCB 221 ; SCALAR, ; ARG = 256 - VALUE, (35d) ;-------------------------------------- ; %CHG ; Deg c ;-------------------------------------- L845E: FCB 242 ; 94.5 -40 FCB 153 ; 59.8 -28 . . .
RBob.
#12
Re: Commanded AFR in Open Loop
It is right before the table:
RBob.
Code:
;======================================= ; OPEN LOOP AIR/FUEL PCT CHANGE vs COOLANT ; ; ; TBL = (PCT CHG * 2.56) * SCALAR ;======================================= L845D: FCB 221 ; SCALAR, ; ARG = 256 - VALUE, (35d) ;-------------------------------------- ; %CHG ; Deg c ;-------------------------------------- L845E: FCB 242 ; 94.5 -40 FCB 153 ; 59.8 -28 . . .
How can I view the code? With Tunerpro, Excel?
Thanks
Thread
Thread Starter
Forum
Replies
Last Post
355tpipickup
Tech / General Engine
9
09-13-2015 11:35 PM