Fuel Trims?
08-03-2010, 12:50 PM
#1
3rd Gear Member
Thread Starter
Join Date: May 2008
Location: NH
Posts: 574
Fuel Trims?
So I finally go around to updating Sniper Recon and now that the fuel trims work, I realized I have no idea what the really mean or how my adjustment will affect them. I can't seem to find a good write up on it, so if someone would like to explain or give me a something to read so I can understand it, that would be must appreciated.
08-03-2010, 01:42 PM
#2
5th Gear Member
Join Date: Mar 2008
Location: Ohio
Posts: 2,994
So I finally go around to updating Sniper Recon and now that the fuel trims work, I realized I have no idea what the really mean or how my adjustment will affect them. I can't seem to find a good write up on it, so if someone would like to explain or give me a something to read so I can understand it, that would be must appreciated.
08-03-2010, 03:31 PM
#4
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
This is from the shop manual:
Overall the fuel trims indicate how accurate the injector low and high slope settings are, and how accurate the MAF transfer function is.
Fuel Trim
Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7 to 1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air/fuel ratio rich and lean to get the oxygen sensor to "switch" around the stoichiometric point. If the time between switches are the same, then the system is actually operating at stoichiometry.
The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometry is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%.
Some calibrations will have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometry. This practice is referred to as using "bias". For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.
Values for SHRTFT1 and 2 may change a great deal on a scan tool when the engine is operated at different rpm and load points. This is because SHRTFT1 and 2 will react to fuel delivery variability that can change as a function of engine rpm and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel, the short term fuel trim corrections can be "learned" by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in Keep Alive Memory (KAM) in tables that are referenced by engine speed and load (and by bank for engines with two HO2S sensors forward of the catalyst). Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:
Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.
Long term fuel trim corrections can be used both while in open loop and closed loop modes.
Long term fuel trim is represented as a percentage, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that is used for each rpm/load point of engine operation.
Long term fuel trim corrections may change depending on the operating conditions of the engine (rpm and load), ambient air temperature and fuel quality (% alcohol, oxygenates, etc.).
When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different rpm and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that rpm/load point.
Short Term Fuel Trim
If the oxygen sensors are warmed up and the PCM determines that the engine can operate near stoichiometric air/fuel ratio (14.7 to 1 for gasoline), the PCM goes into closed loop fuel control mode. Since an oxygen sensor can only indicate rich or lean, the fuel control strategy must constantly adjust the desired air/fuel ratio rich and lean to get the oxygen sensor to "switch" around the stoichiometric point. If the time between switches are the same, then the system is actually operating at stoichiometry.
The desired air/fuel control parameter is called short term fuel trim (SHRTFT1 and 2) where stoichiometry is represented by 0%. Richer (more fuel) is represented by a positive number and leaner (less fuel) is represented by a negative number. Normal operating range for short term fuel trim is +/- 25%.
Some calibrations will have time between switches and short term fuel trim excursions that are not equal. These unequal excursions are used to run the system slightly lean or rich of stoichiometry. This practice is referred to as using "bias". For example, the fuel system can be biased slightly rich during closed loop fuel to help reduce NOx.
Values for SHRTFT1 and 2 may change a great deal on a scan tool when the engine is operated at different rpm and load points. This is because SHRTFT1 and 2 will react to fuel delivery variability that can change as a function of engine rpm and load. Short term fuel trim values are not retained after the engine is turned off.
Long Term Fuel Trim
While the engine is operating in closed loop fuel, the short term fuel trim corrections can be "learned" by the PCM as long term fuel trim (LONGFT1 and 2) corrections. These corrections are stored in Keep Alive Memory (KAM) in tables that are referenced by engine speed and load (and by bank for engines with two HO2S sensors forward of the catalyst). Learning the corrections in KAM improves both open loop and closed loop air/fuel ratio control. Advantages include:
Short term fuel trim does not have to generate new corrections each time the engine goes into closed loop.
Long term fuel trim corrections can be used both while in open loop and closed loop modes.
Long term fuel trim is represented as a percentage, just like short term fuel trim, however it is not a single parameter. There is a separate long term fuel trim value that is used for each rpm/load point of engine operation.
Long term fuel trim corrections may change depending on the operating conditions of the engine (rpm and load), ambient air temperature and fuel quality (% alcohol, oxygenates, etc.).
When viewing the LONGFT1/2 PID(s), the values may change a great deal as the engine is operated at different rpm and load points. The LONGFT1/2 PID(s) will display the long term fuel trim correction that is currently being used at that rpm/load point.
08-03-2010, 11:43 PM
#6
TECH SAVANT
Join Date: Dec 2006
Location: Saint Augustine, FL
Posts: 10,938
When changing fuel ratios in with special forces, is it changing the long term trims then?
When you use Special Forces to change the fueling what gets changed depends on whether you change the RPM based, or the global part throttle (PT) or full throttle (FT) modifiers:
When you change the RPM based modifiers SF makes changes to the Open Loop Stabilised Fuel Table (OLSFT) --in the rpm range specified, for TPS output voltages of 2.44V+ (I.e. > 1/2 throttle or so)--to set the target AFR richer or leaner.
The RPM based drop-down selections in SF add or subtract from the "air" value in the air/fuel ratio. Selecting +0.2 therefore wo9uld add 0.2 to the OLSFT entry and make the mix leaner--E.g. if the value were 13.5:1 0.2 would be added to make it 13.7:1; leaner)
Changing the FT and PT global fuel modifiers changes the high (FT) and low (PT) fuel injector slope values. The high slope is normally set to the same to same value as the fuel injector's lb/h rating, the low slope is set to the lb/h rating +15% or so. This is because at short pulse widths the injector's capacity increases slightly as its opening and closing reaction times become a greater percentage of the total pulse width.
The values selected in the FT and PT drop-downs are used as divisors to modify the high and low injector slopes values.
In the OEM tune for our cars the high slope value is 19.1523lb/h, so if you were to select 0.9 as the FT modifier 19.15 would be divided by 0.9 and the resulting value 21.28 would be saved as the new injector lb/g setting. This tells the PCM that the injector is larger than it really is (assuming you did not actually change the injectors).
The PCM, now believing that the injector is larger will calculate a shorter pulse width to hit the target AFR--but since the injector size didn't really change the shorter pulse = less fuel = leaner.
If 1.1 were selected then 17.40lb/h (19.15/1.1) would have been set as the new injector size--the PCM now thinks the injectors are smaller and calculates a longer pulse. The longer pulse with the actually larger injectors = more fuel = richer.
One last comment. In SF as originally delivered the drop-down selections for the FT and PT fuel modifiers were far to large steps to be useful--absurdly larger in fact, steps of 0.1 in a range of 0.5 to +1.5--from -50% to +33%.
The SF upgrade, available through the Delta Force forum, corrects this--you can read more here for SF v2.x and here for SF v1.x.
The updates are also included in my SFScrape documentation application--here for SF v2.x and here for SF v1.x.
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