However, what if you just want to improve your idle through cruising range and/or want to fine-tune this area and do not want to install wideband sensors in the exhaust. Lucky for us, we have narrowband o2 sensors installed!
Narrowband sensors do not limit ourself with tuning in just the idle through cruise range. If you change the Target AFR table to encompass more of the higher operating range, you can dial in just about your whole operating region of your motorcycle with only the Narrowband sensors! Once done, just restore the Target AFR to normal and safe values, the result will be a well tuned engine that meets EPA requirements, yet runs cool and efficiently.
Though Narrowband sensors have very limited range of measurement, usually .1V to 1V, with .450V being the midpoint that the exhaust gas is at the stoic value of 14.7 (for gasoline). The actual useful range is even more narrow of .2v to .8v, which represents 15.0 to 14.2 AFR.
Here is the response curve for a typical narrow-band sensor. I scaled the AFR similar to the wideband sensor for comparison.
(Click on image to expand)
The narrowband sensor provides three states for the ECM: Rich, Stoichiometric, Lean. With this information, the ECM when operating in closed-loop will maintain zero oxygen content in the exhaust stream by maintaining an average voltage level of .450v. This is called stoichiometric balance.
The average is achieved by the ECM by cycling between rich and lean conditions very quickly. The catalytic converter needs this rapid cycling of AFR for correct operation.
Since the ECM must correct for extreme lean or rich conditions, but can only measure a very small AFR range, the ECM employs a fuel error strategy that integrates the amount of fuel needed (either added or subtracted) to get to the average stoic value. This error is called fuel trim and our Bosch ECM consists of a Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT). Both front and rear cylinders have these trims. The LTFT is the integrated STFT over a period of time. The final trim is the addition of the STFT and LTFT per cylinder.
Example: If the STFT is 2.5% and the LTFT is -.5%, then the final fuel trim error is 2.0%.
There is a direct relationship to the percent error and actual AFR. Since we know the fuel trim error and we know the target AFR, we simply solve for actual AFR and use that as we would a direct measured value from a wideband o2 sensor.
This equation for AFR becomes:
totalerror = stft + ltft
afr= (requested*totalerrort/100+requested)
So taking the example as before:
totalerror = 2.5% + -.5%
totalerror = 2.0%
afr = 14.7 * 2.0 / 100 + 14.7
afr = 14.994
Our error is:
error = Actual AFR / Requeted AFR
error = 14.9994 / 14.7
error = 1.02
So in this example shows we are running lean and the ECM is adding fuel to get to 14.7. The fueling tables need to be corrected by 1.02. So if we are correcting a VE cell at that operating point and the current value is 50.00, then we the new VE cell value is:
VE cell = 50 * 1.02
VE cell = 51
Percent change = ((old value - new value) / old value) * -100
Percent change = ((50-51) / 50) * -100 = 2% increase, which is exactly what the ECM was applying with the fuel trim.
What if you have a negative trim value. This means you have a rich condition and the ECM is taking fuel away. So, if the Short Term Fuel Trim = -2.5% and the Long Term Fuel Trim = .5%,. Using the equations from above:
totalerror = -2.0%
afr = 14.406
error = .98
So the same VE cell would be corrected to:
VE cell = 49
Percent change = ((50-49)/50)*-100 = -2.0%
So this is a very straight forward approach as well as a very accurate way to tune your bike. The downside is not being able to put the engine into closed-loop through out the full operating range. Though you certainly can get well over 95% of the operating range into it. On my Target AFR while tuning I have 154 cells in closed-loop and 134 in open-loop. This is only 54% of the Target AFR in closed-loop, however it represents a very large percentage of my normal operating range >95%.
There are a number of tables we will be discussing as we tune, they are:
Target AFR Table
VE Table
Injector Compensation Factor (Front & Rear) Tables
IPW Comp (Front & Rear) Tables
These are the major tables that determine the fueling of our engine. As we discuss how to tune, please note that tuning is not science (though the math is), but an art. Each tuner has their way of tuning, but the final result should be the same. Whatever your Target AFR calls out, the exhaust gas will reflect that AFR. If that is achieved, then no matter what method you choose, the resulting tune will be optimized for you engine.
This equation for AFR becomes:
totalerror = stft + ltft
afr= (requested*totalerrort/100+requested)
So taking the example as before:
totalerror = 2.5% + -.5%
totalerror = 2.0%
afr = 14.7 * 2.0 / 100 + 14.7
afr = 14.994
Our error is:
error = Actual AFR / Requeted AFR
error = 14.9994 / 14.7
error = 1.02
So in this example shows we are running lean and the ECM is adding fuel to get to 14.7. The fueling tables need to be corrected by 1.02. So if we are correcting a VE cell at that operating point and the current value is 50.00, then we the new VE cell value is:
VE cell = 50 * 1.02
VE cell = 51
Percent change = ((old value - new value) / old value) * -100
Percent change = ((50-51) / 50) * -100 = 2% increase, which is exactly what the ECM was applying with the fuel trim.
What if you have a negative trim value. This means you have a rich condition and the ECM is taking fuel away. So, if the Short Term Fuel Trim = -2.5% and the Long Term Fuel Trim = .5%,. Using the equations from above:
totalerror = -2.0%
afr = 14.406
error = .98
So the same VE cell would be corrected to:
VE cell = 49
Percent change = ((50-49)/50)*-100 = -2.0%
So this is a very straight forward approach as well as a very accurate way to tune your bike. The downside is not being able to put the engine into closed-loop through out the full operating range. Though you certainly can get well over 95% of the operating range into it. On my Target AFR while tuning I have 154 cells in closed-loop and 134 in open-loop. This is only 54% of the Target AFR in closed-loop, however it represents a very large percentage of my normal operating range >95%.
There are a number of tables we will be discussing as we tune, they are:
Target AFR Table
VE Table
Injector Compensation Factor (Front & Rear) Tables
IPW Comp (Front & Rear) Tables
These are the major tables that determine the fueling of our engine. As we discuss how to tune, please note that tuning is not science (though the math is), but an art. Each tuner has their way of tuning, but the final result should be the same. Whatever your Target AFR calls out, the exhaust gas will reflect that AFR. If that is achieved, then no matter what method you choose, the resulting tune will be optimized for you engine.