O2 FeedbackTo control the engines fuel delivery system, the EMS can be programmed to run open loop and/or closed loop O2 feedback. Closed loop uses feedback from the oxygen sensor to make temporary but immediate corrections to the injection to maintain a target AFR. The type of an O2 sensor will determine how O2 feedback can be controlled. Due to the nature of standard narrow band O2 sensors, 14.7:1 is the only air fuel ratio that can be accurately maintained in closed loop. However, wideband O2 sensors can be used in almost every feedback condition.
When the fuel system is open loop, the O2 sensor is ignored and the injector pulse width relies on the fuel map and fuel compensators to adjust injection duration. Open loop is necessary during engine starts when the O2 sensor has cooled below its operating temperature. It may also be necessary when coolant temperatures are low. In this state, the fuel vaporization is poor and the engine will require a richer mixture to properly operate. When under heavy load, the engine typically requires an air fuel ratio that is out of a narrow band O2 sensors standard range and open loop will be necessary. When the accel fuel function is triggered during hard accelerations, open loop may be necessary to help stabilize the O2 feedback. When the decel function cuts fuel completely, O2 feedback will not be necessary.
Tuning O2 FeedbackThe tuning procedure for O2 feedback can be quite tedious. One approach is to use a technique known as the Ziegler Nichols Tuning Method. The Ziegler Nichols closed loop method uses Proportional or P + I control and should produce tuning parameters which will obtain a quarter wave decay. The O2 feedback functions are located in the Fuel pull down menu in the AEMPro software.
1) Use the proportional control (O2 FB proportional table) by itself and set the integral time constant (O2 FB integral table) to zero.
2) Log the parameters AFR#1(or #2) and O2 F.B.#1 (or #2).
3) Increase the value of the proportional gain until the point of instability or a sustained oscillation is reached.
4) From here, the ultimate proportional gain (Gu) is found.
5) From the log, measure the period of oscillation from peak to peak, in seconds, to obtain the critical time constant or ultimate period (Pu).
6) Once the values for Gu and Pu are obtained, the P + I parameters can be calculated from the following equations.
Proportional feedback only (O2 FB integral table = 0)
O2 FB proportional table = 0.5 (Gu)
Proportional and Integral feedback
O2 FB proportional table = 0.45 (Gu)
O2 FB integral table = (Pu)/1.2 Note that these values are not optimal values and additional fine tuning may be required to obtain the best O2 feedback performance.
User Definable Options for O2 Feedback
O2 FB +LimitUnits: Fuel %
Description: The maximum amount of fuel to add when O2 feedback is enabled.
Typical Use: If the proportional and integral controls have not been setup correctly, this value may need to be small to trim the fuel map.
O2 FB Coolant OKUnits: Coolant Temperature
Description: O2 feedback will operate when the coolant temperature has exceeded this value.
Typical Use: This value is typically set near operating temperature.
O2 FB GainUnits: Gain Multiplier
Description: Overall O2 feedback gain
Typical Use: This value will always be one unless the proportional and integral gains are not sufficient in controlling the air fuel ratio.
O2 FB -LimitUnits: Fuel %
Description: The maximum amount of fuel to subtract when O2 feedback is enabled.
Typical Use: If the proportional and integral controls have not been setup correctly, this value may need to be small to trim the fuel map.
O2 FB LoadUnits: Engine Load
Description: O2 feedback will operate when the engine load is below this value.
Typical Use: When using a narrow band O2 sensor, O2 feedback is typically not used at high engine loads because of the sensors inaccuracy outside the stoichiometric range. However, O2 feedback can be used at high engine loads with a quality wideband O2 sensor.
O2 FB RateUnits: mS
Description: O2 feedback base timer for both proportional and integral terms. This is how often the air fuel ratio is looked at to determine the correction.
Typical Use: A smaller number gives a faster response by effectively increasing the gain. This should be the same as the sampling rate of the O2 sensor being used. A typical value is 65.59 mS.
O2 FB SpeedUnits: RPM
Description: O2 feedback will operate when the RPM is below this value but is not in cranking mode.
Typical Use: Limits O2 feedback control above the user-defined rpm.
O2 FB ControlUnits: On/Off
Description: Enables closed loop O2 feedback and the AFR Target parameter.
Typical Use: After the fuel map has been tuned, O2 feedback can be enabled for automatic tuning adjustments.
Accel Fuel MinUnits: microseconds
Description: O2 feedback threshold for accel fuel. O2 feedback will operate when the current accel fuel amount is below this value.
Typical Use: Because accel fuel delivers an abundance of fuel in a very short amount of time, O2 feedback can ignore it to avoid possible air fuel ratio overshooting. However, using this function can still allow O2 feedback when there is a very small amount of acceleration fuel employed.
Accel O2FB inhibitUnits: mS
Description: O2 feedback delay after the accel fuel function has been disabled.
Typical Use: This function allows the mixture to be restored without using O2 feedback in order to prevent air fuel ratio overshooting. A typical value is 196 mS.
Decel O2FB inhibitUnits: mS
Description: O2 feedback delay after the decel fuel function has been disabled.
Typical Use: This function allows the mixture to be restored without using O2 feedback in order to prevent air fuel ratio overshooting. A typical value is 196 mS.
O2 FB ClearUnits: On/Off
Description: Sets O2 feedback to zero after both accel and decel fuel have been implemented.
Typical Use: After utilizing accel or decel fuel, the next condition will typically be different from before. This function is used to allow the O2 feedback to clear its memory and start from zero again.
2-D Tables for O2 FeedbackO2 FB Delay TableUnits: After Start Time vs. Coolant Temperature
Description: Coolant dependant table for O2 feedback after start delay.
Typical Use: This table disables the O2 feedback for a set amount of time to allow a rich engine start and allow time for the O2 heater to warm up. This should never be faster than the time response of the O2 sensors heater.
O2 FB proportional tableUnits: Proportional Gain vs. RPM
Description: Proportional control is a pure gain adjustment acting on the error signal to provide the driving input. The advantage of a proportional-only control is its simplicity. If AFR offsets can be tolerated, the use of a proportional controller may be optimal. However, it will not eliminate the steady-state error that occurs after a set-point change or a sustained AFR disturbance. Note: When tuning the O2 feedback and overshoot occurs, lower this number. If undershooting the AFR target, raise this number.
Typical Use: Used to adjust the speed of the system and reach the AFR target quickly.
O2 FB integral tableUnits: Integral Gain vs. RPM
Description: Integral control is implemented through the introduction of an integrator. This is used to fine tune the O2 feedback at the AFR target once the proportional has acted in getting close to the target. Note: start tuning the O2 feedback with this option at zero, until the proportional has the feedback close to the target, then step this in slowly until the feedback holds the AFR target.
Typical Use: Integral control is used to provide the required accuracy for the control system.
3-D Tables for O2 FeedbackAFR MapUnits: Engine Load vs. RPM vs. AFR
Description: O2 feedback target for specific engine loads and RPM.
Typical Use: The AFR map is used strictly for closed loop O2 feedback.
Parameters (can be viewed or logged)O2 FB DelayUnits: Seconds
Description: Time delay for O2 feedback operation after engine start.
AFR TargetUnits: AFR
Description: Air fuel ratio that directly corresponds with the AFR Map.
AFR #1Units: AFR
Description: Scaled air fuel ratio from Lambda #1.
AFR Error#1Units: AFR
Description: Difference between the actual air fuel ratio and the targeted air fuel ratio from Lambda #1.
O2 F.B.#1Units: Fuel %
Description: The current amount of fuel implemented in order to reach the air fuel ratio target.
AFR #2Units: AFR
Description: Scaled air fuel ratio from Lambda #2.
AFR Error#2Units: AFR
Description: Difference between the actual air fuel ratio and the targeted air fuel ratio from Lambda #2.
O2 F.B.#2Units: Fuel %
Description: The current amount of fuel implemented in order to reach the air fuel ratio target.
