Power to Win
KB1033 - Optimizing your Lambda Control
The accuracy of the air/fuel ratio of your engine can be greatly improved by using the closed loop lambda control function that's available with all of the EFI Technology engine control products. When this feature is enabled the ECU continuously monitors the oxygen content in the exhaust gas via a lambda sensor(s) and then proportionately adjusts the fuel delivery to achieve a predefined target value.
There are two basic sensor types in use, namely narrow band and wide band, and each requires very different signal conditioning circuits. The EFI ECU product range can accept either sensor but usually require an external amplifier when using the wide band type. The X5 and other high end ECU's are available with integrated wide band amplifiers. The following section describes the two sensor types and their operating characteristics.
The narrow band sensor consists of a ceramic body (zirconium dioxide) that is in contact with both the exhaust gas and the ambient air. The ceramic is provided with electrodes made of a gas permeable platimum layer that begins to conduct oxygen ions at approx. 300 deg C. If the oxygen content differs between the two sides of the sensor a voltage is generated between the two electrodes.
The sensor usually has three wires; an output signal and two wires for the heater element. The sensor body is used as the signal ground but some sensors have an additional wire as a dedicated ground. The heating element draws approx. 3 amps when cold and then drops to about 0.5 amps when it reaches its normal operating temperature.
The sensor output signal is typically 0 to 800mV which is proportional to the percentage of oxygen in the exhaust but the relationship is non-linear. The output also varies with the exhaust temperature which changes because of the sensors output resistance. To correctly calibrate this type of sensor you must provide a temperature compensation.
The wide band sensor requires a controller because it is more complex than the standard narrow band sensor. It consists of a narrow band oxygen sensor (the reference cell) coupled to a pump cell and a small diffusion chamber. The pump cell, in conjunction with a catalytic reaction at the surface of the cell's electrodes, can either consume oxygen or excess hydrocarbon fuel in the pump cell cavity, depending on the direction of the current flow.
In normal operation, a small sample of the exhaust gas passes through the diffusion gap into the pump cell. That exhaust gas is either rich or lean and both conditions are sensed by the reference cell which produces a voltage above or below the reference signal.
A rich exhaust will produce a high voltage and the electronics produces a pump current in one direction to consume the free fuel. A lean exhaust produces a low voltage and the electronics sends the pump current in the opposite direction to consume free oxygen.
When the free oxygen or free fuel has been neutralised, the feedback signal goes to about 450 mVolts. The pump current required to produce this equilibrium is a measure of the lambda ratio. The electronics converts the current into a voltage which is the output of the unit. There is a calibration resistor in the sensor connector which compensates for manufacturing variations between sensors.
The operation of the closed loop lambda function is defined by the settings in Lambda Constants of the System Editor. The configuration variable is used to enable the closed loop function and also allows selection for feedback to be from either the lambda 1 or 2 inputs or both if the bank configuration option is checked. The following section describes the functions of each of the lambda control varaibles.
The gain constants are used to adjust the response time of the lambda control loop. The rate limits are used to suspend closed loop operation during transient conditions to allow the engine to stabilize before re-enabling the lambda control.
There are two ways to enable the closed loop control with the system editor. If you wish to permanently activate the closed loop control open the "lambda configuration" constant and click on the "enable button". In this condition the closed loop is always active providing the throttle and rpm enable thresholds are met. To temporarily enable closed loop you can click on the green "Closed Loop" button on the editor toolbar.
Increasing the gain will speed up the overall response of the lambda control but using an excessive amount of gain may induce oscillation. A general rule is to increment the proportional gain by a quarter of the previous value until the output becomes unstable and then back down the gain a small amount.
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