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| Illustration 1 | g01018646 |
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Diagram of the system for the air/fuel ratio control | |
To accurately control the air/fuel ratio, the ECM depends on the following parameters that are programmed via the "Configuration" screen of Cat ET. For details on these parameters, refer to Systems Operation/Testing and Adjusting, "Electronic Control System Parameters".
- "Fuel Quality"
- "Gas Specific Gravity"
- "Fuel Specific Heat Ratio"
- "Desired Oxygen At Full Load"
- "Oxygen Feedback Enable Status"
- "Air/Fuel Proportional Gain"
- "Air/Fuel Integral Gain"
Note: Some fuel metering valves utilize the "Gas Specific Gravity" and the "Fuel Specific Heat Ratio" parameters. The fuel metering valve uses this information to help regulate the fuel flow.
The air/fuel ratio control has two basic modes of operation:
- Open loop
- Oxygen feedback
The mode of operation is determined by the "Oxygen Feedback Enable Status" parameter. Cat ET is used to change the mode of operation.
Select "Disable" in order to activate the open loop mode.
Select "Enable" in order to activate the feedback mode.
The modes of operation are explained below.
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| Illustration 2 | g01093398 |
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Diagram of the open loop mode | |
Open loop - During operation in this mode, the ECM does not use any feedback for determining the fuel command. The open loop mode is intended to be a temporary mode of operation. The open loop mode allows the engine to operate if the feedback system becomes inoperative. In this mode, the ECM cannot maintain consistent exhaust emissions. Input from an operator that is working with an exhaust analyzer is necessary for maintaining the exhaust emissions.
For this mode of operation, the ECM performs several operations in order to determine the fuel command.
Volumetric efficiency - The volumetric efficiency of an engine with a four stroke cycle is the relationship between the quantity of the inlet air and the piston displacement. This is the ratio between the charge that actually enters the cylinder and the amount that could enter the cylinder under ideal conditions. To determine the volumetric efficiency, the volume of the engine's inlet air is compared to the piston displacement.
The ECM uses four factors to calculate the air flow: inlet manifold air temperature, inlet manifold air pressure, engine speed and a map of volumetric efficiency. The engine speed and the calculated engine load are used as references for the map. The corresponding point on the map is used with the other factors in the calculation of the air flow. The ECM assumes that this is the current air flow.
The ECM assumes that the current fuel command is the actual fuel flow: the fuel flow is used to calculate the engine load. The ECM uses the engine speed and the calculated engine load as references for a map of the desired air/fuel ratio. The point on the corresponding point on the map determines the desired air/fuel ratio.
The ECM uses the calculated air flow and the desired air/fuel ratio to calculate the fuel command. The calculation is in Illustration 3.
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| Illustration 3 | g01024465 |
The command for the fuel flow is then adjusted with the "Fuel Quality" parameter which is entered into the ECM with Cat ET.
The command for the fuel flow is finally adjusted with the Fuel Correction Factor (FCF). The ECM sends the corrected fuel command to the fuel metering valve.
Note: The FCF is determined by a proportional integral controller in the ECM. During operation in the open loop mode, the FCF is always 1.
An emissions analyzer must be used to control the emissions levels during operation in the open loop mode. The emissions levels can be adjusted with the "Fuel Quality" parameter.
When the measured NOx is higher than the desired NOx the air/fuel ratio must be leaned in order to bring the NOx emissions to the desired level. To lean the air/fuel ratio, adjust the "Fuel Quality" parameter to a higher value.
When the measured NOx is lower than the desired NOx the air/fuel ratio must be richened in order to bring the NOx emissions to the desired level. To richen the air/fuel ratio, adjust the "Fuel Quality" to a lower value.
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| Illustration 4 | g01093856 |
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Diagram of the feedback mode | |
Oxygen Feedback - During operation in this mode, the ECM uses feedback from the oxygen sensor to help determine the fuel command. The ECM maintains consistent exhaust emissions in this mode.
The control system performs the same functions for the feedback mode that are performed for the open loop mode. The same factors are used for the calculation of the fuel command. However, there are two major differences during operation when the "Oxygen Feedback Enable Status" is enabled:
- Adjustment of the "Fuel Quality" parameter does not affect the exhaust emissions.
- The FCF can change.
The same customer's inputs that are required for operation in the open loop mode are used for the feedback mode. The following additional configuration parameter must be programmed into the ECM:
"Desired Oxygen At Full Load" - To determine this parameter's value, the operator uses an exhaust analyzer to achieve the desired exhaust emissions at full load. This parameter is a relative value. The value may not indicate the amount of oxygen that is in the exhaust. The oxygen value that is displayed on the Cat ET may vary from the value that is indicated by the exhaust analyzer. For more information, refer to Systems Operation/Testing and Adjusting, "Air/Fuel Ratio Control - Adjust".
During operation in the feedback mode, the ECM continues to calculate the desired air/fuel ratio. The result is still used to calculate the fuel command. The result is also adjusted by the inlet manifold air temperature. The ECM uses this adjustment and these other factors to calculate a factor for desired oxygen in the exhaust: "Desired Oxygen At Full Load" parameter, exhaust oxygen, atmospheric pressure and humidity in the inlet air manifold. The calculation for the desired oxygen is used to adjust the FCF.
Note: Some engines do not have the atmospheric pressure sensor and the humidity sensor. The atmospheric pressure sensor and the humidity sensor are usually found in applications that have more stringent requirements for lower exhaust emissions. The two sensors enable the ECM to adjust the air/fuel ratio for ambient changes in the atmospheric conditions.
Because some of the factors for calculating the desired oxygen will vary during operation, the FCF will also vary during operation.
The reaction of the FCF can be adjusted further with the "Air/Fuel Proportional Gain" and the "Air/Fuel Integral Gain" configuration parameters.
Check the exhaust emissions periodically. The "Desired Oxygen At Full Load" parameter may be adjusted in order to bring the measured emissions to the required value. Adjust the "Desired Oxygen at Full Load" in small increments. Allow time for the system to stabilize between adjustments. Repeat this process until the engine operates at the desired NOx emission level.
When the measured NOx is higher than the desired NOx the air/fuel ratio must be leaned in order to bring the NOx value to the desired level. Adjust the "Desired Oxygen at Full Load" to a higher value.
When the measured NOx is lower than the desired NOx the air/fuel ratio must be richened in order to bring the NOx value to the desired level. Adjust the "Desired Oxygen at Full Load" to a lower value.