RETAINER 8N3639 - Caterpillar

The Electronic Control Module (ECM) provides control of the air/fuel mixture for maximum performance and for efficiency at low emission levels. The system includes several factors: maps in the ECM, output drivers in the ECM, sensors and customer's input via the Caterpillar Electronic Technician (ET). Illustration 1 is a diagram of the system's main components.
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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 feedbackThe 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.Open Loop
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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