Two Electronic Control Modules (ECM) are used to control the engine. One module is the master ECM and the other module is the slave ECM. Each module is an environmentally sealed unit that is mounted in a terminal box on the engine.
The master ECM controls most of the functions of the engine. The master ECM monitors various inputs from sensors in order to activate relays, solenoids, etc at the appropriate levels. The master ECM supports the following five primary functions:
- Engine speed governing
- Air/fuel ratio control
- Start/stop sequencing
- Engine monitoring and protection
- Control of the ignition for the left cylinder bank
- Monitoring the level of detonation in the left cylinder bank
- When detonation occurs in the left cylinder bank, the master ECM will adjust the timing in order to control the detonation.
When detonation occurs in the right cylinder bank, the slave ECM will adjust the timing in order to control the detonation.
The master ECM maintains the desired engine speed by controlling the actuator for the throttle. The actuator is located at the outlet from the aftercooler. The actuator is electronically controlled and electrically actuated.
The master ECM issues a throttle command to the actuator via the CAN data link. The throttle command is a percentage. The value of the command can be viewed on the Caterpillar Electronic Technician (ET). The actual position of the throttle can be viewed via a pointer that is built into the throttle's mechanism.
Desired engine speed is determined by the desired speed input (analog voltage or 4 to 20 mA) and parameters such as maximum engine high idle speed that are programmed into the software. Actual engine speed is detected via a signal from the speed/timing sensor. Parameters for the governor gains can be programmed with Cat ET.
The master ECM provides control of the air/fuel mixture for performance and for efficiency at low emission levels. The system consists of an electronic fuel metering valve, output drivers in the master ECM, and maps in the master ECM. The control compensates for changes in the BTU of the fuel in order to maintain desired emission levels.
The ECM maintains the engine's exhaust emissions at the levels that are required at the site. The ECM uses oxygen feedback in order to control emissions.
The following steps describe the basic operation:
- The ECM monitors the amount of oxygen in the exhaust via an oxygen sensor.
- The ECM determines the desired flow rates for the air and for the fuel. The flow rates are determined by these factors:
- Desired engine speed
- Actual engine speed
- Calculated engine load
- Desired engine speed
- The command for the flow of the fuel is sent to the electronic fuel metering valve via the CAN data link.
This process is repeated continuously during engine operation.
The master ECM contains the logic and the outputs for control of starting and of shutdown. The customer programmable logic responds to signals from the following components: engine control, emergency stop switch, remote start switch, data link and other inputs.
When the programmable logic determines that it is necessary to crank the engine, the master ECM supplies +Battery voltage to the air start relay. The master ECM removes the voltage when the programmable crank terminate speed is reached or when a programmable cycle crank time has expired.
The engine must be equipped with an energize-to-run type of gas shutoff valve (GSOV). The source of the voltage to the GSOV depends on the engine's configuration. The GSOV may be energized by the customer's equipment or by the engine's control system.
If the engine's control system controls the GSOV, the master ECM supplies +Battery voltage to the GSOV whenever the programmable logic determines that fuel is required to operate the engine.
For more information on programmable parameters, refer to Troubleshooting, "Programming Parameters".
Engine Monitoring and Protection
The master ECM monitors both the engine operation and the electronic system.
Problems with engine operation such as low oil pressure produce an event code. The master ECM can issue a warning or a shutdown. This depends on the severity of the condition. For more information, refer to Troubleshooting, "Troubleshooting With An Event Code".
Problems with the electronic system such as an open circuit produce a diagnostic code. For more information, refer to Troubleshooting, "Troubleshooting With A Diagnostic Code".
Each ECM provides variable ignition timing that is sensitive to detonation.
Each cylinder has an ignition transformer that is located under the valve cover for the cylinder. To initiate combustion, an ECM sends a pulse of approximately 100 volts to the primary coil of an ignition transformer at the appropriate time and for the appropriate duration. The transformer steps up the voltage in order to create a spark across the spark plug electrodes.
Detonation sensors monitor the engine for excessive detonation. The G3520B Engine has ten detonation sensors. Each sensor monitors two adjacent cylinders. The sensors generate data on vibration that is processed by each ECM in order to determine detonation levels. If detonation reaches an unacceptable level, the appropriate ECM retards the ignition timing of the affected cylinder or cylinders. If retarding the timing does not limit detonation to an acceptable level, the master ECM shuts down the engine.
The master ECM and the slave ECM provide extensive diagnostics for the ignition system. The master ECM also provides a switch for ignition timing in order to allow operation with alternate fuels such as propane that require a timing offset.