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 and detonation of the left cylinder bank
The slave ECM primarily supports the control of ignition and detonation of the right cylinder bank.
The master ECM maintains the desired engine speed by controlling the actuator for the throttle. The actuator is located at the flange of the air inlet manifold.
The throttle actuator is electrically controlled and hydraulically actuated. A high pressure pump provides hydraulic pressure with oil from a system that is separate from the engine oil. The high pressure oil supply is monitored by a pressure switch that generates an event code if the pressure drops below an acceptable level. The throttle position is controlled in open loop mode: there is no feedback from the throttle position.
The master ECM issues a throttle command that represents a percent of the level of electrical current. The output can be viewed on the Caterpillar Electronic Technician (ET). The actual throttle position can be viewed on a mechanical pointer that is built into the mechanism of the throttle.
Desired engine speed is determined by the status of the idle/rated switch, 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 such as governor gain can be programmed with Cat ET.
The master ECM provides control of the air/fuel mixture for performance and efficiency at low emission levels. The system consists of an electronic fuel metering valve, sensors, 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 may use charge density feedback or oxygen feedback in order to control emissions.
Charge Density Feedback - The master ECM monitors the generator's power output, the temperature of the air in the inlet manifold, and the pressure of the air in the inlet manifold. The ECM uses the information in order to adjust the signal for the flow of air and of fuel. This process occurs whenever the engine load is above 25 percent.
Oxygen Feedback - The ECM monitors the amount of oxygen in the exhaust gas via the oxygen sensor. The ECM adjusts the signal for the flow of air and of fuel. This process occurs whenever oxygen feedback is enabled.
The type of feedback that is used depends on the sensors that are installed on the engine and the software that is programmed into the ECM.
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 switch, emergency stop switch, remote start switch, data link and other inputs.
To control the engine at the appropriate times, the master ECM provides +battery voltage to the relays for the starting motor and for the gas shutoff valve.
When the programmable logic determines that it is necessary to crank the engine, the master ECM supplies +battery voltage to the relay for the starting motor. 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 has an energize-to-run type of gas shutoff valve. When the programmable logic determines that fuel is required to start the engine or to run the engine, the master ECM supplies +battery voltage to the valve.
For more information on programmable parameters, see 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, a derating, or a shutdown. This depends on the severity of the condition. For more information, see Troubleshooting, "Troubleshooting With An Event Code".
Problems with the electronic system such as an open circuit produce a diagnostic code. For more information, see 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 gap.
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 contains an input that allows the selection of an alternate base timing. This allows operation with alternate fuels such as propane that require different timing offsets.