The Electronic Control Module (ECM) controls most of the functions of the engine. The ECM is an environmentally sealed unit that is mounted in a terminal box on the engine. The ECM monitors various inputs from sensors in order to activate relays, solenoids, etc at the appropriate levels. The ECM supports the following five primary functions:
- Engine speed governing
- Ignition control
- Air/fuel ratio control
- Start/stop sequencing
- Engine monitoring and protection
The 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 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 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 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, the ECM sends a pulse of approximately 100 volts to the primary coil of each ignition transformer at the appropriate time and for the appropriate duration. The transformers step up the voltage in order to create a spark across the spark plug electrode.
Detonation sensors monitor the engine for excessive detonation. The G3516B Engine has eight detonation sensors. Each sensor monitors two adjacent cylinders. The sensors generate data on vibration that is processed by the ECM in order to determine detonation levels. If detonation reaches an unacceptable level, the ECM retards the ignition timing of the affected cylinder or cylinders. If retarding the timing does not limit detonation to an acceptable level, the ECM shuts down the engine.
The ECM provides extensive diagnostics for the ignition system. The ECM also provides a switch for ignition timing in order to allow operation with alternate fuels such as propane that require a timing offset.
The 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 ECM, and maps in the ECM. The engine may be equipped with a system that senses the amount of oxygen in the exhaust. The ECM compensates for changes in the BTU of the fuel in order to maintain desired emission levels.
The following steps describe the basic operation:
- 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
- The command for the fuel flow is sent to the electronic fuel metering valve via the CAN data link. The 50 Hz applications and the 60 Hz applications use different fuel metering valves.
- On some engines, 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 fuel in order to achieve the desired level of exhaust oxygen.
This process is repeated continuously during engine operation.
The 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 ECM supplies +Battery voltage to the relay for the starting motor. The 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 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 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, see Troubleshooting, "Programming Parameters".
Engine Monitoring and Protection
The ECM monitors both the engine operation and the electronic system.
Problems with engine operation such as low oil pressure produce an event code. The 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".