MI-04 SECM (General Description) The Woodward Small Engine Control Module (SECM) controller has full authority over spark, fuel and air. Utilizing Motorola’s HCS12 micro controller, the SECM has 24 pins of I/O and is fully waterproof and shock hardened (Figure 23). To optimize engine performance and drivability, the SECM uses several sensors for closed loop feedback information. These sensors are used by the SECM for closed loop control in three main categories: MI-04 SECM (Fuel Management) During engine cranking at startup, the SECM provides a low side driver signal to the fuel lock-off, which opens the lock-off allowing liquid propane to flow to the N2001 regulator.
A stall safety shutoff feature is built into the SECM to close the lock-off in case of a stall condition. The SECM monitors three engine states. Crank, when the VR sensor detects any engine revolutions. Stall, when the key is in the ON position but the VR sensor detects no engine revolutions, and the Run state, when the engine reaches pre-idle RPM. When an operator turns on the key switch the lock-off is opened but if the operator fails to crank the engine, the SECM will close the lock-off after 5 seconds. To maintain proper exhaust emission levels, the SECM uses a heated exhaust gas oxygen sensor (HEGO) mounted before the catalyst, to measure exhaust gas content in the LP gas system.
Engine speed is monitored by the SECM through a variable reluctance (VR) sensor. Intake manifold air temperature and absolute pressure is monitored with a (TMAP) sensor. The HEGO voltage is converted to an air fuel ratio value. This value is then compared to a target value in the SECM. The target value is based on optimizing catalyst efficiency for a given load and speed. The SECM then calculates any corrections that need to be made to the air fuel ratio. The system operates in open loop fuel control until the engine has done a certain amount of work. This ensures that the engine and HEGO are sufficiently warmed up to stay in control.
In open loop control, the FTV duty cycle is based on engine speed and load. Once the HEGO reaches operating temperature the fuel management is in closed loop control for all steady state conditions, from idle through full throttle. In closed loop mode, the FTV duty cycle is based on feedback from the HEGO sensor. In order to handle transient loads, engine RPM and load is compared to a threshold used by the SECM. When this threshold is exceeded, the FTV duty cycle will be set to a Feed Forward Adaptive value.
The SECM then makes any necessary corrections to the air fuel ratio by controlling the inlet fuel pressure to the air/fuel mixer Reducing the fuel pressure leans the air/fuel mixture and increasing the fuel pressure enriches the air/fuel mixture. Control is achieved by modulating the fuel trim z Fuel Management z Load/Speed Management z Ignition Management The SECM monitors system parameters and stores any out of range conditions or malfunctions as faults in SECM memory. Engine run hours are also stored in memory.
Stored fault codes can be displayed on the Malfunction Indicator Light (MIL) as flash codes or read by the MI-04 Service Tool software through a CAN (Controller Area Network) communication link. Battery power (12 VDC) is supplied through the fuse block to the main power relay. The ignition key switch is used to energize the main power relay. A main power relay supplies 12 VDC power to the SECM, the heated element of the oxygen sensor, Fuel Lock-Off, Fuel Trim Valve (FTV) and the Smart Coil. The SECM supplies positive voltage to the electronic throttle actuator, oil pressure switch and the coolant temperature sensor.
Transducer or sensor power (+5 VDC) is regulated by the SECM and supplied to the Temperature/Manifold Air Pressure Sensor (TMAP), Throttle Position Sensor (TPS), and the Accelerator Pedal Position Sensors (APP1 & APP2). The SECM provides a constant voltage (VCC) to the Smart Coil Driver, transducer ground for the all sensors, and a low side driver signal controlling the fuel lock-off, MIL and FTV. SECM Figure 23 G420E/G424E Tier LP Engine System Operational Overview 25