Inputs from the inlet air temperature sensor, the boost pressure sensor, and engine rpm are compared with the Fuel Ratio Control (FRC) maps. The output of this circuit is the FRC fuel position, which acts to limit fueling during acceleration until boost pressure rises, to prevent over fueling and black smoke. Once sufficient boost pressure and airflow is attained, the FRC limit on fuel position is normally higher than the desired fuel position and does not affect engine performance.
The software in the personality module contains torque maps, which establish the maximum fuel rates and therefore the power levels at all operating rpms, and set the rated fuel position. The torque maps are established by engine rating software which has been “flashed” into the personality module. In the HEUI ECM, the personality module is permanently wired in place as a part of the ECM, and cannot be physically removed and replaced. Updated software and re-rates must be “flashed” via an electronic service tool. Engine rpm and PTO customer parameters (PTO torque limits) are inputs to the torque maps. These parameters may limit torque during PTO operation as a protection for the driven device.
The electronic governor accepts the signals from torque maps, FRC fuel position, and desired rpm, and sends the fuel position signal to the fuel injection control circuits. Other inputs to fuel injection control are the position of Top Center (TC) for #1 cylinder, and engine rpm, both coming from the speed/timing signal logic circuit.
TC for #1 cylinder is established by signals from the dual speed/timing sensors. These are passive sensors mounted on the upper left side of the engine (behind the air compressor) so as to pick up the magnetic impulses from a series of twenty-five teeth on the back of the camshaft gear. Twenty-four of these teeth are evenly spaced 15 degrees apart, while the twenty-fifth is placed 22 1/2 degrees after TC of #1 cylinder. This allows the ECM to establish the position of #1 cylinder, and fire each unit injector at the correct time and in firing order. The speed/timing sensors are redundant which permits the operator to continue without down time (if either one fails, the engine will remain running). However, an active fault code will be present, and service should be obtained as soon as possible. The sensors must be replaced as a set, as they are NOT identical electrically.
The output of the fuel injection control circuits is a shaped pulse of 110 volts DC which energizes the unit injector solenoid. The solenoid is fast-acting, and allows high-pressure oil to enter the unit injector and act upon an intensifier piston, which multiplies the hydraulic force to several times the input oil pressure. This pressurizes the fuel plunger, causing the nozzle to open. The length of the electrical pulse determines how long fuel will flow and the hydraulic pressure determines the rate of flow. The HEUI unit injector therefore permits flexibility of fuel start, fuel stop, and rate of flow, independent of the limitations of a cam profile.