SEAL AS 5505420 - Caterpillar

Introduction
The Direct Fuel Control (DFC) enables the customer switchgear or silicon controlled regulator to control desired fuel for the engine. The DFC is used to maintain constant engine speed under varying loads.The Electronic Control Module (ECM) controls fuel to the injectors based on the DFC input. The customer control of the fuel value needs to be accurate, controlled, and properly damped in order to maintain stability of the engine rpm.Stability is necessary during steady state and transient electrical loading for the following reasons:
Provide clean power to the switchgear and to the electrical loads
Ensure smooth synchronization when additional generator sets are brought on line
Prevent cyclic oscillations in the engine rpm (hunting)The DFC option removes governing from the engine software. The governing relies on the analog signal (0 to 200 mA) from the controller and a Pulse Width Modulation (PWM) converter. The signal converter converts the standard 0 to 200 mA signal from the switchgear to a pulse width modulated (PWM) analog signal. The PWM signal is the required input for the ECM.Note: Use of the 175-4998 Signal Converter is recommended.The ECM provides a 4 to 20 mA load feedback signal for use by the customer. The analog output from the ECM provides the customer a signal that is proportional to the load on the engine. The 4 mA signal is equal to 0 percent load and the 20 mA signal is equal to 100 percent load.Use of the load feedback into the customer feedback loop can prevent unexpected response in rack adjustment when the engine is loaded 100 percent. The feedback signal can also be used to trigger additional generator sets on-line in order to support the application load requirements.Recommendations for Using Direct Fuel Control
Table 1
PWM Duty Cycle for Maximum Throttle     Current for Maximum Throttle ( 175-4998 Signal Converter )     PWM Duty Cycle for Minimum Throttle     Current for Minimum Throttle ( 175-4998 Signal Converter )     ECM Sample Rate    
75 %     175 mA     13 %     20 mA     15 to 20 ms    PWM Duty Cycle for Maximum Throttle - The ECM uses an analog PWM input signal to determine the desired fuel position. This 500 Hz / 5 V signal varies the on-time (5 V) to the off-time (0 V). The variation establishes the duty cycle and the desired fuel position.Refer to Illustration 1 for an example of the waveforms for a 50 percent and 25 percent duty PWM.The PWM Duty Cycle for Maximum Throttle is the maximum duty cycle that will request the maximum fuel command in DFC. The fuel limit will likely be achieved at some point below maximum throttle.The load feedback signal can be used to determine if maximum fuel command has been achieved. Higher duty cycles of the PWM signal will not increase fuel any further. Increasing the duty cycle further may cause delays in recovering and reducing fuel, based, upon the signal received from the controller. The delays are due to the time required for