F10064 Mercury SHAFT

Control Dynamics
The control algorithms used in the 721 control are designed specifically for reciprocating engine applications. Control dynamics are varied automatically as functions of both speed and load to provide better performance over the operating range of the engine.The 721 control provides two mappings of control dynamics as a function of speed. The control automatically maps gain proportional to engine speed. This provides higher gain at normal operating speeds and lower gain at low speeds. This matches engine control requirements better than a constant-gain control. A second optional dynamics mapping provides additional stability as speed is decreased. This feature is useful particularly on large, low-speed engines where dead time between cylinder firings becomes an important factor in performance.To provide better transient performance, the control can be operated automatically with two gain settings depending on engine speed error (speed error is the difference between the speed setting and the actual engine speed). During steady-state operation with a constant load, the control uses the base gain setting. This gain is adjusted by the user to a value to prevent the control from responding to minor fluctuations in engine speed, a common problem with gas-fueled, spark-ignited engines. This feature eliminates potentially damaging jiggle of the actuator and fuel system. The control automatically increases gain by an adjustable ratio when a speed error exceeding an adjustable window occurs. Operation with base gain is restored once the control senses the return to steady-state speed.The control can also compensate for nonlinear fuel systems and changes in engine dynamics with load. The control dynamics are mapped as a function of actuator current (actuator current is proportional to engine load). This provides optimal dynamics and smooth steady-state operation for all conditions from no load to full engine load.The control also provides two complete sets of dynamic adjustments which are selected when the Circuit Breaker Aux discrete input is activated. The two sets of dynamics are provided for use where engine operating conditions change, such as in systems with clutched-in loads, and electrical power generation where the unit may be operated stand alone or paralleled with an infinite bus.Fuel Limiters
The 721 Digital Speed Control provides an idle fuel limiter to limit overfueling or flooding during start-up. The limiter is set to provide the desired maximum rack position during starts. The control will reduce the fuel when the speed set point is reached as required to control engine speed, but will not exceed the Idle Fuel Limit.A Maximum Fuel Limit set point is provided to limit the maximum control output current to the actuator during normal engine operating conditions. See Figure 6-1.
Figure 6-1. Fuel LimitersSpeed Reference and Ramps
The 721 control provides local control of the speed reference, with inputs to issue raise and lower speed commands. For remote speed setting, the control provides a 4 to 20 mA/1 to 5 Vdc Remote Speed/Load Setting input which is used for the speed reference. Remote is selected by closing both the Raise Speed/Load contact and the Lower Speed/Load contact.This section describes the operation of each of the speed reference and ramp functions and their relation to each other. Read this section carefully to be sure your switchgear sequencing provides the proper operating modes.The control provides idle, lower limit, rated, and raise limit set points, accel and decel times, and raise and lower rates, for local operation. Accel time determines the time required for the engine to ramp from idle (low idle) to rated (fast idle) speed. Decel time determines the time required for the engine to ramp from rated speed to idle speed. Raise and lower rates determine how fast speed is increased or decreased by the raise and lower command inputs.The idle speed set point is provided for engine start-up or cool down speed. Idle speed may be set equal to or less than the rated speed set point. Idle is independent of the lower limit set point and may be set to a lower speed. When idle is selected (Idle/Rated contact open), remote, raise, and lower inputs are all disabled. Idle speed cannot be changed except through adjustment of the idle speed set point. In idle, the Idle Fuel Limit is in effect, and the Maximum Fuel Limit and External Fuel Limit are disabled. This means the maximum fuel available is determined only by the Idle Fuel Limit set point, and the speed reference is determined only by the idle speed set point.When rated speed (high or fast idle) is selected by closing the Idle/Rated contact, the fuel limit is set to the Maximum Fuel Limit set point value. Closing either the Raise or Lower contacts while ramping from idle to rated results in immediate cancellation of the idle to rated ramp.After acceleration to rated speed is completed, the raise and lower commands increase and decrease engine speed based on the raise and lower rate set points. The raise and lower limits determine the limits of these commands.If remote operation is selected after the engine reaches rated speed, the control will ramp speed to the reference value set by the Remote Speed/Load Setting milliamp input based on the raise or lower rate. The Remote Speed/Load Setting operates from 4 to 20 mA (1 to 5 Vdc). The values of the 4 mA and 20 mA remote reference set points must be set between the raise and lower limit set points. The 4 mA Remote Reference set point may be set to a lower or higher speed than the 20 mA set point, providing for either direct or reverse-acting remote speed setting.If a remote input is present when the Idle/Rated contact is closed or during the idle to rated ramp, the speed reference will ramp to the speed reference value determined by the milliamps on the Remote Speed/Load Setting input, based on the raise rate set point. This may not be the desired mode of operation, so be sure to understand the implications of operating the control in this manner.The control treats Remote Speed/Load Setting inputs between 2 and