Patent Publication Number: US-2018051636-A1

Title: System for Controlling the Total Emissions Produced by a Multi-Engine Power System

Description:
TECHNICAL FIELD 
     This disclosure relates generally to a plurality of engines arranged together to generate power, more particularly, to a system and method for controlling the total emissions produced across the plurality of engines. 
     BACKGROUND 
     Certain applications can involve the use of multiple internal combustion engines that are harnessed together to drive one or more loads. One such application can be the generation of electrical power for an electrical load using multiple generator sets (known as “gensets”) with each genset including in combination an engine and an electrical generator or alternator. Marine vessels are another application that can include multiple engines harnessed together to drive one or more primary loads (e.g., propellers) and various auxiliary loads (e.g., HVAC, lighting, pumps, etc.). The engines can be mechanically connected to the loads or electrically connected to the loads by way of generators. In some applications, the loads of a vessel can be driven both mechanically and electrically in a hybrid arrangement. 
     In typical multi-engine applications, all engines are simultaneously operated to produce about the same amount of power. For example, a particular marine vessel may have four identical engines each capable of producing about 5,000 kW. And during operation, all of the engines may be operated at the same level (e.g., at about 20% capacity) to evenly distribute the loads (e.g., to evenly distribute a 4,000 kW load). If different engines have different output capacities than others in the group of engines, the engines may be operated synchronously based in proportion on their individual rated capacities for power output in a manner sometimes referred to a symmetric load sharing or symmetric loading. In symmetric loading, each engine is operated to output power according to its relative capacity in proportion to the total capacity of the combined engines. Hence, the engines are all being operated at the same percentage of their individual, relative capacity, and theoretically should be subjected to the same level of stress and wear, even though some engines may be producing a larger absolute output than other engines. 
     While these load sharing strategies help ensure that each genset of the plurality is operated within its capacity and capabilities, they do not take into account other considerations associated with the operation of the engines, such as the total emissions produced by the engines. Internal combustion engines exhaust a complex mixture of air pollutants. These air pollutants are generally composed of particulates and gaseous compounds including nitrogen oxides (commonly referred to as NO x ) among others. Due to increased awareness of the environment, exhaust emission standards have become more stringent, and the amounts of particulates and NO x  emitted into the atmosphere by an engine may be regulated depending on the type of the engine, size of the engine, class of the engine, and the like as well as the location in which the engine is operating. For example, the engines operating on a marine vessel may be subject to different regulations depending on whether the vessel is in port or at sea. Using a strict symmetric load sharing strategy for a multi-engine application may result in higher than desired total emissions across the plurality of engines. 
     Moreover, in order to comply with the regulation of particulates and NO X , some engine manufacturers have implemented a strategy called selective catalytic reduction (SCR), which is a process where a reagent known as diesel exhaust fluid (DEF), most commonly urea, or a water/urea solution, is selectively injected into the exhaust gas stream of an engine and absorbed onto a downstream substrate in order to reduce the amount of NO X  in the exhaust gases. However, the efficiency and cost of the operation of a SCR system can vary depending upon the load condition of the engine. A strict load sharing strategy for multi-engine applications does not take into account SCR system operating conditions. 
     U.S. Publication No. 2005/0282285 (“the &#39;285 publication”) discloses a strategy for controlling NO X  emissions in an SCR system associated with an internal combustion engine. However, the &#39;285 publication does not address the total emissions produced by a plurality of engines harnessed together to drive a load. 
     SUMMARY 
     In one aspect, the disclosure describes a power system for powering a load. The load presents a power demand. The power system includes a plurality of power sources with each power source including an engine. A SCR system is associated with the engine of at least one of the plurality of power sources. A controller is in communication with the plurality of power sources. The controller is configured to receive engine operation information, emission output information associated with each engine and conversion efficiency information associated with the SCR system and selectively apportion the power demand between each of the plurality of power sources based on minimizing total engine emissions across the plurality of power sources and using the engine operation information, the emission output information and the conversion efficiency information. 
     In another aspect, the disclosure describes a method for controlling a power system. The method includes operating a plurality of engines to power a load with at least one of the engines having an SCR system. A signal indicative of a power demand for the load is received. Engine operation information relating to the plurality of engines is received. Emission output information relating to each engine is received. Conversion efficiency information relating to the SCR system is received. The power demand is selectively apportioned between each of the power sources based on minimizing total engine emissions and using the engine operation information, the emission output information and the conversion efficiency information. 
     In yet another aspect, the disclosure describes a control system for a power system having a plurality of engines that together drive a load. The control system includes a load manager controller configured to determine a power demand for the load and a power system controller. The power system controller is in communication with the plurality of power sources. The power system controller is configured to receive the power demand, engine operation information, emission output information associated with each engine and conversion efficiency information associated with the SCR system and selectively apportion the power demand between each of the plurality of power sources based on minimizing total engine emissions across the plurality of power sources and using the engine operation information, the emission output information and the conversion efficiency information. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of an exemplary marine vessel with various exemplary loads that can be powered by a power system according to the present disclosure. 
         FIG. 2  is a schematic illustration of an illustrative multi-engine power system according to the present disclosure. 
         FIG. 3  is a flowchart embodying an exemplary control system for the multi-engine power system of  FIG. 2   
     
    
    
     DETAILED DESCRIPTION 
     This disclosure relates to a power system including a plurality of internal combustion engines and the control strategies and electronic or digital controllers for regulating operation of the engines. Now referring to the drawings, wherein like reference numbers refer to like elements, there is illustrated in  FIG. 1  a power system  100  that may be arranged to generate power for one or more consumers of power, referred to herein as loads  101  (schematically shown in  FIG. 2 ), of, in the illustrated embodiment, a marine vessel  102  like a freighter or cargo ship. For example, the electrical power system  100  may generate electrical power for the propulsion units  104  of the marine vessel, which may be a plurality of electrically driven azimuth thrusters. The loads  101  driven by the power system  100  may include any device or devices that consume mechanical and/or electrical power, including, but not limited to, motorized cranes  106  for lifting and moving freight, communication equipment  108  for communicating with shore and other marine vessels, navigation controls  110  that may be disposed in the bridge  112  of the marine vessel  102  for directing movement and operation of the marine vessel, electric lights, HVAC systems, water pumps, and other auxiliary loads that are normally found on a conventional marine vessel. 
     Other marine applications for the power system  100 , in addition to the illustrated freighter, include military vessels, passenger liners, tankers, and the like. In addition to being utilized for marine vessels  102 , the power system  100  described herein may be utilized for oil or gas procuring applications, temporary military bases, and other electrical applications where electrical power from a utility-supplied power grid is not readily available or may be interrupted. 
     As can be seen in  FIG. 2 , the power system  100  may include, among other things, a plurality of power sources  120  that can operate in combination with each other to power one or more loads. The power sources  120  may embody any number and type of combustion engines  122 , such as diesel compression ignition engines, some or all of which that are connected to corresponding alternators or generators  124  to form gensets. The mechanical outputs of the combustion engines  122  may be routed directly to loads  101  (e.g., mechanically routed to the propulsion units) and/or indirectly by way of the generators  124  (e.g., electrically routed to motors of the propellers and to the other auxiliary loads). To provide fuel for the engine  122  to combust, the electrical power system  100  may be operably associated with one or more fuel tanks  126  or reservoirs. In the case of gensets, the engines  122  can combust hydrocarbon fuel and air to produce a mechanical force or motive power that rotates a magnetic field in the electrical generator  124  that is converted to electrical power. While the electrical capacity of the gensets described herein may be rated at any suitable quantity, an exemplary genset may produce several kilowatts and the combination of the gensets may together produce several hundred kilowatts. 
     In the illustrated embodiment, the power system  100  includes four different power sources  120 , in this case four different gensets, but in other embodiments any suitable number of power sources  120  may be provided. It is contemplated that a particular vessel  102  could include identical gensets, all different gensets, or any other configuration of gensets, as desired. Similarly, the engines  122  associated with each of the gensets may be identical, all different or any other configuration of engines as desired. For example, the power system  100  may include two larger medium-speed gensets and two smaller high-speed gensets. The larger medium-speed generator sets may be capable of greater power output at higher fuel efficiency (i.e., lower fuel consumption) and/or lower emissions. The smaller high-speed generator sets, however, may be capable of faster transient response and high-efficiency low-load operation. By including a mix of different types and/or sizes of generator sets, benefits associated with the different sets may be realized. It is also contemplated that power sources other than engines and generators may be used to power loads  101  associated with the vessel  102 , for example batteries or other power storage devices. 
     To reduce exhaust emissions, one or more of the engines  122  may be equipped with a SCR system  127  to reduce the amount of nitrogen oxides in the exhaust gases produced by operation of the respective engine  122 . In the illustrated embodiment, the engine  122  associated with each of the four power sources  120  is equipped with a SCR system  127  as shown schematically in  FIG. 2 . However, it is contemplated that one or more of the engines  122  may be equipped with an SCR system  127  while others are not or that none of the engines  122  are equipped with an SCR system  127 . The SCR system  127 , if provided, may be disposed in the exhaust system of the respective engine  122  and be configured to selectively inject DEF, commonly urea or urea/water solution, into the exhaust gas stream of the engine  122 . The DEF may then be absorbed on a downstream substrate in order to reduce the amount of NO X  in the engine exhaust gases. Those skilled in the art will appreciate that the present disclosure is not limited to SCR systems  127  having any particular configuration. 
     To govern operation of the engine  122  and the electrical generators  124 , each power source  120  may include an electronic power source controller  128  that may be a computing device capable of performing typical computing and digital processing functions. To combine the power being generated, the plurality of power sources  120 , which in the illustrated embodiment are configured as gensets, may be electrically connected to a common bus  132  or busbar in a parallel arrangement as shown in  FIG. 2 . In a parallel arrangement, the total current generated by the power system  100  is the sum of the individual currents generated by each of the plurality of power sources  120  or gensets while the potential or voltage is generally the same across each of the gensets. The common bus  132  can be electrically connected through a network or circuit with the electrical equipment of the marine vessel  102 , which may make up part of the load  101  of the vessel. The plurality of power sources  120  therefore may each, as desired, meet a portion of the load  101  of the vessel. While the present disclosure describes power sources  120  configured as gensets that produce an electric current to meet an electric load, as noted previously the mechanical outputs of the engines  122  may be routed directly to the loads  101  without an intervening generator. 
     As shown schematically in  FIG. 2 , the power system  100  may further include a load manager  134  and a power system controller  140 . The load manage  134  may determine a power demand for the power sources  120  based on, among other things, input received from various input devices, referred to generally with reference number  142  in  FIG. 2 , such as provided on the bridge  112  of the vessel  102  (or another location onboard and/or offboard vessel  102 ) and on an actual output or performance of the loads  101  driven by the power system  100 . As described in greater detail below, the power system controller  140  may selectively adjust operation of the power sources  120  or the engines  122  in different ways to meet the power demand from the load manager  134 . 
     The load manager  134  may be configured to compare an actual output of the power system  100  to a desired output (e.g., desired travel speed, desired propeller speed, desired vessel location, etc.), and create a power demand based on the difference. In the disclosed embodiment, the load manager  134  may be a generator controller configured to compare an actual bus voltage to a desired voltage and responsively generate a command for electrical power supply based on the difference. For example, the propulsion units  104  of the vessel  102  may be electrically powered from a common bus and directly controlled from the bridge  112 . The operator of the vessel  102  may move a throttle lever (not shown) to command the vessel  102  (and/or a particular propulsion unit) to move at a particular desired speed. As signals from the bridge  112  cause the propellers to turn on, turn faster, slow down, or turn off, the motors associated with the propulsion units  104  may consume more or less electricity from the common power bus  132 . This change in power consumption may cause a corresponding voltage fluctuation in the bus  132 , and load manager  134  may monitor the voltage fluctuation and responsively generate the command for more or less electrical power to be supplied by the power sources  120  to the bus  132 . 
     In another example, the load manager  134  may be a stand-alone component and configured to compare an actual vessel  102  travel speed or actual propulsion unit  104  speed to a desired travel or desired propulsion unit  104  speed and generate a command for a change in power (mechanical and/or electrical) based on the difference. In yet another example, the load manager  134  may compare an actual vessel  102  position and/or orientation to a desired position or orientation, and generate a command for a change in power based on the difference. Other comparisons may also be instituted by the load manager  134 , and the load manager  134  may take any conventional configuration known in the art for creating the power demand. Signals generated by the load manager  134  indicative of the power demand may be directed to the power system controller  140  for further processing. 
     The power system controller  140 , which may be referred to as multi-engine optimizer (MEO) controller, may include commonly known components that cooperate to apportion the power demand from the load manager  134  among the different power sources  120 . The power system controller  140  may communicate with each of the individual power source controllers  128  that direct operation of the individual power sources  120 . The power system controller  140  may include, among other things, a single or multiple microprocessors, digital signal processors (DSPs), etc. that include means for controlling an operation of the power system  100  and be located onboard and/or offboard vessel  102 . The power system controller  140  may include a processor, an application specific integrated circuit (ASIC), or other appropriate circuitry for performing logic and digital functions, and may have associated memory or similar data storage capabilities. The power system controller  140  may be a discrete, individual unit, or their functions may be distributed over a plurality of distinct components. To the extent multiple electronic controllers are used, the electronic controllers may operate and communicate with each other using digital signals, analog signals, or through any other suitable means. The electronic controllers may communicate with each other through wired connections or may communicate wirelessly through radio frequency or wi-fi mediums. 
     Numerous commercially available microprocessors can be configured to perform the functions of the power system controller  140 . It should be appreciated that the power system controller  140  could readily embody a microprocessor separate from that controlling other vessel- or engine-related functions, or that the power system controller  140  could be integral with a vessel microprocessor and be capable of controlling numerous functions and modes of operation. As a separate microprocessor, the power system controller  140  may communicate with the general vessel microprocessor(s) and/or engine controllers via datalinks or other methods. Various other known circuits may be associated with the power system controller  140 , including power supply circuitry, signal-conditioning circuitry, actuator driver circuitry (i.e., circuitry powering solenoids, motors, or piezo actuators), and communication circuitry. 
     As described above, one method for apportioning the power demand communicated by the load manager  134  is symmetric load sharing in which each power source  120  or engine  122  outputs a proportional share of the total power demand. While symmetrical load sharing accounts evenly for the different capacities of the plurality of power sources  120  or engines  122  of the power system  100 , it may not account for differences between the engines  122  of the individual gensets. For example, each engine  122  may produce exhaust emissions in accordance with an associated exhaust emission curve that determines the emissions produced by the engine  122  in relation to the particular power being output from the engine  122 . This exhaust emission curve may determine or indicate where the individual engine  122  may operate most efficiently in terms of reducing or minimizing engine exhaust emissions. Where the engine  122  operates most efficiently in terms of minimizing exhaust emissions may not correspond to the power output being requested of a power source  120  by the symmetrical load sharing strategy. 
     To allow for the engines  122  to be operated in a manner that minimizes the total exhaust emissions produced by all the operating engines  122  of the power system  100 , the power system controller  140  may be configured to perform an optimization process that determines an optimized apportionment of the power demand to the individual operating engines  122  of the power system  100  based upon minimizing total engine emissions. To this end, the power system controller  140  may apportion the power demand from load manager  134  based on emission output information associated with each power source  120  or engine  122 . Specifically, the power system controller  140  may retrieve and/or receive from each engine  122  (e.g., from the power source controller  128  associated with each engine  122  and/or with each generator  124 ) an emission output map. It is contemplated that the maps may be different for each engine  122  and/or for each different type of engine  122 , as needed. The power system controller  140  may then compare different apportionments of the power demand from the load manager  134  with the emission output map to determine the particular configuration of apportionments that provides the overall lowest exhaust emission output possible from all of the engines  122 . In some embodiments or circumstances, this may result in an equal apportionment of the power demand between the different power sources  120 . In most instances, however, the apportionment may be unequal. In fact, in some instances, one or more of the power sources  120  may be operated to satisfy a majority of the power demand and one or more others of the other power sources  120  may supply little of the demand or even be turned off. 
     Whether the power system controller  140  apportions the power demand based on total engine emissions, may be determined by an operator of the vessel or power system or it may be automatically determined based signals relating to other vessel-related or power system-related functions. Accordingly, the power system controller  140  may be configured to receive emission mode selection information which may communicate to the power system controller  140  whether to enable the engine emission control mode. The emission mode selection information may be input through an input device, for example in the bridge  112 , by an operator of the vessel  102  or the power system  100 . Alternatively or additionally, the emission mode selection information may also include information that may signal an automatic enablement of the engine emission based apportionment of the power demand such as, for example, information relating to the location of the vessel  102  (e.g., in port or at sea) and/or information relating to an operating mode of the engines  122 . Additionally, the emission mode selection information may include information regarding whether the power system  100  is presently in a transient condition (i.e., the speed or load is changing dynamically) in which enablement of the emission mode may not be appropriate or in a steady state condition in which the emission mode may be enabled. 
     In determining the optimized apportionment of the power demand, the optimization process performed by the power system controller  140  may also take into account one or more additional considerations that may constrain the minimization of exhaust emissions. For example, the power system controller  140  may take into account engine operation information. The engine operation information considered by the power system controller  140  may include information relating to any priorities assigned to the power sources  120  or engines  122 . In such a case, the power system controller  140  may be configured to apportion the power demand to the higher priority power sources  120  or engines  122  first. One example of an engine priority situation is where multiple engines are mechanically linked to each other such that they must be run together. The power system controller  140  may also take into account other engine operation information such as which engines  122  may be offline and the current operating status of the engine  122 . The engine operating information may also include a predefined, desired operating range for one or more of the engines  122 . For example, the desired operating range for each of the various engines  122  in the power system  100  may be different. The engine operation information may also include a desired power reserve. Specifically, the operator of the vessel  102  or power system  100  may desire a particular amount of power be left in reserve from particular power sources  120 , and this power reserve may limit the way in which the power system controller  140  can apportion the power demand. The engine operation information may also include information regarding the mode in which the power system  100  is operating such as a dynamic positioning mode or a cruise control mode. 
     After all constraints that may be attributable to engine operation information are applied, the power system controller  140  may then determine the optimized apportionment of the power request to each operating engine  122  that minimizes the total engine exhaust emissions output from the power system  100 . In some embodiments, this determination may involve the use of a particle swarm optimization method. In considering the engine operation information as well as the exhaust emission information, the power system controller  140  may treat identically configured engines  122  separately or apportion an equal share of the power demand to the identical engines  122 . 
     As noted above, and as shown in  FIG. 2 , one or more of the engines  122  may be equipped with an SCR system  127 . To account for the one or more SCR systems  127 , the optimization process performed by the power system controller  140  may include evaluating information relating to operation of the SCR systems  127  in determining the optimized apportionment of the power demand based on the total engine emissions. For example, the power system controller  140  may be configured to retrieve and/or receive (e.g., from the power source controller  128  associated with each SCR-equipped engine) information concerning the efficiency of each SCR system  127  associated with one of the engines in the power system at converting or reducing nitrogen oxides present in the exhaust of the respective engine  122 . More specifically, each engine  122  equipped with an SCR system  127  may have an associated conversion efficiency map that details the conversion rate of the nitrogen oxides in the engine exhaust emissions at different operating conditions of the engine  122  and the SCR system  127 , for example, at different SCR temperatures and different DEF injection rates. The SCR conversion efficiency maps may be different for each SCR-equipped engine  122  in the power system  100 . Upon receiving and/or retrieving the SCR conversion efficiency map, the power system controller  140  may use the SCR conversion efficiency map in determining the NO X  emissions produced by the engines  122  equipped with SCR systems  127  and thereby in determining the optimized apportionment of the power demand that minimizes total engine emissions. 
     Other considerations that may be taken into account by the power system controller  140  in determining the optimized apportionment of the power demand to the operating engines that minimizes the total engine emissions are the costs associated with operating the engines  122  and the associated SCR systems  127  if a balance between operating costs and total engine emissions is desired. For example, operation of the engine  122  consumes fuel and operation of the SCR system  127  consumes DEF. The consumption of fuel and DEF has an associated monetary cost at different engine  122  and SCR system  127  operating conditions and the power system controller  140  can be configured to take this cost into account in determining the optimized apportionment of the power demand. In such a case, the power system controller  140  may direct each engine  122  to an optimized load condition based minimized total engine emissions at a reasonable cost. In balancing total emissions and costs, the power system controller  140  may apply a weight factor to each consideration that reflects the relative importance of emissions versus costs. These weight factors may be predetermined or based on information input by an operator of the vessel  102  or power system  100 . 
     The optimized power distribution to the operating engines  122  for different power demands may be determined offline using the optimization process and stored as a map which is then used in real time by the power system controller  140  to direct apportionment of the power demand to the power sources. The optimization process can also be implemented by the power system controller  140  online to generate the optimized apportionment of the power demand at each time step. 
     In some applications, it may be possible over time for performance of a particular engine  122  or SCR system  127  to drift away from the control maps stored within the corresponding power source controllers  128  or the power system controller  140 . For example, it may be possible for an older engine  122  to have decreased performance due to wear, or for system inputs (e.g., fuel quality, wind current, ocean current, ambient air temperature, etc.) to deviate from assumed or expected values. In these situations, the power system controller  140  may be capable of modifying the existing control maps based on monitored engine  122  and/or SCR system  127  performance. Specifically, the power system controller  140  may be capable of monitoring, processing, and recording engine  122  and SCR system  127  performance for future use in power demand apportioning. 
     The power system controller  140  may rely on different sensors when monitoring engine  122  and SCR system  127  performance and/or modifying the existing control maps. These sensors may include, for example, one or more fuel flow meters associated with each engine  122 , speed sensors, torque sensors, emission sensors (e.g., NOx sensors), temperature sensors, pressure sensors, voltage sensors, current sensors, fuel level sensors, DEF level sensors, DEF flow meters, and other performance sensors. The power system controller  140  may also be capable of computing different aspects of engine  122  and/or SCR system  127  performance based on measured parameters. For example, the power system controller  140  may be capable of computing engine torque, emissions, and/or wear based on measured rpm, fuel flow rates, temperatures, and/or pressures. The power system controller  140  may then update and/or create the required control maps based directly on the measured parameters and/or based on the calculated parameters. 
     INDUSTRIAL APPLICABILITY 
     The disclosed power system  100  and method for controlling a power system  100  may be applicable to any application that may require power provided by multiple engines. For example, the disclosed power system  100  and control method may be applicable to a marine and/or petroleum drilling vessel application, where the power sources cooperate to propel the vessel and to power auxiliary loads under varying conditions. The disclosed power system and method may allow for an optimized apportionment of the power demand to the operating engines that minimizes total engine emissions at, in some embodiments, a reasonable cost. 
     Referring to  FIG. 3 , there is provided a flow chart of a multi-engine control method  200  that may be implemented by the power system controller  140 . The steps of the control method described herein may be embodied as machine readable and executable software instructions, software code, or executable computer programs. The software instructions may be further embodied in one or more routines, subroutines, or modules and may utilize various auxiliary libraries and input/output functions to communicate with other equipment. The control strategy may also be associated with an operator interface through which the strategy may interact with an operator of the marine vessel  102  or power system  100 . 
     In step  210 , a power demand for the experienced load is received. This power demand may be received from the from load manager  134 . Information regarding emission mode selection is received in step  212 . This information relates to whether to implement the emission control mode. As noted above, this information may be communicated by an operator of the vessel  102  or power system  100  through an input device and/or be communicated by other control systems associated with the vessel and/or power system. Engine operation information is received in step  214 . This may include, for example, engine priorities, desired power reserve, engine operating modes, engine online/offline status and/or desired engine operating range 
     In step  216 , the power system controller  140  may determine whether to enable or implement the emission control mode using, for example, the information from steps  210 ,  212  and  214 . Once the emission control mode has been enabled, the power system controller  140  may obtain (i.e., retrieve and/or develop) the associated emission output maps for the operating engines  122  in step  218 . If any engine  122  in the power system  100  has an operating SCR system  127 , the power system controller  140  may retrieve and/or develop the associated SCR conversion efficiency map for that engine  122  and SCR system  127  in step  220 . As discussed above, the power system controller  140  may retrieve the emission and conversion efficiency maps from the power source controllers  128  and when these maps are no longer accurate, the power system controller  140  may develop and update the maps based on monitored performance. If none of the operating engines  122  include an SCR system  127 , then step  220  may not be performed. 
     In step  222 , the power system controller  140  may determine the optimized apportionment of the power demand to the operating engines  122  based on minimizing total engine emissions across the power system  100  as well as the engine operation information from step  214 . In evaluating the total engine emissions, the power system controller  140  may use the emission output maps for each engine from step  218  and the conversion efficiency maps associated with each SCR system from step  220 . As noted previously, this determination may include a particle swarm optimization methodology. Moreover, it should be appreciated that the terms “optimized,” “optimal,” and the like are used herein as relative terms and should not be construed as an absolute or as addressing considerations other than such considerations described herein. 
     If operating costs are to be considered in apportioning the power demand, the DEF cost at the apportioned power demand and the fuel cost at the apportioned power demand may be determined in steps  224  and  226 . In step  228 , These costs are then used along with the emission output maps from step  218  and the SCR conversion efficiency maps from step  220  to determine a final optimized apportionment of the power demand to the operating engines. The apportionment of step  228  may be based on minimizing total engine emissions, the engine operating information (from step  214 ), the DEF cost (from step  224 ) and the fuel cost (step  226 ). In determining the optimized apportionment in step  228 , different weight factors may be applied to the total engine emissions and the total costs to reflect the relative importance of these considerations. If costs are not to be considered, steps  224 ,  226  and  228  may be not be performed. Moreover, it is possible that the power system controller  140  may consider only one of the fluids (DEF or fuel) in evaluating the costs associated with the apportionment of the power demand. 
     In step  230 , the power system controller  140  may apply a limit to the rate of change of the apportionment of the power demand in order to help smooth changes in the apportionment of the power demand to the various engines  122  and thereby prevent any undesirable, significant fluctuations in the operating condition of any of the engines  122 . In step  232 , the operating engines  122  are commanded based on the final apportionment of the power demand (from step  228  if cost evaluation is performed, or step  222  if costs are not evaluated) as limited by any rate limit applied in step  230 . 
     This disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.