Patent Publication Number: US-2013239577-A1

Title: Hybrid gas turbine engine-electric motor/generator drive system

Description:
TECHNICAL FIELD 
     The present disclosure relates generally to a drive system and, more particularly, to a hybrid gas turbine engine—electric motor/generator drive system. 
     BACKGROUND 
     Gas turbine engines are sometimes used to power driven equipment. For example, gas turbine engines are sometimes used to power centrifugal compressors used in the oil and gas industry. In particular, the gas turbine engines are used to power centrifugal compressors used to transport fluids such as, for example, natural gas. Since demand for the fluids fluctuates, the gas turbine engines are not always sized to efficiently power the centrifugal compressors. For example, gas turbine engines that are sized to power the centrifugal compressors during peak demand periods are not sized to efficiently power the centrifugal compressors during low demand periods. Unfortunately, this leads to excessive fuel costs and emissions during low demand periods. 
     One way to reduce fuel costs is disclosed in U.S. Pat. No. 5,689,141 (the &#39;141 patent) issued to Kikkawa et al. on Nov. 18, 1997. The &#39;141 patent discloses a compressor drive system for a natural gas liquefaction plant. The compressor drive system includes a plurality of gas turbines. Additionally, the compressor drive system includes an electric motor associated with each of the gas turbines. Each electric motor serves both as an auxiliary electric motor for generating a startup torque and as an AC generator. When the power requirement of a compressor associated with one of the gas turbines is less than the power output of the gas turbine, the electric motor converts the excess output power of the gas turbine into electric power. This allows the gas turbine to operate efficiently, resulting in reduced operating costs through fuel savings. 
     SUMMARY 
     In one aspect, the present disclosure is related to a method of operating a drive system for a load. The drive system may include an electric motor/generator and a gas turbine engine. The engine may include a combustor, and main and pilot flow paths via which fuel is supplied to the combustor. The engine may be operable in a low emissions mode and a standard emissions mode. A proportion of the fuel that is supplied to the combustor via the pilot flow path may be greater in the standard emissions mode than in the low emissions mode. The method may include determining whether an environmental condition is conducive to operating the engine in the low emissions mode. The method may also include operating the electric motor/generator as a motor if the environmental condition is not conducive to operating the engine in the low emissions mode. 
     In another aspect, the present disclosure is related to another method of operating a drive system for a load. The drive system may include an electric motor/generator and a gas turbine engine. The engine may include a combustor, and main and pilot flow paths via which fuel is supplied to the combustor. The engine may be operable in a low emissions mode and a standard emissions mode. A proportion of the fuel that is supplied to the combustor via the pilot flow path may be greater in the standard emissions mode than in the low emissions mode. The method may include determining an engine power requirement of the load. The method may also include determining whether the engine power requirement of the load is sufficiently large to operate the engine in the low emissions mode. Additionally, the method may include operating the electric motor/generator if the engine power requirement of the load is not sufficiently large to operate the engine in the low emissions mode. 
     In yet another aspect, the present disclosure is related to a turbine-compressor unit. The turbine-compressor unit may include a centrifugal compressor. The turbine-compressor unit may also include a gas turbine engine, which may be coupled to the centrifugal compressor. The engine may include a combustor, and main and pilot flow paths via which fuel is supplied to the combustor. The engine may be operable in a low emissions mode and a standard emissions mode. A proportion of the fuel that is supplied to the combustor via the pilot flow path may be greater in the standard emissions mode than in the low emissions mode. In addition, the turbine-compressor unit may include an electric motor/generator, which may be coupled to the centrifugal compressor. The turbine-compressor unit may also include a controller, which may be in communication with the centrifugal compressor, the engine, and the electric motor/generator. The controller may be configured to determine an engine power requirement of the centrifugal compressor. Additionally, the controller may be configured to determine whether the engine power requirement of the centrifugal compressor is sufficiently large to operate the engine in the low emissions mode. The controller may also be configured to operate the electric motor/generator if the engine power requirement of the centrifugal compressor is not sufficiently large to operate the engine in the low emissions mode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an exemplary disclosed drive system for an exemplary disclosed load; 
         FIG. 2  is a control diagram of the drive system of  FIG. 1 ; 
         FIG. 3  is a flow chart describing an exemplary method of operating, the drive system of  FIGS. 1-2 ; and 
         FIG. 4  is an exemplary demand histogram for the drive system of  FIGS. 1-3 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates a drive system  10  for a load  15 . For example, load  15  may include a centrifugal compressor  20 , which may be used to compress, for example, natural gas or another fluid. Such an arrangement is hereafter referred to as a turbine-compressor unit. Alternatively, load  15  may include another type of compressor (not shown), a generator (not shown), or another type of load known in the art. 
     Regardless of the type of load  15 , drive system  10  may include a gas turbine engine  30 , which may provide power to load  15 . Specifically, engine  30  may be mechanically coupled to load  15  by a coupling  35 , and may rotatably drive load  15 . For example, coupling  35  may include a clutch and/or a gearbox. Alternatively, engine  30  may be coupled directly to load  15  by, for example, a drive shaft. In any case, engine  30  may include a dry low emissions (DLE or DLN) combustion system (not shown). This combustion system may include multiple flow paths (not shown) that deliver different concentrations of fuel and air to a combustor of engine  30  (not shown). 
     For example, the combustion system may include a main flow path and a pilot flow path. The main flow path may deliver a premixed lean fuel-air mixture to the combustor (hereafter the “main fuel stream”). The main fuel stream may burn in the combustor, creating premixed flames. As used herein, premixed flames are flames created when fuel and air are first mixed, and then burned in the combustor. The pilot flow path may deliver to the combustor through a pilot air module a pressurized spray of fuel along with compressed air (hereafter the “pilot fuel stream”). The pilot fuel stream may burn in the combustor, creating a diffusion flame. As used herein, diffusion flames are flames created when fuel and air mix and burn at the same time. 
     It is contemplated that the proportion of fuel supplied to the combustor via each of the multiple flow paths may be adjusted to alter operation of engine  30 . For example, engine  30  may be operated in a standard emissions mode or a low emissions mode. The proportion of fuel supplied to the combustor via the pilot flow path may be greater in the standard emissions mode than in the low emissions mode. For example, in the low emissions mode of one embodiment, almost none of the fuel supplied to the combustor (e.g., less than 5-10%) is supplied via the pilot flow path. In contrast, in the standard emissions mode of this embodiment, a significant amount of the fuel supplied to the combustor (e.g., upwards of 20-30%) is supplied via the pilot flow path. Unfortunately, supplying a greater proportion of the fuel via the pilot flow path may locally increase a combustion temperature of the fuel (diffusion flames burn hotter than premixed flames), and increase emissions of engine  30 . Therefore, it may be desirable to operate engine  30  in the low emissions mode and not in the standard emissions mode (e.g., to reduce an environmental impact of drive system  10 ). 
     It is contemplated, however, that engine  30  may operate in the low emissions mode only under certain conditions. For example, engine  30  may operate in the low emissions mode only when an amount of power required by load  15  from engine  30  (hereafter the “engine power requirement of load  15 ”) is greater than a certain amount of power (e.g., a percentage of a rated power of engine  30 ). As used herein, the rated power of engine  30  is a maximum amount of power that engine  30  may provide. For example, the amount of power that engine  30  may provide may be limited by mechanical features of engine  30 , and may vary based on an altitude of engine  30  (which may affect an inlet air density of engine  30 ) and/or an inlet air temperature of engine  30 . Alternatively or additionally, engine  30  may operate in the low emissions mode only under certain environmental conditions. For example, engine  30  may operate in the low emissions mode only when an inlet air temperature of engine  30  is above a certain temperature. Alternatively or additionally, engine  30  may operate in the low emissions mode only under certain other environmental conditions relating to, for example, humidity, air density, and/or fuel type. 
     It is contemplated, therefore, that drive system  10  may include components for modifying the engine power requirement of load  15  such that engine  30  can operate in the low emissions mode and/or such that engine  30  can be shut down (i.e., not operated). As illustrated in  FIGS. 1 and 2 , these components of drive system  10  may include an electric motor/generator  45 , a controller  50 , an environmental condition sensor  55 , a receiver  60 , a transmitter  65 , and/or another component that may be used to modify the engine power requirement of load  15 . As illustrated in  FIG. 1 , electric motor/generator  45  may be coupled to load  15  by a coupling  70 . For example, coupling  70  may include a clutch and/or a gearbox. Alternatively, electric motor/generator  45  may be coupled directly to load  15  by, for example, a drive shaft. In any case, it should be understood that the relative positions of electric motor/generator  45 , engine  30 , and load  15  need not be as illustrated in  FIG. 1 . For example, electric motor/generator  45  may be positioned between engine  30  and load  15 , and engine  30  may be coupled to load  15  via electric motor/generator  45 . Regardless of the relative positions of electric motor/generator  45 , engine  30 , and load  15 , it should also be understood that electric motor/generator  45  may be connected to an electric grid, and may be operated as a motor or as a generator to modify the engine power requirement of load  15 . 
     For example, electric motor/generator  45  may be operated as a motor to decrease the engine power requirement of load  15 . Specifically, electric motor/generator  45  may be operated as a motor to provide power to load  15 . This power may supplement the power provided to load  15  by engine  30 , decreasing the engine power requirement of load  15 . In particular, the engine power requirement of load  15  may be decreased by the amount of power provided by electric motor/generator  45 . 
     As another example, electric motor/generator  45  may be operated as a generator to increase the engine power requirement of load  15 . Specifically, electric motor/generator  45  may be operated as a generator to receive power from load  15 . This power may be drawn from the power provided to load  15  by engine  30 , increasing the engine power requirement of load  15 . In particular, the engine power requirement of load  15  may be increased by the amount of power received by electric motor/generator  45 . 
     As illustrated in  FIG. 2 , controller  50 , which may include one or more processors (not shown) and one or more memory devices (not shown), may communicate with environmental condition sensor  55 , receiver  60 , transmitter  65 , and/or other components associated with engine  30 , load  15 , and/or electric motor/generator  45  to determine the engine power requirement of load  15 . Based on this determination, controller  50  may modify the engine power requirement of load  15  such that engine  30  can be operated in the low emissions mode and/or such that engine  30  can be shut down, as described below. 
     Environmental condition sensor  55  may be associated with engine  30 , and may be configured to determine an environmental condition that may or may not be conducive to operating engine  30  in the low emissions mode. As used herein, an environmental condition is conducive to operating engine  30  in the low emissions mode if the environmental condition does not prevent engine  30  from operating in the low emissions mode. For example, environmental condition sensor  55  may include a temperature sensor, which may be configured to determine an inlet air temperature of engine  30 . Alternatively, environmental condition sensor  55  may include another type of sensor, which may be configured to determine another environmental condition such as, for example, humidity, air density, and/or fuel type. In any case, environmental condition sensor  55  may generate and communicate to controller  50  a signal indicative of the environmental condition. 
     Receiver  60  may receive, through a communications link, signals from a dispatcher station  72 , a drive system  75 , and/or another offboard system. For example, dispatcher station  72  may include a facility having computers and/or individuals tasked with controlling the operation of drive system  10 , load  15 , and/or electric motor/generator  45 . And, drive system  75  may include a drive system, which may or may not be equivalent to drive system  10 . Receiver  60  may include hardware and/or software that enables receiver  60  to receive the signals through the communications link. The signals may include, for example, satellite, cellular, infrared, radio, and/or other types of wireless signals. Alternatively, the signals may include electrical, optical, and/or other types of wired signals. For example, if load  15  includes centrifugal compressor  20 , the signals received from the offboard system may be indicative of a demand for the fluid compressed by centrifugal compressor  20 . Alternatively, if load  15  includes a generator, the signals received from the offboard system may be indicative of a demand for electricity. In any case, receiver  60  may generate and communicate to controller  50  a signal indicative of the demand. 
     Transmitter  65  may transmit, through a communications link, signals to dispatcher station  72 , drive system  75 , and/or another offboard system. Transmitter  65  may include hardware and/or software that enables transmitter  65  to transmit the signals through the communications link. The signals may include, for example, satellite, cellular, infrared, radio, and/or other types of wireless signals. Alternatively, the signals may include, for example, electrical, optical, and/or other types of wired signals. For example, controller  50  may use transmitter  65  to transmit to the offboard system a signal indicative of the demand, discussed above. Alternatively or additionally, controller  50  may use transmitter  65  to transmit to the offboard system a signal indicative of an amount of additional power needed to meet the demand. 
       FIG. 3  illustrates an exemplary method of operating drive system  10 . And  FIG. 4  illustrates an exemplary demand histogram for drive system  10 .  FIGS. 3 and 4  will be discussed in the following section to further illustrate drive system  10  and its operation. 
     INDUSTRIAL APPLICABILITY 
     The disclosed system may be applicable to gas turbine engines that are operable in low emissions and standard emissions modes, and that are used to drive loads. The system may increase a proportion of an engine&#39;s operating time during which it is operated in the low emissions mode. In particular, the system may use an electric/motor generator to increase and/or decrease the amount of power a load requires from the engine such that the engine can be operated in the low emissions mode and/or such that the engine can be shut down. Operation of the system will now be described. 
     As illustrated in  FIG. 3 , drive system  10 , and more specifically, controller  50  (referring to  FIG. 2 ), may first determine the engine power requirement of load  15  (the amount of power required by load  15  from engine  30 ) (step  300 ). This determination may be based on signals received from dispatcher station  72 , drive system  75 , and/or another offboard system. In particular, these signals may be received by receiver  60 , and may be indicative of a demand. For example, if load  15  includes centrifugal compressor  20 , the demand may be a demand for the fluid compressed by centrifugal compressor  20 . And, the determination may include a calculation involving the demand for the fluid (e.g., a flow rate and/or a discharge pressure of the fluid), a pressure and/or a temperature of the fluid entering centrifugal compressor  20 , a pressure and/or a temperature of the fluid leaving centrifugal compressor  20 , an altitude of centrifugal compressor  20 , an efficiency of centrifugal compressor  20 , and/or another value associated with the engine power requirement of centrifugal compressor  20 . Alternatively, if load  15  includes a generator, the demand may be a demand for electricity. And, the determination may include a calculation involving the demand for electricity, the efficiency of the generator, and/or another value associated with the engine power requirement of the generator. 
     Based on the determined engine power requirement of load  15 , controller  50  may use components of drive system  10  to modify the engine power requirement of load  15  such that engine  30  can be operated in the low emissions mode and/or such that engine  30  can be shut down. In particular, controller  50  may determine whether the rated power of engine  30  is sufficiently large to fulfill the engine power requirement of load  15  (step  310 ). Specifically, controller  50  may compare the engine power requirement of load  15  (as determined during step  300 ) to the rated power of engine  30 . If the engine power requirement of load  15  exceeds the rated power of engine  30 , the rated power of engine  30  may not be sufficiently large to fulfill the engine power requirement of load  15 , and controller  50  may use components of drive system  10  to decrease the engine power requirement of load  15  (step  320 ), as described below. 
     Otherwise, controller  50  may determine whether environmental condition(s) are conducive to operating engine  30  in the low emissions mode (step  330 ). For example, controller  50  may determine whether the inlet air temperature of engine  30  is sufficiently high to operate engine  30  in the low emissions mode. Specifically, controller  50  may compare the inlet air temperature of engine  30  (as determined by environmental condition sensor  55 ) to a minimum temperature at which engine  30  may operate in the low emissions mode. If the inlet air temperature of engine  30  fails to exceed this minimum temperature, the environmental condition(s) may not be conducive to operating engine  30  in the low emissions mode. In such a case, controller  50  may use components of drive system  10  to make operation of engine  30  unnecessary (step  340 ), as described below. 
     Otherwise, controller  50  may determine whether the engine power requirement of load  15  is sufficiently large to operate engine  30  in the low emissions mode (step  350 ). Specifically, controller  50  may compare the engine power requirement of load  15  to the rated power of engine  30 . If the engine power requirement of load  15  fails to exceed a certain percentage of the rated power of engine  30 , the engine power requirement of load  15  may not be sufficiently large to operate engine  30  in the low emissions mode, and controller  50  may use components of drive system  10  to increase the engine power requirement of load  15  (step  360 ), as described below. Alternatively, controller  50  may use components of drive system  10  to make operation of engine  30  unnecessary (step  340 ), as described below. If, however, the engine power requirement of load  15  is sufficiently large to operate engine  30  in the low emissions mode, controller  50  may return to step  300  and again determine the engine power requirement of load  15 . 
     Decreasing the engine power requirement of load  15  during step  320  may include sub-steps. In particular, controller  50  may first determine a minimum modified engine power requirement of load  15  when electric motor/generator  45  is operated as a motor (sub-step  370 ). As previously discussed, the engine power requirement of load  15  may be decreased by the amount of power provided by electric motor/generator  45 . Therefore, controller  50  may determine the minimum modified engine power requirement of load  15  by subtracting from the engine power requirement of load  15  (as determined during step  300 ) a maximum amount of power providable by electric motor/generator  45 . It should be noted that this maximum amount of power providable by electric motor/generator  45  may or may not vary based on a rotational speed of load  15 . Controller  50  may then determine whether the rated power of engine  30  is sufficiently large to fulfill the minimum modified engine power requirement of load  15  (sub-step  380 ). Specifically, controller  50  may compare the minimum modified engine power requirement of load  15  to the rated power of engine  30 . If the minimum modified engine power requirement of load  15  exceeds the rated power of engine  30 , drive system  10  may not be capable of meeting the demand indicated by the signals received by receiver  60 . Therefore, controller  50  may use transmitter  65  to notify an offboard system (e.g., dispatcher station  72  or drive system  75 ) of the demand (sub-step  390 ). Alternatively or additionally, controller  50  may use transmitter  65  to notify the offboard system of the difference between the minimum modified engine power requirement of load  15  and the rated power of engine  30  (the additional amount of power needed to meet the demand). It is contemplated that drive systems  10  and  75  may then operate jointly to meet the demand indicated by the signals received by receiver  60 . Controller  50  may then return to step  300  and again determine the engine power requirement of load  15 . 
     If, however, the minimum modified engine power requirement of load  15  fails to exceed the rated power of engine  30 , controller  50  may operate electric motor/generator  45  as a motor to decrease the engine power requirement of load  15  (sub-step  400 ). In particular, controller  50  may operate electric motor/generator  45  to decrease the engine power requirement of load  15  such that engine  30  can fulfill the decreased engine power requirement of load  15 . Controller  50  may then return to step  300  and again determine the engine power requirement of load  15 . 
     Making operation of engine  30  unnecessary during step  340  may also include sub-steps. In particular, controller  50  may first determine a minimum modified engine power requirement of load  15  when electric motor/generator  45  is operated as a motor (sub-step  410 ). This determination may be equivalent to the determination of sub-step  370 . Controller  50  may then determine whether the minimum modified engine power requirement of load  15  is sufficiently small to shut down engine  30  (sub-step  420 ). Specifically, controller  50  may compare the minimum modified engine power requirement of load  15  to zero. If the minimum modified engine power requirement of load  15  exceeds zero, it may not be possible to shut down engine  30 , and controller  50  may return to step  300  and again determine the engine power requirement of load  15 . 
     If, however, the minimum modified engine power requirement of load  15  fails to exceed zero, it may be possible to shut down engine  30 , and controller  50  may operate electric motor/generator  45  as a motor to decrease the engine power requirement of load  15  (sub-step  430 ). In particular, controller  50  may operate electric motor/generator  45  to decrease the engine power requirement of load  15  such that engine  30  can be shut down (i.e., such that operation of engine  30  is unnecessary). Controller  50  may then return to step  300  and again determine the engine power requirement of load  15 . 
     Increasing the engine power requirement of load  15  during step  360  may also include sub-steps. In particular, controller  50  may first determine a maximum modified engine power requirement of load  15  when electric motor/generator  45  is operated as a generator (sub-step  440 ). As previously discussed, the engine power requirement of load  15  may be increased by the amount of power received by electric motor/generator  45 . Therefore, controller  50  may determine the maximum modified engine power requirement of load  15  by adding to the engine power requirement of load  15  (as determined during step  300 ) a maximum amount of power receivable by electric motor/generator  45 . It should be noted that this maximum amount of power receivable by electric motor/generator  45  may or may not vary based on a rotational speed of load  15 . Controller  50  may then determine whether the maximum modified engine power requirement of load  15  is sufficiently large to operate engine  30  in the low emissions mode (sub-step  450 ). Specifically, controller  50  may compare the maximum modified engine power requirement of load  15  to the rated power of engine  30 . If the maximum modified engine power requirement of load  15  fails to exceed a certain percentage of the rated power of engine  30 , the maximum modified engine power requirement of load  15  may not be sufficiently large to operate engine  30  in the low emissions mode, and controller  50  may proceed to step  340  and use components of drive system  10  to make operation of engine  30  unnecessary, as described above. Alternatively, controller  50  may return to step  300  and again determine the engine power requirement of load  15 . 
     If, however, the maximum modified engine power requirement of load  15  exceeds the certain percentage of the rated power of engine  30 , controller  50  may operate electric motor/generator  45  as a generator to increase the engine power requirement of load  15  (sub-step  460 ). In particular, controller  50  may operate electric motor/generator  45  to increase the engine power requirement of load  15  such that the increased engine power requirement of load  15  allows engine  30  to operate in the low emissions mode. Controller  50  may then return to step  300  and again determine the engine power requirement of load  15 . 
     As previously discussed, controller  50  may return to step  300  after completing steps  320 ,  340 ,  350 , and/or  360 , and again determine the engine power requirement of load  15 . Controller  50  may then repeat steps  310 - 360 . During each repetition, it is contemplated that controller  50  may adjust the operation of drive system  10  to minimize the emissions of drive system  10 . Specifically, if environmental condition(s) are not conducive to operating engine  30  in the low emissions mode, controller  50  may, during step  340 , modify the engine power requirement of load  15  to allow engine  30  to be shut down. If, however, environmental condition(s) are conducive to operating engine  30  in the low emissions mode, controller  50  may, during step  360 , modify the engine power requirement of load  15  to allow engine  30  to operate in the low emissions mode. 
     While the modifications of steps  340  and  360  may instantaneously minimize the emissions of drive system  10 , it is contemplated that the modifications of step  320  may over time minimize the emissions of drive system  10 . This is because the modifications of step  320  may allow engine  30  to be sized for commonly sized demand periods, rather than for peak demand periods. For example, engine  30  may be sized such that its rated power is approximately 80% of the unmodified peak engine power requirement of load  15 , rather than 100% of the unmodified peak engine power requirement of load  15 . As illustrated in  FIG. 4 , this may allow engine  30  to operate in the low emissions mode when the unmodified engine power requirement of load  15  falls within regions iii, iv, and v. Specifically, in region iv, engine  30  may operate in the low emissions mode without modifications to the engine power requirement of load  15 . Additionally, in region iii, the modifications of step  360  may allow engine  30  to operate in the low emissions mode. And, in region v, the modifications of step  320  may allow engine  30  to operate in the low emissions mode. 
     In contrast to an engine  30  that is sized such that its rated power is approximately 80% of the unmodified peak engine power requirement of load  15  (hereafter the “smaller engine  30 ”), an engine  30  that is sized such that its rated power is approximately 100% of the unmodified peak engine power requirement of load  15  (hereafter the “larger engine  30 ”) may only be able to operate in the low emissions mode when the unmodified engine power requirement of load  15  falls within regions iv and v. Specifically, the larger engine  30  may be able to operate in the low emissions mode in region v without modifications to the engine power requirement of load  15 . And, the modifications of step  360  may allow the larger engine  30  to operate in the low emissions mode in region iv. 
     While it may be possible to operate electric motor/generator  45  instead of the differently sized engines  30  in regions i, ii, and/or iii, it should be noted that this may not compensate for emissions differences between the differently sized engines  30 . For example, it should be noted that region iv includes commonly sized demand periods. In other words, the engine power requirement of load  15  falls within region iv most of the time. Thus, the smaller engine  30  may operate in the low emissions mode most of the time without modifications to the power requirement of load  15 . In contrast, the larger engine  30  may only be operable in the low emissions mode in region iv with the modifications of step  360 . Although operating in the low emissions mode with the modifications of step  360  may be preferable to operating in the standard emissions mode, it is contemplated that operating in the low emissions mode without modifications to the power requirement of load  15  may produce less emissions than operating in the low emissions mode with the modifications of step  360 . Therefore, the emissions of drive system  10  may over time be minimized when engine  30  is sized for commonly sized demand periods, rather than for peak demand periods. 
     In addition, it is contemplated that the modifications of step  320  may minimize the emissions of multiple drive systems operating jointly to meet a demand. Specifically, controller  50  may, during step  320 , modify the engine power requirement of load  15  to allow drive system  10  to meet the demand while operating engine  30  in the low emissions mode. In doing so, controller  50  may make operation of an engine of another drive system  75  unnecessary, preventing the engine of drive system  75  from operating in the standard emissions mode. 
     It is also contemplated that the modifications of steps  320 ,  340 , and  360  may reduce operating costs associated with drive system  10 . In particular, the modifications may minimize fuel costs associated with drive system  10 . Specifically, it is contemplated that allowing engine  30  to operate in the low emissions mode may reduce a fuel consumption of engine  30 . This is because engine  30  may operate more efficiently in the low emissions mode. Also, allowing engine  30  to be shut down under certain conditions may make it possible to take advantage of cost differences between electricity and fuel. For example, controller  50  may be configured to proceed to step  340  and operate electric motor/generator  45  as a motor whenever electricity costs are low relative to fuel costs. This may reduce the fuel consumption of engine  30  and increase the electricity consumption of electric motor/generator  45 , thereby minimizing the total fuel and electricity costs. Alternatively, controller  50  may be configured to proceed to step  360  and operate electric motor/generator  45  as a generator whenever electricity costs are high relative to fuel costs. This may increase the fuel consumption of engine  30  and increase the electricity production of electric motor/generator  45 , thereby minimizing the total fuel and electricity costs. In yet another alternative, it is contemplated that the modifications of steps  320 ,  340 , and  360  may minimize emissions costs associated with drive system  10 . For example, these costs may include costs associated with purchasing offsetting emissions credits. And, as previously discussed, the modifications of steps  320 ,  340 , and  360  may minimize the emissions of drive system  10 . 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the method and system of the present disclosure. Other embodiments of the method and system will be apparent to those skilled in the art from consideration of the specification and practice of the method and system disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope of the disclosure being indicated by the following claims and their equivalents.