Patent Publication Number: US-2012042639-A1

Title: Multi-engine system with on-board ammonia production

Description:
RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 11/806,384, filed May 31, 2007, the entire contents of which are incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     This invention was made with Government support under Contract No. DE-FC26-01CH1107 awarded by the Department of Energy. The Government may have certain rights in this invention. 
    
    
     TECHNICAL FIELD 
     The present disclosure is directed to a multi-engine system and more particularly, to a multi-engine system with at least one engine having an on-board ammonia producing capability. 
     BACKGROUND 
     Fossil fuel powered systems for engines, factories, and power plants typically produce emissions that contain a variety of pollutants. These pollutants may include, for example, particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of pollutants in the exhaust stream may be regulated depending on the type, size, and/or class of engine. 
     One method used to reduce emissions is selective catalytic reduction (SCR). SCR provides a method for removing NOx emissions from internal combustion engine systems. During SCR, a catalyst facilitates a reaction between a reductant (e.g., ammonia) and NOx to produce water vapor and nitrogen gas, thereby removing NOx from the exhaust gas. Ammonia that is used for the SCR system may be stored for injection when needed. However, because of the high reactivity of ammonia, storage can be problematic. In addition, machines utilizing SCR systems sometimes operate in remote locations where it may be difficult to replenish the ammonia. On-board ammonia production may provide a safer and more practical alternative to ammonia storage. 
     U.S. Pat. No. 5,964,088 (the &#39;088 patent) issued to Kinugasa et al. on Oct. 12, 1999, discloses two embodiments of a system utilizing on-board ammonia production. One embodiment disclosed in the &#39;088 patent includes a multi-cylinder engine that combusts a lean air/fuel mixture. A first cylinder of the engine is fluidly connected to an exhaust passageway that has an ammonia synthesizing catalyst, while the other cylinders are fluidly connected to an SCR catalytic device. A separate auxiliary engine combusts a rich air/fuel mixture and is fluidly connected to the exhaust passageway with the ammonia synthesizing catalyst. Rich exhaust gas from the auxiliary engine is mixed with lean exhaust gas from the first cylinder, and NOx contained in the mixture reacts with the ammonia synthesizing catalyst to generate ammonia. The ammonia is then directed to the SCR catalytic device where it reacts with the lean exhaust of the remaining cylinders to reduce NOx. 
     In a second embodiment, all of the engine cylinders combust a lean air/fuel mixture and are fluidly connected to an SCR catalytic device. The separate auxiliary engine is replaced with a burner that burns a rich air/fuel mixture and is fluidly connected to an ammonia synthesizing catalyst. NOx in the rich exhaust gas produced by the burner reacts with the ammonia synthesizing catalyst, and the resulting ammonia is directed to the SCR catalytic device. There, the ammonia is mixed with the lean exhaust produced by the engine cylinders and reacts with the SCR catalytic device to remove NOx from the engine emissions. 
     Although the system in the &#39;088 patent may reduce NOx emissions, the utilization of lean exhaust gas or a burner to generate ammonia may limit the NOx reducing capability of the system. In particular, with respect to the first embodiment, the lean exhaust gas contains a large amount of oxygen which adversely affects the production of ammonia. Thus, the combination of the rich exhaust gas with the lean exhaust gas before the exhaust gas is converted to ammonia limits the amount of ammonia that can be produced. With respect to the second embodiment, the burner combusts the rich air/fuel mixture at a temperature that is unfavorable for NOx production, thereby limiting ammonia generation. NOx reduction in both embodiments is limited because only a limited amount of ammonia is available to react with the SCR catalytic device. 
     In addition, the engine system disclosed in the &#39;088 patent consumes a larger amount of fuel to produce a particular mechanical or electrical output than a conventional power system without on-board ammonia production. This is because additional fuel is needed to power the auxiliary engine and the burner, which do not contribute to the production of the mechanical or electrical output. Therefore, energy from the additional fuel is used solely to produce ammonia and is not used to accomplish the task being performed by the main engine. By using the additional fuel, operational costs may increase and the system efficiency may decrease. 
     The disclosed system is directed to overcoming one or more of the problems set forth above. 
     SUMMARY OF THE INVENTION 
     In one aspect, the present disclosure is directed toward a power system that includes a first power source including at least one engine configured to combust a first air/fuel mixture and produce a first exhaust stream. The fuel of the first air/fuel mixture may be liquefied petroleum gas. The system also includes a first exhaust passageway fluidly connected to the first power source and configured to receive the first exhaust stream. In addition, the system includes a second power source having at least one engine configured to combust a second air/fuel mixture and produce a second exhaust stream. Furthermore, the system includes a second exhaust passageway fluidly connected to the second power source and configured to receive the second exhaust stream. The system further includes a first catalyst disposed within the first exhaust passageway to convert at least a portion of the first exhaust stream to ammonia. 
     The present disclosure is also directed to a locomotive that includes a first power source configured to power the locomotive. The first power source may include at least one engine configured to combust a first air/fuel mixture. The locomotive may also include a second power source configured to power the locomotive. The second power source may include at least one engine configured to combust a second air/fuel mixture. The locomotive may further include at least one generator drivingly coupled to at least one of the first power source or the second power source. The at least one generator may be configured to generate electrical energy to power the locomotive. The locomotive may also include a first exhaust passageway fluidly connected to the first power source and configured to receive a first exhaust stream and a second exhaust passageway fluidly connected to the second power source and configured to receive a second exhaust stream. The locomotive may include a first catalyst disposed within the first exhaust passageway to convert at least a portion of the first exhaust stream to ammonia. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic illustration of an exemplary disclosed machine; 
         FIG. 2  is a diagrammatic illustration of an exemplary disclosed power system for use with the machine of  FIG. 1 ; 
         FIG. 3  is a diagrammatic illustration of another embodiment of the exemplary disclosed machine; 
         FIG. 4  is a schematic illustration of another exemplary embodiment of the disclosed power system; and 
         FIG. 5  is a flow chart depicting an exemplary method for operating the power system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an exemplary machine  10  having multiple systems and components that cooperate to accomplish a task. The tasks performed by machine  10  may be associated with a particular industry such as mining, construction, farming, transportation, power generation, or any other industry known in the art. For example, machine  10  may embody a mobile machine such as, a bus, a haul truck, a locomotive, a marine vessel, or any other type of machine known in the art. As shown in  FIG. 1 , machine  10  may include one or more traction devices  12  operatively connected to and driven by a power train  14 . 
     Traction devices  12  may embody wheels located on each side of machine  10  (only one side shown). Alternatively, traction devices  12  may include tracks, belts or other known traction devices. It is contemplated that any combination of the wheels on machine  10  may be driven and/or steered. 
     Power train  14  may be an integral package configured to generate and transmit power to traction devices  12 . In particular, as shown in  FIG. 2 , power train  14  may include a power system  16  operable to generate a power output and a transmission unit  18  connected to receive the power output and transmit the power output to traction devices  12 . 
     Power system  16  may include a first power source  20  configured to combust a rich air/fuel mixture and a second power source  22  configured to combust a lean air/fuel mixture. First power source  20  may include at least one rich engine  24 , while second power source  22  may include at least one lean engine  26 . Rich engine  24  and lean engine  26  may be natural gas powered engines. Rich engine  24  and lean engine  26  may alternatively be any other type of internal combustion engine such as, for example, a gasoline, a diesel, or a gaseous fuel-powered engine. First power source  20  may be operationally connected to second power source  22  by, for example, a countershaft  28 , a belt (not shown), an electrical circuit (not shown), or in any other suitable manner such that first power source  20  and second power source  22  cooperatively contribute to produce a mechanical or electrical output. It is contemplated that in configurations utilizing multiple rich engines  24  and/or multiple lean engines  26 , each rich engine  24  may be operationally connected to other rich engines  24  and lean engines  26  may be operationally connected to other lean engines  26  by, for example, countershaft  28 , a belt (not shown), an electrical circuit (not shown), or in any other suitable manner such that all rich engines  24  and lean engines  26  cooperatively contribute to produce a mechanical or electrical output. It is further contemplated that although first power source  20  and second power source  22  are disclosed as being situated in series, first and second power sources  20  and  22  may be disposed in a parallel configuration. It is yet further contemplated that rich engine  24  may embody an auxiliary power unit. 
     Power system  16  may have multiple subsystems that cooperate to produce a mechanical or electrical power output. Among such subsystems included within power system  16  may be an exhaust system  30  and a control system  32 . 
     Exhaust system  30  may remove or reduce the amount of pollutants in the exhaust produced by power system  16  and release the treated exhaust into the atmosphere. Exhaust system  30  may include an exhaust passageway  34  fluidly connected to an exhaust manifold  36  of first power source  20 , an ammonia-producing catalyst  38  disposed within exhaust passageway  34 , an exhaust passageway  40  fluidly connected to an exhaust manifold  42  of second power source  22 , a merged exhaust passageway  44  fluidly connected to exhaust passageways  34  and  40 , and a selective catalytic reduction (SCR) catalyst  46  disposed within merged exhaust passageway  44  (exhaust passageways  34  and  40  may merge at SCR catalyst  46  or upstream of SCR catalyst  46 ). It is contemplated that exhaust system  30  may further include additional after-treatment devices, such as, for example, one or more oxidation catalysts  48 , an ammonia oxidation catalyst  50 , one or more particulate filters  52 , and/or any other after-treatment device known in the art that is capable of removing or reducing unwanted emissions from the exhaust, if desired. 
     Ammonia-producing catalyst  38  may generate ammonia by facilitating a reaction between NOx and other combustion byproducts in the exhaust-gas stream of first power source  20 . These other combustion byproducts may include, for example, hydrogen gas (H 2 ), propene (C 3 H 6 ), or carbon monoxide (CO). In addition, ammonia-producing catalyst  38  may include a variety of materials, such as, for example, platinum, palladium, rhodium, iridium, copper, chrome, vanadium, titanium, iron, cesium, or any other material capable of generating ammonia. Combinations of these materials may be used, and the catalyst material may be chosen based on the type of fuel used, the air/fuel ratio desired, or for conformity with environmental standards. 
     The efficiency of the ammonia-producing reaction may be improved under rich conditions. Therefore, the air/fuel mixture combusted within first power source  20  may be made rich to generate a rich exhaust favorable for increased ammonia production. Alternatively, a fuel-supply device (not shown) may be fluidly connected to exhaust passageway  34  upstream of ammonia-producing catalyst  38  and configured to supply fuel into exhaust passageway  34 . The injection of fuel into the exhaust of first power source  20  may produce favorable conditions for generating ammonia. 
     SCR catalyst  46  may facilitate a reaction between the ammonia generated by ammonia-producing catalyst  38  and NOx to at least partially remove NOx from the exhaust stream in merged exhaust passageway  44 . For example, SCR catalyst  46  may facilitate a reaction between the ammonia and NOx to produce nitrogen gas and water, among other reaction products. 
     Oxidation catalyst  48  may be situated within exhaust passageway  40  and may regulate the levels of different NOx components in the exhaust of second power source  22  to increase the performance of SCR catalyst  46 . It is contemplated that a plurality of oxidation catalysts  48  may alternatively be situated within each exhaust passageway  40  of second power source  22 , if desired. NOx may include several oxides of nitrogen including nitrogen oxide (NO) and nitrogen dioxide (NO 2 ). However, SCR catalyst  46  may function most effectively with a NO:NO 2  ratio of 1:1. Therefore, oxidation catalyst  48  may be used to oxidize NO into NO 2  to regulate the ratio of NO to NO 2  in the exhaust stream of second power source  22  and increase the performance of SCR catalyst  46 . 
     Ammonia oxidation catalyst  50  may be situated within merged exhaust passage  44  downstream of SCR catalyst  46  and may oxidize or bum any excess ammonia that may pass through SCR catalyst  46 . During the exhaust treatment process, ammonia may be generated and supplied to SCR catalyst  46  at a rate that may exceed the NOx reducing capacity of SCR catalyst  46 . The excess ammonia, known as ammonia slip, may be expelled from SCR catalyst  46  and may contribute to undesired emissions released into the atmosphere. In addition, the excess ammonia may corrode the surfaces of exhaust treatment equipment located downstream of SCR catalyst  46 , which can lead to maintenance issues. Ammonia oxidation catalyst  50  may prevent such issues by converting the excess ammonia to nitrogen gas (N 2 ). 
     Particulate filter  52  may be situated within exhaust passageway  34 , exhaust passageway  40 , and/or merged exhaust passageway  44  to remove particulate matter from the exhaust flow. It is contemplated that particulate filter  52  may include a catalyst for reducing an ignition temperature of the particulate matter trapped by particulate filter  52 , a means for regenerating the particulate matter trapped by particulate filter  52 , or both a catalyst and a means for regenerating. The means for regenerating may include, among other things, a fuel-powered burner, an electrically-resistive heater, an engine control strategy, or any other means for regenerating known in the art. 
     Control system  32  may regulate the air/fuel ratio of an air/fuel mixture combusted by first and second power sources  20  and  22  based on sensed NOx and ammonia levels in exhaust system  30 . By regulating the air/fuel ratio, first and second power sources  20  and  22  may generate an optimal amount of NOx and ammonia for exhaust treatment. Control system  32  may include a NOx sensor  54  situated within exhaust passageway  34  upstream of ammonia-producing catalyst  38  and/or an ammonia sensor  56  situated within exhaust passageway  34  downstream of ammonia-producing catalyst  38 . Control system  32  may also include a NOx sensor  58  situated within exhaust passageway  40  and a controller  60 . It should be understood that although  FIG. 2  discloses that control system  32  includes three sensors, any number of sensors and any combination of sensors may be used. Furthermore, the sensors may be located anywhere within power system  16  that may adequately sense the amount of NOx and ammonia in exhaust system  30 . For example, it is contemplated that control system  32  may include additional NOx sensors situated within merged exhaust passageway  44  either upstream or downstream of SCR catalyst  46 . It is also contemplated that control system  32  may include virtual sensors. 
     NOx sensor  54  may sense the amount of NOx generated by first power source  20  and may be mounted on exhaust passageway  34  upstream of ammonia-producing catalyst  38 . In addition, NOx sensor  54  may be configured to detect the level of NOx in the exhaust flow passing through exhaust passageway  34 . At least a portion of NOx sensor  54  may extend through the wall of exhaust passageway  34  into the exhaust flow. In order to withstand the high temperatures in exhaust passageway  34 , NOx sensor  54  may be constructed, for example, out of ceramic type metal oxides or any other suitable material. NOx sensor  54  may sample the exhaust for NOx, and convert that sensed value into a signal indicative of the NOx level therein. 
     Ammonia sensor  56  may sense the amount of ammonia generated by ammonia-producing catalyst  38  and may be mounted on exhaust passageway  34  downstream of ammonia-producing catalyst  38 . In addition, ammonia sensor  56  may be configured to detect the level of ammonia in the exhaust flow passing through exhaust passageway  34 . At least a portion of ammonia sensor  56  may extend through the wall of exhaust passageway  34  into the exhaust flow. In order to withstand the high temperatures in exhaust passageway  34 , ammonia sensor  56  may be constructed, for example, out of ceramic type metal oxides or any other suitable material. Ammonia sensor  56  may sample the exhaust for ammonia, and convert that sensed value into a signal indicative of the ammonia level therein. 
     NOx sensor  58  may sense the amount of NOx generated by second power source  22  and may be mounted on exhaust passageway  40 . In addition, NOx sensor  58  may be configured to detect the level of NOx in the exhaust flow passing through exhaust passageway  40 . At least a portion of NOx sensor  58  may extend through the wall of exhaust passageway  40  into the exhaust flow. In order to withstand the high temperatures in exhaust passageway  40 , NOx sensor  58  may be constructed, for example, out of ceramic type metal oxides or any other suitable material. NOx sensor  58  may sample the exhaust for NOx, and convert that sensed value into a signal indicative of the NOx level therein. 
     Controller  60  may include one or more microprocessors, a memory, a data storage device, a communication hub, and/or other components known in the art and may be associated only with first and second power sources  20  and  22 . However, it is contemplated that controller  60  may be integrated within a general control system capable of controlling additional functions of power system  16 , e.g. and/or additional subsystems operatively associated with power system  16 , e.g., selective control of transmission unit  18 . 
     Controller  60  may receive signals from NOx sensors  54 ,  58  and ammonia sensor  56  and analyze the data to determine the amount of NOx and ammonia in the exhaust gas. Upon receiving input signals from NOx sensors  54 ,  58  and ammonia sensor  56 , controller  60  may perform a plurality of operations, e.g., algorithms, equations, subroutines, reference look-up maps or tables to determine whether the NOx and ammonia levels are optimal and establish an output to influence the air/fuel ratio of the air/fuel mixture combusted by engines  24  and  26 . Alternatively, it is contemplated that controller  60  may receive signals from various sensors (not shown) located throughout power system  16  instead of NOx sensors  54 ,  58  and ammonia sensor  56 . Such sensors may sense parameters that may be used to calculate the amount of NOx and ammonia in exhaust system  30 . 
     Transmission unit  18  may include numerous components that interact to transmit power from power system  16  to traction device  12 . In particular, transmission unit  18  may be a multi-speed bidirectional mechanical transmission having a neutral gear ratio, a plurality of forward gear ratios, a reverse gear ratio, and one or more clutches (not shown). The clutches may be selectively actuated to engage predetermined combinations of gears (not shown) to produce a desired output gear ratio. It is contemplated that transmission unit  18  may be an automatic-type transmission, with shifting based on a power source speed, a maximum selected gear ratio, and a shift map, or a manual-type transmission, with shifting between each gear directly initiated by an operator. The output of transmission unit  18  may be connected to and configured to rotatably drive traction device  12  via output shaft  62 , thereby propelling machine  10 . 
     It is contemplated that transmission unit  18  may alternately embody a hydraulic transmission having one or more pumps and hydraulic motors, a hydro-mechanical transmission having both hydraulic and mechanical components, an electric transmission having a generator and one or more electric motors, an electro-mechanical transmission having both electrical and mechanical components, or any other suitable transmission. It is also contemplated that transmission unit  18  may alternately embody a continuously variable transmission such as, for example, an electric transmission having a generator and an electric motor, a hydraulic transmission having a pump and a fluid motor, or any other continuously variable transmission known in the art. 
       FIG. 3  illustrates another exemplary embodiment of machine  10 . In the embodiment of  FIG. 3 , machine  10  may embody a land based machine (e.g., a locomotive, a truck, a bus, etc.), a marine vessel, or a stationary power generation application. In this embodiment, rich engine  24  may be powered by liquefied petroleum gas (“LPG”), and lean engine  26  may be powered by diesel fuel. Alternatively, rich engine  24  and lean engine  26  may be powered by gasoline, diesel, ethanol, JP-5, JP-8, gaseous fuel (e.g., propane, butane, dimethyl ether, natural gas, or any combination thereof), or any other type of fuel known in the art. It is contemplated that rich engine  24  may receive fuel (e.g., LPG) from another vehicle  66 . For example, if machine  10  embodies a locomotive, tank  64  may be located on a tender car  66 . Tank  64  may alternatively be located on machine  10 . 
     The power output of rich engine  24  and lean engine  26  may be connected to one or more generators  68  to produce electrical energy. Each generator  68  may be a device configured to produce a power output in response to a rotational input provided by an engine (e.g., rich engine  24  or lean engine  26 ). It is contemplated that generator  68  may embody, for example, a permanent magnet-type generator, an asynchronous generator, or any other type of generator configured to produce either alternating current or direct current electrical energy. Generator  68  may include a rotor (not shown) rotatably connected to an engine (e.g., rich engine  24  or lean engine  26 ) by any means known in the art, such as, for example, a direct crankshaft connection, a driveshaft, a gear train, a hydraulic circuit, or in any other appropriate manner. 
     The electrical energy produced by generators  68  may be used, for example, to power a motor  70  (or multiple motors  70 ) for propulsion of machine  10  and any other vehicles associated with machine  10  (e.g. tender car  66 ). Each motor  70  may be an electric motor configured to receive power from generator  68  and create rotation of traction devices  12 . It is contemplated that motors  70  may be direct current motors, alternating current motors, or any other appropriate type of motors known in the art. In one embodiment, an output of motors  70  may be connected to traction devices  12  via a gear mechanism (not shown). Other electrical components (not shown) may be associated with generator  68  and motors  70 , such as rectifiers, inverters, and other electrical components known in the art. 
     It should be understood that the configuration of exhaust system  30  illustrated in  FIG. 3  may be similar to the embodiment disclosed in  FIG. 2 . 
       FIG. 4  illustrates another exemplary embodiment of power system  16  used in applications such as, for example, powering marine vessels, powering land-based vehicles, and various industrial applications. In the exemplary embodiment of  FIG. 4 , rich engines  24  of first power source  20  may operate independently of each other to produce separate mechanical or electrical outputs. In addition, lean engines  26  of second power source  22  may operate independently of each other to produce separate mechanical or electrical outputs. Furthermore, first power source  20  and second power source  22  may operate independently of each other to produce separate mechanical or electrical outputs. It should be understood that the configuration of exhaust system  30  illustrated in  FIG. 4  may be similar to the embodiment disclosed in  FIG. 2 . 
       FIG. 5 , which is discussed in the following section, illustrates the operation of first and second power sources  20  and  22  utilizing embodiments of the disclosed system. Specifically,  FIG. 5  illustrates an exemplary method for regulating the air/fuel ratio of the air/fuel mixture combusted by engines  24  and  26  for optimal exhaust emission levels. 
     INDUSTRIAL APPLICABILITY 
     The disclosed multi-engine system may reliably and efficiently remove or reduce NOx emissions from exhaust that is released into the atmosphere. In particular, the disclosed multi-engine system may eliminate the need for peripheral equipment such as burners or storage tanks to supply ammonia necessary for NOx reduction to the exhaust treatment system. By designating one engine or set of engines to facilitate the generation of ammonia, the multi-engine system itself may supply the ammonia required to remove or reduce NOx emissions from the exhaust released into the atmosphere. The operation of first and second power sources  20  and  22  will now be explained. 
       FIG. 5  illustrates a flow diagram depicting an exemplary method for generating an optimal level of NOx and ammonia in the exhaust to meet emission standards. The method may begin when the air/fuel mixture to be combusted by second power source  22  is set to a desired air/fuel ratio (step  100 ). The desired ratio may be any ratio capable of producing a desired result related to the operation of first and second power sources  20  and  22 . Such desired results may include, for example, fuel efficiency or maximum mechanical or electrical power generation. In addition, it should be understood that the air/fuel ratio of the air/fuel mixture entering second power source  22  may be leaner than stoichiometric. Furthermore, the air/fuel ratio may be regulated by any method known in the art such as, for example, adjusting the setting of a throttling valve (not shown). 
     Once the air/fuel mixture is set to the desired ratio, controller  60  may receive signals indicative of the amount of NOx and ammonia in exhaust system  30  from NOx sensors  54  and  58  and ammonia sensor  56  (step  102 ). Controller  60  may compare the sensed amount of NOx to tables, graphs, and/or equations stored in its memory to determine whether the sensed amount of NOx is below a predetermined threshold (step  104 ). Such a threshold may be related to government regulated emissions limits or any other threshold related to the amount of emissions released into the atmosphere. If controller  60  determines that the sensed amount of NOx is below the predetermined threshold (step  104 : YES), step  102  may be repeated (i.e. controller  60  may receive new signals from NOx sensors  54 ,  58  and ammonia sensor  56  indicative of new NOx and ammonia levels). However, if controller  60  determines that the amount of NOx is above the predetermined threshold (step  104 : No), controller  60  may determine whether the amount of ammonia in exhaust system  30  is above a predetermined threshold for exhaust treatment (step  106 ). 
     The predetermined threshold may be dependant upon the amount of NOx in exhaust system  30 . For example, the desired amount ammonia may increase when the amount of NOx increases and decrease when the amount of NOx decreases. In addition, controller  60  may determine the desired amount of ammonia for a particular amount of NOx by referencing look-up maps and/or tables and/or performing algorithms, equations, or subroutines. If controller  60  determines that the amount of ammonia in exhaust system  30  is below the predetermined threshold (step  106 : No), controller  60  may adjust the amount of NOx being produced by first power source  20  to reduce the generation of ammonia (step  108 ). For example, controller  60  may decrease the amount of NOx by decreasing the power output of first power source  20 . The power output may be decreased by reducing the amount of air and fuel entering first power source  20 . It should be understood that, regardless of the power output, the air/fuel mixture being combusted by first power source  20  may be maintained at a constant air/fuel ratio that is richer than stoichiometric. It is contemplated that other techniques may be employed to reduce the amount of NOx produced by first power source  20 . Such techniques may include, for example, adjusting the timing of combustion. Once the amount of NOx being produced has been adjusted, step  102  may be repeated (i.e. controller  60  may receive new signals from NOx sensors  54 ,  58  and ammonia sensor  56  indicative of new NOx and ammonia levels). 
     If controller  60  determines the amount of ammonia in exhaust system  30  is below predetermined threshold (step  106 : Yes), then controller  60  may determine whether ammonia-producing catalyst  38  is operating at its maximum capacity (step  110 ). Controller  60  may make this determination by referencing look-up maps and/or tables and/or performing algorithms, equations, or subroutines. If controller  60  determines that ammonia-producing catalyst  38  is operating below its maximum capacity (step  110 : No), controller  60  may adjust the amount of NOx being produced by first power source  20  to increase the generation of ammonia (step  112 ). For example, controller  60  may increase the amount of NOx by boosting the power output of first power source  20 . The power output may be boosted by increasing the amount of air and fuel entering first power source  20 . It should be understood that, regardless of the power output, the air/fuel mixture being combusted by first power source  20  may be maintained at a constant air/fuel ratio that is richer than stoichiometric. It is contemplated that other techniques may be employed to increase the amount of NOx produced by first power source  20 . Such techniques may include, for example, adjusting the timing of combustion. Once the amount of NOx being produced has been adjusted, step  102  may be repeated (i.e. controller  60  may receive new signals from NOx sensors  54 ,  58  and ammonia sensor  56  indicative of new NOx and ammonia levels). 
     If controller  60  determines that ammonia-producing catalyst  38  is operating at its maximum capacity (step  110 : Yes), controller  60  may reduce the amount of NOx being produced by second power source  22  (step  114 ). For example, controller  60  may decrease the amount of NOx by decreasing the power output of second power source  22 . The power output may be decreased by reducing the amount of air and fuel entering second power source  22 . It should be understood that, regardless of the power output, the air/fuel mixture being combusted by second power source  22  may be maintained at a constant air/fuel ratio that is leaner than stoichiometric. It is contemplated that other techniques may be employed to reduce the amount of NOx produced by second power source  22 . Such techniques may include, for example, adjusting the timing of combustion. Once the amount of NOx being produced has been adjusted, step  102  may be repeated (i.e. controller  60  may receive new signals from NOx sensors  54 ,  58  and ammonia sensor  56  indicative of new NOx and ammonia levels). 
     The disclosed system may generate as much ammonia as required to reduce or remove NOx emissions from exhaust released into the atmosphere. Because any oxygen present in the ammonia-producing catalyst may hinder production of ammonia and limit the amount produced, it may be desired to minimize amount of oxygen in the ammonia-producing catalyst. By isolating the rich (low-oxygen) exhaust of the engine or set of engines designated for facilitating the generation of ammonia from the lean (high-oxygen) exhaust generated by the other engine or set of engines the amount of oxygen present in the ammonia-producing catalyst may be minimized. In addition, the engine or set of engines designated for facilitating ammonia production may be configured to combust a rich air/fuel mixture at temperatures that are conducive for NOx production, which is necessary for ammonia generation. 
     In addition, the disclosed system may consume a substantially similar amount of fuel to produce a particular mechanical or electrical output as a conventional multi-engine system without on-board ammonia production. This is because exhaust used to generate the ammonia may be produced by engines that contribute to the production of the mechanical and electrical output of the system. Therefore, an additional separate supply of fuel is not necessary for ammonia production, thereby reducing costs and increasing efficiency of the system. 
     It will be apparent to those skilled in the art that various modifications and variations can be made in the disclosed system without departing from the scope of the disclosure. Other embodiments will be apparent to those skilled in the art from consideration of the specification disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope being indicated by the following claims and their equivalents.