Abstract:
A method includes controlling an engine speed based on: intake manifold air temperature and/or intake manifold pressure one, or more, of the following data parameters: an engine load as a function of a fuel level, a fuel injecting timing, an intake oxygen concentration, a constituent concentration from the exhaust gas flow, an engine power, and an engine torque. The method also recirculates a portion of the exhaust gas flow to the combustion cylinders of the engine via a recirculation channel, as a function of intake manifold temperature and/or intake manifold pressure at which the engine is operated. An engine system, other methods, and a non-transitory computer readable medium encoded with a program, to enable a processor-based control unit to control aspects of the engine are also disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This Continuation-In-Part (C.I.P.) application claims priority to the Oct. 19, 2012 filing date of U.S. application Ser. No. 13/655,764 (Entitled: SYSTEM AND METHOD FOR CONTROLLING EXHAUST EMISSIONS AND SPECIFIC FUEL CONSUMPTION OF AN ENGINE, attorney docket no. 260588-1). This C.I.P. application also claims priority to the Dec. 23, 2013 filing date of U.S. application Ser. No. 14/138,200 (Entitled: SYSTEM AND METHOD FOR CONTROLLING A DUAL FUEL ENGINE, attorney docket no. 271046-1). The contents of both are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    The embodiments of the present invention relate generally to a system and method of operating an engine and, more specifically, to a system and method for controlling exhaust emissions and specific fuel consumption of an engine. 
         [0003]    Compression-ignition engines, such as diesel engines, operate by directly injecting a fuel (e.g., diesel fuel) into compressed air in one or more piston-cylinder assemblies, such that the heat of the compressed air ignites the fuel-air mixture. Compression-ignition engines may also include a glow plug to provide heat to ensure ignition. The direct fuel injection atomizes the fuel into droplets, which evaporate and mix with the compressed air in the combustion chambers of the piston-cylinder assemblies. Typically, compression-ignition engines operate at a relatively higher compression ratio than spark ignition engines. The compression ratio directly affects the engine performance, efficiency, exhaust pollutants, and other engine characteristics. In addition, the fuel-air ratio affects engine performance, efficiency, exhaust pollutants, and other engine characteristics. Exhaust emissions generally include pollutants such as carbon oxides (e.g., carbon monoxide), nitrogen oxides (NO x ), sulfur oxides (SO x ), particulate matter (PM), and smoke. The amount and relative proportion of these pollutants varies according to the fuel-air mixture, compression ratio, injection timing, environmental conditions (e.g., atmospheric pressure, temperature, etc.), and the like. 
         [0004]    In certain applications, the compression-ignition engines are used in relatively extreme environmental conditions, such as high altitudes, particularly in mountainous regions. These environmental conditions can adversely affect engine performance, efficiency, exhaust pollutants, and other engine characteristics. For example, diesel engines operating in mountainous regions are subject to greater loads, lower atmospheric pressures due to higher altitudes, lower temperatures due to colder climate or higher altitude, lower air density due to lower atmospheric pressure, and the like. 
         [0005]    The various engine parameters are particularly susceptible to exceed engine design limits when the engine is operating at a full load at extreme ambient temperature and altitude conditions. It is difficult to adequately account for the impact of ambient conditions to control exhaust emissions and specific fuel consumption of the engine to specific limits. 
         [0006]    An enhanced technique for controlling exhaust emissions and specific fuel consumption of an engine is desired. 
       BRIEF DESCRIPTION 
       [0007]    In accordance with one exemplary embodiment of the present invention, a method is disclosed. The method comprises controlling an engine speed based on: at least one of: intake manifold air temperature and intake manifold pressure; and at least one data parameter of: an engine load as a function of a fuel level, a fuel injecting timing, an intake oxygen concentration, a constituent concentration in at least a portion of an exhaust gas flow, an engine power, and an engine torque; and recirculating at least a portion of the exhaust gas flow to a plurality of combustion cylinder of the engine via a recirculation channel, as a function of at least one of the intake manifold temperature and intake manifold pressure at which the engine is operated. 
         [0008]    In accordance with another exemplary embodiment of the present invention, an engine system is disclosed. The engine system comprises: an engine comprising a plurality of combustion cylinders; a turbine coupled to the engine, and configured to expand a first portion of an exhaust gas generated from the plurality of combustion cylinders; a recirculation channel for recirculating a third portion of the exhaust gas to the plurality of combustion cylinders, as a function of at least one of intake manifold temperature and intake manifold pressure at which the engine operates; and a controller configured to control engine speed based on: at least one of: intake manifold air temperature and intake manifold pressure; and at least one data parameter of: an engine load as a function of a fuel level, a fuel injecting timing, an intake oxygen concentration, a constituent concentration in at least a portion of an exhaust gas flow, an engine power, and an engine torque. 
         [0009]    In accordance with another exemplary embodiment of the present invention, a non-transitory computer readable medium encoded with a program for a processor-based control unit is disclosed. The non-transitory computer readable medium encoded with a program, to enable a processor-based control unit to: control at least one of: at least one of: intake manifold air temperature and intake manifold pressure; and at least one data parameter of: an engine load as a function of a fuel level, a fuel injecting timing, an intake oxygen concentration, a constituent concentration in at least a portion of an exhaust gas flow, an engine power, and an engine torque. 
         [0010]    In accordance with another exemplary embodiment of the present invention, a method comprises: maintaining for an operating engine one of: specific fuel consumption (SFC) within a predefined SFC limit for the operating engine; and an exhaust emission within a predefined emission limit for the operating engine by controlling one of: adjusting a power to a compressor; and recirculating a third portion of the exhaust gas to a plurality of combustion cylinders via a recirculation channel, as a function of an intake manifold air temperature and pressure at which the engine is operated. 
     
    
     
       DRAWINGS 
         [0011]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0012]      FIG. 1  is a diagrammatical representation of a system in accordance with an exemplary embodiment of the present invention; 
           [0013]      FIG. 2  is a diagrammatical representation of a system in accordance with another exemplary embodiment of the present invention; 
           [0014]      FIG. 3  is a diagrammatical representation of a control unit of the system in accordance with the embodiment of  FIG. 2 ; and 
           [0015]      FIG. 4  is a diagrammatical representation of a system in accordance with another exemplary embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The term “Tier 4” or “Tier 4 standards” means the Tier 4 Line Haul Locomotive Emissions Standards as promulgated by the United States Environmental Protection Agency (EPA). The Tier 4 standards are codified at 40 CFR Part 1033, while the de facto standards and Tables are found specifically at 40 CFR 1033.101. Tier 4 standards can also be found at http://www.ecfr.gov/cgi-bin/text-idx?SID=c96b8ff349cc19252400485a86e87e99&amp;mc=true&amp;node=se40.33.1033 — 1101&amp;rgn=div8. The Tier 4 standards are incorporated herein by reference in their entirety. 
         [0017]    Referring to  FIG. 1 , a turbocharged unit  10  having exhaust emission and specific fuel consumption (SFC) control features, is illustrated in accordance with certain embodiments of the present invention. The turbocharged unit  10  includes a turbo-charger  12  and a compression-ignition engine, e.g., a diesel engine  14 . A motor-generator unit (not shown) may be mechanically coupled to the diesel engine  14 . As discussed in further detail below, embodiments of the present invention provide monitoring and control features, such as sensors and control logic, for maintaining a SFC of the engine  14  and a quantity of exhaust emissions in the exhaust gas, within a predefined SFC limit and an emission limit respectively, by controlling feed of a portion of the exhaust gas bypassing a turbine and/or recirculating a portion of the exhaust gas to the engine  14 , as a function of ambient conditions such an intake manifold air temperature and pressure at which the engine  14  is operated. In certain embodiments, fuel injection timing and/or engine speed may also be controlled as a function of ambient conditions. 
         [0018]    The illustrated engine  14  includes an air intake manifold  16  and an exhaust manifold  18 . The turbo-charger  12  includes a first-stage compressor  20 , a second-stage compressor  22 , a first-stage turbine  24 , and a second-stage turbine  26 . Ambient air  27  may be drawn through a filter (not shown) and then compressed to a higher pressure via the first-stage compressor  20 . The temperature of air is increased due to compression. The compressed intake air  28  is cooled via an intercooler  30 , and then further compressed to a further higher pressure via the second-stage compressor  22 . The compressed air is then cooled via another intercooler  32  and then supplied to the intake manifold  16  for combustion within the engine  14 . The compressed air flows through the intercooler  32  such that the temperature of air is reduced prior to delivery into the intake manifold  16  of the engine  14 . In one embodiment, the intercoolers  30 ,  32  may be air-to-water heat exchangers, which utilize a coolant to facilitate removal of heat from the compressed air. In another embodiment, the intercoolers  30 ,  32  may be air-to-air heat exchangers, which utilize ambient air to facilitate removal of heat from compressed air. In yet another embodiment, the intercoolers  30 ,  32  may be a hybrid cooler arrangement that utilizes both air-to-water and air-to-air heat exchangers. 
         [0019]    The first-stage turbine  24  is coupled to the exhaust manifold  18  for extracting energy from exhaust gases for rotating a turbocharger shaft  34  coupled to the second-stage compressor  22 . The second-stage turbine  26  is coupled to the first-stage turbine  24  for extracting energy from expanded gases fed from the first stage turbine  24 , for rotating a turbocharger shaft  36  coupled to the first-stage compressor  20 . The expanded gases from the second-stage turbine  26  may be ejected to the atmosphere. 
         [0020]    In the illustrated embodiment, an exhaust channel  38  is disposed bypassing the first-stage turbine  24 . A bypass control valve  40  is provided to the exhaust channel  38  to control flow through the exhaust channel  38 . In some embodiments, all of the exhaust gas from the exhaust manifold  18  is expanded through the first-stage turbine  24 . In certain other embodiments, a first portion of the exhaust gas from the exhaust manifold  18  is expanded through the first-stage turbine  24  and a remaining portion (also referred to as “a second portion”) of the exhaust gas from the exhaust manifold  18  is fed through the exhaust channel  38  bypassing the first-stage turbine  24 . The second portion of the exhaust gas fed through the channel  38  is expanded via the second-stage turbine  26 . 
         [0021]    The engine  14  includes a plurality of combustion cylinders  42 ,  44 . A first set of cylinders  42  may be referred to as “donor cylinders” (labelled “D” therein) and the other set of cylinders  44  may be referred to as “non-donor cylinders”. In the illustrated embodiment, the engine  14  includes six donor cylinders  42  and six non-donor cylinders  44 . A piston (not shown) is slidably disposed in each cylinder  42 ,  44  and reciprocates between a top dead center and a bottom dead center position. It should be noted herein that the number of cylinders may vary depending upon the application. The combustion cylinders  42 ,  44  are coupled to the intake manifold  16  and receive compressed air via the intake manifold  16 . 
         [0022]    In the illustrated embodiment, the non-donor cylinders  44  are coupled to the exhaust manifold  18 . The exhaust gas from the non-donor cylinders  44  are fed via the exhaust manifold  18  to the first-stage turbine  24  and/or the exhaust channel  38 . The donor cylinders  42  are coupled to an exhaust gas recirculation manifold  46 . 
         [0023]    A first recirculation control valve  50  is provided to control flow through the recirculation channel  48 . The exhaust gas recirculation manifold  46  is coupled to the intake manifold  16  via a recirculation channel  48 . The exhaust gas from the donor cylinders  42  (also referred to as “a third portion of exhaust gas”) is fed via the exhaust gas recirculation manifold  46  to the intake manifold  16 . In the illustrated embodiment, the recirculation channel  48  is coupled to the exhaust manifold  18  via an exit channel  52 . A second recirculation control valve  54  is provided to control flow from the recirculation channel  48  to the exhaust manifold  18 . In some embodiments, the entire exhaust gas from the donor cylinders  42  is fed via the exhaust gas recirculation manifold  46 , the exhaust gas recirculation channel  48  to the intake manifold  16 . In certain embodiments, a portion of the exhaust gas from the donor cylinders  42  is fed via the recirculation channel  48 , the exit channel  52  to the exhaust manifold  18 . A recirculation cooler  56  is provided to the recirculation channel  48 . The recirculation cooler  56  cools the exhaust gas fed through the recirculation channel  48 , prior to feeding to the plurality of combustion cylinders  42 ,  44  via the intake manifold  16 . 
         [0024]    The turbocharged unit  10  also includes a control unit  58 . In the illustrated embodiment, the control unit  58  is an electronic control unit for the turbocharger  12  and the engine  14 . In another embodiment, the control unit  58  is an electronic logic control unit that is programmable by a user. The control unit  58  receives a pressure signal  60  from a pressure sensor  62  provided to detect pressure of intake air fed to the engine  14 . Additionally, the control unit  58  receives a temperature signal  64  from a temperature sensor  66  provided to detect temperature of intake air fed to the engine  14 . The control unit  58  may also receive an oxygen signal  68  from an oxygen sensor  70  provided to detect quantity of oxygen in the intake air fed to the intake manifold  16 . In some embodiments, the control unit  58  may also receive an oxygen signal  71  from another oxygen sensor  73  provided to detect quantity of oxygen from the exhaust gas fed from the exhaust manifold  18 . Further, the control unit  58  may also receive a mass flow signal  72  from a fuel sensor  74  provided to detect mass flow of a fuel fed to the engine  14 . The control unit  58  may also receive a speed signal  11  from a speed sensor  13 , a notch signal  15  from a notch sensor  17 , a load signal  19  from a load sensor  21 , a fuel injection timing signal  23  from an injection timing sensor  25 , a soot signal  95  from a soot sensor  94 , and an exhaust gas recirculation mass flow signal  27  from a mass flow sensor  29 . In the illustrated embodiment, a fuel injector pump  76  drives a plurality of fuel injectors  78  for injecting a fuel  80  into a plurality of cylinders  42 ,  44  of the engine  14 . The soot sensor  94  may be located along the exhaust gas path. 
         [0025]    In accordance with embodiments of the present invention, the control unit  58  receives the signals  11 ,  15 ,  19 ,  23 ,  27 ,  60 ,  64 ,  68 ,  71 ,  72 ,  95  and controls the bypass control valve  40 , and the first and second recirculation control valves  50 ,  54  based on the signals  11 ,  15 ,  19 ,  23 ,  27 ,  60 ,  64 ,  68 ,  71 ,  72 ,  95  so as to control quantity of exhaust gas bypassing the first-stage turbine  24  and recirculated through the recirculation channel  48 . In certain other embodiments, the control unit  58  may additionally control the engine speed and/or fuel mass flow by producing a timing signal  82  to control operation of the fuel injectors  78 . 
         [0026]    As discussed herein, in certain applications, the compression-ignition engines are used in relatively extreme environmental conditions, such as high altitudes. These environmental conditions can adversely affect engine performance, efficiency, exhaust pollutants, and other engine characteristics. Conventional engines do not adequately account for impact of ambient conditions to control exhaust emissions and specific fuel consumption of the engine to specific limits. 
         [0027]    In accordance with the embodiments of the present invention, air-fuel ratio and quantity of exhaust gas recirculation are varied in response to changes in ambient conditions such as intake manifold air temperature and pressure. In other words, set points of the air-fuel ratio and quantity of exhaust gas recirculation are varied in response to changes in intake manifold air temperature and pressure. The use of variable points of the air-fuel ratio and quantity of exhaust gas recirculation in response to ambient conditions, in conjunction with a corresponding fuel injection strategy and control of engine speed, facilitates maintaining SFC and exhaust emission such as NO x  and particulate matter (PM) within specified limits. 
         [0028]    Typically, when an engine is at a higher altitude region or a high temperature region, the airflow delivered to such an engine decreases. Under such a condition, it is required to either maintain the airflow at a required rate or operate at lower airflow rate while still maintaining SFC and emissions within specified limits. In certain such exemplary embodiments of the present invention, the control unit  58  facilitates to increase a quantity of intake air flow to the plurality of combustion cylinders  42 ,  44  by decreasing feed of the exhaust gas from the exhaust manifold  18  via the exhaust channel  38  bypassing the turbine  24 , as a function of the intake manifold air temperature and pressure at which the engine  14  is operated, and vice versa so as maintain a desired air-fuel ratio. In other words, the opening of the bypass control valve  40  is reduced to decrease the flow of exhaust gas through the channel  38  so as to increase airflow to the engine cylinders  42 ,  44 . When the opening of the valve  40  is increased, airflow to the cylinders  42 ,  44  is reduced. 
         [0029]    In some embodiments, to maintain airflow at a predefined rate with increasing altitude, the valve  40  is opened at sea-level operating conditions and closed at a high altitude operating conditions. In certain other embodiments, at high altitude operating conditions, the airflow to the cylinders  42 ,  44  may be reduced by increasing quantity of exhaust gas recirculation flow through the recirculation channel  48  by controlling opening of the first and second recirculation control valves  50 ,  54 . As mentioned above, the quantity of exhaust gas recirculation flow via the channel  48  to the intake manifold  16  may be reduced by diverting a portion of exhaust gas flow from the channel  48  to the exhaust manifold  18  via the exit channel  52 . When the EGR rate is increased, airflow to the cylinders  42 ,  44  is decreased, and vice versa. Further, the control unit  58  may increase engine speed as a function of ambient conditions, to increase airflow to the cylinders  42 ,  44 . Further, the control unit  58  may change the fuel injection timing as a function of the engine ambient conditions. The control of turbine bypass flow and EGR flow is optimized in conjunction with optimized fuel injection strategy and engine speed, as a function of intake manifold air temperature and pressure, to maintaining a specific fuel consumption (SFC) of the engine and a quantity of exhaust emissions in the exhaust gas, within a predefined SFC limit and an emission limit respectively. It should be noted herein that quantity of oxygen in the intake air flow is dependent on the air-fuel ratio and EGR flow rate. 
         [0030]    Referring to  FIG. 2 , the turbocharged unit  10  is illustrated in accordance with a particular embodiment of the present invention. Features and aspects of the embodiment illustrated in  FIG. 2  are similar to those depicted in the embodiment shown in  FIG. 1 . The turbocharged unit  10  includes an exhaust compressor  84  coupled to the exhaust manifold  18  and the exhaust gas recirculation manifold  46 . The exhaust compressor  84  receives a portion of the exhaust gas from the exhaust manifold  18  and compresses the portion of the exhaust gas, prior to feeding the portion of the exhaust gas via the EGR manifold  46 , the recirculation channel  48  to the plurality of the combustion cylinders  44 . In other words, the exhaust compressor  84  operates, or functions, as an “EGR pump” in lieu of having dedicated donor cylinders. The EGR pump (e.g., exhaust compressor  84 ) may be controlled, for example, by control valves (not shown) and/or via controlling power (not shown) supplied to the EGR pump. 
         [0031]    In the illustrated embodiment, all of the exhaust gas from all of the cylinders  44  is fed to the exhaust gas manifold  18 , and subsequently a portion of the exhaust gas is fed from the exhaust gas manifold  18  to the intake manifold  16  via the exhaust gas recirculation manifold  46 , and the exhaust gas recirculation channel  48 . Compared to the embodiment of  FIG. 1 , there is no exit channel  52  between the exhaust gas recirculation channel  48  and the exhaust manifold  18 . 
         [0032]    The control unit  58  may further include a database  86 , an algorithm  88 , and a data analysis block  90 . The database  86  may be configured to store predefined information associated with the turbocharger  12  and the engine  14 . For example, the database  86  may store information relating to temperature, and pressure of the intake air, quantity of oxygen in the intake air, fuel injection timing, engine speed, fuel mass flow, or the like. Furthermore, the database  86  may be configured to store actual sensed/detected information from the above-mentioned sensors  13 ,  17 ,  21 ,  25 ,  29 ,  62 ,  66 ,  70 ,  73 ,  74 , and  94 . The algorithm  88  facilitates the processing of signals from the above-mentioned plurality of sensors  13 ,  17 ,  21 ,  25 ,  29 ,  62 ,  66 ,  70 ,  73 ,  74 , and  95 . 
         [0033]    The data analysis block  90  may include a range of circuitry types, such as a microprocessor, a programmable logic controller, a logic module, and the like. The data analysis block  90  in combination with the algorithm(s)  88  may be used to perform the various computational operations relating to maintaining specific fuel consumption (SFC) of the engine and a quantity of exhaust emissions in the exhaust gas, within a predefined SFC limit and an emission limit respectively. The control unit  58  is operable to control the feed of a portion of the exhaust gas via the exhaust channel  38  bypassing the turbine  24 ; recirculation of a portion of the exhaust gas to the plurality of combustion cylinders  44  via the recirculation channel  48 , as a function of an intake manifold air temperature and pressure at which the engine  14  is operated. 
         [0034]    Referring to  FIG. 3 , the control unit  58  is illustrated in accordance with a particular embodiment of the present invention. In the illustrated embodiment, the database  86  includes a plurality of maps  87 , wherein each map  87  is representative of a data comprising an engine speed, and an engine load as a function of an engine notch, a fuel injecting timing, and an oxygen concentration in an intake air flow as a function of the engine speed, and the engine load. In certain embodiments, the control unit  58  controlling at least one of the engine speed, the engine load, the oxygen concentration in the intake air flow, the fuel injection timing based on a selected map  87 . The map  87  may be selected based on ambient conditions (intake manifold air temperature and pressure) at which the engine is operated. In some embodiments, the control unit  58  controls the valves  50 ,  54  (shown in  FIG. 1 ) to control the recirculation of the exhaust gas to the plurality of combustion cylinders via the recirculation channel, thereby controlling the oxygen concentration in the intake air flow. In certain other embodiments, the control unit  58  controls at least one of an oxygen concentration in the intake air flow, a fuel injection timing based on the intake manifold air temperature detected by the temperature sensor. 
         [0035]    Alternatively to, or concurrently with, using look up tables and maps, the control unit  58  may use model-based or transfer functions calculations to control how the engine is operated. 
         [0036]    Referring to  FIG. 4 , a turbocharged unit  10  is illustrated in accordance with a particular embodiment of the present invention. Aspects of the illustrated embodiment are similar to the embodiment of  FIG. 1 , except that the turbocharger  12  has only a single stage compressor  22  and a single stage turbine  24 . As discussed herein, intake air  28  may be drawn through a filter (not shown) and then compressed to a higher pressure via the single stage compressor  22 . The compressed intake air  28  is cooled via the intercooler  32 , and then supplied to the intake manifold  16  for combustion within the engine  14 . The single stage turbine  24  is coupled to the exhaust manifold  18  for extracting energy from exhaust gases for rotating a turbocharger shaft  34  coupled to the single stage compressor  22 . The expanded gases from the single stage turbine  24  may be ejected to the atmosphere. It should be noted herein that although specific configurations of the turbocharged unit  10  having turbine bypass and EGR features have been shown in  FIGS. 1-4 , it should not be construed as limiting the scope of the invention. Specific features of the turbocharged unit  10  may vary depending upon the application. 
         [0037]    In accordance with the embodiments of the present invention, the SFC and exhaust emissions of the engine  14  are maintained within specific limits over a range of ambient conditions. Different set points of different air-to-fuel-ratio and exhaust gas recirculation levels determined as a function of ambient conditions facilitates to maintain the NO x  and PM levels within specific limits. The specific limits may be predefined limits including, for example, Tier 4 standards. 
         [0038]    In accordance with embodiments of the present invention, the engine  14  when operated under methods herein meets, or exceeds, the Tier 4 standards. Further, as depicted for example in  FIGS. 1 ,  2 , and  4 , various embodiments meet or exceed Tier 4 standards without the need for any aftertreatment systems on the engine  14  and/or turbocharged units  10 . That is various turbocharged units  10 , as shown in  FIGS. 1-4 , meet, or exceed, Tier 4 in the absence of any aftertreatment systems (e.g., filters, Selective Catalytic Reduction (SCR), urea, etc.). Optionally, aftertreatment systems may be additionally be used. 
         [0039]    Although the embodiments depicted in  FIGS. 1 ,  2  and  4  illustrate an exhaust channel  38  for feeding another portion of the exhaust gas bypassing the first stage turbine  24 , other embodiments are available. That is other embodiments are possible that so too adjust or control power to the compressor  22 . For example, a variable geometry turbine casing can be used to adjust power to turbocharger shaft  34  to the compressor  22 . 
         [0040]    The turbocharger  12  embodiments illustrated depict single stage (see e.g.,  FIG. 4 ) and dual stage (see e.g.,  FIGS. 1 and 2 ). Other combinations of stages and quantity of turbine/compressor per stage, may be used in the turbocharger  12 . By example, and not by limitation, the second stage turbine  26  may be instead two turbines (not shown), the first state compressor  20  may be two compressors (not shown). 
         [0041]    The turbocharged unit  10  may be used in a variety of applications including stationary and mobile applications. By example, and not limitation, the turbocharged unit  10  may be used in rail (e.g., locomotive) and marine applications. With rail applications, for example, the turbocharged unit  10  may be used as a source or mechanical energy for a diesel electric locomotive; thereby, allowing for engine speed to be decoupled from the vehicle speed. 
         [0042]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.