Patent Publication Number: US-2022228521-A1

Title: Systems and methods for equalizing backpressure in engine cylinders

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This application claims priority from U.S. Provisional Application No. 62/291,786, filed Feb. 5, 2016, the contents of which incorporated herein by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates generally to aftertreatment systems for use with internal combustion (IC) engines. 
     BACKGROUND 
     Exhaust aftertreatment systems are used to receive and treat exhaust gas generated by engines such as IC engines. Conventional exhaust gas aftertreatment systems include any of several different components structured to reduce the levels of harmful exhaust emissions present in exhaust gas. For example, certain exhaust aftertreatment systems for diesel-powered IC engines include a selective catalytic reduction (SCR) system which includes a catalyst formulated to convert NO x  (NO and NO 2  in some fraction) into harmless nitrogen gas (N 2 ) and water vapor (H 2 O) in the presence of ammonia (NH 3 ). A reductant is often inserted into exhaust conduits communicating the exhaust gas to the SCR system and/or other components of the aftertreatment system. 
     Natural gas as a fuel for heavy duty engines is receiving attention due to its potential to reduce pollutant and greenhouse gas emissions. Generally, natural gas engines comprise diesel engines converted to operate on natural gas, for example operating the diesel engine on natural gas using spark ignition (SI) stoichiometric parameters. For example, some natural gas engines may comprise diesel engines spanning a range from 6.5 L to 12 L in displacement converted to operate as natural gas engines. Such natural gas engines may be operated using stoichiometric combustion with cooled exhaust gas recirculation and three-way catalysis. However, simply converting diesel engines to operate on natural gas may cause the engine to experience high thermal stresses; relatively low efficiency due to low volumetric efficiency and compression ratio; unequal backpressure on engine cylinders, which may cause knock; and poor performance in terms of power and torque density, and transient response. 
     SUMMARY 
     Embodiments described herein relate generally to systems and methods for equalizing exhaust gas backpressure across a plurality of cylinders of an engine and, in particular, to exhaust manifolds structured to equalize backpressure of an exhaust gas across the plurality of cylinders of the engine. 
     In a first set of embodiments, an exhaust manifold comprises a plurality of exhaust intake conduits. Each of the plurality of exhaust intake conduits is structured to be fluidly coupled to an engine and structured to receive exhaust gas from a corresponding cylinder of the engine. At least one of the plurality of exhaust intake conduits may provide a reduction in an exhaust intake conduit cross-sectional area of the respective exhaust intake conduit from an exhaust intake conduit inlet to an exhaust intake conduit outlet of the respective exhaust intake conduits. The exhaust manifold also comprises a plurality of bends. Each of the plurality of bends is defined by a respective one of the exhaust intake conduit outlets. The exhaust manifold also comprises an exhaust intake manifold fluidly coupled to the exhaust intake conduit outlet of at least a portion of the plurality of exhaust intake conduits. Each of the plurality of bends is shaped so as to define an angle of approach of exhaust gas flowing through the respective exhaust intake conduit outlet. A first angle of approach of the first bend relative to the exhaust intake manifold flow axis is smaller than a second angle of approach of a second bend of the plurality of bends. The first bend is structured to receive exhaust gas from a first cylinder of the engine and the second bend is structured to receive exhaust gas from a second cylinder of the engine. The first cylinder is positioned in an outer position on the engine relative to the second cylinder. 
     In another set of embodiments, a system comprises an engine comprising a plurality of cylinders. Each of the plurality of cylinders is structured to burn a fuel so as to produce an exhaust gas. The system also includes an exhaust manifold comprising a plurality of exhaust intake conduits. Each of the plurality of exhaust intake conduits is structured to be fluidly coupled to an engine and structured to receive exhaust gas from a corresponding cylinder of the engine. The exhaust manifold also includes at least one exhaust intake manifold. The exhaust intake conduit outlet of at least a portion of the plurality of exhaust intake conduits is fluidly coupled to the at least one exhaust intake manifold. The exhaust manifold also includes a means for equalizing a pressure pulse amplitude caused by combustion in each of the plurality of cylinders. 
     In another set of embodiments, an exhaust manifold includes a first exhaust intake conduit structured to be fluidly coupled to an engine and structured to receive exhaust gas from a first cylinder of the engine. A second exhaust intake conduit is structured to be fluidly coupled to an engine and structured to receive exhaust gas from a second cylinder of the engine. The engine has a plurality of cylinders with the first cylinder being positioned in an outer position on the engine relative to the second cylinder. The first bend defines an oval-shaped cross-section. The first bend is shaped so as to define a first angle of approach of exhaust gas flowing through the first exhaust intake conduit outlet. A second bend is defined by a second exhaust intake conduit outlets of the second exhaust intake conduit. The second bend is shaped so as to define a second angle of approach of exhaust gas flowing through the second exhaust intake conduit outlet. An exhaust intake manifold is fluidly coupled to each of the first and second exhaust intake conduits. The exhaust intake manifold defines a first cross-sectional area proximate the first exhaust intake conduit and a second cross-sectional area proximate the second exhaust intake conduit. The first cross-sectional area is larger than the second cross-sectional area. The exhaust intake manifold defines an exhaust intake manifold flow axis. The first angle of approach relative to the exhaust intake manifold flow axis is smaller than the second angle of approach. 
     In another set of embodiments, an exhaust manifold includes a plurality of exhaust intake conduits. Each of the plurality of exhaust intake conduits is structured to be fluidly coupled to an engine and structured to receive exhaust gas from a corresponding cylinder of the engine. An exhaust intake manifold is fluidly coupled to an exhaust intake conduit outlet of at least one of the plurality of exhaust intake conduits. Each of the plurality of exhaust intake conduits and the exhaust intake manifold define an exhaust intake manifold core volume. Each of the plurality of exhaust intake conduits and the exhaust intake manifold are shaped so as to define the exhaust intake manifold core volume based on each of the displacement of the engine, the intended operating power of the engine, and the intended flow rate of the exhaust gas through the exhaust manifold. 
     In another set of embodiments, an intake manifold includes a first inlet structured to be fluidly coupled to a turbocharger so as to receive pressurized intake air from the turbocharger. A second inlet is structured to be fluidly coupled to an exhaust gas recirculation (EGR) system so as to receive EGR gas from the EGR system. A third inlet is structured to be fluidly coupled to a fuel line so as to receive fuel from the fuel line. A plurality of outlets are structured to be fluidly coupled to an engine. An intake manifold passage extends between each of the first, second, and third inlets, and the plurality of outlets. The intake manifold passage is shaped so as to cause at least two reversals in flow direction of each of the intake air, the EGR gas, and the fuel through the intake manifold passage so as to improve mixing of each of the intake air, the EGR gas, and the fuel. 
     It should be appreciated that all combinations of the foregoing concepts and additional concepts discussed in greater detail below (provided such concepts are not mutually inconsistent) are contemplated as being part of the subject matter disclosed herein. In particular, all combinations of claimed subject matter appearing at the end of this disclosure are contemplated as being part of the subject matter disclosed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several implementations in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings. 
         FIG. 1  is a schematic illustration of system including an exhaust manifold, according to an embodiment. 
         FIG. 2A  is a side view and  FIG. 2B  is a bottom view of at least a portion of the exhaust manifold of  FIG. 1 . 
         FIG. 2C  is a side view of a portion of the exhaust manifold of  FIGS. 2A and 2B . 
         FIG. 3  is a schematic flow diagram of various operational parameters of a natural gas engine converted from a diesel engine which were altered to obtain high efficiency of the natural gas engine. 
         FIG. 4  is a top view of a system that includes an engine, an intake manifold and an exhaust manifold, according to an embodiment. 
         FIG. 5A  is a top view and  FIG. 5B  is a front view of the intake manifold and the exhaust manifold included in the system of  FIG. 4 . 
         FIG. 6A  is a perspective view of the exhaust manifold of the system of  FIG. 3  fluidly coupled to a turbine and  FIG. 6B  shows the exhaust manifold and turbine with covers assembled thereon. 
         FIG. 7  is a perspective view of an EGR assembly included in the system of  FIG. 3 . 
         FIG. 8A-B  are finite element analysis (FEA) models illustrating heat transfer coefficients in water jackets around valve seats, bridges and an ignitor core of a cylinder included in the engine of  FIG. 3  ( FIG. 8A ) and predicted combustion face temperature and location of max temperature ( FIG. 8B ). 
         FIG. 9  is a schematic illustration of a compressor recirculation system. 
         FIG. 10  is a plot of impact of EGR on gross indicated efficiency of the engine. 
         FIG. 11A  is a plot of burn duration and  FIG. 11B  is a plot of ignition delay vs. crank angle of 50% (CA50) variations. 
         FIG. 12  is a plot of indicated fuel consumption of the engine. 
         FIG. 13  is a plot of premix air/fuel mixture injection and port fuel injection (PFI) in each cylinder of the engine of  FIG. 3 . 
         FIG. 14  is a plot of apparent heat release vs. crank angle of a crank shaft of the engine of  FIG. 3  operated using PFI. 
         FIG. 15  is a computational fluid dynamic (CFD) model showing modeled performance of the intake system included in the system of  FIG. 3 . 
         FIG. 16  are bar charts summarizing turbine loss factors. 
         FIG. 17  is a schematic illustration of an air handling architecture. 
         FIG. 18A  is a plot of fresh air flow and  FIG. 18B  is a plot of EFR Fraction tracking during a federal testing procedure (FTP) cycle. 
         FIG. 19  is a schematic illustration of an air/fuel ratio control system. 
         FIGS. 20A-B  are plots of air estimator performance at speed A ( FIG. 20A ) and speed B ( FIG. 20B ). 
         FIGS. 21A-D  are plots demonstrating the influence of outer loop on constituents of an exhaust gas emitted from the system of  FIG. 3 . 
         FIGS. 22A-C  are plots of NO x , ( FIG. 22A ), methane ( FIG. 22B ) and CO ( FIG. 22C ) emissions during heavy-duty cold FTP transient cycle. The emissions were report at engine out (EO), close-coupled catalyst out (CC) and system out (SO) locations. 
         FIGS. 23A-C  are plots of NOR, ( FIG. 23A ), methane ( FIG. 23B ) and CO ( FIG. 23C ) emissions during heavy-duty warm FTP transient cycle. The emissions were reported at engine out (EO), close-coupled catalyst out (CC) and system out (SO) locations. 
         FIG. 24A  is a plot of cumulative NO x  and  FIG. 24B  is a plot of cumulative methane (CH 4 ) cold FTP transient cycle conversion performance predictions against testing data. 
         FIG. 25  includes plots of a peak cylinder pressure (PCP) and CA50 variations of a baseline and a new engine. 
         FIGS. 26A-B  are FEA models comparing a combustion face temperature of the baseline engine ( FIG. 26A ) and a new engine ( FIG. 26B ). 
         FIG. 27  is a plot of EGR fraction vs. engine pressure change or delta pressure (DP). 
         FIG. 28  is a plot of knock vs. EGR fraction. 
         FIG. 29  is a plot of air flow and EGR fraction vs time. 
         FIG. 30  is a plot of emission results for cold/hot FTP emissions test. 
         FIG. 31  is a plot of cycle brake thermal efficiency (BTE) vs. lower heating value. 
     
    
    
     Reference is made to the accompanying drawings throughout the following detailed description. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative implementations described in the detailed description, drawings, and claims are not meant to be limiting. Other implementations may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented here. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the figures, can be arranged, substituted, combined, and designed in a wide variety of different configurations, all of which are explicitly contemplated and made part of this disclosure. 
     DETAILED DESCRIPTION 
     Embodiments described herein relate generally to systems and methods for equalizing exhaust gas backpressure across a plurality of cylinders of an engine and, in particular, to exhaust manifolds structured to equalize backpressure of an exhaust gas across the plurality of cylinders of the engine. 
     Natural gas as a fuel for heavy duty engines is receiving attention due to its potential to reduce pollutant and greenhouse gas emissions. Generally, natural gas engines comprise diesel engines converted to operate on natural gas, for example operating the diesel engine on natural gas using SI stoichiometric parameters. For example, some natural gas engines may comprise diesel engines spanning a range from 6.5 L to 12 L in displacement converted to operate as natural gas engines. Such natural gas engines may be operated using stoichiometric combustion with cooled exhaust gas recirculation and 3-way catalysis. 
     However, simply converting diesel engines to operate on natural gas may cause the engine to experience high thermal stresses; relatively low efficiency due to low volumetric efficiency and compression ratio; unequal backpressure on engine cylinders which may cause knock; and poor performance in terms of power and torque density, and transient response. For example, the backpressure exerted by the exhaust gas flowing out of individual cylinders of engines (e.g., natural gas engines converted from diesel engines) may vary across the plurality of cylinders. This may lead to inconsistent temperatures across the plurality of cylinders, which may cause at least a portion of the plurality of cylinders included in the engine to run “hot” (e.g., at a temperature exceeding a design temperature of the respective cylinder). This can cause engine “knock” or pre-ignition, which further reduces the efficiency of the engine. 
     Various embodiments of the systems and methods described herein may provide benefits over conventional engine systems, including, for example: (1) equalizing pressures across cylinders, such as via an exhaust manifold structured to equalize a backpressure exerted by the exhaust gas on each of a plurality of cylinders of an engine; (2) maintaining a consistent temperature across all of the plurality of cylinders of the engine so as to reduce knock, thereby increasing engine efficiency; (3) providing continuous area reduction of exhaust gas flow to a turbine fluidly coupled to the engine so as to maintain exhaust gas momentum and reduce flow losses; (4) aligning a trajectory of the exhaust gas flow into an EGR system so as to maximize momentum recovery into the EGR flow path; and (5) defining cross-sections structured to maintain attachment of the exhaust gas flow to walls of the exhaust manifold or components thereof, thereby reducing turbulence and/or momentum losses. 
       FIG. 1  is a schematic illustration of a system  100 , according to an embodiment. The system  100  comprises an engine  102 , an exhaust manifold  110 , and optionally a turbine  160  and an EGR assembly  170 . 
     The engine  102  comprises an engine block  104  within which a plurality of cylinders  106  are defined. Each of the plurality of cylinders  106  is structured to burn fuel (e.g., natural gas) so as to produce an exhaust gas. The engine  102  may include a diesel engine, a natural gas engine, a gasoline engine, a biodiesel engine, an LPG engine, a dual-fuel engine, or any other suitable engine. In particular embodiments, the engine  102  may include a diesel engine converted to operate on natural gas. In other embodiments, the engine  102  is specifically designed to operate on natural gas. 
     The exhaust manifold  110  is fluidly coupled to the engine  102  and structured to receive the exhaust gas from the engine  102 . The exhaust manifold  110  is structured to equalize a pressure pulse amplitude caused by combustion in each of the plurality of cylinders  106  of the engine  102 . This is also referred to herein as equalizing a backpressure exerted by the exhaust gas on each of the plurality of cylinders  10 . As used herein, the term “equalize” refers to achieving less than 10% variation in pressure pulse amplitudes in the exhaust manifold  110  caused by combustion in each of the plurality of cylinders  106 . In particular implementations, the exhaust manifold  110  is structured such that equalizing pressure pulse amplitude achieves less than 5% variation between cylinders  106 . In further implementations, the exhaust manifold  110  is structured such that equalizing pressure pulse amplitude achieves less than 3% variation between cylinders  106 . In some embodiments, the exhaust manifold  110  may also cause a temperature of each of the plurality of cylinders  106  to be substantially the same (e.g., within +/−5% to +/−10% of each other, inclusive of all ranges and values therebetween). The consistent pressure and temperature across the plurality of cylinders  106  may reduce knock, thereby minimizing losses in the efficiency of the engine  102 . 
     Expanding further, the exhaust manifold  110  may comprise a plurality of exhaust intake conduits  112 . Each of the plurality of exhaust intake conduits  112  is structured to be fluidly coupled to the engine  102  and structured to receive exhaust gas from a corresponding cylinder  106  of the engine  102 . Each of the plurality of exhaust intake conduits  112  may provide a reduction in an exhaust intake conduit cross-sectional area of the exhaust intake conduit  112  from an exhaust intake conduit inlet to an exhaust intake conduit outlet thereof. The exhaust manifold  110  also comprises at least one exhaust intake manifold  114 . The exhaust intake conduit outlet of at least a portion of the plurality of exhaust intake conduits  112  is fluidly coupled to the at least one exhaust intake manifold  114 . 
     For example, as shown in  FIG. 1  and  FIGS. 2A-C , the exhaust manifold  110  may comprise a first exhaust intake manifold  114   a  and a second exhaust intake manifold  114   b  (collectively referred to as “the exhaust intake manifolds  114 ”). The exhaust manifold  110  may also comprise a first set of exhaust intake conduits  112   a  and a second set of exhaust intake conduits  112   b  (collectively referred to herein as the “exhaust intake conduits  112 ”). The first set of exhaust intake conduits  112   a  is fluidly coupled to the first exhaust intake manifold  114   a  and structured to receive exhaust gas from a first portion of the plurality of cylinders  106 . Furthermore, the second set of exhaust intake conduits  112   b  are fluidly coupled to the second exhaust intake manifold  114   b  and structured to receive exhaust gas from a second portion of the plurality of cylinders  106 . 
     A cross-section of the exhaust intake conduit inlet may be larger than a cross-section of the exhaust intake conduit outlet, thereby causing the exhaust intake conduit cross-sectional area of each of the exhaust intake conduits  112  to decrease from the exhaust intake conduit inlet to the exhaust intake conduit outlet thereof. This may accelerate the exhaust gas flow towards the exhaust intake manifolds  114 , thereby preventing any loss in momentum or pressure of the exhaust gas as it flows into the exhaust intake conduits  112 . 
     In some embodiments, the exhaust intake conduit outlet of each of the plurality of exhaust intake conduits  112  comprises a bend  113  where it is coupled to the corresponding exhaust intake manifold  114 . Moreover, in some embodiments, the exhaust intake conduit outlet of at least one the plurality of exhaust intake conduits  112  defines a non-circular cross-section (e.g., an elliptical or oval-shaped cross-section), for example at the bend  113 . The non-circular cross-section may prevent the exhaust gas from separating from inner surfaces of sidewalls of the exhaust intake conduits  112  as the exhaust gas enters the exhaust intake manifolds  114 , thereby preventing flow losses. 
     The reduction in the cross-sectional area of the exhaust intake conduits  112  and/or the bends  113  provided therein may serve to equalize a backpressure exerted by the exhaust gas on each of the plurality of cylinders  106 . This may also cause a temperature in each of the plurality of the cylinders  106  to be substantially the same, thereby reducing knock. 
     In some embodiments, the exhaust intake manifolds  114  may also define a cross-sectional area that reduces from a portion where the exhaust gas enters the exhaust intake manifolds  114  to a portion where the exhaust gas exits the exhaust intake manifold  114 . The reducing cross-sectional area of the exhaust intake manifold  114  may further facilitate equalizing of a backpressure exerted by the exhaust gas on each of the plurality of cylinders  106 , for example by preventing momentum losses of the exhaust gas. 
     A first outlet port  116   a  and a second outlet port  116   b  (collectively referred to herein as “the outlet ports  116 ”) may be fluidly coupled to the first exhaust intake manifold  114   a  and the second exhaust intake manifold  114   b . Each of the outlet ports  116  defines an outlet port flow axis  117 , which is positioned orthogonal (e.g., at an angle in the range of 60 degrees to 120 degrees inclusive of all ranges and values therebetween) to an exhaust intake manifold flow axis  120  of the exhaust intake manifolds  114 . In some embodiments, the outlet port flow axis  117  may be parallel to and/or in line with an exhaust intake conduit flow axis  123  of the plurality of exhaust intake conduits  112  so as to minimize the number of turns the exhaust gas experiences from the exhaust intake conduits  112  to the turbine  160 . 
     The outlet ports  116  may provide a reduction in an outlet port cross-sectional area of the outlet ports  116  from an outlet port inlet to an outlet port outlet of each of the outlet ports  116 . Furthermore, each of the outlet ports  116  may define a non-circular (e.g., an elliptical or oval-shaped) cross-section. The reduction in cross-sectional area and/or the elliptical or oval cross-section of the outlet ports  116  may also serve to equalize the backpressure exerted by the exhaust gas on each of the plurality of cylinders  106 . 
     The outlet ports  116  may be fluidly coupled to the turbine  160  (e.g., a turbine included in a turbocharger). The reducing cross-sectional area and/or the elliptical or oval cross-section of the outlet ports  116  may provide uniform flow of the exhaust gas into the turbine  160 . The first outlet port  116   a  and the second outlet port  116   b  may provide a fully divided flow of the exhaust gas received from the respective first set and the second set of the plurality of cylinders  106  to the turbine  160 , which may also serve to equalize the backpressure of the exhaust gas on each of the plurality of cylinder  106 . 
     In some embodiments, at least one pull-off conduit may be fluidly coupled to the at least one exhaust intake manifold  114 . At least a portion of the at least one pull-off conduit may define a pull-off conduit flow axis  126  positioned orthogonal to each of the exhaust intake manifold flow axis  120  and the outlet port flow axis  117 . In various embodiments, a pull-off conduit first portion of the at least one pull-off conduit may define a reducing pull-off conduit cross-sectional area from a pull-off conduit first portion inlet to a pull-off conduit first portion outlet of the pull-off conduit first portion. 
     For example, as shown in  FIG. 1 , the exhaust manifold  110  may include a first pull-off conduit  118   a  and a second pull-off conduit  118   b  (collectively referred to herein as “the pull-off conduits  118 ”) fluidly coupled to the first exhaust intake manifold  114   a  and the second exhaust intake manifold  114   b , respectively. 
     At least a portion of the pull-off conduits  118  which is fluidly coupled to the exhaust intake manifolds  114  may be positioned orthogonal (e.g., at an angle in the range of 60 degrees to 120 degrees, inclusive of all ranges and values therebetween) to each of the exhaust intake manifold flow axis  120  of the exhaust intake manifolds  114  and the outlet port flow axis  117  of the outlet ports  116 . For example, the pull-off conduits  118  may be positioned orthogonal to the exhaust intake manifolds  114  in a first plane (e.g., in an X-Y plane) and orthogonal to the outlet ports  116  in a second plane (e.g., in a Y-Z plane). 
     A pull-off conduit first portion  119   a/b  of the pull-off conduits  118   a/b  may define a reducing pull-off conduit cross-sectional area from a pull-off conduit first portion inlet to a pull-off conduit first portion outlet of the pull-off conduit first portion  119   a/b . The reducing cross-sectional area may serve to maintain the momentum of the exhaust gas flowing through the pull-off conduit first portion  119   a/b , thereby reducing flow losses. 
     The pull-off conduit first portion  119   a/b  of the pull-off conduits  118   a/b  are fluidly coupled to each other at a joint  121  so as to define a single flow path for the exhaust gas downstream of the joint  121 . The single flow path reduces in cross-sectional area until it reaches a pull-off conduit first portion outlet  122  or throat. The sidewalls of the first portion  119   a/b  of the pull-off conduits  118   a/b  are joined with each other at the joint  121  at a sufficiently small angle (e.g., less than 5 degrees) so that the portions of the exhaust gas flowing into the joint  121  towards the pull-off conduit first portion outlet  122  from each of the pull-off conduit first portions  119   a/b  may experience minimal turbulence and smoothly mix with each other. A cross-sectional area of the pull-off conduit first portion outlet  122  may be optimized so as to prevent the exhaust gas from experience sudden momentum of flow losses, which may change the backpressure exerted by the exhaust gas on one or more of the plurality of cylinders  106 . 
     The exhaust manifold  110  may also include a diffuser  128 . The diffuser  128  may have a larger cross-sectional area relative to a cross-sectional area of the pull-off conduits  118  so as to reduce a velocity of the exhaust gas flowing therethrough, expand the exhaust gas, and/or reduce a temperature thereof. The diffuser  128  may be coupled to an EGR assembly  170 , which may be structured to communicate the portion of the exhaust gas entering the pull-off conduits  118  to the plurality of cylinders  106 , for example, to cool the combustion temperature of the air/fuel mixture therein (e.g., to reduce knock). 
     The pull-off conduits  118  may include a pull-off conduit second portion  124  fluidly coupled to each of the diffuser  128  and the pull-off conduit first portion outlet  122 . The pull-off conduit second portion  124  may define an expanding cross-sectional area from the pull-off conduit first portion outlet  122  to a pull-off conduit second portion outlet of the pull-off conduit second portion  124 . The pull-off conduit second portion outlet is fluidly coupled to the diffuser  128 . 
     The expanding cross-sectional area of the pull-off conduit second portion  124  may provide smooth reduction in pressure and flow velocity of the exhaust gas from the pull-off conduits  118  to the diffuser  128 . This may prevent vortices, flow losses, or sudden variations in backpressure of the exhaust gas. The pull-off conduit second portion  124  may also include a first bend  125  and a second bend  127  leading to the diffuser  128 . The first bend  125  and the second bend  127  may define an elliptical or oval cross-section which may cause the exhaust gas flow to remain attached to an inner surface of the sidewalls of the pull-off conduit second portion  124 , thereby preventing flow losses. 
     In some embodiments, an upstream portion of the pull-off conduit second portion  124  may define a smaller change in cross-sectional area from an inlet to an outlet thereof, relative to a downstream portion of the pull-off conduit second portion  124 . The smaller change in cross-sectional area of the upstream portion relative to the downstream portion may provide a controlled reduction in exhaust gas momentum and velocity leading to the diffuser  128  so as to prevent sudden changes in backpressure of the exhaust gas. 
       FIG. 2A  is a side view of at least a portion of the engine  102  and the exhaust manifold  110  of  FIG. 1 .  FIG. 2B  is a bottom view of at least a portion of the exhaust manifold  110  of  FIG. 1 . As illustrated in  FIG. 2A , the cylinders  106  of the engine  102  include a first cylinder  130 , a second cylinder  132 , a third cylinder  134 , a fourth cylinder  136 , a fifth cylinder  138 , and a sixth cylinder  140 . All of the cylinders  106  are arranged in the engine  102  in a line, with the first and sixth cylinders  130 ,  140  being positioned in an outer-most position on the engine  102 , the third and fourth cylinders  134 ,  136  being positioned in an inner-most position on the engine  102 , and the second and fifth cylinders  132 ,  138  being positioned in an intermediate position on the engine  102  between the outer-most and inner-most cylinders  106 . As used herein, the terms “outer” and “inner,” in regard to the position of the cylinders  106  on the engine  102 , refers to the position of each of the cylinders  106  on the engine relative to the other cylinders  106 . An outer-most cylinder  106  (e.g., the first cylinder  130 ) is positioned adjacent one other cylinder (e.g., the second cylinder  132 ). Inner cylinders (e.g., the second cylinder  132 ) are positioned adjacent two other cylinders (e.g., the first and third cylinders  130 ,  134 ). 
     Similarly, the exhaust intake conduits  112  of the exhaust intake manifolds  114  include a first exhaust intake conduit  142 , a second exhaust intake conduit  144 , a third exhaust intake conduit  146 , a fourth exhaust intake conduit  148 , a fifth exhaust intake conduit  150 , and a sixth exhaust intake conduit  152 . The first exhaust intake conduit  142  is structured to be fluidly coupled to the first cylinder  130 ; the second exhaust intake conduit  144  is structured to be fluidly coupled to the second cylinder  132 ; the third exhaust intake conduit  146  is structured to be fluidly coupled to the third cylinder  134 ; the fourth exhaust intake conduit  148  is structured to be fluidly coupled to the fourth cylinder  136 ; the fifth exhaust intake conduit  150  is structured to be fluidly coupled to the fifth cylinder  138 ; and the sixth exhaust intake conduit  152  is structured to be fluidly coupled to the sixth cylinder  140 . Accordingly, the first and sixth exhaust intake conduits  142 ,  152  are positioned in an outer-most position on the engine  102 ; the third and fourth exhaust intake conduits  144 ,  148  are positioned in an inner-most position on the engine  102 ; and the second and fifth exhaust intake conduits  146 ,  150  are positioned in an intermediate position on the engine  102  between the outer-most and inner-most exhaust intake conduits  112 . As mentioned above, at least one of the exhaust intake manifolds  114  define a cross-sectional area that reduces from a portion where the exhaust gas enters the respective exhaust intake manifolds  114  to a portion where the exhaust gas exits the respective exhaust intake manifolds  114 . Additionally, in some embodiments, the exhaust intake manifolds  114  define cross-sectional areas that are different based on the intended position of the exhaust intake manifolds  114  on the engine  102 . For example, in some embodiments, the exhaust intake manifolds  114  define a cross-sectional area that reduces from an outer-most position to an inner-most position, as defined based on the intended configuration of the exhaust intake manifolds  114  when installed on the engine  102 . This is most clearly shown in  FIG. 2B . In other words, the exhaust intake manifolds  114  define a larger cross-sectional area proximate an outer cylinder  106  than proximate an inner cylinder  106 . For example, in some embodiments, the first exhaust intake manifold  114   a  defines a first cross-sectional area proximate the first exhaust intake conduit  142  and a second cross-sectional area proximate the third exhaust intake conduit  146 , the second cross-sectional area being smaller than the first cross-sectional area. 
       FIG. 2C  is a side view of a portion of the exhaust manifold  110  of  FIGS. 2A and 2B . As mentioned above, the exhaust manifold  110  includes several design features that are implemented to achieve various design objectives, such as equalizing a backpressure exerted by the exhaust gas on each of a plurality of cylinders of the engine  102  or equalizing a pressure pulse amplitude at a point in the exhaust manifold  110  (e.g., proximate the outlet ports  116 ) caused by combustion in each of the plurality of cylinders of the engine  102 . Another design objective is to maximize the total pressure of the exhaust gas so as to optimize operation of the turbine  160  and the EGR assembly  170 . For example, in some embodiments, the shape of various portions of each of the exhaust intake conduits  112  and the exhaust intake manifolds  114  is defined so that exhaust gas flowing through the respective exhaust intake conduits  112  and exhaust intake manifolds  114  causes the same pressure pulse amplitude at a point in the exhaust manifold  110 . In other words, the exhaust gas “acts the same” regardless of the cylinder from which it was expelled. Additionally, the shape of various portions of each of the exhaust intake conduits  112  and the exhaust intake manifolds  114  is defined so as to maximize the pressure of the exhaust gas flowing through the respective exhaust intake conduits  112  and exhaust intake manifolds  114 . 
     As will be explained in further detail below, in some embodiments, at least three design parameters are defined so as to equalize the pressure pulse amplitude in the exhaust manifold  110 . First, at least one of the bends of the respective exhaust intake conduit outlets has a non-circular (e.g., oval or elliptical) cross-section. Second, at least one of the exhaust intake conduits defines a cross-sectional area that reduces from the exhaust intake conduit inlet to the exhaust intake conduit outlet. Third, each of the plurality of bends are shaped so as to define particular angles of approach of exhaust gas flowing through the respective exhaust intake conduit outlet. As will be appreciated, each of these design parameters was defined so as to achieve the design objectives. 
     As illustrated in  FIG. 2C , each of the exhaust intake conduits  112  includes an exhaust intake conduit inlet and an exhaust intake conduit outlet. In operation, exhaust gas flows from the exhaust intake conduit inlet, through the exhaust intake conduit  112 , out of the exhaust intake conduit outlet, and into the exhaust intake manifold  114 . For example, as illustrated in  FIG. 2A , the first exhaust intake conduit  142  includes a first exhaust intake conduit inlet  172  and a first exhaust intake conduit outlet  174 ; the second exhaust intake conduit  144  includes a second exhaust intake conduit inlet  176  and a second exhaust intake conduit outlet  178 ; and the third exhaust intake conduit  146  includes a third exhaust intake conduit inlet  179  and a third exhaust intake conduit outlet  182 . 
     As also mentioned above, each of the exhaust intake conduit outlets  174 ,  178 ,  182  define a bend where the respective exhaust intake conduit outlets  174 ,  178 ,  182  is coupled to the exhaust intake manifold  114 . For example, as illustrated in  FIG. 2C , the first exhaust intake conduit outlet  174  defines the first bend  113 , the second exhaust intake conduit outlet  178  defines a second bend  184 , and the third exhaust intake conduit outlet  182  defines a third bend  186 . 
     In some embodiments, at least one of the first, second, and third bends  113 ,  184 ,  186  defines a non-circular (e.g., oval or elliptical) cross-section. For example, in some embodiments, the first bend  113  defines a non-circular cross-section. In some embodiments, the second and third bends  184 ,  186  do not define a non-circular cross-section. In other embodiments, each of the first, second, and third bends  113 ,  184 ,  186  defines a non-circular cross-section. 
     In some embodiments, at least one of the exhaust intake conduits  112  defines a cross-sectional area that reduces from the exhaust intake conduit inlet to the exhaust intake conduit outlet. For example, in one embodiment, the third exhaust intake conduit  146  defines a cross-sectional area that reduces from the third exhaust intake conduit inlet  180  to the third exhaust intake conduit outlet  182 . In some embodiments, the first exhaust intake conduit  142  defines a cross-sectional area that does not reduce from the first exhaust intake conduit inlet  172  to the first exhaust intake conduit outlet  174 . In some embodiments, the exhaust intake conduits  112  that are configured to be positioned proximate inner cylinders on the engine  100  define a cross-sectional area that reduces from the exhaust intake conduit inlet to the exhaust intake conduit outlet to a greater extent than those exhaust intake conduits  112  positioned on outer cylinders of the engine  100 . 
     In some embodiments, each of the exhaust intake conduits  112  include a cross-sectional area that defines an area schedule between the exhaust intake conduit inlet and the exhaust intake conduit outlet. For example, the first exhaust intake conduit  142  includes a first cross-sectional area that varies along its length, thereby defining a first area schedule between the first exhaust intake conduit inlet  172  and the first exhaust intake conduit outlet  174 ; the second exhaust intake conduit  144  includes a second cross-sectional area that defines a second area schedule between the second exhaust intake conduit inlet  176  and the second exhaust intake conduit outlet  178 ; and the third exhaust intake conduit  146  includes a third cross-sectional area that defines a third area schedule between the third exhaust intake conduit inlet  179  and the third exhaust intake conduit outlet  182 . In some embodiments, the area schedules are defined by both the exhaust intake conduits  112  and the exhaust intake manifold  114 . For example, in some embodiments, the first area schedule is defined by the cross-sectional diameter of the first exhaust intake conduit  142  from the first exhaust intake conduit inlet  172  to the first exhaust intake conduit outlet  174 , and further to the first exhaust intake manifold  114   a  to a point proximate (e.g., upstream of) the first outlet port  116   a.    
     In some embodiments, the first area schedule is linear. In other words, the cross-sectional area of the first exhaust intake conduit  142  decreases at a linear rate from a first cross-sectional diameter at the first exhaust intake conduit inlet  172  to a smaller second diameter at the first exhaust intake conduit outlet  174 . In some embodiments, the second and third area schedules are non-linear. In other words, for example, the cross-sectional area of the second exhaust intake conduit  144  decreases from a first cross-sectional diameter at the second exhaust intake conduit inlet  176  to a smaller second diameter at the second exhaust intake conduit outlet  178  at a non-linear rate. The non-linear area schedule is most clearly shown by the third exhaust intake conduit  146 . As shown in  FIG. 2C , the “necking” at the third exhaust intake conduit outlet  182  causes the third area schedule to be non-linear due to the sharp reduction in cross-sectional diameter proximate the third bend  186 . 
     The area schedules also define an exhaust intake manifold core volume. For example, in one embodiment, a first exhaust intake manifold core volume is the internal volume of the structure that defines the plurality of exhaust intake conduits  112   a  and the first exhaust intake manifold  114   a  of the exhaust manifold  110 . In one embodiment, the first exhaust intake manifold core volume is the volume of the plurality of exhaust intake conduits  112   a  and the first exhaust intake manifold  114   a  upstream of the first outlet port  116   a . In some embodiments, the exhaust manifold  110  is sized so as to define the exhaust intake manifold core volume relative to the displacement of the engine  100 , based on a volume ratio. In other embodiments, the exhaust manifold  110  is sized based on other factors, such as intended operating power of the engine  100  or intended flow rate of exhaust gas through the exhaust manifold  110 . For example, in some embodiments, the exhaust manifold  110  is sized larger for larger engine displacement, higher intended operating power, and/or higher intended exhaust gas flow rate. 
     Each of the first, second, and third bends  113 ,  184 ,  186  are also shaped so as to define an angle of approach of exhaust gas flowing through the respective exhaust intake conduit outlet  174 ,  178 ,  182 . The angles of approach may be defined, for example, relative to the exhaust intake manifold flow axis  120 . For example, the first bend  113  is shaped so as to define a first angle of approach  188 ; the second bend  184  is shaped so as to define a second angle of approach  190 ; and the third bend  186  is shaped so as to define a third angle of approach  192 . The angles of approach  188 ,  190 ,  192  are defined so as to minimize recirculation caused by the exhaust gas impacting the walls of the exhaust intake manifold  114 . In some embodiments, the first angle of approach  188  is smaller than each of the second and third angles of approach  190 ,  192 . In other words, in some embodiments, the angle of approach is smaller for exhaust intake conduits  112  structured to be positioned in outer positions on the engine  100 . While  FIGS. 1 and 2A-2B  show an exhaust manifold structured to reduce exhaust gas backpressure which may lead to reduced knock and increase in efficiency of the engine (e.g., a diesel engine converted into a natural gas engine) various other parameters and structures of the engine may also be structured to improve an efficiency of the engine. For example,  FIG. 3  shows a family tree of various parameters of an engine that may be structured to increase an efficiency of an engine, for example a diesel engine converted into a natural gas engine. 
       FIG. 4  is a top perspective view of a system  200 , according to another embodiment. The system  200  includes an engine  202 , an exhaust manifold  210 , an intake manifold  250  and a turbine  260 . In some embodiments, the engine  202  may include a 15 liter engine having six in-line cylinders having a bore of 137 mm and stroke of 169 mm, a power of up to 447 kW, and torque of up to 2.779 Nm at 1,200 rpm. 
     The exhaust manifold is fluidly coupled to the engine  202 . The exhaust manifold  210  may be substantially similar to the exhaust manifold  110  ( FIG. 1 ) and, therefore, not described in further detail herein. Various portions of the system  200  and their novel features which lead to an increase in efficiency of the engine  202  are described below. 
     Intake Manifold and Port Breathing 
     One objective of increasing engine  202  efficiency is to reduce variations in operational parameters from cylinder-to-cylinder and from cycle-to-cycle. The major contributors impacting efficiency of the engine  202  include the intake manifold  250 , the exhaust manifold  210  and ports for more efficient air handling. The intake manifold  250  and the ports are structured so as to increase the efficiency of the engine  202 . For example, the intake manifold  250  is structured so as to receive each of pressurized intake air from the turbocharger, EGR gas, and fuel injection. As shown in  FIG. 4 , the intake manifold  250  is “S-shaped” such that the intake charge air including EGR gas and fuel is subjected to at least two flow reversals before entering the engine  202 , which improves constituent mixing of the intake charge air, the EGR gas, and the fuel. 
     In one embodiment, the intake manifold  250  includes first, second, and third inlets. The first inlet is structured to be fluidly coupled to a turbocharger so as to receive pressurized intake air from the turbocharger. The second inlet is structured to be fluidly coupled to an EGR system so as to receive EGR gas from the EGR system. The third inlet is structured to be fluidly coupled to a fuel line so as to receive fuel from the fuel line. The intake manifold  250  also includes a plurality of outlets structured to be fluidly coupled to the engine  202 . The intake manifold further includes an intake manifold passage extending between each of the first, second, and third inlets, and the plurality of outlets. The intake manifold passage is shaped so as to cause at least two reversals in flow direction of each of the intake air, the EGR gas, and the fuel through the intake manifold passage so as to improve mixing of each of the intake air, the EGR gas, and the fuel. 
     The intake ports may have a patterned design, and are exactly the same and all exhaust ports are exactly the same. The intake manifold  250  includes individual, drop-down runners from the plenum thereof to the intake ports thereof. All cylinders of the engine  202  pull off charge flow in exactly the same manner and there is no crosstalk between cylinders. 
     The intake manifold  250  provides a long mixing length so as to achieve flow uniformity. The intake ports of the intake manifold  250  are sufficiently large so as to reduce flow losses into the cylinder. Furthermore, exhaust ports of the exhaust manifold  210  are smaller for higher velocity flow to support the pulse EGR system  270  (see  FIG. 7 ). The exhaust manifold  210  provides a fully divided, pulse capture flow to the EGR  270  and isolates the front bank from the rear bank. Moreover, the trajectory and area schedule of exhaust and EGR system components is optimized to reduce backpressure of the exhaust gas. 
     The cylinder head of the engine  202  comprises a Big Intake Small Exhaust (BISE) diamond valve pattern. The diamond pattern allows for generation of swirl and the bigger intake valves enable bigger intake ports, contributing to improved engine breathing. The intake ports have high flow capability and low losses. The intake manifold may comprise a front end-inlet design with the plenum above the intake port center line, as shown in  FIGS. 5A-B . Individual, equal length runners may pull off of the plenum to feed each cylinder of the engine  202  for consistent charge distribution. The runners may be angled towards the incoming charge where they connect to the plenum in order to help direct flow down the runners to feed each cylinder. The individual runners offer additional benefits such as further separation of cylinders, so as to reduce or eliminate fuel cross-talk with a Port Fuel Injection (PFI) architecture. 
     The exhaust ports also include the BISE diamond valve pattern. Smaller exhaust valves and ports result in higher exhaust flow velocities that positively affect performance of the pulse EGR system  270 . The pulse EGR system  270  performance may be improved by efficient exhaust flow passages and junctions in order to minimize losses. The exhaust manifold  210  provided a drastically lower loss factor to the turbine  260  and EGR system  270 . 
     Turbomachinery 
       FIG. 6A  is a perspective view of the exhaust manifold  210  of the system of  FIG. 3  fluidly coupled to a turbine  260  and  FIG. 6B  shows the exhaust manifold  210  and turbine  260  assembled with covers positioned thereon. The turbine  260  is structured to provide every cylinder of the engine  202  the same experience and minimize loss coefficient. The loss coefficient may be minimized by optimizing the trajectory and the area schedule of the exhaust manifold  210 . 
     Each cylinder of the engine  202  may be provided the same exhaust gas backpressure by maintaining fully divided flows up to the turbine  260 . In some embodiments, the turbine may comprise a twin port electronically controlled waste gate to enable intake of exhaust gas flow from both banks of the exhaust manifold  210  (e.g., the plurality of outlet ports  116 ), further providing an identical breathing experience for all cylinders. 
     Exhaust Gas Recirculation 
       FIG. 7  is a perspective view of an EGR system  270  included in the system  200  of  FIG. 3 . The engine  202  operates with a stoichiometric air/fuel ration (AFR) and charge diluted with up to 25% EGR, while minimizing residuals. In addition to the exhaust manifold features already discussed, a very efficient flow junction to combine the flows may be implemented. The hot components of the EGR system  270  were developed concurrently and coupled to the exhaust manifold  210 . Concurrent with CFD analysis of the flow domain, thermal mechanical fatigue analysis of the designs was also performed to ensure durability. 
     Oil Control 
     A stable spark-ignited architecture may be achieved by eliminating sources of knock and pre-ignition. One cause of knock and pre-ignition may include oil control and particularly from piston rings and valve stem seals. To address oil intrusion from the valve stem, a seal which is four times drier than conventional seals is used. The seal is also rated for vacuum, which may be beneficial on a throttled engine (e.g., the engine  202 ) which frequently operates with a vacuum in the intake manifold (e.g., the intake manifold  250 ). Oil intrusion past the piston rings of the cylinders was prevented using a three piece oil ring which has improved ring dynamics (compared to a two piece oil ring), therefore providing improved oil control. 
     Cylinder Head and Spark Plug Cooling 
     A potential source of pre-ignition addressed in various embodiments is an overheated spark plug.  FIG. 8A-B  are finite element analysis (FEA) models showing heat transfer coefficients in water jackets around valve seats, bridges and an ignitor core of a cylinder included in the engine of  FIG. 3  ( FIG. 8A ) and predicted combustion face temperature and location of max temperature ( FIG. 8B ). 
     As seen in  FIGS. 8A-B , the heat transfer coefficients are highest in the bridges and surrounding the entire ignitor bore. The engine ignitor bore is structured such that all of the coolant flows past the ignitor bores, thereby providing exceptional cooling to the spark plug. Furthermore, the combustion face, particularly edges, are also effectively cooled so as to prevent hot spots and pre-ignition. As shown in  FIG. 8B , the combustion face is relatively cool. This not only prevents pre-ignition, it also ensures a durable and robust cylinder head with tolerance to brief periods of knock and temperature rise. 
     Fuel System 
     The engine  202  hardware allows placement of port fuel injectors in two possible locations and using two possible injectors. The injectors may be placed in the intake manifold  250  runners and/or above the intake ports in the cylinder head of the engine  202 . In some embodiments, the engine  202  operates with 100% single point fuel injection (SPFI) or upstream mixed, or 100% multi point fuel injection (MPFI) injected in the individual ports. Additionally, in various embodiments, the engine  202  may run with any mix of SPFI and MPFI. Combustion CFD predicted a benefit to using a port fuel distribution tube and engine  202  testing proved this to be the case. 
     An additional benefit of locating the fuel injectors above the port is intake manifold over pressure (IMOP), that is, intake backfire mitigation. With a single point injection near the intake throttle, the entire intake manifold  250  contains a stoichiometric combustible mixture. With a MPFI system, most of the intake manifold  250  may be filled with air or possibly a very lean mixture beyond the ignition limits. 
     Compressor Bypass Valve 
     Stoichiometric gas engines are throttled by air (opposed to a diesel engine throttled by fueling) which may pose a challenge for the turbine  260  (e.g., a turbocharger compressor). When the engine  202  quickly transitions from a high boost condition to a no boost condition (tip-out), the throttle plate may slam shut and the high speed, high pressure charge between the compressor and the throttle may have to be relieved, otherwise the pressure may spike and find a low pressure path back through the compressor. This is known as compressor surge and may cause a loading reversal of the compressor blades, which may quickly lead to fatigue failures. 
     In order to prevent this, an electronically controlled compressor recirculation valve (CRV) was implemented.  FIG. 9  is a schematic illustration of a compressor recirculation system  300 , according to an embodiment. The compressor recirculation system  300  includes a CRV  302  that is controllably actuated to prevent back flow of the intake air to a compressor  304 . The CRV  302  is structured to be controllably opened to provide a path to bleed off the high pressure air to prevent compressor surge, particularly during tip-out events or at any suitable time to help prevent surge. The CRV  302  is controlled based on various parameters, such as air pressure, compressor operating parameters, engine operating parameters, etc. 
     Engine Optimization 
     A stoichiometric with cooled EGR combustion system was coupled to the engine  202  as it has the capability to deliver high BMEP, extremely low emissions and robust operation. The performance optimization and development of the engine  202  subsystems was split into three critical areas: combustion system, fuel system, and charging system. 
     Combustion System 
     Development of the combustion system was focused on improvements in closed cycle efficiency, reduced heat transfer and capability of short burn durations under highly dilute conditions and short ignition delay times. High dilution was chosen in order to control component temperatures and to realize closed cycle efficiency improvements through reduced heat transfer as shown in  FIG. 10 . 
     Initial combustion system work was done using a full combustion cycle analysis on a calibrated combustion CFD model. A baseline combustion system delivered 10% to 90% burn durations capable of tolerating high levels of EGR dilution. The system  200  was structured to maintain that burn duration with improved efficiency and ignition delay.  FIG. 10  is a plot of impact of EGR on gross indicated efficiency of the engine.  FIG. 11A  is a plot of burn duration and  FIG. 11B  is a plot of ignition delay vs. crank angle of 50% (CA50) variations. 
     The progression from iteration # 1  to iteration # 4  as shown in  FIG. 10  represents the investigation of swirl level along with charge motion development during the cycle and the impact to combustion. Iteration # 1  represents the best efficiency that was achieved because charging penalties were minimized representing the entitlement for efficiency. Iteration # 2 -# 4  represent iterations to improve the burn duration with minimal impact to efficiency by influencing the in cylinder charge motion. Trends in burn duration and ignition delay are shown in  FIGS. 11A-B . Iteration # 4  was chosen as the combustion system for this engine as it was the best tradeoff for key deliverables. Fuel consumption trends are shown in  FIG. 12  for constant EGR levels. 
     Fuel System 
     The benefit of a premix combustion system may comprise homogeneity of the fuel and air mixture. The disadvantages may comprise transport delays, catalyst dither amplitude attenuation, and mitigation techniques in the event of cylinder misfire. Challenges of port fuel injection are mixture stratification, number of physical parts and injection pressure requirements. The benefits to MPFI may comprise cost, fuel control cylinder by cylinder, transient response time and three-way catalyst (TWC) control.  FIG. 13  are plots of premix air/fuel mixture injection and port fuel injection (PFI) in each cylinder of the engine of  FIG. 3 . Degraded combustion due to PFI was observed as shown in  FIG. 13 . 
     Further work was done to understand if MPFI stratification could be improved to match performance of the premixed combustion.  FIG. 14  is a plot of apparent heat release vs. crank angle of a crank shaft of the engine of  FIG. 3  operated using PFI. Many iterations of mixing devices as well as injection strategies were assessed via combustion CFD in order to compare combustion performance as shown in  FIG. 14 . The MPFI mixing observations yielded a design and injection strategy that was transparent to the premix injection strategy allowing for advantages of PFI to be realized. 
     Charging System 
     The charging system was structured to provide a uniform and equal mixture of air and EGR to each cylinder of the engine  202 . In addition, the charging system was designed such that it could minimize the trapped residuals. Additionally the turbine  260  was sized such that it could accommodate the lower flow rates of a stoichiometric engine  202  with EGR  270  and provide the necessary pressure balance to drive the desired EGR levels. The intake system was assessed for charge delivery and mixture uniformity. With stringent requirements for charge uniformity and charge distribution a final intake configuration as shown in the CFD model of  FIG. 15  was chosen. 
     The exhaust system was extensively tested and developed in order to ensure good cylinder balance as well as facilitate an efficient exhaust event.  FIG. 16  shows a summary of iterations assessed using a novel modeling approach in CFD to determine turbine loss factors. The iterations were focused on geometric changes that improved losses in the manifold and balanced the losses cylinder by cylinder. Iteration # 8  was chosen as the exhaust manifold  210  configuration for the engine  202 . 
     Controls 
     Air Handling Control 
       FIG. 17  is a schematic diagram of an air handling (AH) system  310 , according to an embodiment. The air handling system  310  includes the engine  202   FIG. 2  and the CRV  302  of  FIG. 9 . The fresh air coming from the atmosphere enters the system  310  through an air filter  312 . Pressure is raised by a compressor  314  of a turbocharger  316  and temperature reduced by a charge air cooler (CAC)  318 . This air reservoir together with an Intake Air Throttle (IAT)  320  is used to control the air flow going into the intake manifold  250 . Similarly, the EGR flow diverted from the exhaust manifold  210  is controlled by an EGR Valve (EGV)  322 . Both air and EGR are mixed in the intake manifold  250  at rates controlled by the valves. Lastly, the compressor  314  is maintained away from the surge region by actuating the CRV  302 . 
     The exhaust gas not diverted to the intake manifold  250  is communicated to a waste-gated turbine  324  of the turbocharger  316  where a waste gate valve (WG)  326  is used to control what portion of the flow is bypassed. By doing so, the energy going to the turbine  324  and consequently the boost can be controlled within certain limits. The control comprises calculating the IAT  320 , EGV  322 , and WG  326  actuator commands to achieve the target engine Fresh Air Flow (FAF), EGR fraction and/or boost. 
     In stoichiometric engines, FAF is directly related to engine power so the target FAF is calculated from the driver torque request and engine speed. EGR fraction, on the other hand, is used to reduce knock and PMEP and NO x . The EGR fraction target is usually calibrated as a function of (at least) load and engine  202  speed. Finally, having three actuators allows the tracking of three references (when feasible). The last target for the air handling control is turbocharger boost, which allows to trade off transient performance with pumping efficiency. A common target to exercise this tradeoff is the pressure drop on the IAT  320 , which may be stored, for example as a function of engine  202  load and speed. 
     The control was designed using physical models of the AH components, which significantly reduced the need for empirical table lookups to address system nonlinearities and changes in environmental conditions. The FAF and EGR fraction control performance is shown in  FIG. 18 .  FIG. 18A  is a plot of fresh air flow and  FIG. 18B  is a plot of EFR Fraction tracking during a federal testing procedure (FTP) cycle. Overall, the FAF and EGR Fraction remain on top of the reference with few exceptions, most of them related to turbo spooling. The large EGR fraction deviation during idle regions corresponds to EGR flow measurement errors at low flows. The actual tracking error is zero since the EGR valve  322  remains close during those regions. 
     Air Fuel Ratio Control 
     The conversion efficiency of the TWC is directly related to the AFR. Therefore, AFR is a strong lever to control the system out emissions.  FIG. 19  is a schematic illustration of an air/fuel ratio control system, according to an embodiment. The AFR control system comprises a cascade control system with two loops (inner loop and outer loop). The inner loop adjusts the on-time of the fuel injectors to precisely track the AFR target, while the outer loop determines the AFR target based on the catalyst states for best conversion efficiency of the emission constituents. 
     The inner loop consists of feedforward and feedback fuel scheduling. The feedforward fueling schedules fuel injector on-time based on the estimated air mass in the cylinder, while the feedback trims the feedforward calculation based on a wideband lambda sensor located after the turbocharger. Due to the slow response of the feedback loop, the transient performance of the AFR control is mainly determined by the feedforward fueling. The dynamics of the injectors can be neglected due to its fast response time compared to the air dynamics. Therefore, accuracy of the air estimator plays the most important role in defining the inner loop control performance. A physics-based approach, which utilizes a charge virtual sensor and an EGR flow sensor, was developed to accurately predict air flow to the cylinder.  FIG. 20A-B  shows the on-engine validation of the air estimator at different engine speeds. 
     The outer loop consists of a feedforward mean AFR target table. A feedback control trims the AFR target based on a wideband O 2  sensor located at midbed (between the first catalyst TWC 1  and second catalyst TWC 2 ). The feedforward mean lambda values are pre-determined via steady state catalyst characteristic testing at various engine  202  operating conditions. The objective is to maintain a constant AFR target at the midbed location that optimizes the conversion of all the emissions constituents.  FIGS. 21A-D  are plots demonstrating the influence of outer loop control on constituents of an exhaust gas emitted from the system  200  of  FIG. 3 .  FIGS. 21A-D  shows the influence of the outer loop control on the system out emissions. The benefit of having the outer loop control can be seen clearly from the individual plots. However, to meet the ultra-low NO x  requirement, a trade-off among the emissions constituents may also be observed. 
     Aftertreatment 
     Various embodiments include a close-coupled after treatment architecture to meet system out ultralow NO x  emission requirement. The after treatment architecture consists of both a close-coupled TWC and underfloor TWCs. This architecture provides a suitable compromise between high system efficiency and packaging constraints. The close coupled aftertreatment architecture demonstrates excellent performance in managing both cold-start and warm-start transient emissions control. Furthermore, through benchmarking evaluation, “TWC technology A” was selected to achieve both high NO x  conversion and methane (CH 4 ) conversion at near stoichiometric lambda (i.e., air/fuel ratio). The platinum-group metal (PGM) loading of the after treatment system was engineered differently between the close-coupled and underfloor TWCs. 
       FIGS. 22A-C  are plots of NO x , ( FIG. 22A ), CH 4  ( FIG. 22B ) and CO ( FIG. 22C ) emissions during heavy-duty cold FTP transient cycle. The emissions were reported at engine out (EO), close-coupled catalyst out (CC) and system out (SO) locations. During the cold FTP transient cycle as shown in  FIGS. 22A-C , the close-coupled TWC effectively managed the first 0-50 seconds NO x  emissions control before warming up the underfloor catalyst. CH 4  emissions was largely controlled through close-coupled TWC for the first 380 seconds. The close-coupled TWC successfully converted over 70% of the cumulative engine out NO x  emissions and over 60% of the cumulative engine out CH 4  emissions during the cold FTP cycle. 
       FIGS. 23A-C  are plots of NO x , ( FIG. 23A ), methane ( FIG. 23B ) and CO ( FIG. 23C ) emissions during heavy-duty warm FTP transient cycle. The emissions were reported at EO. CC and SO locations. During the warm FTP transient cycle as shown in  FIGS. 23A-C , the close-coupled architecture converted over 70% of the cumulative engine out NO x  emissions and over 65% of the cumulative engine out CH 4  emissions during the warm FTP cycle. 
     Various embodiments include a TWC model that is capable of closely predicting the application cycles emissions for natural gas application. Key challenges of developing such a model include dynamic oxygen storage mechanism, complex CH 4  oxidation and reforming kinetics and its interaction with the oxygen storage dynamics, and the highly transient nature of the air-fuel ratio control during the TWC application. The accuracy of such a TWC model may also depend on obtaining the right kinetic mechanisms through well-designed tests and reliable data collection. 
     A global-kinetic TWC model was developed and validated using a production natural gas engine with an underfloor only after treatment system during transient emissions cycles (e.g. cold Federal Test Procedure cycle, warm Federal Test Procedure cycles and World Harmonized Transient Cycles).  FIG. 24A  is a plot of cumulative NO x  and  FIG. 24B  is a plot of cumulative CH 4  cold FTP transient cycle conversion performance predictions against testing data. As shown in  FIGS. 24A-B , the model has a high predictability of aftertreatment CH 4  and NO x  performance during the cold FTP cycle against engine bench testing results. 
     System Integration 
     A system approach was used to develop a system that was capable of 90% reduction in NO x  below current standards and has equivalent efficiency to a diesel engine. With the intake manifold  250 , EGR assembly  270 , exhaust manifold  210  and combustion systems as described herein, the robustness of the engine  202  was dramatically improved. Robustness of the engine  202  is depicted in  FIG. 25  which includes plots of a peak cylinder pressure (PCP) and CA50 variations of a baseline and the engine  202  (also referred to as “the new engine”) and show improvements in cylinder to cylinder and cycle to cycle variation at peak torque with three sigma error bars for reference. The reduction in variation across the engine  202  allows for better control of the engine  202 , enables lower emissions capability, improved robustness/operating range, higher engine efficiency and capability for increased power density. 
     In addition to variation reduction, component durability was also addressed. An important component involved in engine durability is the cylinder head. A comparison of the head temperatures is shown in  FIGS. 26A-B  which are FEA models comparing a combustion face temperature of the baseline engine ( FIG. 26A ) and the new engine ( FIG. 26B ). The head temperatures were improved through a revised design so as to reduce the maximum temperatures as well as provide uniform cooling of the combustion face. A comparison of the baseline and the new engine are shown below. 
     In addition to the cylinder head the exhaust system utilized improved high temperature materials for durability as well as revised designs to improve the loss coefficient of the manifold and improve the relationship between EGR fraction and engine delta p as shown in  FIG. 27 . 
     The ability to drive large amounts of EGR with low engine delta p may allow for reduced residuals supporting a wide operating range for EGR at high load conditions which provides further robustness to knock. An example of the EGR Range at high load is shown in  FIG. 28  for the peak torque condition. 
     With a significantly improved engine design, controls were redesigned as well to enable improved air handling, combustion and air/fuel ratio controls. The control system is capable of delivering the transient response, robustness and efficiency while at the same time delivering tight control for NO x  emissions reduction. The tracking performance of the air handling system is shown in  FIG. 29 . 
     Using the system described herein 90% reduction NO x  emissions below current standards was achieved. The emission results are shown in  FIG. 30 . A cold/hot FTP emissions test was utilized to demonstrate compliance with the objectives of 0.02 g/hp-hr, according to various embodiments. In addition to the reduced NO x  emissions the fuel economy was significantly improved over the baseline engine satisfying another target to demonstrate equivalent fuel consumption to a diesel engine. Results are shown in  FIG. 31 . 
     It should be noted that the term “example” as used herein to describe various embodiments is intended to indicate that such embodiments are possible examples, representations, and/or illustrations of possible embodiments (and such term is not intended to connote that such embodiments are necessarily extraordinary or superlative examples). 
     The terms “coupled,” and the like as used herein mean the joining of two members directly or indirectly to one another. Such joining may be stationary (e.g., permanent) or moveable (e.g., removable or releasable). Such joining may be achieved with the two members or the two members and any additional intermediate members being integrally formed as a single unitary body with one another or with the two members or the two members and any additional intermediate members being attached to one another. 
     It is important to note that the construction and arrangement of the various exemplary embodiments are illustrative only. Although only a few embodiments have been described in detail in this disclosure, those skilled in the art who review this disclosure will readily appreciate that many modifications are possible (e.g., variations in sizes, dimensions, structures, shapes and proportions of the various elements, values of parameters, mounting arrangements, use of materials, colors, orientations, etc.) without materially departing from the novel teachings and advantages of the subject matter described herein. Additionally, it should be understood that features from one embodiment disclosed herein may be combined with features of other embodiments disclosed herein as one of ordinary skill in the art would understand. Other substitutions, modifications, changes and omissions may also be made in the design, operating conditions and arrangement of the various exemplary embodiments without departing from the scope of the present invention. 
     While this specification contains many specific implementation details, these should not be construed as limitations on the scope of any inventions or of what may be claimed, but rather as descriptions of features specific to particular implementations of particular inventions. Certain features described in this specification in the context of separate implementations can also be implemented in combination in a single implementation. Conversely, various features described in the context of a single implementation can also be implemented in multiple implementations separately or in any suitable subcombination. Moreover, although features may be described above as acting in certain combinations and even initially claimed as such, one or more features from a claimed combination can in some cases be excised from the combination, and the claimed combination may be directed to a subcombination or variation of a subcombination.