Patent Publication Number: US-2021189982-A1

Title: Powertrain with Continuously Variable Transmission and Aftertreatment System

Description:
TECHNICAL FIELD 
     This patent disclosure relates generally to operation of a powertrain including an internal combustion engine and a continuously variable transmission and, more particularly, to a system and method of the engine and CVT to regulate an aftertreament system. 
     BACKGROUND 
     Powertrains are the assemblies that transmit the rotational power produced by an internal combustion engine to the point of application or load. Powertrains may include various components and devices to manipulate and adjust the rotational power being transmitted, for example, by changing the angular direction, the torque, or the rotational speed. Transmissions are a major component of a powertrain in which the rotational speed and, inversely, the torque can be changed from input to output. Traditional transmissions typically increased or reduced speed through a series of fixed gear ratios, however, continuously variable transmissions (CVTs) have been developed that enable speed and torque to be adjusted through a continuous range of input rotation to output rotation. Because of the adaptability associated with CVTs, they have been used in heavy industrial applications and large scale mobile machines for construction, mining, agriculture, and other industries 
     Also included in powertrains are internal combustion engines, which may be operatively associated with emission control technologies such as aftertreatment systems that function by reducing or converting emissions produced by the internal combustion process. One example of an aftertreatment system is selectively catalytic reduction (SCR) in which the exhaust gases are chemically reacted in the presence of a catalyst with an introduced reductant fluid to convert nitrogen oxides (NO x ) to nitrogen (N 2 ) and water (H 2 O). Aftertreatment system have also been operated in conjunction with the powertrain to achieve advantageous results in power generation. For example, U.S. Pat. No. 8,073,610 (“the &#39;610 patent”) describes a system in which a transmission and an aftertreatment catalyst may be used together to improve operative efficiency of the system. However, as the operative state or output of the internal combustion engine changes, it may affect the aftertreatment process. The present disclosure is directed to novel systems and methods for cooperatively operating an aftertreatment system in combination with a powertrain including a CVT. 
     SUMMARY 
     The disclosure describes, in an aspect, a drivetrain including an internal combustion engine with a plurality of combustion chambers in which to combust a fuel. An exhaust system may be in fluid communication with the plurality of combustion chambers to direct exhaust gases away from the internal combustion engine. Disposed in the exhaust system can be a selective catalytic reduction (SCR) catalyst to reduce nitrogen oxides (NO x ) in the exhaust gases to nitrogen (N 2 ) and water (H 2 O). The internal combustion engine can be operatively associated with a continuously variable transmission (CVT) coupled to a driveshaft. An electronic controller may also be associated with the internal combustion engine and with the CVT to inversely adjust the engine speed and a CVT output to selectively regulate a catalyst temperature of the SCR catalyst. 
     In another aspect, the disclosure describes a method of operating a powertrain to regulate temperature of a selective catalytic reduction (SCR) catalyst. The method measures a catalyst temperature of the SCR catalyst disposed in an exhaust system of an internal combustion engine and regulates the engine speed of the engine in an inverse relation to the catalyst temperature. The method also regulates a CVT output of a continuously variable transmission (CVT) coupled to the internal combustion engine in a direct relation to the catalyst temperature to offset the adjustment to engine speed. 
     In yet another aspect, the disclosure describes a powertrain including an internal combustion engine with a plurality of combustion chambers in which the combustion of fuel occurs. An exhaust system communicates with the plurality of combustion chambers to remove the exhaust gases. To reduce nitrogen oxides (NO x ) in the exhaust gases to nitrogen (N 2 ) and water (H 2 O), a selective catalytic reduction (SCR) catalyst is disposed in the exhaust system. Coupled to the driveshaft of the internal combustion engine is a continuously variable transmission operatively (CVT) for adjusting speed and/or torque in the powertrain. A electronic controller associated with the powertrain is configured to receive and compare the catalyst temperature to a catalytic threshold. If the catalyst temperature is below the catalytic threshold, the electronic controller increases the engine speed and restricts a CVT output to warmup the SCR catalyst. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a powertrain including an internal combustion engine operatively associated with a continuously variable transmission and with an aftertreatment system. 
         FIG. 2  is a schematic representation of a chart illustrating a variable range of related operating conditions of the internal combustion engine and the continuously variable transmission in accordance with the disclosure. 
         FIG. 3  is a flow diagram illustrating an example of a computer implemented methodology or process for regulating the catalyst temperature of an aftertreatment catalyst through selective operation of the internal combustion engine and the continuously variable transmission. 
     
    
    
     DETAILED DESCRIPTION 
     Now referring to the drawings, wherein whenever possible like elements refer to like reference numbers, there is illustrated a powertrain  100  for transmission of rotational power produced by an internal combustion engine  102  to a point of application or a load  104  such as a propulsion device. The internal combustion engine  100  is configured to combust a mixture of an oxidizer such as air and a hydrocarbon-based fuel to convert the chemical energy therein to a motive mechanical power in the form of rotational motion that can be applied through a driveshaft  1 - 106  of the engine for other work. The internal combustion engine  100  may be any size, but the present application is particularly suited to large-scale heavy industrial engines on the magnitude of several hundred horsepower or kilowatts. Internal combustion engines of these scales are used in a variety of industrial machines including mobile machines used in construction, mining, agriculture, and other industries such as wheel loaders, dozers, dumb trucks, and the like. Moreover, while the internal combustion engine  102  can combust any suitable fuel and can operate on any suitable combustion cycle, the present disclosure may be particularly applicable to diesel burning, compression-ignition engines. 
     To deliver fuel for the combustion process, the internal combustion engine  102  can be operatively associated with a fuel system  110 . The fuel system  110  may include a plurality of fuel injectors  112  that are operatively disposed to deliver fuel to a respective plurality of combustion chambers in the internal combustion engine  102 , with at least one fuel injector associated with each combustion chamber. The fuel injectors  112  can inject a desired quantity of fuel into the combustion chamber where it is ignited and the resulting combustion reciprocally drives a piston attached to and rotating a crankshaft. In diesel-burning compression ignition engines, the fuel auto-ignites upon introduction to the highly pressurized conditions in the cylinder  104  resulting from the compression stroke, and accordingly, the fuel injectors  112  may be timed to increase efficiency and power generation. To store the fuel, the fuel system  110  can include a fuel reservoir or fuel tank  114  that is in fluid communication with the plurality of fuel injectors  112  through one or more fuels lines  114 , which may also be associated with fuel pumps, fuel rails and the like. 
     To deliver air for use as an oxidizer in the combustion process, the internal combustion engine  102  can be operatively associated with an air intake system  120 . The air intake system  120  can receive air from the surrounding environment, which may be the atmosphere, through an air filter  122  to remove contaminants, dust, and debris. The intake air is delivered from the air filter  122  through an intake conduit  124  to an intake manifold  126  on the internal combustion engine  102 . The intake manifold  126  is in fluid communication with and can direct the intake air to the plurality of combustion chambers. The intake air can be selectively admitted to the combustion chambers through the selective actuation of one or more intake valves associated with each chamber. 
     To remove the byproducts of the combustion process from the combustion chambers, the internal combustion engine  102  can be operatively associated with an exhaust system  130 . The exhaust system  130  can include an exhaust manifold  132  included with the internal combustion engine  102  and in fluid communication with the plurality of combustion cylinders via selectively actuated exhaust valves. As the piston disposed in the combustion chamber reciprocally moves upwards with the exhaust valve open, the exhaust gases are forcibly discharged to the exhaust manifold and can be directed by an exhaust conduit  134  to the atmosphere. 
     In an embodiment, to increase the efficiency of the internal combustion engine  102 , a turbocharger  140  can be operatively associated with the intake system  120  and the exhaust system  130 . The turbocharger  140  can include a turbine  142  disposed in the exhaust conduit  134  that is coupled to a compressor  144  disposed in the intake conduit  124 . The turbine  142  and the compressor  144  can each include a plurality of appropriately shaped vanes that are attached to a rotating hub  146  coupling the turbine and compressor. As pressurized exhaust gases are directed through and expand in the turbine  142  past the vanes, the pressurized flow may drive the rotating hub  146  which in turn rotates the vanes in the compressor  144 . The compressor  144  therefore compresses the intake air increasing the flow delivered to the internal combustion engine  102 . 
     To treat emissions in the exhaust gases, the internal combustion engine  102  can be operatively associated with an aftertreatment system  150  including one or more aftertreatment devices disposed in the exhaust conduit  134  downstream of the engine. For example, to reduce nitrogen oxides like NO and NO 2 , sometime referred to as NOR, the aftertreatment system  150  can conduct a selective catalytic reduction (SCR) process in which the NOx in the exhaust gases is converted to nitrogen (N 2 ) and water (H 2 O). In the SCR process, the exhaust gases are directed through an SCR catalyst  152  disposed in the exhaust conduit  134  and interact with a reductant agent, referred to as diesel exhaust fluid (DEF), with a common DEF being urea. The DEF may include ammonia (NH 3 ), which in the presence of the SCR catalyst  152  reacts with the NO x  converting it to Na and H 2 O. To deliver DEF to the exhaust gases, a DEF injector  154  may be in fluid communication with the exhaust conduit  134  upstream of the SCR catalyst  152 , although it may possibly be disposed directly into the SCR catalyst  152 . The DEF injector  154  can be an electromechanically operated injector configured to introduce measured amounts of pressurized DEF as a spray into the exhaust conduit  134  in a process sometimes referred to as dosing. The DEF itself may be retained in a refillable DEF tank  156  or reservoir on the machine associated with the internal combustion engine  102 . 
     In addition to the SCR catalyst  152 , the aftertreatment system  150  can include other devices to treat the exhaust gasses. For example, to reduce carbon monoxide (CO) and hydrocarbons (C x H x ) attributable to unburned fuel in the exhaust gases, a diesel oxidation catalyst (DOC)  158  can be disposed in the exhaust conduit  134  to initiate an oxidation reaction converting those components to carbon dioxide (CO 2 ) and water (H 2 O). As another example, to remove particulate matter and soot from the exhaust gases, a diesel particulate filter (DPF) may be disposed to receive and filter the exhaust flow. Because the filter physically traps and accumulates particulate matter, it may require periodic regeneration or cleaning before its starts to impede exhaust flow. 
     In addition to the internal combustion engine  102  and its support systems, the powertrain  100  can also include a transmission  160  to change the rotational speed and, in an inverse relation, the torque being produced by the engine. The transmission  160  can be operatively coupled to the driveshaft  106  projecting from the internal combustion engine  102  and directly receives the rotational motion therefrom. In an embodiment, the transmission  160  may be a continuously variable transmission (CVT) configured to operate over a continuous range of input speed and torque to output speed and torque rather than stepping through fixed gear ratios. In a more particular embodiment, the CVT  160  may be a split torque hydro-mechanical transmission in which the rotational motion and torque from the internal combustion engine  102  is transmitted through a hydrostatic transmission  162  and a mechanical transmission  164 . The hydrostatic transmission  162  can receive rotational power through an input  166  to the CVT  160 , that is used to drive a variable displacement pump  170 . The hydrostatic transmission  162  can also include a variable displacement motor  172  in fluid communication with the variable displacement pump  170  through a hydrostatic fluid circuit  174 . The variable displacement pump  170  and motor  172  can be variably adjusted to alter the pressures and flowrates in the fluid circuit so that turns or strokes of the pump can drive quantitatively different turns or strokes that result in the motor. 
     The mechanical transmission  164  can also be directly coupled to the input  166  of the CVT  160 , thus splitting the torque input, and can have any suitable configuration including a plurality of adjustably intermeshable gears. In a particular embodiment, the mechanical transmission  164  can include one or more planetary gear sets  180 . The planetary gear set  180  may include a central sun gear  182  surrounded by one or more revolving planet gears  184  that can move around the sun gear  182 . The planet gears  184  mesh with and are surrounded by a ring gear  186 . By selectively restricting or releasing one set of gears of the planetary gear set  180 , the other sets of gears can be made to rotate or revolve in varying speeds and directions. The outputs of the hydrostatic transmission  162  and the mechanical transmission  164  can be combined and directed through an output  168  of the CVT  160  and transmitted onto the load  104 . In addition to the hydrostatic transmission  162  and mechanical transmission  164 , the CVT  160  can include other gears, clutches and the like to facilitate transmission and adjustment of rotational power from the input  166  to the output  168 . 
     To coordinate and regulate operation of the powertrain  100 , an electronic controller  190  can be included, which may also be referred to as an electronic control unit (ECU), or as an engine control module (ECM), or possibly just controller. The electronic controller  190  can be a programmable computing device and can include one or more microprocessors  192 , non-transitory computer readable and/or writeable memory  193  or a similar storage medium, input/output interfaces  194 , and other appropriate circuitry for processing computer executable instructions, programs, applications, and data to regulate performance of the powertrain  100 . The electronic controller  190  may be configured to process digital data in the form of binary bits and bytes. The electronic controller  190  can communicate with various sensors to receive data about powertrain operation and performance characteristics and can responsively control various actuators to adjust that operation. 
     To send and receive electronic signals to input data and output commands, the electronic controller  190  can be operatively associated with a communication network having a plurality of terminal nodes connected by data links or communication channels. For example, as will be familiar to those of skill in the art of automotive technologies, a controller area network (“CAN”) can be utilized that is a standardized communication bus including physical communication channels conducting signals conveying information between the electronic controller  190  and the sensors and actuators. However, in possible embodiments, the electronic controller  190  may utilize other forms of data communication such as radio frequency waves like Wi-Fi, optical wave guides and fiber optics, or other technologies. In an embodiment, the electronic controller  190  may be a preprogrammed, dedicated device like an application specific integrated circuit (ASIC) or field programmable gate array (FPGA). To possibly interface with an operator or technician, the electronic controller  190  can be operatively associated with an operator interface display that may be referred to as a human-machine interface (HMI). 
     In an embodiment, the electronic controller  190  can responsively regulate operation of the powertrain  100  such that the internal combustion engine  102 , the aftertreatment system  150 , and the CVT  160  cooperatively interact together. Therefore, the electronic controller  190  can be operatively associated and in electrical communication with sensors, actuators and control devices associated with the three assemblies. For example, to control and adjust operation of the internal combustion engine  102 , the electronic controller  190  can control devices thereon such as the plurality of fuel injectors  112 . In addition, to determine the operating speed of the engine  102 , the electronic controller  190  can be associated with an engine speed sensor  196 . In an embodiment, the engine speed sensor  196  can be in physical contract with the driveshaft  106  to measure revolutions per minute (RPM), or can operate on magnetic or optical principles to sense the rotational speed of the driveshaft. 
     To control operation of the aftertreatment system  150  and, in particular, the SCR process, the electronic controller  190  can be associated with a SCR sensor  197  disposed proximate to the SCR catalyst  152 . The SCR sensor  197  may measure variables and parameters related to the SCR process such as, for example, the temperature of the SCR catalyst  152 . For the reaction of DEF with NOx to occur, the SCR catalyst  152  must be at an elevated temperature, for example, approximately 200° C. and higher, depending upon the catalytic materials and catalyst size. The SCR sensor  197  may also sense other properties important to the SCR process, such as NOx content of the exhaust gases. To determine the exhaust temperature and flowrate, the electronic controller  190  can be associated with an exhaust sensor  198  that may be disposed in or immediately downstream of the exhaust manifold  132 . The flowrate of the exhaust gases can be measured in terms of volume, time, and/or pressure. To variable adjust the CVT  160  to change the ratio of speed and/or torque between the input  166  and the output  168 , the electronic controller  190  can be associated with a CVT controller  199  that operatively adjusts the hydrostatic transmission  162  and the mechanical transmission  164 . 
     In an embodiment, the electronic controller  190  can control operation of the powertrain  100  to regulate temperature of the SCR catalyst  152  as needed to conduct the SCR process. As stated above, the SCR catalyst  152  must be at elevated temperatures to convert NO x to Na and H 2 O, typically above 200° C. Such a temperature may be referred to as the activation temperature or catalytic threshold. Depending upon whether the SCR catalyst  152  is above or below the catalytic threshold, the electronic controller  190  may be programmed to implement and switch between a warmup mode and a keep warm mode. In the warmup mode, the SCR catalyst  152  may be below the catalytic threshold and the electronic controller  190  may operate the powertrain  100  to rapidly raise the catalyst temperate to the catalytic threshold. Warmup mode may be implemented when the internal combustion engine  100  is initially started or has been running in idle for a period of time. In keep warm mode, the SCR catalyst  152  may be at or above the catalytic threshold and the electronic controller  190  may operate the powertrain  100  to maintain that temperature. The aftertreatment system  150  may be designed and disposed with respect to the exhaust system  130  so that the keep warm mode may be implemented during normal or routine operating conditions of the internal combustion engine  102 . 
     To implement and switch between the warmup mode and the keep warm mode while maintaining the prevailing operation and settings for the powertrain  100 , the electronic controller  190  can adjust operation of the internal combustion engine  102  and the CVT  160  in a related and inverse manner. For example, referring to  FIG. 2 , the engine  102  and CVT  160  can be operated to maintain a set speed or torque desired of the powertrain  100  at the load  104  while utilizing the exhaust gases to regulate temperature of the SCR catalyst  152 .  FIG. 2  is an illustrative graph  200  depicting the relation between the catalyst temperature  202  along the X-axis, the engine speed  204  and CVT speed  206  in, for example, RPM on the left Y-axis, and the engine temperature  208  and CVT output torque  210  on the right Y-axis. 
     When the catalyst temperature  202  is low and insufficient to conduct the SCR process, the electronic controller  190  can increase the engine speed  204  which results in increasing the exhaust gases produced and thus the exhaust flowrate. In  FIG. 2 , the increase in engine speed  204  may be represented by the solid curve  212 . The engine speed  204  may, for instance, be increased above a set or desired speed. In heavy duty or large scaled applications, the internal combustion engine  102  may be set at a constant speed and power output at or near its peak efficiency or peak power output and any desired variation in rotational speed and/or torque may be addressed by adjusting the transmission or similar assembly. However, in the warmup mode, to increase the flowrate of hot exhaust gases directed to the SCR catalyst  152 , the engine speed  204  is increased resulting in more exhaust gases and an increased exhaust flowrate. This effectively increases the enthalpy or heat energy directed to the SCR catalyst  152  to rapidly increase or raise the catalyst temperature to the catalytic threshold. In a diesel combustion engine, engine speed  202  can be increased by increasing the quantity of fuel introduced to the combustion chambers per combustion cycle. In the embodiment of  FIG. 1 , the electronic controller  190  may direct the plurality of fuel injectors  112  accordingly to increase the fuel injection quantities. To adjust for or offset the increased engine speed  204 , the electronic controller  190  can inversely adjust the CVT  160 . In particular, the ratio of the CVT speed  206  between the CVT input  166  to the CVT output  168  can be decreased in an inverse proportion to the increase in engine speed  204 . The inversely proportional decrease in CVT speed  206  can be represented by the location of the dashed curve  214  under the warmup mode. Accordingly, the overall speed output of the powertrain  100  remains constant. 
     As the SCR catalyst  152  rises in catalyst temperature  202  toward the catalytic threshold, the electronic controller  190  can switch to the keep warn mode in which it attempts to maintain the catalyst temperature  202 . Accordingly, the engine speed  204  can be decreased to a desired or set speed, as indicated by the location of the solid curve  212  in the keep warm region. The decrease in engine speed  204  results in a decrease in exhaust flowrate to the SCR catalyst  152 ; however even the lower flowrate may be sufficient to maintain the catalyst temperature at or in excess of the catalytic threshold. Also, in diesel combustion engines, decreasing engine speed results in a decrease in the air/fuel ratio in the combustion chambers. A decrease in engine speed results in a decrease in intake air mass flow directed the combustion chamber, for example, due to a decrease in the efficiency of the turbocharger. Thus, although the decrease in engine speed is caused by decreasing the quantity of fuel introduced to the combustion chambers, the decrease in intake air mass flow occurs at a greater rate thereby resulting in an air/fuel ratio closer to stoichiometric and richer combustion conditions. Rich combustion conditions typically result in higher temperatures and result in hotter exhaust gases to maintain the catalyst temperature  202  of the SCR catalyst  152  above the catalytic threshold. To maintain constant powertrain output, the CVT speed  206  can be inversely increased as indicated by the location of the dashed curve in the keep warm region. 
     In an embodiment, as indicated by the solid and dashed curves  212 ,  214 , the inverse adjustments between the engine speed  204  and CVT output  206  may be proportionally scaled and may occur across a range of catalyst temperatures  202  as a continuum or spectrum. Accordingly, the transition between warmup mode and keep warm mode may not be explicitly defined. Moreover, the electronic controller  190  may direct engine and CVT operation between the warmup or keep warm modes as a continuously responsive process to account for increases and decreases in the catalyst temperature  202 . The engine speed sensor  196  can measure the instantaneous engine speed  204  which can be converted by the electronic controller  196  to the appropriate CVT speed  206  in a related but inverse relation so that the output of the powertrain remains consistent. Further, the electronic controller may attempt to balance between the warmup and keep warm modes for the instant catalyst and other conditions to optimally regulate the temperature of the SCR catalyst. It should be noted that  FIG. 2  is exemplary only, and should not be construed as indicating specific values or direct relations between values of the internal combustion engine  102  or CVT  160 . 
     INDUSTRIAL APPLICABILITY 
     Referring to  FIG. 3 , there is illustrated a flow diagram  300  of an exemplary routine or algorithm for operating the disclosed powertrain  100 . The flow diagram  300  can include a series of steps, including actions and decisions, that can be implemented as computer-executable software instructions or code in the form of an application or program that can be executed by the processor  192  associated with the electronic controller  190 . Further, the flow diagram  300  in software form may be stored in a non-transitory state in the memory  193  associated with the electronic controller  190 . 
     The process disclosed in the flow diagram  300  can be initiated with a measurement step  302  measuring the catalyst temperature  304  of the SCR catalyst  152 . The measurement step  302  can be accomplished with the SCR sensor  197  operably associated with the SCR catalyst  152 . In the embodiment of the flow diagram  300 , the catalyst temperature  304  can be compared to a catalytic threshold  306  to determine if the SCR catalyst is at a catalyst temperature sufficient to conduct the SCR process. The catalytic threshold  306  can be determined in part by the material of the SCR catalyst, the size of the SCR catalyst, and other information such as exhaust flow rate and exhaust temperature and, for example, may be approximately 200° C., which may be the activation temperate of a typical SCR catalyst. In contrast to the procedure described above with respect to  FIG. 2 , where the relative speeds or outputs of the internal combustion engine  102  and CVT  160  are cooperatively adjusted over a continuum of catalyst temperatures  202 , the flow diagram  300  represents a more decisive and binary determination of operating between the warmup and keep warm modes based on the catalytic threshold  306 . The catalytic threshold  306  may be stored as electronic data in the memory  193  of the electronic controller  190 . 
     In the event the catalyst temperature  304  is below the catalytic threshold  306 , the flow diagram  300  may proceed to a warmup mode  310 . The catalyst temperature  304  may be below the catalytic threshold  306  because the internal combustion engine  102  is just starting up or has been in idle. In large scale internal combustion engines  102  used on mobile machines associated with the mining industry or on industrial pumps and generators, the engines may be placed in idle for several hours to conserve fuel but enable the SCR catalyst  152  to cool below the catalytic threshold  306 . In warmup mode, to rapidly increase the catalyst temperature  304 , an increase fuel step  312  may be conducted to increase the fuel quantity introduce to the plurality of combustion chambers. In diesel combustion engines, this results in an increase in an engine speed/exhaust flowrate step  314 . Particularly, increasing the fuel quantity accelerates the engine speed resulting in an increased exhaust flowrate discharged from the combustion chambers. In an exhaust direction step  316 , the increased exhaust flowrate is directed to the SCR catalyst  152  to rapidly heat it to the catalytic threshold. In particular, because there is a significant volume of hot exhaust flow through the SCR catalyst  152 , more enthalpy or heat energy is quickly transferred to the materials of the SCR catalyst than during lower engine speeds. 
     To compensate for the increased engine speed, which may be above a desired or commanded engine speed under which the internal combustion engine  102  is governed, the warmup mode  310  can in a restriction step  318  restrict the CVT output of the CVT  160 . For example, the CVT  160  may decrease the CVT output speed relative to the CVT input speed by adjusting the hydrostatic transmission  162  and mechanical transmission  164 . Accordingly, adjusting the CVT  160  compensates for the increased engine speed such that the output of the powertrain  100  remains consistent. 
     In the event the catalyst temperature  304  is at or above the catalytic threshold  306 , the flow diagram  300  may proceed to a keep warm mode  320 . In the keep warm mode  320 , the process represented in the flow diagram  300  attempts to maintain the catalyst temperature  304  above the catalytic threshold  306  so the SCR process can proceed unabated. For example, in a decrease fuel step  322 , the fuel quantity introduced to the plurality of combustion chambers is decreased, for example, to a fueling rate that may be more efficient or operate the internal combustion engine  102  closer to its peak power point. The decrease fuel step  322  results in a decrease in engine speed/exhaust flowrate step  324  as the engine speed slows due to the decrease in fuel quantity per combustion cycle. In diesel combustion engines, this result is due to the engine speed being determined by the quantity of fuel combusted. In an exhaust direction step  326 , the decreased exhaust flowrate is directed to the SCR catalyst  152  to maintain the catalyst temperature  304  above the catalytic threshold  306 . Because of the lower volume of exhaust flowrate, less enthalpy or heat energy may be transferred per unit time to the SCR catalyst  152 . In addition, because of the reduced volume of exhaust flow through the SCR catalyst  152 , less heat will be transferred away especially when operating under low load conditions with reduced exhaust temperatures. But because of the rich burn conditions in the internal combustion engine  102 , the exhaust flow may be at higher temperatures and may be sufficient to maintain the catalyst temperature  304  above the catalytic threshold  306 . 
     To compensate for the decreased engine speed, the keep warm mode  320  may, in an adjustment step  328  increase the CVT output of the CVT  160 . For example, the CVT output speed may be increased relative to the CVT input speed by adjusting the hydrostatic transmission  162  and mechanical transmission  164 . In addition, the CVT output torque may be adjusted to maintain the load on the powertrain  100 . Accordingly, the overall output of the powertrain  100  remains consistent despite adjustments made to the internal combustion engine  102  and the CVT  160 . 
     Both the warmup  310  mode and the keep warm mode  320  may conduct a NOx reduction step  330  in which the NOx in the exhaust gases is reduced in the SCR catalyst  152  by the SCR process. The flow diagram  300  can also return to the measurement step  302  to continue measuring the catalyst temperature  304  of the SCR catalyst  152  to determine whether switching between warmup and keep warm modes  310 ,  320  is advantageous at a particular instance. Accordingly, the flow diagram  300  represents a continuing, ongoing process assessing the present operating conditions of the powertrain  100 . It should be noted that the flow diagram  300  is exemplary only and that a different order or arrangement of the steps, additional steps, or omission of step is possible. An advantage of the foregoing disclosure is that the internal combustion engine  102  and the CVT  160  can be cooperatively utilized to rapidly heat a SCR catalyst  152  that is below the catalytic threshold and maintain the catalyst temperature when it is above the catalytic threshold. These and other possible advantages and features of the disclosure will be apparent from the foregoing description and accompanying drawings. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and technique. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.