Abstract:
A system and method for improving exhaust gas recirculation performance is provided to induce improved exhaust gas recirculation flow during engine operating transients, including transients in which exhaust gas flow conditions are unfavorable. The apparatus includes an exhaust line including a mechatronic exhaust brake valve, an intake system including a PBS compressed air injection system, an exhaust gas recirculation passage between the exhaust and intake lines, and a controller which coordinates operation of the PBS and MEB. The controller is programmed to command the MEB to close for a period before the compressed air injection is initiated so as to build exhaust line backpressure pressure and maintain a desired pressure differential across the EGR passage so that recirculated exhaust gas flow continues to enter the intake during to PBS injection event to suppress formation of undesired excess NO x , particulate and other emissions.

Description:
[0001]    The present invention relates to an apparatus for improving control of emissions from internal combustion engines, in particular improvement in control of NO x , particulate and other emissions in vehicles equipped with turbocharged diesel engines and compressed air injection systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    In the field of vehicle emissions controls, it is well known that during certain operating states of the engine undesired combustion products such as oxides of nitrogen (“NO x ”) may be minimized by introducing a portion of the exhaust gases leaving the engine&#39;s combustion chambers back into the engine&#39;s intake manifold. The recirculated exhaust gas dilutes the incoming fresh intake air, resulting in a mixture to the engine that provides two primary mechanisms for reducing NOx formation. The first mechanism is the mixture reducing the peak in-cylinder combustion temperatures where the exhaust gas acts as a heat sink. The second mechanism is the dilution of the fresh air stream, displacing some of the oxygen which would have otherwise been drawn into the combustion chamber. The lower oxygen content results in fewer constituent oxygen atoms that feed the creation of NOx and results in an overall reduction of NOx formation. 
         [0003]    In conventional internal combustion engines, such as for example the engine  1  shown schematically in  FIG. 1 , an exhaust gas recirculation passage  2  is provided between an exhaust line  10  leading away from the engine&#39;s combustion chambers  3  to the engine&#39;s intake manifold  20 . The exhaust gas recirculation line is often provided with a cooler  22  for cooling the portion of exhaust gas being recirculated into the intake manifold, and a flow control valve  23 . The flow control valve  23  may be opened, closed and/or throttled to control the amount of exhaust gas being recirculated and thereby better match the engine&#39;s recirculated exhaust gas need to the current engine operating state. If the engine is equipped with a turbocharger  30 , the exhaust gas recirculation passage  2  is typically provided downstream of the turbocharger&#39;s compressor section  31 , intercooler  40  and/or any intake flow control device  50 , and upstream of the turbocharger&#39;s turbine  32  and exhaust gas treatment devices  60 . 
         [0004]    A well known problem with exhaust gas recirculation systems is the tendency for recirculating exhaust gas flow from the exhaust to the intake manifold to decrease or even halt during certain engine operating conditions, i.e., when there exists an unfavorable pressure ratio between the exhaust and the intake lines, or low exhaust mass flow rate conditions are present. For example, in response to a sudden increase in engine torque demand, there may be too little exhaust gas flow available in the exhaust to supply the intake manifold with sufficient recirculated exhaust gas to match the sudden increase in oxygen and fuel being supplied to the engine&#39;s cylinders. In such situations, the lack of sufficient recirculated exhaust gas may result in an inability to adequately suppress NO formation during the transient condition, and a corresponding potential to exceed NO emissions requirements. 
         [0005]    Previous attempts to improve exhaust gas recirculation flow primarily have concentrated on building backpressure in the downstream exhaust piping, such as by at least partially closing a downstream exhaust brake valve located upstream or downstream of the turbine side of a turbocharger, or by using a costly variable geometry turbocharger whose vanes may be adjusted to reduce flow through the turbocharger and thus build backpressure. Such approaches increase the pressure differential across the exhaust gas recirculation line between the exhaust line and the intake manifold. However, even with the assistance of such exhaust line components, adequate exhaust gas recirculation flow to the intake manifold cannot be assured in many transient engine operating conditions which occur on too short a time scale to for prior mechanical devices to adequately respond. 
         [0006]    In view of the these and other problems in the prior art, it is an objective of the present invention to provide enhanced exhaust gas recirculation flow in all operating engine conditions, including in particular transient engine operating conditions. 
         [0007]    This and other objectives are addressed by a system and method for real-time adjustment of fresh air induction and exhaust gas recirculation with an internal combustion engine equipped with a mechatronic exhaust brake (“MEB”), an air injection system (commonly referred to as pneumatic boost system, “PBS”) and a high speed controller which receives inputs from various sensors and/or CANbus signals, and outputs control signal for real-time coordination of MEB and PBS operations. 
         [0008]    A mechatronic exhaust brake is an electronically actuated valve used to vary the position of a throttle flap in the exhaust line of an engine. The variable position control of the MEB flap provides the ability to vary the back pressure characteristics of the exhaust stream of an engine. The MEB in particular is a rapidly responding electro-mechanical device, including a fast-response torque motor is coupled with a controller which monitors a vehicle controller area network (“CAN”) for signals used to determine the vehicle&#39;s operating conditions. 
         [0009]    A PBS system provides the capability of significantly enhancing the torque response of an internal combustion engine, particularly in response to an increase in torque demand at a time when the engine is operating at low speed and/or light load. In such conditions, there is a notable lag between the time the increase torque demand is made and the engine&#39;s turbocharger develops sufficient pressurized air in the fresh air intake to produce increased torque output. This is primarily due to the turbocharger being driven by exhaust gas flow, and there being a delay between the start of the increased torque demand and the build-up of a sufficient volume of exhaust gas flow to increase the rotational speed of the turbocharger (and thereby increase the fresh air intake pressure). 
         [0010]    During transient conditions, injecting compressed air into an engine equipped with a PBS system produces a near-instantaneous increase in torque output from the engine, providing improved vehicle drivability, potential fuel savings and several other benefits. A problem with the near-instantaneous nature of PBS compressed air injection, however, is the potential a negative effect on NO emissions which primarily results from an unfavorable pressure difference across the exhaust line and the intake line. This is because the mass flow of the compressed air in a PBS event is very high, and it may take several combustion cycles before the exhaust gas flow from the engine builds up in the exhaust line. During this initial period the sudden high air pressure in the intake manifold from the PBS injection may slow or halt exhaust gas recirculation flow needed to reduce NO levels during combustion, at least until the resulting increase in exhaust gas mass flow is capable of overcoming the unfavorable pressure difference between the exhaust and the intake. The result of insufficient EGR flow during this initial period may be large increases in NO x  formation due to lack of sufficient combustion temperature suppression by an appropriate amount of recirculated exhaust gas and increased oxygen received in the cylinder due to lack of displacement of intake air by the recirculated exhaust gas. 
         [0011]    In the present invention, the MEB and PBS systems are provided and coordinated, preferably using a CAN bus communications network to provide favorable exhaust gas recirculation pressure conditions to maintain sufficient EGR gas flow into the intake of the engine to avoid excess NO emissions. For example, in a situation in which the PBS system determines that a compressed air injection is needed and that conditions are appropriate for such an injection, the PBS may delay initiation of the compressed air injection operation by a minimum wait time, while communicating via the CAN bus to command the MEB to move to and/or maintain a partially closed position for the duration of the wait time. The restriction provided by the MEB operation provides an increase in exhaust back pressure which assists in increasing EGR gas flow from the exhaust line to the intake line, with the timing being adapted to provide for the increased EGR gas flow to reach the intake line at approximately the moment the delayed compressed air injection is initiated. As the PBS system begins the compressed air injection, the MEB is then opened to reduce the amount of restriction (e.g., to a second partially-closed position which is more open than the first partially closed position). This partial opening of the MEB throttle flap is intended to provide an appropriate back pressure for the increase in exhaust gas flow which results immediately upon the injection of compressed air from the PBS system into the engine&#39;s cylinders. Preferably, the rate by which the MEB is moved from the first partially opened position to the second partially opened position may be varied (for example, based on changing engine conditions) to more closely match the desired EGR flow from the exhaust line to the intake line to the actual requirements of the engine. As the PBS completes the compressed air injection event, the MEB may be moved back to a more fully opened position commensurate with the engine&#39;s current operating state. The amount of MEB throttle flap opening may also be controlled based on other parameters, such as control to maintain a desired differential pressure or differential pressure profile across the EGR line between the exhaust and intake lines. 
         [0012]    Alternatively, rather than separate control electronics for the PBS and MEB systems, the functions of these control systems may be integrated into a single module. 
         [0013]    The present invention thus provides a highly rapid and responsive interactive system for controlling exhaust back pressure in coordination with compressed air injection events to more precisely maintain accurate control of NO emissions by providing a favorable pressure difference across an exhaust gas recirculation system during virtually any engine operating condition. Moreover, because the use of a PBS system may reduce the length of a transient event (for example, by increasing engine torque output enough that the engine reaches a more efficient operating point and the vehicle reaches a desired speed more quickly), the total NO x  emissions potentially produced during a given drive cycle may be smaller than that of a non-PBS-, non-MEB-equipped engine. 
         [0014]    Further synergistic benefits may also be obtained with the present invention. For example, the emissions control accuracy provided by the coordinated system provides the ability to design a vehicle powertrain which, as a result of the superior emissions performance, may dispense with undesirable costly and maintenance-intensive exhaust gas after-treatment equipment and related control systems. This in turn offers further savings in vehicle weight and enhanced fuel efficiency. For example, the present invention may enable a vehicle to provide emissions performance to meet increasingly stringent government emissions requirements at levels as low as 0.2 g/bhp*hr without the need to include selective catalytic reduction (“SCR”) equipment on the vehicle. The invention may also eliminate any need to resort to costly variable-geometry turbochargers. 
         [0015]    The control of the MEB throttle flap, including flap position, opening timing and ramp rate (i.e., the rate at which the flap is moved, either linear or higher-order curve) need not be directly from the PBS system. For example, in an alternative embodiment of the present invention the MEB control may be based on the engine ECU using inputs such as accelerator pedal position or torque requests to initiate operation. This approach has an advantage of direct connectivity to the engine. 
         [0016]    It should be understood that the coordinated timing of build-up of exhaust back pressure of the present invention does not require a particular form of MEB valve, and may be implemented with a sufficiently responsive exhaust valve or similar back pressure device controlled in response to a preset or lookup-table-governed position(s) based on initial operating parameters. 
         [0017]    In a further embodiment of the present invention, the system may provide for response to engine transients which may generate an unfavorable exhaust line-to-intake line pressure difference by activating the exhaust back pressure control device quickly enough to maintain EGR flow, independent of whether a PBS compressed air injection event is initiated or whether the vehicle is equipped with a PBS system. Thus, the present invention&#39;s improved emissions performance may permit development of emissions-compliant engines for markets in which only PBS=equipped engines were believed to be suitable. 
         [0018]    Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic illustration of a previously known turbocharged engine with an exhaust gas recirculation passage. 
           [0020]      FIG. 2  is a schematic illustration of an exhaust gas recirculation system arrangement in accordance with an embodiment of the present invention. 
           [0021]      FIG. 3  is a flow chart illustrating an example operating logic for the exhaust gas recirculation system arrangement of  FIG. 2 . 
           [0022]      FIG. 4  is a graph illustrating a typical operation of the MEB during the operations illustrated in the  FIG. 3  flow chart. 
           [0023]      FIG. 5  is a graph illustrating the typical effects on exhaust gas flows during the coordinated operation of the MEB and PBS system during the operations illustrated in the  FIG. 3  flow chart. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 2  is a schematic illustration of an embodiment of the present invention, in which engine  100  is provided with a fresh air intake manifold  102  which receives air for combustion in the engine cylinders from an upstream intake line  106 , and an exhaust manifold  104  which conveys combustion exhaust gases from the engine cylinders to exhaust line  108 . 
         [0025]    Extending between the exhaust line  108  and the intake line  106  is an exhaust gas recirculation path  110 . The exhaust gas recirculation path includes an EGR throttle valve  112  which may be set to control the rate of EGR flow from the exhaust line  108  to the intake line  106 . The exhaust gas recirculation path  110  also includes an EGR heat exchanger  114  provided to cool the recirculating exhaust gas, A back-flow-prevention check valve  116  may also be provided. The exhaust gas recirculation path  110  conveys recirculating exhaust gases to the intake line  106  via an EGR injection point  118 , in this embodiment a venturi arrangement which employs acceleration of the fresh air intake flow to assist in extracting exhaust gas from the recirculation path  110 . Preferably, the EGR injection point  118  is in the form of a venturi configured to make use of the Coand{hacek over (a)} effect to enhance the exhaust gas flow, as described in U.S. patent application Ser. No. ______. 
         [0026]    The exhaust line  108  also includes the exhaust gas-driven turbines of a first stage turbocharger  120  and a second stage turbocharger  122 . The turbines drive corresponding compressor wheels  124 ,  126  to sequentially increase the pressure and mass flow rate of the fresh air being delivered via intake line  106  from intake air filter  127  to the engine  100 . Between the turbocharger compressor stages is a first fresh air heat exchanger  128  which removes heat from the air compressed by the first compressor wheel  124 . A second fresh air heat exchanger  130  (also known as a “charge air cooler)” is located downstream of the second compressor wheel  126  to further remove heat from the air compressed by turbochargers. 
         [0027]    The exhaust line  108  further includes an MEB  134  downstream of the first turbine  120 . The electronics of MEB  134  communicate with other electronics of the vehicle via a CAN bus network. Because CAN bus technology is well known, the details of the CAN bus connections are not further illustrated. After passing through the MEB  134 , the exhaust gases pass through a particulate filter  136  which removes particulate combustion byproduct particles from the exhaust flow, followed by passing through an exhaust stack  138  to reach the atmosphere. 
         [0028]    Between the second intake air heat exchanger  130  and the EGR injection point  118  is an intake air throttle valve assembly  140  and a PBS compressed air injection module  142 . Alternatively, if the PBS module is capable of handling the throttle valve assembly&#39;s functions, the throttle valve assembly  140  may be omitted. In this embodiment the PBS module  142  includes a plurality of rapid-acting solenoid valves  144  which control the flow of compressed air from reservoir  146  into the intake line  106  (the reservoir  146  being supplied with compressed air from compressor  147  and air drier unit  149 ). The compressed air is injected into intake line  106  downstream from flow control valve  148 , which is closed in conjunction with the compressed air injection during a PBS event in order to prevent backflow of compressed air upstream of PBS module  142 . The operation of the PBS system is controlled by electronics unit  150 , in this embodiment integrated into the PBS module  142  and connected to the vehicle&#39;s CAN bus to communicate with other modules, including the control electronics for the MEB  134 . 
         [0029]    An example operation of the above embodiment is described with the aid of the flow chart shown in  FIG. 3  and the  FIGS. 4-6  graphs illustrating system responses. 
         [0030]    The operating logic shown in  FIG. 3  begins at the start point  300 . At step  302  the system electronics (whether embodied in a stand-alone controller module or a combined module, such as a combined engine, PBS and MEB electronic control unit (“ECU”)) determines whether the present acceleration demand can be satisfied by the engine, without the assistance of a PBS compressed air injection event. The acceleration demand may be inputted to the system via input  301  from a demand source, such as a signal from a physical sensor such as a throttle pedal position sensor, or a signal from an electronic control module which has calculated a target acceleration demand (i.e., an engine torque output demand) based on evaluation of vehicle sensors and operating conditions such as engine speed, road speed, intake and/or exhaust manifold pressure and/or temperature, transmission state, stored compressed air amount, exhaust treatment device operating state (e.g., whether in regeneration mode), and/or anticipated road conditions derived from GPS position data. If no PBS injection is deemed needed (“yes”) control is returned to the beginning of the program logic. 
         [0031]    If the system electronics determines that the engine will not be able to meet the present torque demand without the assistance of a PBS injection event (“no”) control shifts to step  304 . In step  304  the system electronics determine, based on vehicle sensor and other inputs, whether the prerequisite conditions for executing a PBS injection are met (for example, determining there is sufficient compressed air in the reservoir  146  to conduct the anticipated compressed air injection while maintaining a sufficient reserve of compressed air to operate essential compressed air consumers on the vehicle, such as pneumatic brake actuators). If the PBS injection conditions are not met (“no”) control is returned to the beginning of the program logic. If the PBS injection prerequisite conditions are met (“yes”) control shifts to step  306 . 
         [0032]    In step  306  the system electronics initiates operation of the mechantronic exhaust brake  134  to position the MEB&#39;s throttle flap to a position which results in generation of increased back pressure upstream in the exhaust line  108 . The rate at which the throttle flap is moved into the desired position and the target angular position of the flap may be determined from vehicle operating parameters in order to match the pressure back pressure level and the timing of the arrival of the back pressure at the EGR line  110  to achieve a desired exhaust gas recirculation mass flow rate at the intake injection point  118  when PBS compressed air injection is initiated. This tailoring of the position, angular velocity and/or acceleration curve of the MEB throttle flap to the projected PBS flow provides an increase in EGR flow at or near exactly the correct timing to highly accurately matched EGR flow to the increased intake air flow arriving at the engine&#39;s cylinders when the PBS injection is initiated. This highly accurate matching helps to maintain a desired minimal level of NOx emissions. The amount of increase in EGR flow may be managed to maintain a desired differential pressure across the EGR line, or by other approaches, such as maintaining a desired differential pressure across the MEB. 
         [0033]    The MEB throttle flap&#39;s position, angular velocity and/or acceleration curve may be determined by any of the associated system electronics, including at the MEB electronics, at the PBS electronics, or in a combination ECU. The throttle flap&#39;s position, angular velocity and/or acceleration curve may be determined by a variety of techniques, including by reference to a look-up table defining flap movement as a function of vehicle operating parameter such as engine rpm, current exhaust gas flow rate in exhaust line  108 , differential pressure between the exhaust line  108  and the intake line  106 , etc. Alternatively, the throttle flap movement may be determined in accordance with calculations implementing flow control equations in the system logic, either in the MEB electronics or elsewhere, based on vehicle sensor signals and/or vehicle component operating states. During the time the MEB is activated, the position of the MEB&#39;s throttle flap may be varied as needed to any intermediate position between fully closed and fully open so as to refine its restriction of exhaust gas flow, and hence the exhaust line backpressure, to provide optimal upstream conditions during a PBS injection event. For example, rather than being held in a fixed partially closed position, the MEB throttle flap may be adaptively opened or closed as necessary to maintain a desired differential pressure across the exhaust gas recirculation path  110  between the exhaust line  108  and the intake line  106 . Alternatively, the MEB throttle flap position may be varied to obtain a desired recirculating exhaust gas mass flow rate or to increase or decrease the EGR mass flow rate to match intake operating parameters. 
         [0034]    Immediately following the signaling in step  306  for the MEB  134  to move its throttle flap to increase exhaust line  108  back pressure, in step  308  a timer is started. In step  310  the timer counts until a desired delay period between the operation of the MEB  134  and the initiation of PBS injection pulses. The desired delay period may be fixed, or may be variable to accommodate different vehicle operating states and/or operating conditions. When the timer has reached the end of the programmed time (“yes” in step  310 ), the control logic advances to step  312 . A typical desired delay period may be on the order of 200-400 milliseconds, but also may be very short, for example, 50 milliseconds. 
         [0035]    The PBS compressed air injection is commanded to be initiated in step  312  following the delay period. Essentially simultaneously, within the PBS module  142  the PBS electronics  150  commands at least one of the compressed air injection flow control valves  144  to open, while intake line backflow prevention valve  148  is closed to prevent backflow of compressed air upstream toward the turbochargers. The backflow prevention valve  148  typically remains closed at least until the increased exhaust gas flow in exhaust line  108  resulting from the PBS injection accelerates the turbocharger compressor wheels  124 ,  126  enough to build sufficient pressure in intake line  106  to “take over” supply of fresh air to the engine from the PBS injection system. 
         [0036]    Following the initiation of the PBS compressed air injection in step  312 , the control logic proceeds along two paths in parallel. 
         [0037]    In the path shown on the left side of the lower portion of  FIG. 3 , in step  314  the system electronics determine whether the conditions for discontinuing PBS injection have been met, for example, reaching the end of the desired duration of compressed air injection, or the identification of a parameter which requires PBS injection terminations such as reaching a compressed air reservoir  146  low pressure limit. If the PBS injection termination conditions have not been met (“no”) the control in this parallel branch repeatedly returns to step  312  until the termination conditions are met (“yes”). 
         [0038]    Once the conditions for deactivating the PBS system to discontinue compressed air injection have been met, the control logic shifts to step  316 , whereby the control electronics command deactivation of the PBS injection. This is followed in step  318  by a determination as to whether the MEB  134  had been deactivated (i.e., the MEB throttle flap has been moved to a position which results in a decrease in throttle flap-generated exhaust backpressure). If the MEB  134  has not been deactivated (“no”) control in this branch repeatedly returns to step  318  until the MEB has been deactivated (“yes”). 
         [0039]    In parallel with the PBS injection deactivation steps, in step  315  the system electronics determines whether the conditions for repositioning the MEB  134  have been met, for example, upon the build-up of sufficient exhaust gas flow in exhaust line  108  as a result of the PBS compressed air injection to build sufficient pressure to drive sufficient exhaust gas recirculation flow into the intake without the flow restriction of the MEB throttle flap. If the MEB deactivation conditions have not been met (“no”) the control in this second parallel branch repeatedly returns to step  315  until the termination conditions are met (“yes”). 
         [0040]    Once the conditions for MEB deactivation have been met, the control logic shifts to step  317 , whereby the control electronics command the MEB throttle flap to move to the next desired position. Next, in step  319  the system electronics check to see whether the PBS injection is still active, i.e., a determination is made as to whether the PBS injection has been deactivated. If the PBS injection system has not been deactivated (“no”), in step  321  the system electronics determines whether the MEB should be reactivated by determining whether, with the PBS injection still ongoing, the conditions for re-activating the MEB are present. If the MEB activation conditions are not present, this branch of the control logic repeatedly returns to step  319  until either the PBS system has been deactivated (“yes” in step  319 ) or the conditions for reactivating the MEB have been met (“yes” in step  321 ). If the conditions for reactivating the MEB are present at step  321 , in step  323  the MEB is activated by operating the throttle flap to increase backpressure in the exhaust line  108 . 
         [0041]    Following reactivation of the MEB the control logic shifts to step  325 , wherein the system electronics determines whether the MEB should again be deactivated. If the MEB deactivation conditions do not exist, the control logic (“no”) repeatedly returns to step  325 . If the system electronics determine the MEB deactivation conditions do exist, the control logic returns to step  317 , whereupon the system electronics again command deactivation of the MEB, and then again assesses whether the PBS injection has been terminated in step  319 . 
         [0042]    Once both the PBS injection and the MEB have been deactivated (“yes” to either step  318  or step  319 ), the control logic returns to the beginning of the control algorithm. 
         [0043]      FIG. 4  provides an example of a typical response of the MEB during the operations illustrated in the  FIG. 3  flow chart. At time T 1  the MEB initiates movement of its throttle flap (step  306 ). Because of the extremely high speed of the mechatronic exhaust valve unit, within approximately 100 milliseconds the throttle flap has reached a position more than 90% closed at time T 2 . After an initial period T 2 -T 3  during which the MEB throttle flap is maintained at its initial partially closed position (for example, on the order of 200-500 milliseconds), at time T 3  the MEB is commanded to reduce the degree of restriction (i.e., degree of MEB closure), in this example to maintain a desired amount of pressure difference across the EGR path  110 . After a reaction time of approximately 100 milliseconds, the throttle flap reaches a second slightly more open position at time T 4  and begins a controlled opening period at a rate of approximately 0.1% of closure per millisecond until reaching a desired degree of opening at time T 5 . The decay rate of the opening is coordinated with the PBS injection to ensure the pressure in the exhaust line  108  remains higher than in intake line  106  in order to maintain a favorable pressure distribution for exhaust gas recirculation over the course of the PBS event. 
         [0044]    Over the course of approximately 250-440 milliseconds between times T 4  and T 5 , the throttle flap reaches the desired degree of restriction (in this embodiment, a degree of opening of approximately 65%), where the flap is held until time T 6 . In coordination with the termination of PBS injection and at a time at which a favorable EGR pressure differential can be self-maintained, the system electronics at time T 6  command the MEB throttle flap to the full open position (step  317 ), which it reaches at time T 7  in approximately 10 milliseconds. 
         [0045]    The position of the MEB throttle flap may be controlled in a manner different than above-described pattern of “closed to steep angle and gradually opened. For example, after the initial closure of the valve, subsequent throttle flap opening position commands may either further close the throttle flap, momentarily open the throttle flap a certain amount then move the throttle flap back in the closed position. Other alternative throttle flap movement patterns may include a ramped or stair-stepped movement from an open position toward a closed position to provide a slower back pressure increase rate, a simple “close-then-open” sequence (i.e., a square-wave pattern), or a closure and opening pattern which follows variations in system input parameters to “follow” variable back-pressure demands during transient engine operating conditions. The MEB throttle flap control patterns may also be adapted to individual engine and/or vehicle configurations as needed. 
         [0046]      FIG. 5  provides an example of a typical operational responses occurring during the PBS injection event, along with illustration of the actuation of the PBS system and the MEB during the operations illustrated in the  FIG. 3  flow chart. 
         [0047]    The first of the four graphs in  FIG. 5  correspond to the MEB throttle flap actuation pattern shown in  FIG. 4 . The bottom-most graph illustrates the PBS system&#39;s compressed air injection pattern. The two center graphs respectively illustrate the exhaust gas pressure immediately upstream of the MEB in the exhaust line  108 , and the exhaust gas pressure at the point of entry of exhaust gas from exhaust line  108  to EGR path  110 . 
         [0048]    As noted in the discussion of  FIGS. 3 and 4 , upon determining in step  302  that the engine will need assistance in meeting the torque demand, at time T 1  the MEB throttle flap is commanded to a first position. After a brief delay (approximately 250-450 milliseconds) to allow build-up of sufficient exhaust gas pressure in the exhaust line  108  upstream of the MEB throttle flap and at the inlet to the EGR passage  110  (corresponding to points T 2   a  and T 2   b  on the second and third  FIG. 5  graphs, respectively), the PBS system at time T 4   a  initiates compressed air injection into the intake line  106 , approximately simultaneously with the beginning of the gradual opening of the MEB throttle flap. 
         [0049]    Due to the influence of the high pressure compressed air injected by the PBS system, the exhaust gas pressure in exhaust line  108  at the entrance to the EGR passage  110  will immediately begin to rise, potentially resulting in an over-pressure condition unless the MEB throttle flap begins to open to increase the exhaust gas flow rate. As shown in the second graph in  FIG. 5 , the exhaust gas pressure at the MEB thus follows the gradual opening of the throttle flap between times T 4  and T 5 . The objective is to maintain a relatively constant exhaust gas pressure gradient at the entrance to the EGR passage  110  by balancing the increased exhaust gas flow from the PBS compressed air injection with the decreasing restriction of the exhaust line by the MEB throttle flap. The relatively constant EGR inlet pressure gradient results in a relatively constant recirculating EGR exhaust gas mass flow rate which is well matched to the intake air mass flow rate from essentially the beginning of the transient engine operation, avoiding EGR-deprived combustion cycles which can generate undesired excessive NO x  emissions. This relatively constant EGR inlet pressure gradient effect is visible in the latter portion of the third  FIG. 5  graph. 
         [0050]    The system electronics at time T 6  command the MEB throttle flap to fully re-open, followed very shortly thereafter at time T 6   a  commanding the PBS compressed air injection control valves to close. By this time, the increased exhaust gas flow has caused the turbocharger compressors to increase speed to the point that the turbochargers are supplying sufficient pressure in intake line  106  to sustain the engine&#39;s increased output in response to the torque demand, and therefore the pressure in the exhaust line  108  at the inlet to the EGR passage  110  remains relatively stable following the termination of PBS injection. 
         [0051]    Alternative embodiments of the present invention may use a variable-geometry turbine on a turbine side of a turbocharger to assist in varying back pressure. Similarly, and exhaust throttling device may be located upstream or downstream of a turbocharger or, in the presence of more than one turbocharger, between turbochargers. 
         [0052]    The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Because such modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.