Patent Publication Number: US-8527185-B2

Title: Energy-based closed-loop control of turbine outlet temperature in a vehicle

Description:
TECHNICAL FIELD 
     The present invention relates to the control of a turbine outlet temperature in a vehicle which uses a turbine to drive an air compressor within an air intake assembly. 
     BACKGROUND 
     Particulate filters are used in vehicle exhaust systems to efficiently capture microscopic particles of soot, ash, metal, and other suspended matter which is generated during the fuel combustion process. However, over time the accumulated particulate matter increases the differential pressure across the filter. In order to extend the useful operating life of the filter and to further optimize engine performance, some particulate filters can be selectively regenerated using heat. Exhaust gas temperature is temporarily elevated by injecting and igniting fuel upstream of the filter above a calibrated light-off temperature. This process is often referred to as post-hydrocarbon injection or HCI. 
     In addition to the particulate filter, various catalysts may be used during the HCI process to further cleanse the exhaust gas. For example, palladium, platinum, or another suitable catalyst can work in conjunction with the regenerative heat to break down accumulated matter in the filter via a simple exothermic oxidation process. Additionally, the vehicle may use an exhaust gas recirculation (EGR) valve to direct a portion of the exhaust gas back into the engine&#39;s cylinders to further reduce vehicle emissions. 
     Within a turbocharged air intake compressor system, a variable geometry turbocharger, turbine, or other suitable device is driven by the exhaust gas that is discharged by the engine. The turbine rotates to drive an air compressor, which feeds the compressed intake air into the engine to boost engine power. Overall vehicle emissions performance is thus largely dependent on the temperature and mass flow of the exhaust gas and intake air at various stages of the combustion and exhaust cleaning processes. 
     SUMMARY 
     A vehicle is disclosed herein that includes a controller which automatically maintains a predetermined temperature at an outlet of the turbine noted above, to thereby control vehicle emissions and particulate filter regeneration. The controller operates in a closed loop using values which are measured with respect to the turbine and a turbine-driven compressor of an air intake assembly. These values are used by the controller to calculate an engine thermal efficiency value, and to adjust the air mass entering the engine and/or the fueling rate at which fuel is injected into the exhaust stream. In this manner, the controller maintains a desired turbine outlet temperature. 
     In particular, a vehicle includes an internal combustion engine, an exhaust system, a turbine, a turbine-driven air compressor, sensors, and a controller. The air compressor is operable for compressing intake air, and for delivering the compressed intake air to the engine. The turbine converts the exhaust gas from the engine into mechanical energy sufficient for powering the air compressor. 
     The sensors include a first sensor for measuring a temperature of the intake air entering the air compressor, a second sensor for measuring a temperature of the exhaust gas exiting the turbine, and a third sensor which measures a mass flow rate of the compressed intake air entering the engine. The controller calculates an engine thermal efficiency value as a function of the temperature and mass flow rate values from the various sensors. The controller uses the engine thermal efficiency value to execute a control action and thereby maintain a temperature of the stream of exhaust gas downstream of the turbine above a calibrated threshold temperature. 
     The controller uses the engine thermal efficiency value to calculate a required adjustment parameter, i.e., a change in a rate of injection of non-torque forming fuel into the injector and/or a change in the mass flow rate of compressed intake air entering the engine. 
     A control system is also disclosed herein for use aboard the vehicle described above. The control system includes the first temperature sensor, the second temperature sensor, and the mass flow sensor. A host machine calculates an engine thermal efficiency value as a function of the inlet temperature, the outlet temperature, and the mass flow rate of the compressed intake air. Thereafter, the host machine uses the engine thermal efficiency value maintains a temperature of the exhaust gas at the outlet of the turbine above a calibrated threshold temperature. 
     A method for maintaining a temperature of the exhaust gas in the vehicle noted above includes measuring the inlet and outlet temperatures of the air compressor and the turbine, respectively, and measuring a mass flow rate of the compressed intake air entering the engine from the compressor. The method additionally includes using a host machine to calculate an engine thermal efficiency value as a function of the inlet temperature, the outlet temperature, and the mass flow rate. The host machine then uses the engine thermal efficiency value to automatically maintain a temperature of the exhaust gas at the outlet of the turbine above a calibrated threshold temperature. 
     The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of a vehicle having an internal combustion engine and a controller adapted for maintaining a desired turbine outlet temperature; and 
         FIG. 2  is a flow chart describing a method for maintaining the desired turbine outlet temperature in the vehicle shown in  FIG. 1 . 
     
    
    
     DESCRIPTION 
     Referring to the drawings, wherein like reference numbers refer to like components, a vehicle  10  is shown in  FIG. 1  having an engine control module or other suitable controller  50 . The controller  50  maybe embodied as a host machine which executes an algorithm  100  programmed or recorded on a computer-readable medium in order to maintain a desired temperature within an exhaust system  20 . Algorithm  100  is explained below with reference to  FIG. 2 . 
     Vehicle  10  includes an internal combustion engine  12 . The engine  12  may be embodied as a multi-cylinder torque generating device which operates in a compression-ignition configuration, although other engine designs may also be used. Torque generated by engine  12  is transmitted to drive wheels through a transmission, with the drive wheels and transmission omitted from  FIG. 1  for simplicity. Engine  12  draws diesel, gasoline, or other suitable fuel  16  from a fuel tank  17 . A stream of exhaust gas  18  is generated as a byproduct of the combustion process occurring within the engine  12 . The exhaust gas  18  passes through the exhaust system  20 , where it is ultimately discharged as purified exhaust gas  118  into the surrounding atmosphere via a tail pipe  25 . 
     The exhaust system  20  includes an air intake manifold  14 , the exhaust manifold  15 , an intake air compressor assembly  22 , and an exhaust after-treatment system  40 . Intake air, which is represented in  FIG. 1  by arrow  11 , is drawn into the engine  12  via the air intake assembly  22 . The exhaust system  20  as a whole is monitored by controller  50 , and is configured to cleanse or purify the exhaust gas  18  before it is ultimately discharged to atmosphere as purified exhaust gas  118 . 
     To that end, after-treatment system  40  may include one or more of an oxidation catalyst  30 , a particulate filter  32 , and a selective catalytic reduction (SCR) device  34 . System  40  further includes a set of fuel injectors  43  in fluid communication with the tank  17  to receive fuel  16 , with the injectors providing post hydrocarbon injection (HCI) of the non-torque generating fuel into the exhaust gas  18  during regeneration of the filter. The order of the various devices within system  40  may vary from the order shown in  FIG. 1  and described above. 
     Particulate filter  32  may be constructed of a suitable substrate constructed of ceramic, metal mesh, pelletized alumina, or any other temperature and application-suitable material(s). As understood in the art, an SCR device such as SCR device  34  converts nitrogen oxide (NOx) gasses into water and nitrogen using an active catalyst. The SCR device  34  may be configured as a ceramic brick or a honeycomb structure, a plate, or any other suitable catalyst design. 
     Still referring to  FIG. 1 , air intake compressor assembly  22  includes an air compressor  36 , an aftercooler  37  for cooling the compressed air, and a turbine  38 , e.g., a variable position turbocharger (VGT) device as noted above according to one possible embodiment. The controller  50  may selectively adjust an angular position of such a VGT, e.g., by adjusting the position of its vanes. Turbine  38  is driven by exhaust gas  18 , and thus rotates a compressor input member  39  to thereby energize or drive the air compressor  36 . The vehicle  10  may also include an exhaust gas recirculation or EGR valve  41 , which likewise can be controlled as needed to selectively direct a portion of the exhaust gas  18  back into the intake manifold  14  as needed. 
     Physical sensors includes a mass flow sensor  42  positioned at the outlet side of air intake compressor assembly  22 , and a pair of temperature sensors  44  and  46 . Temperature sensor  44  is positioned to measure the temperature of the exhaust gas  18  as it enters the turbine  38 . The temperature sensor  46  measures the temperature of the exhaust gas  18  as it exits the turbine  38 . A bank of other sensors  48  are used to measure the manifold pressure, air temperature, and mass flow of intake air (arrow  11 ) as the intake air enters the air compressor  36 . A sensed or modeled value for the temperature entering the turbine  38  may be used by the controller  50 , e.g., for protecting the turbine. However, for the present control system, the inlet and outlet temperatures of the air intake compressor assembly form the boundary conditions as noted below. 
     Mass flow sensor  42  generates a mass flow rate signal  21 , temperature sensor  44  generates a temperature signal  19 , and temperature sensor  46  generates a signal  23 . Signals  19 ,  21 , and  23  are relayed to the controller  50  for use in regulating the actual turbine outlet temperature via a set of control and feedback signals  60 . The controller  50  uses the signals  19 ,  21 , and  23 , as well as a set of signals  27  from the bank of sensors  48  as needed, in calculating an engine thermal efficiency value for engine  12 . Controller  50  also uses the engine thermal efficiency value to execute one or more control actions, thereby maintaining the outlet temperature of turbine  38  above a calibrated threshold. The threshold should sufficiently exceed the calibrated light-off temperature noted above, which may be otherwise difficult to achieve with a modern lean-burn engine. 
     In one embodiment, the turbine  38  may be configured as a turbocharger device having multiple vanes, each with a variable geometry or turbine angle as indicated by arrow  28  in  FIG. 1 . Controller  50  can control the angle of the vanes using signals  60  to thereby direct exhaust gas  18  onto the blades of the turbine at a specific angle, e.g., by energizing an actuator which moves the vanes, as is well understood in the art. At low engine speeds, for example, the vanes may be at least partially closed to reduce lag. At higher engine speeds the vanes may be fully opened. The turbine  38  thus converts exhaust gas  18  into mechanical energy suitable for driving the air compressor  36 , and helps to regulate the volume and rate of air (arrow  11 ) being compressed and fed into the engine  12 . 
     Controller  50  may be configured as an engine control module or a host machine programmed with or having access to algorithm  100 . The controller  50  may be configured as a digital computer acting as a vehicle controller, and/or as a proportional-integral-derivative (PID) controller device having a microprocessor or central processing unit (CPU), read-only memory (ROM), random access memory (RAM), electrically-erasable programmable read only memory (EEPROM), a high-speed clock, analog-to-digital (A/D) and/or digital-to-analog (D/A) circuitry, and any required input/output circuitry and associated devices, as well as any required signal conditioning and/or signal buffering circuitry. A standard PID controller can be used having gains determined as a function of exhaust flow and temperature, and/or speed and load, in order to control the desired temperature with the desired response time. 
     During regeneration of particulate filter  32 , the temperature within the oxidation catalyst  30  is maintained by controller  50  above the calibrated light-off temperature. The controller  50  also maintains a desired temperature in the particulate filter  32 . This is done in order to ensure the accuracy of open-loop fueling quantities provided during the post HCI process. Calibrated or desired temperatures and other values such as the lower heating value (LHV) of exhaust gas  18  may be stored for reference by controller  50  in a lookup table  80 . Additionally, the conversion efficiency of the SCR device  34  is highly dependent on the temperature in the oxidation catalyst  30  and the SCR device. Execution of warm up and/or temperature maintenance modes to pre-heat SCR device  34  and/or the oxidation catalyst  30  may be required in order to attain the desired temperatures for regeneration of the particulate filter  32 . 
     Desired temperatures in the oxidation catalyst  30  may be achieved by throttling intake air  11  and by additional early post injection of hydrocarbons into the exhaust gas  18 . Due to part-to-part variation of certain engine components, namely the mass air flow sensor  42 , it may be common to see a variation in a desired vs. an actual temperature. At partial load conditions, a drop off may occur below the light-off temperature which forces an interruption of late post injection or HCI process, thereby extending regeneration times and reducing overall fuel economy. The conversion of the SCR device  34  may be limited in such a situation. 
     Therefore, the controller  50  shown in  FIG. 1  is adapted to execute algorithm  100  of  FIG. 2  in order to provide a solution for a closed-loop system around the measurements of temperature sensor  46 . The solution adjusts intake air flow into the engine  12 , and/or adjusts the volume or rate of fuel used in the early post injection/HCI process in order to attain and maintain a desired turbine outlet temperature. In the event temperature sensor  46  is located a substantial distance away from the turbine outlet, information from lookup table  80  can be utilized to determine a desired temperature of the exhaust gas  18  exiting the turbine  38 . 
     Referring to  FIG. 2 , algorithm  100  begins with step  102 . Certain operating conditions may exist in lean-burn internal combustion engines, e.g., the engine  12  shown in  FIG. 1 , which may result in relatively low exhaust gas temperatures. This may cause some vehicles to operate continuously below the calibrated catalyst light-off temperature during certain vehicle drive cycles. The controller  50  solves this problem by executing algorithm  100 , which ultimately results in adjustment by the controller of the air mass flow and/or early HCI rates or volumes. Algorithm  100  may be used to control exhaust temperatures at any point in the vehicle  10 , e.g., at an inlet to the SCR device  34 . 
     Beginning with step  102 , the controller  50  calculates the thermal efficiency of the engine  12  as follows: 
               η   ⁢           ⁢   th     =     1   -         (       m   .       gas   ,   out       )     ⁢     (     T     gas   ,   out       )     ⁢     (     cp     gas   ,   out       )             (       m   .       gas   ,   in       )     ⁢     (     T     gas   ,   in       )     ⁢     (     cp     gas   ,   in       )       +       (       m   .     fuel     )     ⁢     (     L   ⁢           ⁢   H   ⁢           ⁢     V   fuel       )                   
In this equation, {dot over (m)} represents the mass flow rate of the fluid indicated in the subscript, e.g., of the exhaust gas  18 , at the inlet ({dot over (m)} gas, in ) and outlet ({dot over (m)} gas, out ) of the air compressor  36  and turbine  38 , respectively, or of the fuel  16  ({dot over (m)} fuel ). LHV is the lower heating value of the fuel  16  as noted above.
 
     At step  104 , with the calculated engine thermal efficiency value (ηth) from step  102  describing the current operating point, controller  50  substitutes the value for T gas,out , i.e., the turbine outlet temperature, with T gas,out,des , i.e., a calibrated or desired outlet temperature from turbine  38 . The controller  50  then solves for the intake air mass flow {dot over (m)} gas,in , i.e., into the compressor  36 , which now becomes the desired air mass flow. {dot over (m)} gas,in,des , {dot over (m)} gas,out,des  in order to achieve the desired temperature exiting the turbine  38 . 
     That is: 
                 m   .       gas   ,   in   ,   des       =     [             (       m   .       gas   ,   out       )     ⁢     (     T     gas   ,   out   ,   des       )     ⁢     (     cp     gas   ,   out       )         1   -     η   ⁢           ⁢   th         -       (       m   .     fuel     )     ⁢     (     L   ⁢           ⁢   H   ⁢           ⁢     V   fuel       )             (     T     gas   ,   in       )     ⁢     (     cp     gas   ,   in       )         ]           
The desired adjustment to air mass setpoint is then equal to: {dot over (m)} gas,in −{dot over (m)} gas,in,des . Such a setpoint is referred to herein as a required adjustment parameter.
 
     At step  106 , the controller  50  can also solve for {dot over (m)} fuel,des  in exactly the same fashion to determine the amount of additional early post injection quantity to achieve the desired temperature. 
     
       
         
           
             
               
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     At step  108 , the controller  50  executes a suitable control action using the values calculated in the preceding steps. For example, controller  50  may adjust both the mass flow of the air (arrow  11 ) entering the air compressor  36  shown in  FIG. 1 , and the post injection quantity from the injectors  43  of the same figure, in order to achieve a desired temperature out of the turbine  38  without the use of calibration maps. The determination of which method and/or quantities can be decided beforehand during calibration. For instance, the adjustment to a post injection quantity of fuel  16  into the exhaust gas  18  may occur only after a maximum allowed adjustment to air quantity is first made, or a split may be made between the two. 
     While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.