Abstract:
A method of setting actuator position of a variable geometry turbine linked to drive a compressor for a compression ignition engine using exhaust gas recirculation. The method includes detecting an engine transient event; determining current mass air flow through the compressor and exhaust temperature; resetting variable geometry turbine position to maximize mass air flow through the compressor; adding the maximum allowable quantity of fuel; determining exhaust temperature increase; adjusting exhaust pressure to allow an increase in mass air flow and exhaust temperature; and returning to the resetting step until a limit is reached.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The technical field relates generally to turbo-charged compression ignition engines having exhaust gas recirculation and, more particularly, to methods of controlling a variable geometry exhaust turbine in the turbo-charger sub-system. 
         [0003]    2. Description of the Technical Field 
         [0004]    Exhaust turbines have long been used to recover energy from high pressure exhaust gas produced by most internal combustion engines. One use of the recovered energy is to drive a compressor/supercharger on the intake side of the engine and thereby increase the mass of the air charge delivered to engine combustion chambers. This is referred to as turbo-supercharging, or more commonly just turbo-charging. The use of turbo-charging to increase the mass of air delivered to the engine allows more fuel to be burned per combustion stroke thereby increasing the work done with each combustion stroke. The system is a self reinforcing loop as the engine serves as the pump which drives the exhaust turbine. 
         [0005]    Variable geometry turbines (VGT) are used with turbo-charged engines to reduce engine pumping losses. U.S. Pat. No. 6,067,800 describes use of a VGT for improving torque response, fuel economy and emission levels. 
         [0006]    Exhaust gas recirculation (EGR) is commonly employed on compression ignition engines to reduce nitrous oxide emissions. An EGR valve connects the exhaust manifold of an engine to its intake manifold. Exhaust gas displaces fresh air in the combustion cylinders and functions as an inert gas in the cylinder reducing cylinder temperature during combustion and thus reducing the formation of nitrous oxides. The concurrent use of EGR and turbo-charging with an engine complicates control over the engine. In a typical engine control system where both turbo-charging and EGR are present, a VGT has been used to control mass airflow (MAF) in the intake manifold and the EGR valve has been used to control emissions. 
       SUMMARY 
       [0007]    A method of setting actuator position of a variable geometry turbine linked to drive a compressor that drives air into an intake manifold for a compression ignition engine is described. The method allows for exhaust gas recirculation from an exhaust manifold to the intake manifold. The method comprises the steps of: detecting an engine transient event from steady state conditions; determining current mass air flow through the compressor and exhaust temperature; resetting variable geometry turbine position to maximize mass air flow through the compressor and minimize pumping losses across the engine; adding the maximum allowable quantity of fuel; determining exhaust temperature increase; adjusting exhaust pressure to allow an increase in mass air flow and exhaust temperature; and returning to the resetting step until a limit is reached. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic view of a compression engine system having exhaust gas recirculation and a turbo-charging sub-system incorporating a variable geometry turbine. 
           [0009]      FIG. 2  is a feedback control diagram for obtaining variable geometry turbine position. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. 
         [0011]    Referring now to the figures, and in particular to  FIG. 1 , there is shown a simplified schematic diagram of a compression ignition engine system  10  equipped with an exhaust gas recirculation (EGR) system  12  and a variable geometry turbocharger (VGT)  14 . A representative engine block  16  is shown having four combustion chambers  18 . Each of the combustion chambers  18  includes a direct-injection fuel injector  20 . The duty cycle of the fuel injectors  20  is determined by the engine control unit (ECU)  24  and transmitted along signal line  22 . Air enters the combustion chambers  18  through the intake manifold  26  and combustion gases are exhausted through the exhaust manifold  28  in the direction of arrow  30 . 
         [0012]    To reduce the level of NOx emissions, the engine is equipped with an EGR sub-system  12 . The EGR sub-system  12  comprises a conduit  32  connecting the exhaust manifold  28  to the intake manifold  26 . This allows a portion of the exhaust gases to be circulated from the exhaust manifold  28  to the intake manifold  26  in the direction of arrow  31 . An EGR valve  34  regulates the amount of exhaust gas recirculated from the exhaust manifold  28 . The recirculated exhaust gas acts as an inert gas in the combustion chambers  18 . This lowers the flame and in-cylinder gas temperature with a consequential reduction in the formation of NOx compounds. The recirculated exhaust gas also displaces fresh air and reduces the air-to-fuel ratio of the in-cylinder mixture. During operation of engine system  10  the exhaust manifold back pressure (EXMP) in the exhaust manifold  28  is kept high enough to drive exhaust gas through EGR valve  34  into the intake manifold  26 . 
         [0013]    The turbocharger  14  recovers exhaust gas energy to increase the mass of the air charge delivered to the engine combustion chambers  18 . The exhaust gas flowing in the direction of arrow  30  drives the turbo-charger  14 . This larger mass of air can be burned with a larger quantity of fuel, resulting in more torque and power as compared to naturally aspirated, non-turbocharged engines. 
         [0014]    The turbo-charger  14  includes a compressor/supercharger  36  and an exhaust turbine  38  coupled to one another by a common shaft  40 . Exhaust gas  30  drives the exhaust turbine  38  which, in turn, drives the compressor  36  which, in turn, compresses ambient air  42  and directs it (arrow  43 ) into an intercooler  52 . Intercooler  52  extracts heat from the compressed ambient air thereby increasing its density and reducing pressure at the outlet from the compressor  36 . Air is discharged from the intercooler  52  into the intake manifold  26 . The VGT  14  can be configured during engine operation to vary the turbine flow area and the angle at which the exhaust gas  30  is directed at the turbine  38  blades. This is accomplished by changing the angle of the inlet guide vanes  44  on the turbine  38 . 
         [0015]    Operation of all of the engine sub-systems, including the EGR  12 , VGT  14  and fuel injectors  20  are controlled by the ECU  24 . For example, signals on signal line  46  from the ECU  24  regulates the EGR valve  44  position, and a signal on signal line  48  regulates the position of the VGT guide vanes  44 . 
         [0016]    Commands on signal lines  46 ,  48  are calculated from measured engine operating variables and operator inputs by means of a control algorithm. Sensors provide the ECU  24  with engine operating information. For example, an intake manifold pressure (MAP) sensor  50  provides a signal to the ECU  24  indicative of the pressure in the intake manifold  26 . Likewise, exhaust manifold pressure (EXMP) sensor  54  provides a signal to the ECU  24  indicative of the pressure in the exhaust manifold  28 . An intake manifold temperature sensor  58  provides a signal to the ECU  24  indicative of the intake air temperature. A mass airflow (MAF) sensor  64  also provides a signal indicative of the compressor mass airflow to the ECU  24 . Additional sensory inputs are also received by the ECU  24  along signal line  62  such as engine coolant temperature, engine speed, and EGR valve position. Operator inputs  68  applied to the ECU  24  include accelerator pedal position or other fueling request inputs. 
         [0017]    The engine control methods described herein apply to all turbocharged compression ignition engines equipped with EGR systems, regardless of the type of fuel used. While operation is described with reference to a diesel engine, the methods are also applicable to other compression ignition engines as well. 
         [0018]    More detailed consideration of ECU  24  management of the engine system  10  is now given. ECU  24  provides control signal levels for opening, closing and progressively restricting or freeing gas flow through EGR valve  34 . ECU  24  provides a control signal for positioning vanes  44  on the VGT  14  inlet stator. Values for the position signals for the EGR valve  24  and the VGT  28  vanes are calculated with reference to the instantaneous load on engine  16 . Data which relate to engine load under steady state conditions include fuel flow, which is calculated by ECU  24 , operator inputs  68  such as accelerator pedal position, vehicle speed and engine speed (rpm) data. These data have conventionally been used to interrogate a look up table (LUT)  60  for values for the EGR valve  34  and VGT  14  vane  44  position signals which are selected to deliver EXMP setpoints and optimized Brake Specific Fuel Consumption (BSFC). 
         [0019]    In contrast to steady state conditions, placing engine  22  under transient operating conditions, particularly transient conditions involving an increase in power demanded, the practice has been to position VGT  14  vanes  44  to maximize EXMP thereby increasing pressure ratio across the turbine  38  to deliver more power to the compressor  36 . Increased EXMP also increases engine system  10  pumping losses. If VGT  14  movement makes the turbine  28  operate in an inefficient region, a decline in MAF can result. Falling MAF results in low fuel flow and lower torque. Available power for acceleration from engine system  10  can decline markedly. 
         [0020]    EXMP may be controlled during engine system  10  transient conditions to increase MAF while still minimizing engine pumping losses. As a starting point it may be initially assumed that mass flow rate through the EGR valve  34  is constant and a VGT  14  vane  44  position may be chosen that will maximize engine  16  work over a given time period. Available work is maximized by burning as much fuel as possible with each combustion stroke. If more fuel is burned at a constant emission level with a given mass flow rate through the EGR valve  34 , VGT  14  vane  44  position is chosen to maximize MAF transported through the engine block  16  each combustion cycle. Sensors to supply for some variables, such as enthalpy of the exhaust, are provided including: an intake manifold pressure sensor  50  (“MAP”); an exhaust manifold pressure sensor  54  (EXMP or Pin); and an exhaust manifold temperature sensor  56  (Tin). 
         [0021]    The following equation gives turbine power, Power(turb): 
         [0000]      Power(turb)=mdot* Cp *Tin*eta*( 1 −(Pout/Pin)̂(gamma− 1 /gamma))   (Eq. 1)
 
         [0000]    where, 
         [0022]    mdot=mass flow through the turbine 
         [0023]    Cp=heat capacity at constant pressure 
         [0024]    Tin=exhaust manifold temperature (temperature at the turbine inlet) 
         [0025]    Pout=Turbine outlet pressure 
         [0026]    Pin=Exhaust manifold pressure (pressure at turbine inlet) 
         [0027]    Gamma=Ration of specific heats 
         [0028]    Eta=Total or static efficiency of the turbine 
         [0029]    Mass storage in the engine block  16  combustion chambers  18  is disregarded. The equation shows how turbine work can be maximized when MAF is large and when exhaust temperature is large. Accommodating mass storage in the engine block  16  should not change the basic result. 
         [0030]    Assuming turbine  38  recovered energy equals compressor  36  work, maximum compressor work is proportional to faster spinning of the compressor which in turn leads to faster air flow rate. However, current MAF and exhaust temperature are the result of past operation of engine system  10 . VGT  14  vane  44  position is chosen so that exhaust manifold pressure allows: 
         [0031]    Turbine power to be maximized given the current state of MAF, Tin 
         [0032]    (a) so that MAF is as large as possible a short time in the future; 
         [0033]    (b) allowing more fuel to be added to that MAF; 
         [0034]    (c) producing an increase in exhaust temperature due to higher fuel burning; 
         [0035]    (d) which allows Pin to be adjusted so that MAF and Tin are increased and the process continues. 
         [0036]    MAF, Cp, Tin fall out of the optimization and act as inputs to the optimization process. The problem simplifies to one of choosing Pin for a given MAF and Tin at each time Eq. 1 is maximized. The quantity has an effective maximum since the value within the parenthesis in Eq. 1 increases monotonically with Pin to a limit. 
         [0037]    Power output has an upper limit because the value in brackets in Eq.  1  increases monotonically with Pin and asymptotically converges to  1 . The efficiency of the turbine which is a function of pressure ratio, MAF and Tin reaches a maximum and then decreases. This multiplied by the term in brackets should give a single global maximum that applies until the MAF or turbine speed change sufficiently. For a given MAF through the turbine and exhaust temperature there should exist a Pin where turbine operates more efficiently delivering higher power. In other words, VGT  14  inlet guide vane  44  position can be changed from its prior steady state location upon entry to transient conditions to increase efficiency where there is power demand. 
         [0038]    Though the turbine could operate efficiently with higher pressure ratio across it, this does not imply that engine brake torque is high. With a high EXMP and lower intake manifold  26  air pressure (MAP) there exists a point where increasing engine system  10  pumping losses negates all the excess air that is generated by operating the turbine efficiently. All of the energy obtained by burning the fuel with the excess air is lost compensating the pumping loss across the engine. With Pin closing on MAP, EGR transport from the exhaust manifold to the intake manifold declines and as a result higher torque is obtained at the cost of increasing NOX. The EXMP set point should be selected to provide minimum EGR driving capability and at the same time lower pumping losses. Pumping is typically a function of EXMP and MAP. Hence during the process of selecting exhaust back pressure, current MAP should be taken into consideration to minimize pumping losses. 
         [0039]    The strategy is to take the product of MAF and exhaust temperature (which is the enthalpy) as one of the input and intake manifold pressure as the other input for determining a optimal EXMP that minimizes pumping losses and maximizes MAF. Referring to  FIG. 2 , with a change in engine load, produced by changes in EGR and fuel injection control signals  46 ,  22 , current MAP and enthalpy are used to select an EXMP setpoint, here referred to as the EXMPtarget (module  51 ). Measured EXMP is then subtracted (module  53 ) from EXMPtarget to obtain an error value (ERR) which is applied to a proportional-integral-derivative control module  55  to generate an adjustment in VGT  14  vane  44  position by varying its input control signal (VGTpos). VGTpos is applied to the compression engine system  10 , or more precisely, an actuator associated with VGT  14 .