Abstract:
A method for controlling an exhaust gas recirculation system of an internal combustion engine includes: determining a setpoint power to be output by the internal combustion engine; and limiting an exhaust gas flow conducted through the exhaust gas recirculation system based on the setpoint power.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a control device and a method for controlling exhaust gas recirculation systems in internal combustion engines. 
         [0003]    2. Description of the Related Art 
         [0004]    An internal combustion engine having a catalytic converter for a vehicle is known from Published German Patent Application document DE 10 2008 043 487 A1, whose fresh air charge may be increased via a turbocharger and whose catalytic action may be improved via an exhaust gas recirculation system. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    According to one aspect of the present invention, a method for controlling an exhaust gas recirculation system of an internal combustion engine includes the following steps:
       determining a setpoint power to be output by the internal combustion engine; and   limiting an exhaust gas flow conducted through the exhaust gas recirculation system based on the setpoint power.       
 
         [0008]    The above-mentioned method is based on the consideration that, in the internal combustion engine mentioned at the outset, the turbocharger and the exhaust gas recirculation system basically derive their operating energy from the enthalpy of the exhaust gas of the internal combustion engine. Based on this fundamental consideration, it is recognized as part of the method that an activated exhaust gas recirculation system could deprive the turbocharger of the energy required for its operation, so that the internal combustion engine generates suboptimal output power. 
         [0009]    This problem could be addressed by setpoint value specifications for the exhaust gas recirculation system and for the turbocharger, which are determined in advance under steady-state conditions on the engine test bench under various optimization criteria. The interaction between the exhaust gas recirculation system and the turbocharger would then result automatically when the setting occurs based on the setpoint value specifications. 
         [0010]    However, it is recognized as part of the above-mentioned method that the setpoint value specifications determined under steady-state conditions on the engine test bench would be ineffective with a dynamic behavior of the internal combustion engine, such as that which may be found during transient processes, for example. In this case, both actuators, i.e., the turbocharger and the exhaust gas recirculation system, would mutually influence each other in the internal combustion engine and at a minimum noticeably delay the achievement of a steady state with the internal combustion engine. 
         [0011]    This is where the above-mentioned method comes in since the basic problem remains unchanged. The exhaust gas recirculation system withdraws a portion of the exhaust gas, whose enthalpy would be required to drive the turbocharger. However, if the exhaust gas flow is limited by the exhaust gas recirculation system, in particular during a dynamic behavior of the internal combustion engine, accordingly more enthalpy from the exhaust gas is available for operating the turbocharger. A steady state, in which both the exhaust gas recirculation system and the turbocharger may be operated again with the above-mentioned setpoint value specifications determined on the engine test bench, may be achieved more quickly with the internal combustion engine as a result of the turbocharger now moving more freely. 
         [0012]    In one specific embodiment of the above-mentioned method, the internal combustion engine has a turbocharger, the limitation of the exhaust gas flow through the exhaust gas recirculation system being dependent on a setpoint exhaust gas flow through a turbine of the turbocharger. 
         [0013]    The setpoint exhaust gas flow through the turbine drives it and increases the fresh air charge in a combustion chamber of the internal combustion engine with the aid of a supercharger driven by the turbine. In this way, the required dynamics may be ensured during a charge buildup in the combustion chamber of the internal combustion engine in order to then also set a required exhaust gas recirculation rate with the aid of an improved scavenging gradient, so that, as was already mentioned, a steady state is achieved more quickly. 
         [0014]    In one additional specific embodiment of the above-mentioned method, the setpoint exhaust gas flow through the turbine of the turbocharger is dependent on a power of the turbocharger which is required to generate the setpoint power to be output by the internal combustion engine. The setpoint power to be output by the internal combustion engine may be composed of a sum of partial powers which are influenced by various actuators in the internal combustion engine, such as a throttle valve, for example. As part of the above-mentioned method, it is possible to consider in the limitation of the exhaust gas flow only the partial power which is contributed by the turbocharger to the output of the total power of the internal combustion engine. It is thus ensured that the exhaust gas flow is limited only when this is required by the dynamics of the internal combustion engine. The limitation is then just high enough so that a required charging air pressure may be set for sufficient dynamics. 
         [0015]    In one particular specific embodiment, the above-mentioned method includes the step of calculating the setpoint exhaust gas flow based on the required power of the turbocharger and a temperature of the exhaust gas. This specific embodiment is based on the consideration that the above-mentioned partial power dependent on the exhaust gas flow is a thermodynamic power, and hence an enthalpy flow, which is to be transmitted from the exhaust gas flow to the fresh air flow, taking into account certain losses, with the aid of the turbocharger in the internal combustion engine. At a constant temperature, this enthalpy flow is only dependent on the mass of the exhaust gas arriving at the turbocharger. Proceeding from the fact that the temperature of the exhaust gas changes at most negligibly during the dynamic behavior of the internal combustion engine, the exhaust gas mass flow required at the turbocharger may be calculated directly when the aforementioned thermodynamic setpoint power is adjusted for the temperature of the exhaust gas. If, moreover, the exhaust gas mass flow discharged by the internal combustion engine is known, it is possible, by balancing all mass flows, to calculate the exhaust gas mass flow to be set in the exhaust gas recirculation system which would be needed to ensure the exhaust gas mass flow which is required at the turbine for the partial power which is to be contributed by the turbocharger to the setpoint power to be output by the internal combustion engine. 
         [0016]    As an alternative or in addition, the setpoint exhaust gas flow may be calculated based on one or multiple of the following variables: exhaust gas temperature, ambient temperature, ambient pressure, and pressure of the exhaust gas ( 32 ) downstream from the turbocharger ( 22 ). 
         [0017]    The temperature of the exhaust gas may be determined in any arbitrary manner, for example based on an estimation, a measurement or a predefined value. 
         [0018]    In one further specific embodiment, the internal combustion engine is operated in a lean operating mode. This specific embodiment is based on the consideration that the exhaust gas recirculation system could be used to lower untreated NOx emissions. To achieve a predefined dynamics during acceleration processes of the above-mentioned motor vehicle, it may be useful to transgress a required exhaust gas recirculation rate in the dynamics for a short period to then achieve the required setpoint charge in the internal combustion engine as quickly as possible, in order to then be able to set the exhaust gas recirculation rate to the target operating point. This shortens the transient states, which positively affects the emissions, the fuel consumption and driving dynamics. 
         [0019]    In one preferred specific embodiment, the internal combustion engine may be used to drive a motor vehicle, the setpoint power being dependent on a driver&#39;s input torque for the motor vehicle. 
         [0020]    According to one further aspect of the present invention, a control device is provided which is designed to carry out one of the indicated methods. 
         [0021]    In one specific embodiment of the above-mentioned control device, the device has a memory and a processor. One of the above-mentioned methods is stored in the memory in the form of a computer program, and the processor is provided for carrying out the method when the computer program has been loaded from the memory into the processor. 
         [0022]    According to one further aspect of the present invention, a computer program includes program code means to carry out all steps of one of the above-mentioned methods when the computer program is being executed on a computer or one of the above-mentioned devices. 
         [0023]    According to one further aspect of the present invention, a computer program product includes a program code which is stored on a computer-readable data carrier and which, when the code is being executed on a data processing device, carries out one of the above-mentioned methods. 
         [0024]    According to one further aspect of the present invention, an internal combustion engine having an exhaust gas recirculation system and a turbocharger includes an indicated control device. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]      FIG. 1  shows a schematic illustration of an internal combustion engine. 
           [0026]      FIG. 2  shows a diagram in which a torque is plotted over the time. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0027]    In the figures, elements having identical or comparable functions are denoted by identical reference numerals and are described only once. 
         [0028]    Reference is made to  FIG. 1 , which shows a schematic illustration of an internal combustion engine  2 . 
         [0029]    Internal combustion engine  2  may be designed as a gasoline engine or a diesel engine. Internal combustion engine  2  includes one or multiple cylinders, each having a combustion chamber  4 . Fuel is injected into combustion chamber  4  in accordance with a four-stroke operation to provide a torque  10  for driving a wheel  6  of a motor vehicle which is not shown in greater detail. 
         [0030]    For this purpose, internal combustion engine  2  takes in a fresh air flow  14 , which is regulated with the aid of a throttle valve  16 , via a fresh air filter  12 . Regulated fresh air flow  18  is compressed with the aid of a supercharger  20  of a turbocharger  22  and mixed with an exhaust gas recirculation flow  24  from an exhaust gas recirculation system  26 . Mixed air flow  28  enters combustion chamber  4  and is combusted in a manner known to those skilled in the art. 
         [0031]    The combusted fresh air exits combustion chamber  4  in the form of an exhaust gas flow  30 , a portion of exhaust gas flow  30  being conducted as above-mentioned exhaust gas recirculation flow  24  to compressed regulated fresh air flow  18  in a manner that will be described below. Remaining exhaust gas flow  32  is relaxed via a turbine  34  of turbocharger  22 , turbine  34  taking up the internal energy of remaining exhaust gas flow  32 , i.e., enthalpy  35 , as part of the relaxation and thereby driving supercharger  20  for compressing regulated fresh air flow  18 . Relaxed exhaust gas flow  36  is then discharged into the surroundings as exhaust gas  42  via a lambda probe  38  and a catalytic converter  40 . 
         [0032]    Exhaust gas recirculation system  26  has an exhaust gas recirculation valve  44 , with the aid of which exhaust gas recirculation flow  24  is regulatable. A cooling device  46  is situated downstream in the exhaust gas recirculation system  26  and may be used to cool exhaust gas recirculation flow  24  in a manner known to those skilled in the art for optimal combustion in combustion chamber  4 . 
         [0033]    As mentioned above, the regulation of regulated fresh air flow  14  is carried out with the aid of throttle valve  16 , which is activated by an engine controller  48  designed as an arithmetic unit using an appropriate throttle valve signal  50  based on a driver&#39;s input torque  52 . Driver&#39;s input torque  52  corresponds to a value for torque  10  at which wheel  6  is to rotate. 
         [0034]    Moreover, exhaust gas recirculation valve  44  is activated via an exhaust gas recirculation valve signal  54  based on a lambda signal  56  from lambda probe  38  in a manner known to those skilled in the art to minimize a nitrogen oxide rate in relaxed exhaust gas flow  36 . 
         [0035]    This means that turbocharger  22  and exhaust gas recirculation system  26  assume two tasks in internal combustion engine  2  which are independent of each other. 
         [0036]    However, enthalpy  35  transmitted by turbocharger  22  is dependent on remaining exhaust gas mass flow  32  in a manner known to those skilled in the art. This means that the higher remaining exhaust gas mass flow  32  is, the higher is consequently also enthalpy  35 . Consequently, remaining exhaust gas mass flow  32  is dependent on how high exhaust gas recirculation flow  24  is. During operation of internal combustion engine  2 , exhaust gas recirculation system  26  and turbocharger  22  thus act counter to each other, whereby they could interfere with each other in their operation. 
         [0037]    This problem may generally be addressed in a steady-state operation of internal combustion engine  2  by storing operating points for generating throttle valve signal  50  and exhaust gas recirculation valve signal  54  in engine controller  48 , as part of which the operation of turbocharger  22  and of exhaust gas recirculation system  26  are matched to each other. 
         [0038]    In a dynamic operation of internal combustion engine  2 , i.e., when driver&#39;s input torque  52  and thus also the operating point of internal combustion engine  2  change, turbocharger  22  and exhaust gas recirculation system  26  must first adjust to one of the operating points stored in engine controller  48 . This may take a very long time, depending on the circumstances, due to the two tasks of turbocharger  22  and exhaust gas recirculation system  26  being independent of each other. 
         [0039]    To still address the above-described problem mentioned as part of the dynamic operation of internal combustion engine  2 , internal combustion engine  2  should adjust to a new operating point as quickly as possible. To make this possible, the present embodiment provides for limiting the degree of freedom of internal combustion engine  2  in the short term and for limiting the freedom of one of the two actuators (turbocharger  22  or exhaust gas recirculation system  26 ). Since turbocharger  22  has influence on the adjustment to an operating point, while exhaust gas recirculation system  26  does not, the freedom of exhaust gas recirculation system  26  should be limited. 
         [0040]    To minimize this limitation of the freedom of exhaust gas recirculation system  26  to the extent possible, it is provided as part of the present embodiment to limit exhaust gas recirculation valve signal  54  as a function of enthalpy  35  which is necessary to ensure that internal combustion engine  2  freely adjusts to a new operating point during dynamic operation. For example, this necessary enthalpy  35  may be determined by balancing a setpoint enthalpy for regulated fresh air flow  18  for implementing driver&#39;s input torque  52  and an actual enthalpy of regulated fresh air flow  18 , which is dependent on a throttle valve signal  50 , for example. 
         [0041]    If necessary enthalpy  35  is known, the same may be converted into a required value for remaining exhaust gas flow  32  based on a temperature  58  of remaining exhaust gas flow  32  detected with a temperature sensor  60  or in any other arbitrary manner. If, finally, combusted mass flow  64  which is discharged from combustion chamber  4  is detected via a mass flow sensor  62 , it is possible, once again with the aid of balancing this mass flow  64  with the required value for remaining exhaust gas flow  32 , to determine how high the maximum exhaust gas recirculation flow  24  may be so that required enthalpy  35  may be transmitted to supercharger  20 . 
         [0042]    Engine controller  48  may then accordingly generate exhaust gas recirculation valve signal  54  to limit exhaust gas recirculation flow  24  accordingly to this maximum value using exhaust gas recirculation valve  44 . 
         [0043]    Reference is made to  FIG. 2  which illustrates torque  10  over time  66 . 
         [0044]    The curves shown in  FIG. 2  represent the progression of torque  10  over time  66  following a change in the operating point of internal combustion engine  2 , for example as part of a changed driver&#39;s input torque  52 . 
         [0045]    Curve  68  in the dotted line shows the chronological progression of torque  10  if exhaust gas recirculation flow  24  is not limited to new driver&#39;s input torque  52  as part of the adaptation of torque  10 , while curve  70  in the solid line shows the chronological progression of torque  10  if torque  10  is limited to new driver&#39;s input torque  52  as part of the present embodiment. 
         [0046]    As soon as torque  10  has stabilized again to new driver&#39;s input torque  52 , the limitation of exhaust gas recirculation flow  24  may be lifted.