Abstract:
A method and apparatus for operating an internal combustion engine, in particular for commercial vehicles, having a fuel/air feed device and a downstream exhaust system, wherein, to achieve improved efficiency, the exhaust gas enthalpy in the exhaust gas flow of the internal combustion engine is used to operate a heat engine, in particular a Stirling engine, which produces mechanical energy.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of DE 10 2013 009 219.1 filed May 31, 2013 which is incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for operating an internal combustion engine, to an apparatus for carrying out the method and to an internal combustion engine suitable for the method. 
         [0003]    Internal combustion engines, particularly in motor vehicles or commercial vehicles, are often designed as diesel engines for reasons of efficiency and, to meet environmental regulations, have exhaust gas aftertreatment devices, such as SCR catalytic converters and/or exhaust gas recirculation devices. In this case, it is not possible to make optimum use of the exhaust gas enthalpy contained in the exhaust gas of the internal combustion engine. Moreover, high exhaust gas recirculation rates impose a load on the cooling circuit of the internal combustion engine, which are generally liquid-cooled internal combustion engines, due to the required re-cooling of the exhaust gases by means of appropriate exhaust gas coolers, if appropriate in combination with additional low-temperature circuits. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    It is the object of the invention to specify a method for operating an internal combustion engine that improves exhaust gas enthalpy and raises the efficiency of the internal combustion engine. It is another object of the invention to provide an apparatus for carrying out the method as well as an internal combustion engine suitable for the method, in particular for commercial vehicles. 
         [0005]    According to the invention, it is proposed that the exhaust gas enthalpy in the exhaust gas flow of the internal combustion engine is used to operate a heat engine, in particular a Stirling engine, which produces mechanical energy. 
         [0006]    In this way, it is possible essentially to achieve two advantages, namely, on the one hand, use of the waste heat contained in the exhaust gas and, on the other hand, lowering of the exhaust gas temperatures downstream of the point of removal of the heat, which may improve the thermal economy of the internal combustion engine, making it possible, for example, to dispense with a low-temperature circuit. 
         [0007]    The use of a Stirling engine (also referred to as a hot gas engine) as a heat engine, which can be of any construction known per se with a closed circuit and which converts heat into mechanical energy, is particularly advantageous. For the sake of simplicity, reference will always be made below to a Stirling engine as a preferred heat engine. However, it is self-evident that the term “Stirling engine” is then to be understood here expressly in a broad sense and is in each case representative of any heat engine suitable for carrying out the method according to the invention in which a working medium, in particular a working gas (e.g. air, helium or hydrogen), is heated and cooled in order to perform mechanical work. 
         [0008]    The Stirling engine can be used to drive an electric machine (preferably a generator) and thus to produce electric current; however, the machine can also be connected as a starter, if appropriate. 
         [0009]    As an alternative or in addition, the drive shaft of the heat engine can be coupled to the crankshaft of the internal combustion engine and can thus feed in a superimposed drive torque, particularly in part-load and/or full-load operation of the internal combustion engine. In this case, the speed of the drive shaft of the Stirling engine can be adapted by means of a speed governor to the instantaneous speed of the crankshaft of the internal combustion engine for optimum efficiency. 
         [0010]    The exhaust gas enthalpy in the exhaust gas flow of the internal combustion engine is preferably transferred as driving heat flow Q ZU  to the heat engine, either indirectly, in particular via at least one interposed intermediate heat exchanger, or directly. In an advantageous development thereof, at least one heat pipe, preferably a plurality of heat pipes, can be inserted into an exhaust line of the exhaust system of the internal combustion engine (for reasons of clarity, the plural “heat pipes” will always be used below by way of example), supplying the Stirling engine, preferably a heat exchanger or heat exchange region assigned to the working piston of the Stirling engine, with the driving heat flow Q ZU  and hence with driving energy, either indirectly, in particular via at least one interposed intermediate heat exchanger, or directly. This makes possible heat transfer to the Stirling engine in a manner that is well adapted in terms of construction and is efficient. As an alternative, however, it is also possible for there to be an exhaust line branching off, the heat flow of which flows through the Stirling engine or a heat exchanger or heat exchange region of the Stirling engine. 
         [0011]    In the case of an internal combustion engine having an exhaust gas recirculation device, a particularly preferred possibility is for the heat pipes to be arranged upstream of the exhaust gas branch point for the exhaust gas recirculation device, with the result that the associated drop in temperature of the exhaust gas means that less exhaust gas cooling is required, which may even eliminate the need for an exhaust gas cooler in the recirculation line, 
         [0012]    Moreover, in the case of a catalytic exhaust gas aftertreatment device in the exhaust system of the internal combustion engine, the heat pipes can be provided upstream of the aftertreatment device, in particular in order in this way to be able to divert a larger amount of driving energy or a higher driving heat flow to the Stirling engine when required. If appropriate, temperature control of the exhaust gas (e.g. by means of bypass lines) could also be provided in the exhaust system in order to bypass the heat pipes and preferentially supply the aftertreatment device in the case of low exhaust gas temperatures (cold starting), for example. 
         [0013]    A particularly preferred apparatus for carrying out the method on an internal combustion engine having a fuel/air feed device and a downstream exhaust system consists in that the exhaust system is coupled for heat transfer, via at least one heat pipe, in particular via a plurality of heat pipes integrated into said system, to a heat engine or a Stirling engine, which produces electrical and/or mechanical energy, In this case, the Stirling engine can drive an electric machine and/or the drive shaft of said Stirling engine can be coupled to the crankshaft of the internal combustion engine. 
         [0014]    The heat sink of the Stirling engine can furthermore preferably be cooled by relative wind of the motor vehicle and/or be connected via a heat exchanger to the liquid cooling system of the internal combustion engine. 
         [0015]    In another embodiment of the invention it is possible, in the case of an internal combustion engine with an exhaust gas aftertreatment device and/or an exhaust gas recirculation device, for the heat pipes for the Stirling engine to be integrated into an exhaust casing of the exhaust system, which is positioned upstream of the exhaust gas aftertreatment device and/or of the exhaust gas recirculation discharge line. Here, the exhaust gas aftertreatment device can be formed by an SCR catalytic converter. 
         [0016]    Finally, an internal combustion engine, in particular for commercial vehicles, having the features presented above, and a vehicle, in particular a commercial vehicle, are preferably proposed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    An illustrative embodiment of the invention is explained in greater detail below by means of the attached, roughly schematic drawing, 
           [0018]      FIG. 1  is a block diagram of an internal combustion engine with an apparatus according to an embodiment of the claimed invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0019]    In  FIG. 1 , a multi-cylinder reciprocating piston internal combustion engine  1  such as a diesel engine for a commercial vehicle which, where not described, can be of conventional construction has a crankshaft  2  and the output from the crankshaft  2  drives the other driving devices in the motor vehicle. 
         [0020]    The internal combustion engine  1  has a fuel/air feed device, merely indicated, having an intake manifold  3  and an exhaust system  4  carrying the exhaust gases away, which, where not described, is of known construction. If appropriate, an exhaust gas turbocharging system having at least one exhaust turbine could also be provided in the exhaust system  4 , A Stirling engine  6  is coupled to the internal combustion engine  1  or to the crankshaft  2  thereof via a clutch and a speed governor  5 , for example, the drive shaft  7  of said Stirling engine on the one hand being connected in terms of drive to the crankshaft  2  and on the other hand driving an electric machine  8 , e.g. a three-phase generator. 
         [0021]    In the illustrative embodiment, the Stirling engine  6  is designed as an alpha type, for example, having a working piston  9  and a displacer piston  10 , the cylinders (not shown) of which, which are offset by 90° to one another, correspond to one another in a known manner in a gas circuit closed to a greater or lesser extent, wherein the cylinder of the displacer piston  10  is cooled in a known manner, being embodied as a heat sink (Q AB ) (merely indicated by the arrow  11 ), while the cylinder of the working piston  9 , or a heat exchanger  22  connected thereto, is heated by supplying heat (Q ZU ). However, the Stirling engine  6  could also be embodied in a known manner as a beta type with coaxial cylinders or as a gamma type. 
         [0022]    Cooling Q AB  is accomplished either by relative wind, e.g. by arranging cooling ribs on the cylinder of the displacer piston  10 , and/or by liquid cooling, with the cylinder  10  or a corresponding heat exchanger being connected to the cooling liquid system (not shown) of the internal combustion engine  1 . 
         [0023]    To heat the cylinder of the working piston  9  of the Stirling engine  6  (arrow  12 ), a plurality of heat pipes  15  is inserted into the exhaust system  4  or into an exhaust line  13  coming out of the combustion chambers of the internal combustion engine  1  in an exhaust casing  14 , said pipes transferring the exhaust gas enthalpy or waste heat of the exhaust gas to the Stirling engine  6 , either directly or indirectly, e.g., by means of an intermediate heat exchanger  21 , 
         [0024]    A diversion point or exhaust gas recirculation valve  16  of an exhaust gas recirculation device (not shown specifically) of the internal combustion engine  1 , followed in the direction of flow by an exhaust gas aftertreatment device having an SCR catalytic converter  17  (selective catalytic reduction), is arranged in the exhaust system  4  downstream of the exhaust casing  14  with the heat pipes  15 . 
         [0025]    Exhaust gas can be recirculated to the intake manifold  3  of the internal combustion engine  1  at defined rates via the exhaust gas recirculation valve  16  and an exhaust gas recirculation line  18 , If appropriate, an exhaust gas cooler connected to the liquid cooling system of the internal combustion engine  1  can be provided in the exhaust gas recirculation line  18 . 
         [0026]    Given the arrangement of appropriate clutches (not shown) on the drive shaft  7 , the Stirling engine  6  can be used to supply the driving energy produced by thermal coupling to the crankshaft  2  and/or to the electric machine  8  for producing electric power and into the on-board electrical system of a motor vehicle. If appropriate, the machine  8  can also be connected as an electric motor and serve as a starter motor or can supply additional driving energy in a boost mode. 
         [0027]    If appropriate, means, such as a controllable bypass line  19  (indicated in broken lines), which effect bypassing of the heat pipes  15 , can be provided in the exhaust system  4  or exhaust line  13 . It is thereby possible to achieve a rise in the temperature for the SCR catalytic converter  17  with a higher priority when cold starting the internal combustion engine  1 , for example. 
       LIST OF REFERENCE SIGNS 
       [0000]    
       
           1  internal combustion engine 
           2  crankshaft 
           3  intake manifold 
           4  exhaust system 
           5  speed governor 
           6  Stirling engine 
           7  drive shaft 
           8  electric machine 
           9  working piston 
           10  displacer piston 
           11  heat dissipation Q AB  (heat sink of the Stirling engine) 
           12  heat supply Q ZU  (heat source of the Stirling engine) 
           13  exhaust line 
           14  exhaust casing 
           15  heat pipes 
           16  exhaust gas recirculation valve 
           17  SCR catalytic converter 
           18  exhaust gas recirculation line 
           19  bypass line