Abstract:
The invention relates to a device and a method for the recovery of waste heat from an internal combustion engine ( 2 ), according to which a feed pump ( 6 ), a heat exchanger ( 8 ), an expansion engine ( 10 ) and a capacitor ( 12 ) are arranged in a circuit ( 4 ) containing a circulating working medium. A bypass connection ( 14 ) is mounted in parallel to the expansion engine ( 10 ), in the circuit ( 4 ), the expansion engine ( 10 ) being coupled to the circuit ( 4 ), or decoupled therefrom, according to an operating situation of the internal combustion engine ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a device and a method for waste-heat utilization. 
     DE 10 2006 057 247 A1 has already disclosed a supercharging device which serves for utilizing the waste heat of an internal combustion engine. At least one heat exchanger of a circuit of a working medium is accommodated in the exhaust tract of the internal combustion engine. A turbine part and a delivery assembly are also arranged in the circuit. A compressor part arranged in the intake tract of the internal combustion engine is driven by means of the turbine part. 
     SUMMARY OF THE INVENTION 
     The device according to the invention for utilizing the waste heat of an internal combustion engine and the associated method according to the invention having the features of the independent claims has the advantage that vaporous working medium is conducted past the expansion machine through a bypass connection which is connected in parallel with the expansion machine. As a result of the control by means of a bypass pressure regulating valve, it is possible to manipulate the thermodynamic process for waste-heat utilization in a targeted manner. Depending on an operating situation of the internal combustion engine, the expansion machine can be coupled into or decoupled from a line circuit. If there is no load demand on the expansion machine, the steam can be conducted past the expansion machine. 
     It is advantageous for a bypass pressure regulating valve and/or a pressure limiting valve to be arranged in the bypass connection because, by means of said regulating elements, the steam can be controlled so as to be conducted past the expansion machine as required. The pressure limiting valve is opened when a predefined pressure is exceeded and can thus protect components in the line circuit and the expansion machine against destruction by excess pressure. The bypass pressure regulating valve may supply steam to the expansion machine or conduct said steam past the expansion machine as a function of the respective load demands. 
     A pressure regulating valve in a line between the feed pump and heat exchanger is advantageous because the pressure level for the evaporation and thus the evaporation temperature are adjusted by means of the pressure regulating valve. Alternatively or in addition, a pressure relief valve may also be provided. Here, only a single pressure level can be set, but lower costs are incurred for acquisition and regulation. 
     It is advantageous for the condenser to be connected to a cooling circuit of the internal combustion engine. Heat from the heat exchanger can be dissipated via the bypass connection and the condenser to the cooling circuit of the internal combustion engine. Said energy may be utilized, before the start-up of the expansion machine, for a faster warm-up of the internal combustion engine. 
     It is particularly advantageous for at least one line of the bypass connection to run through a housing or in the vicinity of the housing of the expansion machine. If ice or residues are situated in the expansion machine as a result of a frozen working medium, it is possible by means of the opening of the bypass pressure regulating valve for heated steam to be conducted through the housing or conducted past in the vicinity of the housing of the expansion machine. The ice and residues are removed by means of the heated steam and damage upon start-up of the expansion machine is avoided. 
     If the internal combustion engine is being operated at reduced power or the load demand on the internal combustion engine is briefly reduced, it is advantageous for the working medium to flow through the bypass connection as a result of the opening of the bypass pressure regulating valve, and for the power output of the expansion machine to thus be reduced. As a result of the simultaneous power reduction of the internal combustion engine and expansion machine, it is possible to prevent mechanical energy, for which there is no consumer, from being produced with a certain time delay owing to the thermodynamic inertia. 
     It is advantageous for pressure pulsations and pressure oscillations in the heat exchanger and/or in the adjoining lines to be reduced by means of a cyclic opening of the bypass pressure regulating valve, because no costs are incurred for further components for preventing pressure oscillations. 
     To protect the expansion machine against damage by water droplets generated during the condensation of inadequately superheated steam, the bypass pressure regulating valve may be opened in the event of excessively low quality of the steam, such that the steam is conducted past the expansion machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are illustrated in the drawing and will be explained in more detail in the following description, in which: 
         FIG. 1  is a schematic illustration of a device for waste-heat utilization according to a first exemplary embodiment, and 
         FIG. 2  is a schematic illustration of a device for waste-heat utilization according to a second exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1 and 2  show a device for utilizing the waste heat of an internal combustion engine  2 , having a line circuit  4 , in which a working medium circulates. At least one heat exchanger  8 , an expansion machine  10 , a condenser  12  and a feed pump  6  are arranged in the line circuit  4 . 
     The internal combustion engine  2  may in particular be in the form of an air-compressing, auto-ignition or mixture-compressing, applied-ignition internal combustion engine  2 . The device for waste-heat utilization is suitable in particular for applications in motor vehicles. The device according to the invention for waste-heat utilization is however also suitable for other applications. 
     The internal combustion engine  2  burns fuel in order to generate mechanical energy. The exhaust gases generated in the process are discharged via an exhaust system in which an exhaust-gas catalytic converter may be arranged. A line portion  22  of the exhaust system leads through a heat exchanger  8 . Heat energy from the exhaust gases or from the exhaust-gas recirculation arrangement is dissipated via the line portion  22  in the heat exchanger  8  to the working medium, such that the working medium can be evaporated and superheated in the heat exchanger  8 . 
     The heat exchanger  8  of the line circuit  4  is connected via a line  26  to the expansion machine  10 . The expansion machine  10  may be in the form of a turbine or piston machine. The evaporated working medium flows via the line  26  to the expansion machine  10  and drives the latter. The expansion machine  10  has a drive shaft  11  via which the expansion machine  10  is connected to a load. In this way, mechanical energy may for example be transmitted to a drivetrain or serve for driving an electrical generator, a pump or the like. After flowing through the expansion machine  10 , the working medium is conducted via a line  28  to a condenser  12 . The working medium which is expanded by means of the expansion machine  10  is cooled in the condenser  12 . The condenser  12  may be connected to a cooling circuit  20 . Said cooling circuit  20  may for example be a cooling circuit of the internal combustion engine  2 . The working medium liquefied in the condenser  12  is transported via the line  29  into the line  24  by a feed pump  6 . 
     In the line  24  there is situated a pressure regulating valve  27  which serves for regulating the pressure of the working medium in the inlet to the heat exchanger  8 . The evaporation temperature of the working medium can be regulated by means of the predefined pressure in the inlet to the heat exchanger  8 . Furthermore, a bypass connection  31  may be provided in parallel with the feed pump  6 , in which bypass connection is situated a pressure relief valve  30 . The maximum admissible pressure of the working medium between the feed pump  6  and heat exchanger  8  can be set by means of the pressure relief valve  30 . 
     The line  24  leads directly into the heat exchanger  8 , in which the working medium is evaporated and if appropriate superheated. The evaporated working medium passes to the expansion machine  10  again via the line  26 , and the working medium flows again through the line circuit  4 . A flow direction of the working medium through the line circuit  4  is defined by the feed pump  6  and the expansion machine  10 . It is thus possible for heat energy to be extracted continuously from the exhaust gases and the constituent parts of the exhaust-gas recirculation arrangement of the internal combustion engine  2  by means of the heat exchanger  8 , which heat energy is released in the form of mechanical energy to the shaft  11 . 
     As working medium, use may be made of water or some other liquid which meets the thermodynamic requirements. As it flows through the line circuit  4 , the working medium undergoes thermodynamic changes in state. In the liquid phase, the working medium is raised to the pressure level for the evaporation by the feed pump  6 . The heat energy of the exhaust gas is subsequently dissipated to the working medium by means of the heat exchanger  8 . Here, the working medium is evaporated isobarically and subsequently superheated. The steam is subsequently expanded adiabatically in the expansion machine  10 . Here, mechanical energy is gained and transmitted to the shaft  11 . The working medium is then cooled in the condenser  12  and supplied to the feed pump  6  again. 
     In the line circuit  4 , there is situated a bypass connection  14  which is connected in parallel with the expansion machine  10 . The bypass connection  14  produces a connection between the line  26  between the heat exchanger  8  and expansion machine  10  and the line  28  between the expansion machine  10  and condenser  12 . In the bypass connection  14  there is arranged a further bypass pressure regulating valve  16 . Instead of the further bypass pressure regulating valve  16 , a pressure limiting valve  32  may be situated in the bypass connection  14 . A parallel connection of the bypass pressure regulating valve  16  and the pressure limiting valve  32  in the bypass connection  14  is also possible. 
     By means of the opening of the bypass pressure regulating valve  16  or of the pressure limiting valve  32 , it is possible for the working medium to flow directly from the heat exchanger  8  to the condenser  12  and to be conducted past the expansion machine  10 . 
     The pressure limiting valve  32  may be set to a maximum pressure, upon the exceedance of which it opens and allows the working medium to flow through the bypass connection. Said maximum pressure should be adapted to the configuration of the system and prevent an excess pressure in the line circuit  4 . By virtue of the pressure limiting valve  32  being opened in good time, damage to components in the line  26  and to the expansion machine  10  can be prevented. 
     By means of a targeted opening of the bypass pressure regulating valve  16 , the power of the expansion machine can be reduced in parallel with the power of the internal combustion engine  2 . If there is no load demand on the expansion machine, the steam can be conducted past the expansion machine  10  in a targeted manner by means of the opening of the bypass pressure regulating valve  16 . 
     Furthermore, the bypass pressure regulating valve  16  may serve for reducing pressure oscillations that may arise in the heat exchanger  8  and the adjoining lines  24 ,  26 . By means of a brief, cyclic opening and closing of the bypass pressure regulating valve  16 , the pressure oscillations can be reduced and damped. 
     If the internal combustion engine  2  is started at low ambient temperatures, the heat absorbed in the heat exchanger  8  from the exhaust gas or the exhaust-gas recirculation arrangement can be conducted directly via the bypass connection  14  to the condenser  12 . As a result of the opening of the bypass pressure regulating valve  16 , the heated steam flows past the expansion machine  10  and is conducted directly to the condenser  12 , which can transfer the thermal energy directly to the cooling circuit  20  of the vehicle. Since no expansion of the working medium in the expansion machine  10  has taken place, a particularly large amount of energy is available for the fast warm-up of the internal combustion engine  2 . 
     If insufficient superheating of the steam takes place in the heat exchanger  8  owing to an excessively low availability of heat from the exhaust system, the steam has a low quality. In this case, there is the risk of droplets forming in the expansion machine  10  owing to condensation during the expansion of the steam. Said droplets may lead to damage in the expansion machine  10 . To protect the expansion machine against said damage by condensation, the bypass pressure regulating valve  16  may be opened in the event of low quality of the steam, such that the steam is conducted past the expansion machine  10 . 
     A further exemplary embodiment is illustrated in  FIG. 2 , in which a line portion  15  of the bypass connection  14  situated downstream of the bypass pressure regulating valve  16  or the pressure limiting valve  32  is conducted through a housing  33  of the expansion machine  10 . Alternatively, said line portion  15  of the bypass connection  14  situated downstream of the bypass pressure regulating valve  16  or the pressure limiting valve  32  may also be conducted past the expansion machine  10  in the vicinity of the housing  33 . 
     By means of the exemplary embodiment shown in  FIG. 2 , at temperatures below the freezing point or close to the freezing point of the working medium, before the start-up of the expansion machine  10 , steam heated in the heat exchanger  8  can be conducted through the housing  33  of the expansion machine  10 , or conducted past in the vicinity of the housing  33  of the expansion machine  10 , by means of the opening of the bypass pressure regulating valve  16 . Ice or frozen residues in the expansion machine  10  can be dissolved by the heated steam.