Abstract:
A method for operating an internal combustion engine ( 10 ) having a steam power plant ( 12 ), in which the exhaust gas of the internal combustion engine ( 10 ) at least indirectly heats the steam and this steam drives a turbine ( 26 ), the steam power plant ( 12 ) being sometimes, in particular at the beginning of operation, initially operated in a protection operating mode, in which the protection of the turbine ( 26 ) from damage by water droplets has priority.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The invention relates to a method for operating an internal combustion engine having a steam power plant, in which the exhaust gas of the internal combustion engine at least indirectly heats the steam and this steam drives a turbine, and to a control and/or regulating unit, a computer program, and a storage medium. 
         [0002]    Steam power plants, which utilize the waste heat in the exhaust gas of an internal combustion engine, are known. DE 10 2006 057 247 A1 describes a steam power plant, a heat exchanger of a circuit of an operating medium being housed in an exhaust system. The heat exchanger is connected downstream from a conveyor assembly in the circuit of the operating medium. The circuit of the operating medium contains a turbine part, via which at least one compressor part situated in the intake system of the internal combustion engine is driven. 
       SUMMARY OF THE INVENTION 
       [0003]    The invention has the advantage that in a steam power plant which utilizes the waste heat in the exhaust gas of an internal combustion engine, a steam turbine is protected from damage by liquid droplets, in particular water droplets, in the steam jet. In addition, rapid operational readiness of the steam power plant is produced in order to provide steam circuit power as early as possible and thus contribute to reducing the fuel consumption. 
         [0004]    For example, a steam power plant in the exhaust system of the internal combustion engine is constructed as follows: The exhaust gas flows through a primary system of the heat exchanger. In the secondary system, the condensed water conveyed by a feed pump is heated in such a way that steam arises. The steam generated in the heat exchanger is conducted into a turbine, where it expands in a nozzle and drives a rotor of the turbine. An outlet of the turbine conveys the expanded steam into a condenser. The expanded steam is cooled in the condenser, so that it condenses to form water. This water is sucked in by the feed pump and supplied again to the heat exchanger. 
         [0005]    The invention proceeds from the consideration that steam turbines may be damaged if water droplets condensed from the steam hit the blading of the rotor of the steam turbine at high velocity. This danger exists especially during a startup procedure, when parts of the steam circuit and the steam turbine itself have not yet reached their respective operating temperature. It is therefore proposed that the steam power plant be operated during a startup procedure in a protection operating mode, in which the protection of the turbine from damage by water droplets has priority. In such a protection operating mode, for example, it can be ensured as much as possible that the vapor generated in the heat exchanger cannot condense on cold pipe walls downstream. The danger is therefore at least reduced that individual droplets will be entrained by the steam flow and strongly accelerated, whereby damage could occur upon their impact on the rotor. 
         [0006]    A first bundle of measures has the purpose that a relative velocity between an entering steam jet and a rotor of the turbine is reduced in the protection operating mode in relation to a normal operating mode. The relative velocity characterizes the impact velocity of the water droplets on the rotor of the turbine. The lower the relative velocity, the lower the probability of damage to the rotor. In this way, the durability of the turbine and therefore of the steam power plant as a whole is advantageously increased. 
         [0007]    For this purpose, the method according to the invention provides that the relative velocity between the incoming steam jet and the rotor of the turbine is reduced, in that at least sometimes a cross section of a nozzle for controlling the incoming steam jet is enlarged and/or the turbine is operated without a load and/or the rotation of the turbine is supported by a motor. If the cross section of the nozzle is enlarged, a lower pressure ratio results via the nozzle, which in turn results in a lower acceleration and a lower exit velocity of the steam jet. Additionally or alternatively, the relative velocity between the steam jet and the rotor of the turbine can be reduced further if the turbine is operated without a load, i.e., can rotate freely. Additionally or alternatively, the relative velocity can in turn be reduced further if the turbine is supported by a motor. For example, the rotor can be coupled to an electric motor which synchronizes the rotor of the turbine at least approximately to the steam jet velocity. Damage to the rotor can thus be avoided in three ways. The electric motor can advantageously operate as a generator to generate electrical energy in the following normal operating mode of the steam power plant. 
         [0008]    A second bundle of measures has the purpose that a temperature of the steam generated in a steam generator is increased up to a limiting temperature above a normal operating temperature in the protection operating mode in relation to the normal operating mode. While the above-described first measures reduce the relative velocity of the water droplets condensed from the steam in relation to the rotor, the second measures have the effect that water droplets in the steam jet are formed either not at all or only in a reduced quantity, i.e., a back condensation of the steam jet essentially does not occur. Both bundles of measures may supplement one another and advantageously add their effects together. 
         [0009]    For this purpose, the method according to the invention provides that the temperature of the steam generated in the heat exchanger (“steam generator”) is increased up to a limiting value above the normal operating temperature, in that an exhaust-side bypass of a heat exchanger is closed or is reduced in its flow rate, if an operating temperature of the heat exchanger has not exceeded a maximum permissible value, and/or a delivery quantity of a feed pump is adapted, and/or the cooling of a condenser is stopped or reduced, if an operating pressure of the steam power plant has not exceeded a maximum permissible value. It is thus possible to “superheat” the steam in relation to the normal operating mode, so that the probability of the occurrence of the harmful water droplets is less. The limiting value for the temperature of the steam is preferably selected in such a way that no part of the steam power plant is excessively strained or the safety is endangered. If the exhaust gas heat exchanger has an exhaust-side bypass, the bypass is initially closed during the protection operating mode, so that the entire exhaust gas stream can flow through the heat exchanger. A temperature of the heat exchanger is simultaneously monitored. As soon as an upper permissible operating temperature of the heat exchanger is reached or even exceeded, the bypass is opened, with the result that the exhaust gas quantity flowing through the heat exchanger is reduced. An inner heat exchanger temperature is preferably used as a control variable for adjusting the bypass, which can also be ascertained, for example, by a thermal heat exchanger model on the basis of input variables, such as for example an exhaust gas temperature, an exhaust gas mass flow, a fluid temperature, and a fluid mass flow. 
         [0010]    Alternatively or additionally, a delivery quantity of the feed pump can be regulated or adjusted. The feed pump conveys water into a fluid inlet of the heat exchanger, so that the steam exit temperature from the heat exchanger can be adjusted using the delivery quantity. For example, in the normal operating mode of the steam power plant, a fluid mass flow supplied to the heat exchanger is between 8 g/s and 60 g/s (grams per second). During the protection operating mode it can be advantageous to reduce the fluid mass flow accordingly in relation to these values, since energy is required for heating the heat exchanger, which is not available for the vaporization and superheating. The fluid mass flow can thus be adjusted and/or regulated using a speed change of the feed pump in such a way that the steam temperature is increased in relation to the normal operating mode. The temperature of the steam generated in the heat exchanger is between 270° C. and 360° C. in the normal operating mode, for example. 
         [0011]    Alternatively or additionally, the cooling of the condenser can be adjusted. For example, the cooling of the condenser can be stopped, whereby less water condenses, and a higher steam temperature is subsequently reachable in the heat exchanger. The operating pressure of the steam power plant is monitored simultaneously. If the maximum permissible operating pressure is reached or exceeded, the cooling of the condenser is turned on again and/or continuously increased. For example, depending on the operating point of the internal combustion engine, the required cooling power of the condenser can be 19 kW to 140 kW. The cooling power of the condenser is preferably not switched, but rather regulated. The permissible operating pressure of the steam power plant is the control variable. In contrast, if as much exhaust gas heat as possible is to be transferred into the cooling system of the internal combustion engine, to which the condenser is connected, the condenser is already cooled at the beginning. This can be advantageous for shortening the warm-up or reducing the emissions of the internal combustion engine. In this case, for example, the cooling power on the condenser is regulated in such a way that an operating pressure of, for example, 2 bar results. It is also possible to execute the regulation of the condenser cooling with respect to the protection operating mode, on the one hand, and shortening the warm-up and reducing the emissions, on the other hand, i.e., to combine both requirements. 
         [0012]    Furthermore, the method provides that the protection operating mode is ended when the steam power plant has reached a normal operating temperature. The transition from the protection operating mode to the normal operating mode can thus occur as rapidly as possible, because no damage of the turbine by water droplets is to be expected at the normal operating temperature. The criterion for changing over can either be obtained by various temperature and/or pressure sensors at various points of the steam circuit, or can be computed by a thermal model. In normal operation, process control of the steam circuit which is optimized in efficiency is achieved. 
         [0013]    Furthermore, the method provides that at least an operating pressure of the steam power plant, a pressure ratio between an inlet and an outlet of the turbine, a fluid mass flow of a feed pump, a temperature of a heat exchanger between exhaust gas and steam, a temperature of a condenser, and/or a temperature of the turbine is monitored. Therefore, on the one hand, a reliable differentiation can be made between the protection operating mode and the normal operating mode, and, on the other hand, the protection operating mode can be adjusted or regulated optimally. In this way, damage to the steam power plant can also be particularly reliably prevented. 
         [0014]    In addition, the method provides that when the pressure ratio between the inlet and the outlet of the turbine reaches or exceeds a threshold value, the delivery quantity of a feed pump is reduced. For example, if the pressure ratio exceeds a threshold value, the delivery quantity of the feed pump is reduced enough that the pressure ratio is again below the threshold value. This can prevent the steam exit velocity out of the nozzle of the turbine from being so great that the rotor of the turbine could be damaged by water droplets. The threshold value for the pressure ratio is specified accordingly. 
         [0015]    Furthermore, the method provides that a superheating temperature of the steam—if water is used as the operating means—in normal operation is between 270° C. and 360° C. Using a superheating temperature of the steam determined by these limits, particularly good efficiency of the steam power plant can be achieved in the normal operating mode. Proceeding therefrom, it is possible to increase the steam temperature generated in the heat exchanger at the beginning of operation—i.e., during the protection operating mode—in order to reduce or prevent the occurrence of the harmful water droplets. The upper limit of the superheating temperature of the steam is determined by the temperature compatibilities of the components of the steam power plant and by safety requirements. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    Exemplary embodiments of the invention are explained hereafter with reference to the drawing. In the drawing: 
           [0017]      FIG. 1  shows a schematic of a steam power plant; and 
           [0018]      FIG. 2  shows a flowchart of a sequence of the method in a protection operating mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    The same reference numerals are used for functionally equivalent elements and variables in all figures, even in the case of different embodiments. 
         [0020]      FIG. 1  shows a steam power plant  12 , which is supplied with energy by the exhaust system of an internal combustion engine  10 . The exhaust gas of the internal combustion engine  10  is supplied via an exhaust pipe  14  and a bypass valve, implemented in the present case as a three-way valve  16 , to a heat exchanger  18 , which has the task of a steam generator  17 . A third connection of the three-way valve  16  conducts exhaust gas via a bypass  19  past the heat exchanger  18 . The exhaust gas flows through a primary system of the heat exchanger  18  and is then exhausted together with the component conducted through the bypass  19  through an outlet pipe  20 . 
         [0021]    A secondary system of the heat exchanger comprises a fluid entry  21  and a steam exit  22 . The steam exit  22  leads to an inlet  24  of a turbine  26 . The turbine  26  is implemented as an impulse turbine and comprises a nozzle  28  and a rotor  30 . An outlet  32  of the turbine  26  leads to a condenser  34 . An outlet  36  of the condenser leads to an inlet  38  of a feed pump  40 . The feed pump  40  operates at the fluid entry  21  of the heat exchanger  18 , so that the steam circuit is closed. 
         [0022]    The turbine  26  operates at a load  42  which is controlled by an actuator  44 . A dashed line  46  indicates that the load  42  can support a drive of the internal combustion engine  10 . For example, the load  42  can be a compressor part, which increases a pressure in the intake system of the internal combustion engine  10 , so that during an intake stroke of the internal combustion engine  10 , a greater quantity of air required for the combustion can reach the cylinder. Furthermore, an electric motor  48  is coupled to the turbine  26 . A control and/or regulating unit  50 , which comprises a storage medium  52  and a computer program  54 , is located on the lower left in the illustration of  FIG. 1 . Dashed arrows  56  indicate that the control and/or regulating unit  50  is connected to various components of the steam power plant  12 . 
         [0023]    Inter alia, the following are measured in operation of the steam power plant  12 : A temperature of the heat exchanger  18  using a temperature sensor  58 , a pressure at the inlet  24  of the turbine  26  using a pressure sensor  60 , and a pressure at the outlet  32  of the turbine  26  using a pressure sensor  62 . Furthermore, an actuator  64  for adjusting the cooling of the condenser  34  and an actuator  66  for adjusting a speed of the feed pump  40  are provided. 
         [0024]    If it is established by the control and/or regulating unit  50  that the steam power plant  12  or the internal combustion engine  10  is in a startup state, a changeover is performed into a protection operating mode. The control and/or regulating unit  50  obtains the information for this purpose, inter alia, from the data of the pressure sensors  60  and  62 , the temperature sensor  58 , and from the models of the steam circuit stored in the control and/or regulating unit  50 . Multiple arrows shown (without reference numerals) in the circuit of the steam power plant  12  indicate the direction of the fluid stream, the steam stream, and the exhaust gas stream. In other words, under certain circumstances, the steam power plant is operated in the protection operating mode. 
         [0025]    A plurality of measures is performed simultaneously in the protection operating mode: A cross section of the nozzle  28  of the turbine  26  is enlarged, so that the velocity of the incoming steam jet is reduced. A relative velocity of the incoming steam jet in relation to the rotation of the rotor  30  is thus reduced. The actuator  44  decouples the load  42  from the turbine  26 . The electric motor  48  is energized and drives the turbine  26  in such a way that the rotor  30  is essentially synchronized with the velocity of the incoming steam jet. 
         [0026]    Furthermore, various measures are performed in order to keep the temperature of the steam at the steam exit  22  of the heat exchanger  18  as high as possible. Firstly, the three-way valve  16  is set in such a way that the exhaust gas stream exiting from the internal combustion engine  10  is conducted completely through the primary system of the heat exchanger  18 , as long as a temperature monitored by the temperature sensor  58  does not exceed a threshold value. If this threshold value is reached or exceeded, the three-way valve at least partially opens the route via the bypass  19 , in order to guide less exhaust gas through the heat exchanger  18  and thus keep its temperature below the threshold value. 
         [0027]    Secondly, using the actuator  66 , the delivery quantity of the feed pump  40  is adjusted in such a way (i.e., kept comparatively low at the beginning of operation), that a desired steam temperature results at the steam exit  22  of the heat exchanger  18 . The delivery quantity of the feed pump  40  is adjusted in the present case by a speed of the feed pump  40 . In addition, the delivery quantity of the feed pump  40  is adjusted as a function of a pressure difference between the inlet  24  and the outlet  32  of the turbine  26 . The pressure difference is determined by a difference in the pressures ascertained in the pressure sensors  60  and  62 . 
         [0028]    Thirdly, the steam circuit is rapidly heated up in that the cooling of the condenser  34  is stopped or reduced as long as a permissible operating pressure of the steam power plant  12  has not yet been reached. If this operating pressure is reached, the cooling of the condenser  34  is turned on and/or amplified. 
         [0029]    If it is recognized on the basis of the temperature sensor  58  and the two pressure sensors  60  and  62  and a model stored in the control and/or regulating unit  50  that the startup procedure of the steam power plant  12  is completed, a switch is made into a normal operating mode. The load  42  is coupled to the turbine  26  and can support the drive of the internal combustion engine  10 . The electric motor  48  is either turned off or switched over into the generator operation. The remaining actuators of the steam power plant  12  are operated according to the normal operating mode. 
         [0030]      FIG. 2  shows a flowchart of the sequence of the method using a computer program  54  in the control and/or regulating unit  50 . Starting from a starting block  70 , it is queried in a block  72  whether the requirements for a protection operating mode are fulfilled. If not, the sequence branches into an end block  74 , in which a normal operating mode of the steam power plant  12  is produced and the illustrated flowchart is thus exited. 
         [0031]    However, if so, various variables of the steam circuit are ascertained in a following block  76 . These include, inter alia, the temperature of the temperature sensor  58 , the pressures of the pressure sensors  60  and  62 , and various model variables  78  of the steam power plant  12  stored in the control and/or regulating unit  50 . In a following block  80 , various measures are performed to reduce the relative velocity between the incoming steam jet in the turbine  26  and the rotor  30  of the turbine. For this purpose, an actuator  82  (not shown in  FIG. 1 ) is actuated in order to enlarge the cross section of the nozzle  28 . Furthermore, an actuator  44  is actuated by the block  80  in order to decouple the load  42  from the turbine  26 . Furthermore, the block  80  switches the electric motor  48  via the actuator  48  in such a way that it is operated as a motor and adapts the speed of the rotor  30  to the velocity of the incoming steam jet through the nozzle  28 . 
         [0032]    Various measures are performed in a following block  84  in order to increase the temperature of the incoming steam jet in the turbine  26  over a normal value. For this purpose, the three-way valve  16  is adjusted using an actuator  86  in such a way that an exhaust gas stream through the bypass  19  is reduced or blocked. Furthermore, the delivery quantity of the feed pump  40  is adapted via the actuator  66 . The cooling power of the condenser  34  is reduced via the actuator  64  as long as a permissible operating pressure of the steam power plant  12  is not exceeded. Subsequently, the program branches back to the block  72 , where it is checked again whether the condition for the protection operating mode is fulfilled. If so, the sequence branches to block  76  again in order to adjust or regulate the various variables, as is required for the protection operating mode. 
         [0033]    The method is terminated as soon as it is established in block  72  that the startup state has been ended. The sequence can then switch into the normal operating mode in block  74 .