DE102012211578A1 - Apparatus for utilizing waste heat of internal combustion engine of passenger car, has turbine switched between different combinations of inlet nozzles, where different combinations form switching circuit for current mass flow of medium - Google Patents

Abstract

The apparatus has an evaporator (4) transferring waste heat to a working medium (M). A driven turbine (6) is provided with the working medium. The turbine is provided with multiple inlet nozzles with a fixed predetermined geometry of the working medium. The turbine is switched between different combinations of the inlet nozzles, where the different combinations form a switching circuit for different current mass flow of the working medium. A mass flow region is formed between a minimum mass current and a maximum mass flow. An independent claim is also included for a method for utilizing waste heat of a combustion engine of a motor vehicle.

Description

The invention relates to a device for utilizing waste heat of an internal combustion engine, in particular of a motor vehicle, with a heat exchanger for transmitting the waste heat to a working medium and with a turbine drivable by the working medium. The invention further relates to such a turbine and a corresponding method.

Such a device is known from DE 10 2007 003 801 A1 refer to. The device is used to obtain electrical energy from the exhaust heat of a motor vehicle internal combustion engine. Here, the exhaust heat of the internal combustion engine is used with the aid of a heat exchanger for evaporating water, which is supplied as a vapor via a Laval nozzle a tangentially flowed through Pelton constant pressure turbine. This is connected to a generator for generating electrical energy.

Due to the in a motor vehicle, especially in passenger cars, highly fluctuating operating conditions is for steam generation a widely varying waste heat available. This leads to greatly varying steam mass flows for the steam supplied to the turbine. Depending on the current operating state, this leads to different efficiencies in the conversion of the energy contained in the steam into rotational energy of the turbine.

Proceeding from this, the object of the invention is to improve the efficiency of such systems in the automotive sector.

The object is achieved according to the invention by a device having the features of claim 1. The object is further achieved by a turbine for such a device and by a method for using waste heat of an internal combustion engine with the aid of such a device. The preferred embodiments and advantages cited with regard to the device are also to be transferred analogously to the turbine and the method.

Thereafter, it is provided that the turbine has a plurality of inlet nozzles for the working medium and that further preferably a control device is arranged, which switches depending on the currently applied mass flow of the working medium between different combinations of inlet nozzles. The individual combinations in each case form switching stages for different currently applied mass flows.

By virtue of this measure, the inlet nozzles can each be operated in the region of their optimum efficiency and, at the same time, a very wide range of varying mass flows can be covered by the various switching stages.

As a turbine here is preferably a constant pressure turbine and as inlet nozzles Laval nozzles are used with fixed predetermined, non-adjustable geometry so with a non-adjustable nozzle cross-section. Such Laval nozzles generally have a convergent inflow, which is reduced to a minimum nozzle cross-section and followed by a divergent Ausströmbereich. At its optimum operating point, ie at a defined, optimum working pressure at a given temperature of the working medium (steam), the pressure of the working medium is preferably completely converted into kinetic flow energy and the working medium exits the Laval nozzle at supersonic speed.

In such a device, the working medium is preferably guided in a circuit, wherein the energy transfer from the waste heat into the mechanical rotational energy of the turbine takes place by means of the so-called Rankine process. In this case, in addition to the evaporator, a capacitor is required, is discharged through the heat back to the environment. The device is preferably used in a motor vehicle, in particular in passenger cars. The waste heat is recovered either selectively or in combination from the exhaust gas or from the coolant by heat exchange in the evaporator. The arrangement of the evaporator has a retroactive effect on the entire system of the internal combustion engine. In an arrangement of the heat exchanger in the exhaust system, this leads to an increased back pressure in the exhaust system, which is particularly relevant for higher engine performance. When connected to the cooling system, this is additionally burdened by the extra heat generated during the cycle.

When switching between different nozzle combinations, ie when switching between switching stages, there is also the problem that this leads to pressure changes in the system, which can have a negative effect on the stability of the evaporation process in the evaporator.

According to a preferred embodiment, it is therefore provided that the individual switching stages, ie the individual inlet nozzles and their combination for the respective switching stage, are selected such that a predetermined maximum pressure jump when switching between two successive switching stages is not exceeded. The pressure jump is preferably only in a range between 1 × 10 ⁵ to 3 × 10 ⁵ Pa. Generally, based on a nominal operating pressure for which the entire system is designed, the pressure jump is preferably in the range of 5% to 20% of the nominal operating pressure. Furthermore, the nominal operating pressure is preferably in the middle of the pressure range for the pressure jump. Thus, a symmetrical deviation upwards and downwards from the nominal operating pressure is permitted.

By arranging a plurality of inlet nozzles, therefore, not only the efficiency and the efficiency of the turbine itself is improved by the individual inlet nozzles are each designed such that they are operated at or as close as possible to their optimum operating point. In addition, the feedback when switching to the evaporation process is kept low.

In a preferred embodiment, the inlet nozzles and their combination in the respective switching stages are selected such that the number of inlet nozzles is minimized, taking into account a predetermined mass flow range to be expected. As a result, on the one hand, the manufacturing effort is kept low overall for the turbine. At the same time thereby the required space for the turbine is kept low, which is particularly important in the usually confined spaces in a passenger car essential.

For the design of the inlet nozzles, the number of inlet nozzles is therefore minimized, taking into account a predetermined minimum and maximum mass flow.

Overall, the turbine is designed for a mass flow range which in a motor vehicle preferably corresponds to only about 20% to 30% of the available waste heat of the internal combustion engine due to the effects on the overall system of the internal combustion engine. For example, the maximum mass flow for cars is typically in the range of 10kW and for trucks in the range of up to 50kW isentropic steam expansion capacity. Isentropic steam expansion performance is understood to mean the power of the steam during its adiabatic (isentropic) expansion in the turbine, ie its work per unit of time performed during this change of state (energy transferred to the turbine). The minimum mass flow depends on a technically useful lower amount, which in a car preferably corresponds to about an isentropic steam expansion capacity of 0.1 kW. When using water as a working medium, the limits for the mass flow in a car are preferably about 1 g / s ( corresponds to about 0.3kW isentropic steam expansion capacity) and at about 15g / s (equivalent to about 8.5kW isentropic steam expansion capacity) at a steam temperature of about 300 ° C. In principle, other working media suitable for the operating conditions can also be selected. Depending on the specific evaporation enthalpy of the working medium, the mass flows for which the turbine is designed vary.

The steam temperature of the working medium is preferably generally in the range of 200 ° C to 300 ° C. Generally, the temperature is selected to provide superheated steam without liquid droplets.

The values given here for the system parameters such as temperature of the working medium and operating pressure of the working medium (steam) relate respectively to the conditions prevailing at the inlet side of the nozzles, ie the pressure or the temperature of the steam supplied to the nozzles, unless explicitly stated otherwise ,

Preferably, the inlet nozzles are designed for different mass flows, based on a nominal operating pressure and a nominal temperature of the working medium for which the system is designed. Conveniently, each of the inlet nozzles is designed for a different mass flow. Due to the different design of the different inlet nozzles can be realized with a small number of inlet nozzles as many gradations and combination options for the individual switching stages.

In a preferred embodiment, a plurality of successive switching stages are predetermined for the predetermined mass flow range between a minimum mass flow and a maximum mass flow of the working steam, wherein each of the switching stage has a fixed predetermined combination of inlet nozzles. These firmly defined combinations per switching stage are enabled in operation by the control device at firmly defined switching points, so acted upon by the working steam. These firmly defined Switching points are given values of the operating pressure. If these are exceeded or fallen short of, switching takes place to the adjacent higher or lower switching stage. , The switching is generally done preferably via individually controllable control valves, via which the entire mass flow, which is supplied to the turbine, is divided among the various inlet nozzles of a respective switching stage. Each inlet nozzle is preferably associated with a control valve. A flow path to the respective inlet nozzle is in each case released via the control valves. The control valves are preferably activated by the control device.

In order to achieve the greatest possible variability and any combinability of the various inlet nozzles, they are designed in an expedient embodiment all for the same optimum operating point and thus optimum working pressure. Under optimal working pressure here is the working pressure at the operating point of the nozzle understood, in which this reaches its theoretical maximum efficiency. By choosing an equal optimum working pressure any combinations between the individual inlet nozzles are possible. All switching stages can therefore be operated in the same pressure range. It is used in each switching stage always the same pressure range regardless of the specific combination of the individual inlet nozzles.

Conveniently, it is particularly provided that when switching to the next higher switching stage, ie in the designed for a higher mass flow combination of inlet nozzles, the input side of the inlet nozzles pending operating pressure drops to the optimum working pressure for which the respective nozzle is formed. The optimum working pressure therefore preferably forms a lower threshold for the operating pressure of the working medium over the entire bandwidth over all switching stages. This ensures that the individual inlet nozzles are operated optimally.

The overall system is preferably designed for a nominal operating pressure with a permissible deviation thereof in the region of the maximum pressure jump, ie a lower and an upper threshold for the operating pressure are specified, which are not exceeded when switching between the switching stages. Preferably, the control device for the lowermost switching stage as well as for the uppermost switching stage allows a lower or exceeded these thresholds up to a minimum or maximum operating pressure. For the lowermost shift stage, a permissible minimum operating pressure in the range of preferably from 2 to 5 × 10 ⁵ Pa and for the uppermost shift stage is preferably a maximum operating pressure in the range of about 8 to 20 × 10 ⁵ Pa allowed. Based on the nominal operating pressure, the minimum operating pressure is generally 50% to 85% and the maximum operating pressure is generally 115% to 200% of the nominal operating pressure. This allows the system to operate at very low mass flows and very high mass flows until the minimum or maximum operating pressure is reached. Since no switching occurs in these outer switching stages, the requirement of limiting the maximum pressure jump remains.

In this embodiment, the inlet nozzle of the lowermost switching stage - deviating from the other inlet nozzles - designed for an optimal working pressure (at a given temperature and given working medium) in the range of the minimum operating pressure. The further inlet nozzles are designed for an optimal working pressure in the range of the lower switching threshold. Alternatively, the inlet nozzle of the lowermost switching stage is also designed for an optimum working pressure in the region of the lower switching threshold.

Generally, the system is preferably designed for a nominal operating pressure ranging from 5 to 20 x 10 ⁵ Pa.

In the dimensioning and design of the individual inlet nozzles, the first switching stage provided for the minimum mass flow is preferably formed by a single inlet nozzle, which is designed for the minimum mass flow at a lower threshold value for the operating pressure. This lower threshold is in particular the optimum working pressure of the inlet nozzle.

Furthermore, expediently, the last switching stage, which is designed for the maximum mass flow, formed by the combination of all available inlet nozzles. As a result, the number of inlet nozzles is kept low at the same time low pressure jumps between the individual switching stages.

For the different design of the increments of the inlet nozzles, the mass flow difference, for which two successive inlet nozzles are designed, preferably in the range of 1/4 to 4/4 of the minimum mass flow. This comparatively fine gradation ensures that the pressure increase and thus the pressure jump when switching between two switching stages remains within the desired range.

• – das verwendete Arbeitsmedium,
• – die Dampftemperatur am Eintritt der Turbine, insbesondere liegt die Dampftemperatur T im Bereich von 200°C bis 300°C,
• – der nominelle Auslegungs- oder Betriebsdruck p_nom des Systems, insbesondere des Verdampfers, welcher beispielsweise im Bereich von 5 bis 20·10⁵ Pa und insbesondere bei etwa 7·10⁵ Pa liegt,
• – der maximale (Dampf-)Massestrom m_max, der beispielsweise bei etwa 15g pro Sekunde bei Wasser als Arbeitsmedium liegt,
• – ein maximal zulässiger Betriebsdruck p_max vor der Turbine, der beispielsweise bei etwa 10·10⁵ Pa liegt und welcher insbesondere einen oberen Grenzwert für die letzte Schaltstufe angibt,
• – der minimale Massenstrom m_min, der sich in technisch sinnvoller Weise von der Turbine nutzen lässt und der beispielsweise zwischen 1 und 2 g pro Sekunde bei Wasser als Arbeitsmedium liegt,
• – der minimale Betriebsdruck p_min, der beispielsweise im Bereich von etwa 4·10⁵ Pa liegt und welcher insbesondere einen unteren Grenzwert für die erste Schaltstufe angibt
• – der maximale Drucksprung ∆p_max, wobei bevorzugt eine symmetrische Unter- bzw. Überschreitung des nominellen Betriebsdrucks p_nom vorgesehen ist.

With regard to the design of the inlet nozzles, therefore, the following parameters are generally used in combination for a corresponding design method:

• - the working medium used,
• The steam temperature at the inlet of the turbine, in particular the steam temperature T is in the range of 200 ° C. to 300 ° C.,
• The nominal design or operating pressure p _{nom of} the system, in particular of the evaporator, which is, for example, in the range of 5 to 20 × 10 ⁵ Pa and in particular approximately 7 × 10 ⁵ Pa,
• - The maximum (steam) mass flow m _max , which is for example at about 15g per second with water as the working medium,
• A maximum permissible operating pressure p _{max upstream} of the turbine, which is, for example, about 10 × 10 ⁵ Pa and which in particular indicates an upper limit value for the last switching stage,
• - The minimum mass flow m _min , which can be used in a technically meaningful way of the turbine and, for example, between 1 and 2 g per second with water as the working medium,
• - The minimum operating pressure p _min , which is for example in the range of about 4 · 10 ⁵ Pa and which in particular indicates a lower limit for the first switching stage
• - The maximum pressure jump Δp _max , preferably a symmetrical under or exceeding the nominal operating pressure p _{nom is} provided.

Furthermore, special restrictions with respect to a switching sequence between different combinations or restrictions with respect to the combination of nozzles can be specified for the design method.

The turbine is generally suitably designed for speeds in the range of tens of thousands of revolutions per minute (RPM), especially for speeds in the range of 40,000 to a maximum of 100,000 RPM. The turbine is designed in particular as a so-called Curtis turbine, which is a two-stage constant pressure turbine with a first impeller, a subsequent to this stator and a second impeller. In principle, the constant pressure turbine can be designed in one or more stages.

The turbine expediently comprises on the inflow side a cylindrical nozzle body with the at least one adjoining impeller, wherein the inflow nozzles are formed on the shell side of the cylindrical nozzle body. Cylindrical nozzle body is generally understood to mean a rotational nozzle body with an approximately cylinder jacket-shaped surface. Conveniently, this nozzle body is disc-shaped, thus has a height of the cylinder, which is preferably significantly smaller than the diameter of the cylinder. The nozzle body is in this case arranged fixed, whereas the at least one impeller is rotatable. Distributed around the circumference of the nozzle body, the inlet nozzles are incorporated in particular on the outside of the nozzle body. They are preferably radially outwardly open and are completed by a housing component radially outward. The nozzle body and thus the turbine as a whole are flowed through in the axial direction, ie in the longitudinal direction of a turbine axis, on which the impeller is mounted.

An embodiment of the invention will be explained in more detail with reference to FIGS.

1 a graph of the Rankine process,

2A a schematic, fragmentary view of a turbine,

2 B a schematic diagram of the course of the flow velocity and the pressure of the working medium when flowing through the turbine and

3 a diagram for the exemplary design of the inlet nozzles of the turbine with multiple switching stages, in which the input side is applied to the voltage applied to the turbine operating pressure compared to the mass flow.

To use the waste heat of an internal combustion engine, especially in a car, the in 1 illustrated Rankine process used. The installed in the motor vehicle device for waste heat use includes a pump 2 , one as a steam generator or evaporator 4 trained heat exchanger, a turbine 6 and a capacitor 8th , For converting the heat energy into electrical energy in the embodiment of the turbine 6 a generator 10 associated with the particular turbine 6 forms a compact unit. The turbine 6 itself includes a nozzle body on the inflow side 12 to which a truck assembly 13 followed. To control the entire device is a control unit 14 arranged. This also controls a particular the nozzle body 12 upstream valve unit 15 with several control valves not shown here.

The components pump 2 , Evaporator 4 , Turbine 6 as well as capacitor 8th be in the flow direction 16 flows through a working medium M. Due to the horizontal dashed line in the 1 becomes the separation of a high pressure part 18 from a low pressure part 20 illustrated. In the sub-area between the capacitor 8th and evaporator 4 the working medium M is liquid, whereas in the partial area between the evaporator 4 and the capacitor 8th is vaporous and is present as vapor D. In the evaporator 4 There is a heat input and through the condenser heat dissipation, as indicated by the arrows 22 is shown.

For the heat input is the evaporator 4 to a heat pipe 24 connected, which is traversed by a hot medium H. This hot medium H is, for example, the exhaust gas of the internal combustion engine or else a heated coolant for the internal combustion engine. In the heat conduction 24 It may therefore be directly to an exhaust pipe of the motor vehicle or to a coolant line. The evaporator 4 is accordingly shown either as exhaust and / or coolant heat exchanger. Exhaust gas and coolant are preferably used in parallel to the heat input.

The working medium M is due to the heat input in the evaporator 4 vaporized, the steam D thus obtained is the turbine 6 fed into the turbine 6 relaxes and thus generates a mechanical power by a turbine shaft is driven. In the embodiment described here, this power is used directly to drive an electric generator for generating electrical energy. This is further used in the motor vehicle and fed into the motor vehicle own electrical system and cached, for example, in an energy storage. The electrically generated power is preferably in the range of a few kW and in particular up to 5kW. After flowing through the turbine forming an expansion machine, the steam at low pressure and with heat release in the condenser 8th liquefied. The working fluid M is using the pump 2 circulated within the circuit described, this being the working medium M between the condenser 8th and evaporator 4 brings back to a higher pressure level.

The technically feasible in a motor vehicle from the waste heat technically useful steam mass flows are relatively low and are in the range of a few grams of steam per second in the case of water as the working medium M. To this steam expansion efficiency provided efficiently in the turbine 6 to convert into mechanical power is the turbine 6 designed in particular as a constant pressure turbine.

A preferred embodiment of the turbine 2 is fragmentary and schematic in 2A illustrated. Generally, in a constant-pressure turbine, the steam D is expanded via Laval nozzles, that is, the pressure energy is converted into kinetic energy. The relaxed steam D is then at least one impeller 28 fed and braked there, creating a torque of the impeller 28 is produced.

In the exemplary embodiment, the turbine 6 designed as an axially flowed through, two-stage constant-pressure turbine. On the input side, it has the nozzle body 12 with several differently shaped inlet nozzles 26 on. On the nozzle body 12 closes in the axial direction 30 the turbine 6 initially a first rotatably mounted impeller 28 , then a stationary stator 32 and finally a second rotatably mounted impeller 28 at.

The associated course of the speed v of the steam D and the pressure p of the steam D while flowing through the turbine 6 arises from the 2 B , As can be seen from this, is through the inlet nozzles 26 the vapor pressure p is converted into flow velocity v, preferably at supersonic velocity, which is converted into rotational energy of the turbine shaft at a constant vapor pressure p.

The turbine 6 is formed overall in the manner of a cylindrical, cylindrical body made of individual cylindrical discs, namely the nozzle body 12 , the two wheels 28 and the stator 32 is formed. On the shell side, the individual elements each have structures for forming the inlet nozzles 26 or the running or guide vanes. These structures are formed for example by milling in solid material. The in 2A shown roller body is an inner component, which is installed in a cylindrical housing, not shown here, so that on the housing a radial Limitation of the inlet nozzles 26 and the individual flow spaces in the wheels 28 and the stator 32 is reached.

The waste heat provided for the use of the internal combustion engine depends greatly on the current operating state of the internal combustion engine and varies in a passenger car in a range of 1 KW to several 100 KW. One in the evaporator 4 generated steam mass flow m is proportional to this of the evaporator 4 absorbed heat output. The turbine 6 must therefore be adapted for a wide range of mass flows m. At the same time, the operating pressure p in the circuit should not be subject to excessive fluctuations.

To allow a high efficiency and an efficient operation are the inlet nozzles 26 according to a first aspect of the valve unit 15 and about the control unit 14 depending on the current mass flow m in different combinations can be switched. This is understood to mean that the current mass flow m is divided into different nozzle combinations as a function of the mass flow m. As a result, over a large mass flow range of the operating pressure p and thus the evaporation pressure in the evaporator 4 regulated and kept within a desired bandwidth. The operating pressure p fluctuates, for example, within a permissible fluctuation range by a nominal operating pressure p _nom , which is, for example, in the range between 5 and 20 bar.

Under nozzle combinations here is both a single inlet nozzle 26 as well as a plurality of inlet nozzles 26 up to all inlet nozzles 26 Understood. Each of the different nozzle combinations forms a switching stage. At defined, fixed predetermined switching points, which are formed by defined mass flows m, is switched between the successive switching stages. Each of the inlet nozzles 26 has a nozzle cross-section A, so that each inlet nozzle 26 is designed for a particular mass flow in its operating point at an optimal working pressure. The nozzle cross section is the cross section at the bottleneck of the respective inlet nozzle. A respective nozzle combination has an overall nozzle cross section which is the sum of the nozzle cross sections A of the individual inlet nozzles 26 is formed.

For a given nozzle cross-section A increases with increasing mass flow m of the operating pressure p. When switching to a nozzle combination with enlarged total nozzle cross-section then takes a sudden pressure drop, so generally referred to as a pressure jump pressure change. This pressure jump of the operating pressure p can negatively affect the stability of the evaporation process in the evaporator 4 to have.

According to a second aspect, therefore, the individual inlet nozzles 26 designed and combined in the individual switching stages such that the pressure jumps remain when switching between two switching stages as possible within a predetermined range of a maximum pressure jump Δp _max . In the case of a large number of inlet nozzles, the pressure jumps can be kept low by correspondingly small graduations of the total nozzle cross section.

However, a large number of inlet nozzles on the one hand leads to a high manufacturing outlay and thus to high costs. On the other hand, this increases the required installation space of the turbine.

The pressure jumps within the permissible maximum pressure jump Δp _max with at the same time minimum number of inlet nozzles 26 to hold, are the inlet nozzles 26 designed in a defined manner, as described below with reference to 3 is explained.

In the method for the design of the individual optimized switching stages, so optimal nozzle combinations, operating characteristics of the internal combustion engine and the evaporator 4 considered. It is specified as the maximum allowable pressure jump Δp _max a maximum value of the pressure change when switching between two switching stages. In particular, a permissible overshoot and undershooting of the desired nominal operating pressure p _{nom is} specified. The deviation from the nominal operating pressure p _nom should be as low as possible. For example, it is preferably in the range of 0.5 to 1.5 × 10 ⁵ Pa. In the embodiment described below, the deviation is 1 × 10 ⁵ Pa. By setting the maximum allowable pressure jump Δp _max switching thresholds S _min , S _max are therefore defined in 3 are represented by solid lines.

An undershooting of the lower switching threshold S _min is at the lowest switching stage at low mass flows up to a minimum operating pressure p _min (at minimum mass flow m _min ) allowed. This is in the exemplary embodiment at about 4 · 10 ⁵ Pa. Correspondingly, an exceeding of the upper switching threshold S _max at the highest switching stage for large mass flows m up to a maximum operating pressure p _max (at the maximum mass flow m _max ) is permissible. This is in the exemplary embodiment at about 9 · 10 ⁵ Pa. In general, for the lowest or highest switching stage, it is therefore permissible to undershoot or exceed the switching thresholds S _min , S _max by, for example, 0.5 times to a maximum of 1 times the maximum pressure jump Δp _max .

Taking into account the maximum expected mass flow m _max and taking into account a technically meaningful minimum mass flow m _min then the nozzle cross sections A of the individual inlet nozzles 26 determined under the premise that the lowest possible number of inlet nozzles 26 to cover the entire mass flow range between the minimum mass flow m _min and the maximum mass flow m _{max is} achieved.

To determine the individual nozzle cross-sections A, the system parameters are considered, namely which working medium M is used, which temperature T of the steam D at the inlet of the turbine 6 is to be expected. Furthermore, the nominal operating pressure p _nom , the maximum permissible deviation thereof, the maximum mass flow m _max , the useful minimum or minimum mass flow m _min and the maximum permissible pressure jump Δp _max between two switching stages are taken into account as input parameters for the design process. Furthermore, the minimum operating pressure p _min and the maximum operating pressure p _{max are} predetermined (corresponding to the minimum or maximum mass flow).

κ ist hierbei der Isentropenkoeffizient und ρ ist die Dichte des Arbeitsmediums M. Beide Größen hängen funktionell von der Wahl des Arbeitsmediums M, dem aktuell herrschenden Druck p sowie der aktuell herrschenden Temperatur T ab. κ, ρ = f(M, p, T). From the values for the temperature T, for the minimum mass flow m _min and for the minimum operating pressure p _min , the minimum nozzle cross section A _{min is determined} for the first switching stage. This minimum nozzle cross section A _{min also} corresponds to the nozzle cross section A of the first inlet nozzle 26 , The first switching stage is therefore exclusively by this one inlet nozzle 26 established. The calculation is based on the design of the inlet nozzles 26 assumed as Laval nozzles. Based on the model of a critically flown nozzle, the minimum (total) nozzle area A _{min is determined} according to the following formulas:

κ here is the isentropic coefficient and ρ is the density of the working medium M. Both variables depend functionally on the choice of the working medium M, the currently prevailing pressure p and the currently prevailing temperature T. κ, ρ = f (M, p, T).

The required maximum total nozzle cross section A _max is calculated according to the same formulas, for which purpose the values for the maximum mass flow m _max and the maximum operating pressure p _max are used.

In exceptional cases, with only a slight variability of the mass flow m, the boundary conditions mentioned above can already be achieved with a fixed nozzle cross-section.

Otherwise, based on the determined values for the minimum nozzle cross-section A _min and for the maximum nozzle cross-section A _max in the course of an iteration process, the parameters total number of inlet nozzles 26 as well as cross sections A of the inlet nozzles 26 the nozzle cross-sections of the inlet nozzles optimized to obtain a minimum number of inlet nozzles. It is first with the number of nozzles 2 started and the number of inlet nozzles 26 increased until the entire mass flow range between the minimum mass flow m _min and the maximum mass flow m _{max is covered} under the given conditions.

Assuming that always at least one inlet nozzle 26 open must be at N nozzles maximum N different nozzle cross-sections A and thus a total of a maximum of 2 ^N-1 combinations. When using inlet nozzles 26 same cross-section A reduce the possible combinations accordingly. Preferably, therefore, the nozzle cross sections A of the inlet nozzles 26 all different.

The result of such an iteration process is in the 3 as well as the following table:

In this case there are a total of 5 inlet nozzles 26 are provided, which are designed at their operating point (optimum working pressure) for different mass flows (here with water as the working medium M, steam temperature T = 300 ° C), namely for 1 g / s (nozzle 1), 2.0 g / s ( Nozzle 2), 2.5 g / s (Nozzle 3), 3.3 g / s (Nozzle 4) and 1.0g / s (Nozzle 5). Generally, about 3 to 8 inflow nozzles are preferably provided. The individual nozzles 26 are combined to a total of 8 switching stages. The nozzle combinations and switching steps result in particular from the above table. As can be seen from this, the first four switching stages are each by a single inlet nozzle 26 which cover a mass flow range of 1 g / s to 4.3 g / s in their operating point. The later switching stages for larger mass flows m are then by combinations of 2, 3, 4 or all nozzles 26 educated. The minimum mass flow m _min is 1 g / s and thus corresponds to the design of the inlet nozzle 26 for the first shift stage in the optimal working pressure for which the inlet nozzle 26 is designed.

The individual switching stages are determined in the exemplary embodiment such that the operating pressure p drops when switching to a next higher switching stage in each case to the pressure of the lower switching threshold S _min . At the same time, the individual inlet nozzles 26 all designed for the same optimum working pressure, this optimum working pressure preferably corresponds to the pressure of the lower switching threshold S _min , in the exemplary embodiment about 6 · 10 ⁵ Pa.

The switching between the switching stages takes place in each case at fixed predetermined switching points, which are defined by the input side of the turbine operating pressure (= inlet pressure). The switching is preferably carried out in each case at least approximately at the same operating or switching pressure. The deviation from a common switching pressure is preferably less than or equal to 5%. In the embodiment, an upper switching pressure is about 8 × 10 ⁵ Pa and a lower switching pressure at about 6 × 10 ⁵ Pa.

LIST OF REFERENCE NUMBERS

2

pump

4

Heat exchanger / evaporator

6

turbine

8th

capacitor

10

generator

12

nozzle body

13

truck assembly

14

Control unit!

15

valve unit

16

flow direction

18

High-pressure part

20

Low-pressure part

22

arrows

24

heat conduction

26

Inlet

28

Wheel

30

axially

32

stator

M

working medium

D

steam

H

hot medium

A

Nozzle area

A _min

minimum nozzle cross-section

A _max

maximum nozzle cross-section

m

mass flow

m _max

maximum mass flow

_min

minimum mass flow

p

operating pressure

p _nom

nominal operating pressure

p _min

minimum operating pressure

p _max

Maximum operating pressure

Δp _max

maximum pressure jump

T

steam temperature

S _min

lower switching threshold

S _max

upper switching threshold

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102007003801 A1 [0002]

Claims (15)

Device for utilizing waste heat of an internal combustion engine, in particular of a motor vehicle, with an evaporator ( 4 ) for transferring the waste heat to a working medium (M) and with a turbine drivable by the working medium (M) ( 6 ), characterized in that the turbine ( 6 ) several inlet nozzles ( 26 ) with fixed geometry for the working medium (M) and between different combinations of inlet nozzles ( 26 ), wherein the different combinations each form a switching stage for different current mass flows (m) of the working medium (M). Apparatus according to claim 1, characterized in that it is designed for a nominal operating pressure (p _nom ) and the inlet nozzles ( 26 ) and their combination in the respective switching stages are selected such that a predetermined maximum pressure jump (Δp _max ), in particular in the range of 5% to 20% of the nominal operating pressure (p _nom ) is not exceeded when switching between two successive switching stages. Apparatus according to claim 2, characterized in that the number of inlet nozzles ( 26 ) is minimized taking into account a predetermined mass flow range. Device according to one of the preceding claims, characterized in that the inlet nozzles ( 26 ) are designed for a given operating pressure (p) of the working medium (M) for different mass flows (m), and in particular each of the inlet nozzles ( 26 ) is designed for a different mass flow (m). Device according to one of the preceding claims, characterized in that for a given, expected mass flow range between a minimum mass flow (m _min ) and a maximum mass flow (m _max ) successive switching stages with a fixed predetermined combination of inlet nozzles ( 26 ) are given. Device according to one of the preceding claims, characterized in that it is designed for a mass flow range (m) of the working medium (M), which corresponds to an isentropic steam expansion capacity between 0.1 kW and 50 kW. Device according to one of the preceding claims, characterized in that several, preferably all inlet nozzles ( 26 ) are designed for the same optimum operating pressure (p) of the working medium (M). Device according to one of the preceding claims, characterized in that it is designed for a nominal operating pressure (p _nom ) and a lower switching threshold (S _min ) and an upper switching threshold (S _max ) for the switching between the individual switching stages are given, wherein the upper switching threshold (S _max ) also has an uppermost switching stage for a higher maximum operating pressure (p _max ) in the range of 115% to 200% of the nominal operating pressure (p _nom ) and above the lower switching threshold (S _min ) a lowest switching stage for a minimum operating pressure (p _min ) in the range in particular from 50% to 85% of the nominal operating pressure (p _nom ) is designed. Device according to one of the preceding claims, characterized in that a minimum mass flow (m _min ) is predetermined and the first switching stage by a single inlet nozzle ( 26 ), which is designed at a minimum operating pressure (p _min ) for a mass flow (m) corresponding to the minimum predetermined mass flow (m _min ). Device according to one of the preceding claims, characterized in that the last switching stage, which is designed for a maximum mass flow (m _max ), the combination of all inlet nozzles ( 26 ) having. Device according to one of the preceding claims, characterized in that the mass flow difference, for which two successive inlet nozzles are designed, is in the range of 1/4 to 4/4 of the minimum mass flow (m _min ). Device according to one of the preceding claims, characterized in that it is in the turbine ( 6 ) is an axially flowed constant pressure turbine. Device according to one of the preceding claims, characterized in that the turbine ( 6 ) an inflow-side cylindrical nozzle body ( 12 ) with at least one adjoining impeller ( 28 ), wherein the inlet nozzles ( 26 ) on the shell side of the cylindrical nozzle body ( 12 ) are formed. Turbine ( 6 ) for a device according to one of the preceding claims, characterized in that it comprises a plurality of inlet nozzles ( 26 ) for a working medium (M) and between different combinations of inlet nozzles ( 26 ), wherein the different combinations each form a switching stage for different current mass flows (m) of the working medium (M). Method for using waste heat of an internal combustion engine, in particular of a motor vehicle, in which a working steam (D) is generated from the waste heat and with this a turbine ( 6 ), characterized in that the turbine ( 6 ) several inlet nozzles ( 26 ) and depending on the respective current mass flow (m) between different combinations of inlet nozzles ( 26 ) is switched.

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