DE102020215579A1 - Process for reducing the amount of water required to protect components in Otto engines with water injection systems - Google Patents

Abstract

Method for operating a supercharged, spark-ignited internal combustion engine (10) having a combustion chamber (22) in which combustion takes place, a drive shaft (28) and an exhaust system (13), in one step the internal combustion engine ( 10) is operated at an operating point (P0) at a speed (n28; n28P) of the drive shaft (28) at which an air ratio (lambda) is set in a combustion chamber (22), with a torque ( M28.0) that is just achieved with a stoichiometric air ratio (lambda) and at the same time a maximum permissible temperature (Tzul,max) of the exhaust system (13) is just maintained without water being fed to the combustion chamber (22). - wherein the charger (31) is operated at its lowest possible power in this operating point (P0), - wherein measures are taken to reduce a temperature (T13) in the exhaust system (13).

Description

State of the art

It is known from the prior art that mixture enrichment takes place as soon as a certain engine performance can no longer be achieved because the exhaust gas temperatures are too high. Internal combustion engines are also known from the prior art whose combustion processes in the combustion chambers are influenced by the use of water injection instead of mixture enrichment. An advantage of this water injection is that a temperature of the exhaust gas is reduced over a temperature of the exhaust gas from combustion without the introduction of water through water injection. As a result, components in an exhaust line, for example the turbine, a catalytic converter or a particle filter, can be protected from excessive temperatures and the resulting damage. In addition, as a result of the reduced temperatures, emissions can be reduced and the performance of an internal combustion engine can be increased.

The injection of water is used to replace the previously used mixture enrichment for the purpose of component protection and thus to achieve stoichiometric operation. Mixture enrichment has the disadvantage that there are operating points at which a mixture is considered “rich” and accordingly has a lambda value of less than 1. The exhaust gas temperature is reduced by the mixture enrichment. Sub-stoichiometric operation means that the function of the 3-way catalytic converter is no longer guaranteed, and there are greatly increased hydrocarbon and carbon monoxide emissions and greatly increased fuel consumption

The mode of operation of the water injection is primarily based on the effect of cooling the cylinder charge, in which the high enthalpy of vaporization of water is used. On the one hand, the temperature level of the process is reduced and the tendency to knock is reduced. Both have the effect of reducing the exhaust gas temperature.
Of particular importance in this process is that the amount of water required should be reduced. On the one hand, the water to be injected is another medium that has to be carried in the vehicle and therefore has to be refilled. In addition, a corresponding tank influences the use of space within the body (“packaging”). It is therefore desirable to provide a sensibly small tank size for the water. A reduced water consumption thus leads to an extension of the interval for refueling and, on the other hand, to an increased range in water operation with one water tank filling. Other reasons for reducing the water rate are also the reduction of a so-called blow-by quantity and thus the entry of water into the engine oil. It is therefore intended to increase the map range that can be operated stoichiometrically and to reduce the map range that is at even higher power levels and in which water must be injected, and on the other hand to reduce the amount of water in the map range with water injection.

According to a first aspect of the invention, a method of operating a supercharged, spark-ignited internal combustion engine having a combustion chamber is provided. The internal combustion engine is operated at an operating point at a speed of the drive shaft at which an air ratio is set in a combustion chamber, with a torque being set at this operating point that is just achieved with a stoichiometric air ratio and at the same time a maximum permissible temperature of the exhaust system is just is still maintained without water being supplied to the combustion chamber, with the charger being operated at its lowest possible power at this operating point, and measures being taken to reduce a temperature in the exhaust system. This has the advantage that the elements of the exhaust system already mentioned, such as the turbine, the catalytic converter or a particle filter, are protected from excessive temperatures and thus destruction.

According to a first variant of a corresponding method, so-called artificial throttling is provided. For this purpose, in a first step, the power of a charger is increased, thereby increasing the pressure in an intake manifold between the charger and a throttle valve. This is accompanied by an increase in turbine power. This reduces the enthalpy in the exhaust gas after the turbine and lowers the exhaust gas temperature. This has the advantage that when the operating point of the internal combustion engine is already such that a temperature of the exhaust gas after the turbine assumes a maximum permissible temperature value, the distance from the maximum permissible temperature value is regained in terms of temperature. This means that even with this approach - despite the approach of changing the operating point to higher outputs - cooling of the combustion by water can potentially be avoided. The approach is only possible if the maximum permissible turbine inlet temperature has not yet been reached.

However, in the event that an operating point is not to be shifted towards higher power levels, provision is made for an associated immediate increase in the pressure in the intake manifold between the charger and the throttle valve is reduced again for the subsequent part of the intake manifold between the throttle valve and an inlet into a combustion chamber through the throttle valve. As a result, an air charge in the combustion chamber does not increase in the ideal case, an air-fuel ratio lambda equal to 1 is set again, and as a result a fuel mass in the combustion chamber is also the same as before this measure. A torque then does not increase at the drive shaft.

If after a first iteration (increasing the air mass through the charger, increasing the pressure through the charger, reducing the pressure through the throttle valve), there is still a difference between the exhaust gas temperature at the critical point and the maximum permissible temperature at the critical point, this can Procedures as just described are repeated. This makes it possible to increase engine performance until the exhaust gas temperature limit is reached. In the final state, the turbine power, compressor power and the maximum engine power output at lambda = 1 is increased compared to the initial state, but the throttle valve is closed further than before.

According to a further aspect of the invention, when a maximum permissible temperature can no longer be maintained in the exhaust system without the supply of water, provision is made for water to be injected into the combustion chamber in a further step. This enables a margin to be restored between the exhaust gas temperature at the critical point and the maximum allowable temperature at the critical point. Such a result after water injection can be used to again provide an increase in pressure through the supercharger and then a decrease in pressure through the throttle valve.

According to an alternative method, provision can be made to shift a focus of combustion in the combustion chamber in one step and thus reduce an exhaust gas temperature at a critical point in the exhaust system. It is intended to shift the center of combustion to an earlier point beyond the center of combustion that is optimal in terms of efficiency. If shifting the focus of combustion forward no longer lowers the exhaust gas temperature at the critical point or alternatively the risk of knocking during combustion becomes too great, injecting water into the combustion chamber can cool the combustion and thus both Lowering the exhaust gas temperature as well as reducing the risk of knocking, which in turn enables the ignition angle to be advanced. In addition, the two methods mentioned above can be combined with one another in order to further reduce the drop in exhaust gas temperature at a critical point.

Furthermore, a computer program is provided that executes all the steps of one of the methods. In addition, a machine-readable storage medium is optionally provided, on which the computer program is stored. Furthermore, a control device should be designed in such a way that it can carry out all the steps of one of the methods.

• 1 eine Darstellung eines typischen Systems aus Brennkraftmaschine, Abgassystem und Luftzufuhrsystem,
• 2 eine Darstellung eines beispielhaften prinzipiellen Verlaufs eines maximalen Drehmoments,
• 3 eine Darstellung einer benötigten Wasserrate in Abhängigkeit von einer Last der Brennkraftmaschine,
• 4 einen Verfahrensablauf eines ersten Ausführungsbeispiels,
• 5 einen Verdichter mit elektrischer Maschine,
• 6 ein zweites prinzipiell dargestelltes Verfahren zum Beeinflussen einer Abgastemperatur.

The invention is explained in more detail with reference to the figures described below. Show it:

• 1 a representation of a typical system of internal combustion engine, exhaust system and air supply system,
• 2 a representation of an exemplary basic course of a maximum torque,
• 3 a representation of a required water rate depending on a load of the internal combustion engine,
• 4 a process flow of a first embodiment,
• 5 a compressor with an electric machine,
• 6 a second method shown in principle for influencing an exhaust gas temperature.

In the 1 A typical system including an internal combustion engine 10, an exhaust system 13 connected to the engine 10, and an air supply system 16 is shown. In this example, this system is located in a motor vehicle. The internal combustion engine 10 typically has a plurality of cylinders 19 . In each cylinder 19 there is a combustion chamber 22 in which combustion of fuel with air takes place. In each cylinder 19 there is also a piston 25 which drives a drive shaft via a drive mechanism, not shown but well known, consisting of a connecting rod and a crank. A pressure acting on the piston 25, which is caused by the combustion of fuel with air, causes, among other things, a compressive force on the connecting rod, so that the drive shaft 28 rotates. The drive shaft 28 subsequently drives with a drive torque M28. In this case, a drive wheel or a plurality of drive wheels is typically driven via a clutch (not shown), a manual transmission and a differential gear, so that a motor vehicle can be driven by means of the internal combustion engine 10 . By means of at least one camshaft 30 are in detail non-illustrated valves (intake valves and/or exhaust valves) are actuated in order, for example, to let fresh air through an inlet into a combustion chamber 22 (intake valves) and to let out exhaust gas into the exhaust system 13 (exhaust valves).

The exhaust system 13 serves to discharge exhaust gas discharged from a combustion chamber 22 into the environment. For this purpose, it has a pipe system which—typically beginning with an exhaust manifold—feeds the exhaust gas to a charger 31 . The exhaust gas is fed to an exhaust gas side 34 of the charger 31 . Within this exhaust side 34, the exhaust gas drives a turbine wheel 37, which is connected via a shaft 38 to a compressor wheel 40 in a torque-transmitting manner. After the exhaust gas has passed through the exhaust gas side 34, the exhaust gas enters a pipe area 43 which feeds the exhaust gas to an exhaust gas reactor 47 (e.g. a three-way catalytic converter). In this exhaust gas reactor 47, the chemical composition of the exhaust gas is changed as intended. This exhaust gas reactor 47 can, for example, be followed by another exhaust gas reactor, not shown here. This can be a particle filter, for example.

A bypass valve 50 is connected in parallel with the charger 31 or its exhaust gas side 34 in a bypass flow path 49 . This bypass valve 50 (also called bypass valve or wastegate) serves to change the pressure of the exhaust gas in front of the turbine wheel 37 and thus to change the pressure of the compressor, the charge pressure. If the bypass valve 50 is closed, for example, and the bypass path 49 is closed as a result, the entire quantity of the exhaust gas is routed via the turbine wheel 37 . If the bypass valve 50 is partially or even fully open, part of the exhaust gas is routed past the exhaust side 34 of the charger 31 in order to avoid an undesirable increase in the pressure in front of the turbine and thus ultimately the boost pressure.

The air delivery system 16 begins upstream with an inlet 60 through which air is drawn from the environment. This air is sucked into a compressor side 63 of the charger 31 and from there transported further through the compressor wheel 40 . First, this compressed air is pressed into an intercooler 66 and cooled there. From there this air is pressed into an intake manifold 69 . A quantity of air which is to reach the combustion chambers 22 is precisely metered by a throttle flap 72 . For example, partially closing the throttle valve 72 reduces the amount of air and an air pressure between the throttle valve 72 and an intake valve.

In 2 an exemplary basic illustration of a curve of a maximum torque M28max(n28) of an internal combustion engine 10 as a function of a speed n28 of the drive shaft 28 is shown. Furthermore, a line Lllim is also shown as an example and in principle. This line Lllim represents a specific limit for an air ratio lambda. On the left and along this course of this line Lllim, it is just possible to generate an exhaust gas during operation of internal combustion engine 10, which is formed from an air-fuel mixture with an air ratio lambda equal to 1 and whose temperature T maximum corresponds to a maximum permissible temperature Tzul,max and furthermore no cooling measures have to be taken in order not to exceed this maximum permissible temperature Tzul,max. In this general description, this maximum permissible temperature Tzul,max is initially only a maximum permissible temperature, which is determined, for example, for any component of the exhaust system 13 . This maximum permissible temperature Tzul,max can be, for example, a maximum permissible temperature at an inlet 52 on the exhaust gas side 34 of the charger 31 or, for example, at an outlet 53 on the exhaust gas side 34 of the charger 31 or also, for example, at an inlet 54 of an exhaust gas reactor 47 or in the exhaust gas reactor 47 .

A first exemplary method V1 is based on the 1 , 2 , 3 and 4 explained in more detail. In 3 a representation of a relationship between the required water rate and a load of the internal combustion engine is shown. 4 shows the individual process steps. As part of this first exemplary embodiment, what is referred to here as artificial throttling, in particular throttling in the air supply system 16, is provided. This throttling aims to lower the temperature of the exhaust gas. This lowering of the temperature of the exhaust gas by throttling has the advantage that a line Lllim is shifted in this example and now occupies the position of the line L1lim' by way of example. While without these special process steps at an operating point beyond line L1lim, ie within the area described by points A, B, C and D, cooling measures are required in order to keep the temperature of the exhaust gas at the maximum permissible temperature Tzul,max hold, it is made possible by the special method steps that it would only be necessary to take special cooling measures such as enriching the mixture and/or adding water to the combustion chamber 22 when the new line L1lim' is exceeded.

This first exemplary method assumes a system as described in 1 represent is posed. The charger 31 shown there has the bypass valve 50 already mentioned. The internal combustion engine 10 shown there has a conventional camshaft 30. A conventional camshaft 30 is understood to mean a camshaft 30 which, among other things, has the features that the valve opening duration that can be achieved by it, based on a valve lift of 0.5 mm, is in the range between ~190 and ~220 °KW lies. In particular, it is a camshaft 30 that is neither a Miller camshaft nor an Atkinson camshaft.

An operating point P0, to which the speed n28P and the torque M28,0 are assigned, is initially assumed. At this operating point P0, it is just about possible to maintain the maximum permissible temperature Tzul,max in accordance with what was mentioned above. In addition, at this operating point P0 the air/fuel ratio lambda is equal to 1, which is why this operating point P0 lies on the line Lllim or this point on the line Lllim results from this property. At this operating point P0, the bypass valve 50 has a specific closed position p50,0. In addition, at this operating point P0 it is not yet necessary to take special cooling measures, i. H. which can just about get by without the injection of water into the combustion chamber 22 . Above this operating point P0, water would have to be injected or the mixture enriched in order to continue to be able to maintain the maximum permissible temperature Tzul,max. A charging pressure of the charger 31 now corresponds to the charging pressure p31.0.

Based on this situation, the bypass valve 50 is brought into a new closed position p50,1 as part of the method, ie in general a power P31 of the charger 31 is increased, step S1, ie the bypass valve 50 is further closed as a result of this new closed position p50,1 in the closed position p50.0. This further closing of the bypass valve 50 results in an increase in the output of the charger 31 . As in the comparison with the representation after 1 As can be seen, this further closing of the bypass valve 50 results in the proportion of the exhaust gas which is routed through the supercharger 31 being increased. The proportion of the exhaust gas flowing through the sub-flow path 49 is reduced. This leads to an increase in the charge pressure to the charge pressure p31,1, step S2. This increased boost pressure p31,1 is above the pressure p required for the load to be set. To ensure that the temperature of the exhaust gas does not rise in this situation under the condition that lambda equals 1, the increase in boost pressure p31,1 in the intake manifold 69 results in an inflow of air to one or more combustion chambers 22 and thus also the pressure in the intake manifold between the throttle valve 72 and to throttle an intake through the throttle valve 72, ie to reduce a flow cross-section through the throttle valve 72, step S3. This allows a load to be kept constant. With appropriate tuning, the operating point P0 can be maintained. Due to the above-described further closing of the bypass valve 50 and the resulting increase in the output of the charger 31, energy is extracted from the exhaust gas (enthalpy drop across the turbine wheel 37). This reduced energy of the exhaust gas leads to a reduction in the temperature of the exhaust gas after the turbine wheel 37 and thus a reduced temperature T53 at the outlet 53 of the exhaust gas side 34 of the charger 31. Furthermore, this reduced temperature T53 leads to a reduced temperature T47 in the exhaust gas reactor 47. If For example, if the temperature T47 is determined as the critical temperature of the exhaust gas system 13, this means that the measures described above mean that the practically reduced temperature T47 is now at a greater distance from a limit value of the critical temperature in the exhaust gas reactor 47, the temperature of the temperature limit T47,lim, having. That is, related to 2 that, for example, the power output at the speed n28P can be increased by increasing the torque, starting from the torque M28,0 at the operating point P0, to a torque M28,1 at an operating point P1, for example, without reaching the temperature limit T47,lim and straight not having to take any additional internal engine measures, such as injecting water into the combustion chamber 22.

In this example after 2 with a further closing of the bypass valve 50 and the resulting further increase in the output of the supercharger 31, the operating point P2 is assumed and the temperature limit T47,lim is reached again. In this operating point P2, the position of the throttle valve 72, ie a flow cross-section through the throttle valve 72, vs. the position in the operating point P1, further reduced and thus a flow resistance through the throttle valve 72 increased. At this operating point P2, a value of 1.0 for the lambda air ratio can just about be set without reaching the temperature limit T47,lim, with additional measures being taken, namely the injection of water into the combustion chamber 22.

Of course, it is not necessary for a speed n28P of the drive shaft 28 to be kept at a constant value as part of the method, as is shown in 2 is shown. Rather, the speed n28P can be increased or decreased. For the operating points to be assumed - provided they are within the ACD field or within the ACEA field - the above applies analogously.

In the 3 the already mentioned water rate mH20 is shown over the current load of the internal combustion engine 10 . Torque M28 is used here as the load. A curve K1 is an example of a required water rate at lambda=1 and an unthrottled exhaust gas flow. A curve K2 stands for a required water rate at lambda=1, the exhaust gas flow being throttled as much as possible in accordance with method V1 while maintaining the maximum permissible temperature. The curve K3 stands for a water rate using a method which combines the methods V1 and V2, ie in addition to the method V1 the displacement of the combustion center of gravity position is combined further according to the method V2, the description of which follows below.

In summary, the temperature limit T47,lim can be maintained below or up to and including the connecting line between points A-D, Lllim without having to take additional cooling measures within the engine. For this exemplary method, it applies that above the connecting line between points A-D, Lllim and up to and including the connecting line between points A-E, L1lim', the temperature limit T47,lim can be maintained, with no additional cooling internal engine measures having to be taken (no injection of water). This exemplary method means that additional cooling internal engine measures only have to be taken when the connecting line between points AE, L1lim' is crossed, i. H. water must be injected in order to comply with the temperature limit T47,lim. When the connecting line between points AC, L1lim" is reached, i.e. with an exemplary closing of the bypass valve 50 and an adapted closing of the cross section of the throttle valve 72, as well as cooling internal engine measures by injecting water, the temperature limit T47,lim can just about be maintained. Above this line between points A-C, L1lim" the temperature limit T47 can no longer be maintained with the measures described here. This means that by shifting the limit from L1,lim to L1,lim', an area of the characteristics map in which water has to be injected into a combustion chamber 22 in order to maintain the temperature limit T47,lim is reduced. Accordingly, an amount of water to be carried can be expectedly reduced. Or to put it another way: with the same amount of water, a longer distance can be covered as expected using the method proposed here.

The method only reaches its limits when the following limitations occur: when a maximum permissible temperature in front of the exhaust gas side 34 or the turbine wheel 37 and a maximum permissible temperature in the exhaust gas reactor 47 are reached at the same time. Alternatively, this applies when a maximum permissible pressure in front of the exhaust gas side 34 or the turbine wheel 37 is reached, or when a maximum permissible speed of the charger 31 is reached, or when a maximum permissible temperature is reached at the outlet of the compressor side 63, or when a maximum permissible charging pressure is reached.

For this purpose, a function is created in an engine controller of the internal combustion engine 10 which makes it possible to specify a pressure difference across the throttle flap 72 as a function of speed and load in order to achieve the desired temperature reduction. This pressure difference is then to be added to the required and specified target charging pressure. A desired boost pressure and the intake manifold pressure p69 required for the load are then regulated by the engine control. The desired boost pressure p52 at the inlet 52 of the exhaust gas side 34 is set or regulated via the position of the bypass valve 50, and the cylinder charge is set or regulated via the position of the throttle valve 72. This functionality for lowering the temperature T47 is coordinated. If, for example, a required pressure difference can no longer be set, a corresponding increase in temperature must be compensated for by an increased water injection rate. As an alternative, the function can also be implemented in that a position of the throttle valve 72 is permanently preset via speed/load (operating point). The load is then regulated purely by setting the required boost pressure. This approach also requires some form of coordination. If the required boost pressure cannot be provided, the throttle valve 72 must be opened further and the water rate increased accordingly.

In the event that instead of a charger 31 with a fixed geometry, a charger 31 with variable turbine geometry without bypass valve is used, instead of changing a setting of the closed position of the bypass valve, a change in the variable turbine geometry is made and in this way generally a power P31 of the charger 31 increased. The steps described above in relation to the bypass valve are readily transferrable to the supercharger 31 with variable turbine geometry.

In another embodiment based on the first embodiment, a camshaft 30 of a special design is used. In this case, two different alternative designs are possible. the The first alternative embodiment is the already mentioned Miller camshaft, which has an opening duration or an opening angle of less than or equal to 190° crankshaft angle in relation to a combustion chamber 22 . This opening angle is based on a stroke of more than 0.5 mm. An internal combustion engine 10 with such a camshaft 30 would have in its exhaust system 13 - as already described above - either a charger 31 with a fixed geometry, parallel to which a bypass path 40 with a bypass valve 50 is connected, or a charger 31 with variable turbine geometry without bypass valve. Thus, while an increase in charge pressure p52, as already mentioned above, takes place either by controlling the bypass valve 50 or by changing the variable turbine geometry, in this exemplary embodiment the throttling takes place primarily with the Miller camshaft. This does not increase the load. In this context, throttling means that the throttling takes place by advancing the camshaft 30, ie the Miller camshaft. Advancing means that the camshaft 30 closes an inlet into a combustion chamber 22 earlier than before at a lower pressure p69.2 in order to reduce the filling of air and thus also of an air-fuel mixture. If throttling to the desired extent is not possible with such a procedure, ie only by adjusting the camshaft 30, the throttling is supplemented by reducing a free cross section of the throttle flap 72--as in the exemplary embodiments described above. It is particularly preferred that the throttling takes place via the throttle valve 72 only to a lesser extent. For the purpose of throttling the air flow through the intake valve, a lift of an intake valve can also be reduced, for example by adjusting the camshaft 30.

The camshaft 30 should not be used for load control here. The position of the camshaft 30 to be advanced is preset during application in the development phase; however, this is entirely dependent on external boundary conditions such as ambient pressure, intake temperature, engine temperature, etc. In this case, too, there must be a functionality in the control of internal combustion engine 10 that allows a differential pressure to be retained via throttle valve 72 .

In the second alternative embodiment, the already mentioned Atkinson camshaft is used as the camshaft 30 . In relation to a combustion chamber 22, this has an opening duration or an opening angle of greater than or equal to 220° crankshaft angle. This opening angle is also based on a stroke of at least 0.5 mm. As already described in the embodiment above, such an internal combustion engine 10 with such a camshaft 30 also has in its exhaust system 13 either a charger 31 with a fixed geometry, parallel to which a bypass flow path 40 with a bypass flow valve 50 is connected, or a charger 31 with variable turbine geometry without bypass valve.

Here, too, boost pressure p52 is increased, as already mentioned above, by either activating bypass valve 50 or changing the variable turbine geometry, and in this exemplary embodiment throttling takes place primarily with the Atkinson camshaft. This does not increase the load. In this case, throttling means that the throttling takes place by retarding the camshaft 30, ie the Atkinson camshaft. Retarding means that an inlet into a combustion chamber 22 is closed later by the camshaft 30 than at a lower pressure p69,2. This takes place when the pressure p69,2 in the intake manifold 69 between the throttle valve 72 and the inlet into the combustion chamber 22 has increased. If such a procedure, i. H. only by adjusting the camshaft 30, throttling to the desired extent is not completely possible, as in the previously described exemplary embodiments, the throttling is supplemented by reducing a free cross section of the throttle flap 72. It is particularly preferred that the throttling takes place via the throttle valve 72 only to a lesser extent. For the purpose of throttling the air flow through the intake valve, a lift of an intake valve can also be reduced, for example by adjusting the camshaft 30.

Another exemplary embodiment is described below in two variants. It is based on the first embodiment and, in a first variant, can use the in 1 have described exhaust system with a charger 31 with fixed geometry and a bypass path 49 including bypass valve 50. In a second variant—as before—a charger 31 with variable turbine geometry without a bypass valve is used instead of a charger 31 with a fixed geometry. Instead of changing a setting of the closed position of the bypass valve, the variable turbine geometry is changed as a substitute, and in this way a power P31 of the charger 31 is generally increased. The steps described above in relation to the bypass valve are readily transferrable to the supercharger 31 with variable turbine geometry. The camshaft 30 used allows a different adjustment of the maximum intake valve lift of an intake valve (variable valve lift).

In both cases, the method proceeds in such a way that the bypass valve 50 or the variable turbine geometry of the supercharger 31 continues to be closed within the respectively desired or necessary framework. In this case, however, the boost pressure or intake manifold pressure p69 is not throttled by adjusting (further closing or opening) the throttle flap 72 in order to maintain the required air charge in the combustion chamber 22 . Rather, the desired air charge is set and the air flow is thus throttled by combined actuation of camshaft 30. This combined actuation of camshaft 30 can have an advance adjustment of the intake camshaft and also a reduction in the lift of an intake valve. The measures proposed here also reduce the temperature after the turbine wheel 37 and thus also reduce the temperature T47 in the exhaust gas reactor 47 . As already described for the first exemplary embodiment, this makes it possible to increase the load and reduce water consumption.

In the case of an internal combustion engine 10 with variable valve lift, the functionality or control functionality can be implemented in two different ways. Either the valve lift is preset with a lower valve lift, analogous to the control times, depending on the engine speed or load. In this context, the load is then regulated directly via the regulation of a required boost pressure p69 by appropriate adjustment of the variable turbine geometry or appropriate adjustment of the bypass valve 50 . At this point it should be noted that the control time or the control times of the camshaft 30 with variable valve lift are not used for load regulation and are defined during the application. Alternatively, the throttling functionality can be implemented in such a way that a desired boost pressure p69 is set or preset. A filling of a cylinder 19 or a combustion chamber 22 is then controlled via the valve lift.

Some form of coordination is also required in this exemplary embodiment. If a required boost pressure p69 cannot be provided, a valve lift must be increased and a water rate increased accordingly.

Another embodiment based on the first embodiment described herein provides a charger 31 that is electrically assisted. This loader 31, 5 , is electrically assisted. This means that an electrical machine 75 acts on the function of the charger 31 via the shaft 38 thereof. In connection with the reduction in temperature T53 at the outlet 53 of the turbine wheel 37 or the reduction in the temperature 47 in the exhaust gas reactor 47, the flow resistance of the charger 31—and here particularly the exhaust gas side 34 of the charger 31—is increased. Especially related to 2 , of the characteristics map shown there and in particular relating to the area ACDA and the operating points potentially located there, the electric machine 75 is used. The ultimate goal is to use the drag of the supercharger 31 to extract energy from the exhaust gas (enthalpy drop across the turbine wheel 37) and thereby lower the temperature after the supercharger 31 as described above. For this purpose, especially after closing the bypass valve 50 or changing the variable turbine geometry, the electric machine 75 is operated in such a way that it causes a braking torque on the shaft 38 of the charger 31 . This is done by what is known as recuperation, ie the electric machine 75 is operated as a generator, so that this current is preferably supplied to an on-board network of the motor vehicle. In the ideal case, this means that despite the closing of the bypass valve 50 or the closing of the variable turbine geometry, there is no increase in boost pressure of the pressure p69. If recuperation is not possible or only possible to a limited extent, the method described can be supplemented by closing the throttle valve 75 and thus reducing the pressure p69 in the intake manifold 69.

Another basic method V2 for reducing an exhaust gas temperature is described below with reference to 6 described. In this method, provision is made for shifting a focal point of the combustion, step S10, by setting an ignition point earlier than before, and thus an ignition spark ignites the fuel-air mixture in combustion chamber 22 earlier than before. In this case, a center of gravity that is optimal with respect to an operating point with regard to the consumption of internal combustion engine 10 is to be shifted in the “advanced” direction, step S11. Internal combustion engines 10 are usually not knock-limited in a map range well below a rated output. In this power range, a relatively large amount of heat is dissipated via a combustion chamber wall, for example the cylinder wall or the cylinder head, so that the exhaust gas temperature drops. A limitation is that knock occurs when the ignition angle is too early. There must therefore be a gap between the efficiency-optimal ignition angle and the knock limit in order to be able to advance the ignition angle. Due to the early onset of combustion, early ignition times lead to relatively high ignition points relatively early on combustion chamber pressures. Increases in pressure and high temperatures in the combustion chamber, but a reduced exhaust gas temperature.

In this power range, which is well below a nominal power, an ignition angle is applied in such a way that an optimal combustion focus is set in terms of fuel consumption. This center of gravity is at approx. 8° crankshaft after ignition TDC (TDC = top dead center). Nevertheless, due to the set operating points in this power range, exhaust gas temperatures could already set in that would exceed a permissible maximum temperature upstream of a charger 31 or a permissible maximum temperature in an exhaust gas reactor 47 . For this reason, advancing an ignition timing would be recommended, since this would lead to a reduction in exhaust gas temperatures. In accordance with the mechanism of promoting knocking described above, it would be disadvantageous that significant increases in temperature and pressure would occur earlier. One of the main mechanisms of action of water injection is to reduce the tendency of the fuel-air mixture to knock by cooling this mixture at the ignition point or even before the ignition point. This means that when the ignition point is advanced, it may already be necessary in this power range or it would be necessary to influence the combustion process in a combustion chamber 22 - for example by injecting water into the combustion chamber 22, step S12 - in such a way that the The temperature of the combustion process is reduced in order to avoid knocking and a temperature of the exhaust gas would be reduced at the same time. If a shift in the center of combustion was limited to 8° crankshaft after ignition TDC at the earliest, a main effect of the water injection would also be limited. The controlled procedure would be as follows: When the temperature limit is reached, the ignition angle is advanced up to the knock limit, with the load then being increased. If the knock limit is reached, water can then be injected to further increase the load. The ignition angle then to be set can always be earlier than the ignition angle that is optimal for efficiency, in order to keep the exhaust gas temperature correspondingly low.

As already indicated, within the scope of this method it is provided that the shift in the center of combustion is also advanced further beyond a consumption-optimal point (step 11) in order to reduce the exhaust gas temperature. A limit of advance is reached when one of the following limits is reached: the knock limit, the maximum allowable cylinder pressure, the maximum allowable cylinder pressure rise. For this purpose, a requested ignition angle is stored in the controller of the internal combustion engine 10 in a format which takes into account a current speed, a load to be called up or set, control times and a water rate as influencing factors.

The two methods V1 “artificial throttling” and V2 “advanced shift in the center of combustion beyond the consumption-optimal point” described here in principle can be combined with one another in any form. In order to obtain a minimized or minimum water rate as an optimum, detailed data must be determined as part of an application.

Claims (21)

Method for operating a supercharged, spark-ignited internal combustion engine (10) having a combustion chamber (22) in which combustion takes place, a drive shaft (28) and an exhaust system (13), in one step - the internal combustion engine (10) is operated at an operating point (P0) at a speed (n28; n28P) of the drive shaft (28) at which an air ratio (lambda) is set in a combustion chamber (22), wherein at this operating point (P0 ) a torque (M28,0) is set that is just achieved with a stoichiometric air ratio (lambda) and at the same time a maximum permissible temperature (Tzul,max) of the exhaust system (13) is just maintained without the combustion chamber (22 ) water is supplied, - the charger (31) being operated at its lowest possible power at this operating point (P0), - wherein measures are taken to reduce a temperature (T13) in the exhaust system (13). procedure after claim 1 , characterized in that in a first step (S1) the power of the supercharger (31) is increased and thereby a pressure (p69,1) in an intake manifold (69) between the supercharger (31) and a throttle valve (72) is increased. procedure after claim 2 , characterized in that the pressure (p69,2) in the intake pipe (69) between the throttle valve ( 72) and an inlet into a combustion chamber (22) in a step (S2). procedure after claim 3 , characterized in that the pressure (p69,2) in the intake manifold (69) between the throttle valve (72) and an inlet into a combustion chamber (22) is lowered in a step (S3). procedure after claim 4 , characterized in that to reduce the pressure (p69,2) in the intake manifold (69) between the throttle valve (72) and the inlet into the combustion chamber (22), a flow resistance of the throttle valve (72) is increased. Procedure according to one of claims 2 until 4 , characterized in that the steps (S1; S2; S3) are repeated one after the other. Procedure according to one of Claims 1 until 3 , characterized in that a camshaft (30) is used in an embodiment as a Miller camshaft, through which an increased pressure (p69.2) in the intake manifold (69) between the throttle valve (72) and the inlet into the combustion chamber (22). an adjustment of the camshaft (30) is throttled. procedure after claim 6 , characterized in that the intake into the combustion chamber (22) is closed earlier by the camshaft (30) than at lower pressure (p69,2). procedure after claim 6 or 7 , characterized in that for the purpose of throttling an air flow through the intake valve whose lift is reduced. Procedure according to one of Claims 1 until 3 , characterized in that a camshaft (30) is used in an Atkinson camshaft design, through which an increased pressure (p69,2) in the intake manifold (69) between the throttle valve (72) and the inlet into the combustion chamber (22). an adjustment of the camshaft (30) is throttled. procedure after claim 9 , characterized in that the intake into the combustion chamber (22) is closed later by the camshaft (30) than when the pressure is lower (p69,2). procedure after claim 9 or 10 , characterized in that for the purpose of throttling an air flow through the intake valve whose lift is reduced. Procedure according to one of Claims 1 until 5 , characterized in that the charger (31) is electrically supported in that an electric machine (75) increases a flow resistance of the charger (31) via a shaft (38) of the charger (31) in order to extract energy from the exhaust gas by recuperation, by the electric machine (75) is operated as a generator. Procedure according to one of claims 2 until 12 , characterized in that when a maximum permissible temperature (Tmax, perm) can no longer be maintained in the exhaust system (13) without the supply of water, water is injected into the combustion chamber (22) in a step (S4). procedure after claim 1 , characterized in that in a step (S10) a focal point of the combustion in the combustion chamber (22) is shifted. procedure after Claim 14 , characterized in that the focal point of the combustion is shifted to the efficiency-optimal focal point earlier, step (S11). procedure after Claim 14 or 15 , characterized in that when the center of gravity of the combustion is shifted in a step (S12), the course of the combustion in the combustion chamber (22) is influenced by injecting water into the combustion chamber (22). Procedure according to one of Claims 14 until 16 , characterized in that this method with the method according to any one of claims 2 until 12 is combined. Computer program which is designed to carry out all the steps of one of the methods according to one of Claims 1 until 17 to execute. Machine-readable storage medium on which the computer program Claim 18 is saved. Control unit that is designed to carry out all the steps of one of the methods according to one of Claims 1 until 17 to execute.

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