EP3290796B1 - Method for controlling a fuel/air ratio in a heating system and a control unit and a heating system - Google Patents

Description

The invention relates to a method for controlling a fuel-air ratio in a heating system. The invention also relates to a control unit which is designed to carry out the method according to the invention and to a heating system with the control unit according to the invention.

State of the art

In order to ensure optimal combustion, it is necessary to ensure the correct fuel-air ratio when operating gas burners. For this, the correct functioning of the sensors used to determine the fuel-air ratio must be guaranteed. Gas burners are known from the prior art, which for this purpose execute methods for calibrating the corresponding sensors. In such calibration procedures, the gas burner is largely operated over its entire power range. This has the disadvantage that more pollutants can be emitted during such a calibration. The duration of such a calibration is in the range from several seconds to minutes. This has the additional disadvantage that the gas burner is not available for normal operation during this time. A method according to the preamble of claim 1 is described in the document DE102010055567A1 disclosed.

Disclosure of the invention benefits

• Erzeugen einer vorübergehenden, zeitlichen Fluidzufuhränderung einer Fluidzufuhrkenngröße,
• Versuch, ein relatives Signalmaximum einer mit der zeitlichen Fluidzufuhränderung korrelierten zeitlichen Änderung von mindestens einer Verbrennungskenngröße zu ermitteln,
• Feststellung eines Fehlzustandes, falls das relative Signalmaximum eine Signaluntergrenze unterschreitet, falls ein relatives Signalmaximum ermittelt wurde,
• Kalibrieren des Heizsystems, falls ein Fehlzustand festgestellt wird,

und dadurch gekennzeichnet ist, dass die Fluidzufuhränderung abhängig von einem Brennerleistungsparameter gewählt wird, hat den Vorteil, dass das tatsächliche Brennstoff-Luft-Verhältnis weitestgehend ohne zusätzliche Emissionen überprüft wird. Das Brennstoff-Luft-Verhältnis wird auch als Lambdawert bezeichnet. Nur bei einer Abweichung vom vorgesehenen Brennstoff-Luft-Verhältnis wird das Heizsystem geregelt. Auf diese Weise wird der Schadstoffausstoß minimiert. Das erfindungsgemäße Verfahren hat den zusätzlichen Vorteil, dass es während des normalen Betriebs des Heizsystems ausgeführt werden kann. Das Verfahren stellt nur einen kurzzeitigen Eingriff in die Regelung des Heizsystems dar, bei dem nur kleine Fluidzufuhränderungen vorgenommen werden im Vergleich zu möglichen gesamten Fluidzufuhränderungen im Betrieb des Heizsystems. Dass die Fluidzufuhränderung abhängig von einem Brennerleistungsparameter gewählt wird, hat den Vorteil, dass die im Allgemeinen von einer Brennerleistung abhängende Korrelation zwischen der mindestens einen Verbrennungskenngröße und dem Brennstoff-Luft-Verhältnis berücksichtigt wird. Auf diese Weise wird eine präzise und besonders zuverlässige Feststellung eines Fehlzustandes ermöglicht.The method according to the invention for controlling and regulating a fuel-air ratio in a heating system, which comprises the following steps:

• Generating a temporary, temporal fluid supply change of a fluid supply parameter,
• Attempt to determine a relative signal maximum of a time change of at least one combustion parameter that is correlated with the change in fluid supply over time,
• Detection of a fault condition if the relative signal maximum falls below a signal lower limit, if a relative signal maximum has been determined,
• Calibrating the heating system if a fault condition is detected,

and characterized in that the change in fluid supply is selected as a function of a burner output parameter, has the advantage that the actual fuel-air ratio is checked as far as possible without additional emissions. The fuel-air ratio is also known as the lambda value. The heating system is only regulated if there is a deviation from the intended fuel-air ratio. In this way, pollutant emissions are minimized. The method according to the invention has the additional advantage that it can be carried out during normal operation of the heating system. The method represents only a short-term intervention in the regulation of the heating system, in which only small fluid supply changes are made compared to possible total fluid supply changes during operation of the heating system. The fact that the change in fluid supply is selected as a function of a burner output parameter has the advantage that the correlation, which generally depends on a burner output, between the at least one combustion parameter and the fuel-air ratio is taken into account becomes. In this way, a precise and particularly reliable determination of a fault condition is made possible.

"Heating system" is understood to mean at least one device for generating thermal energy, in particular a heating device or heating burner, in particular for use in a building heating system and / or for generating hot water, preferably by burning a gaseous or liquid fuel. A heating system can also consist of several such devices for generating thermal energy and other devices that support heating, such as hot water and fuel storage tanks.

A "fluid supply parameter" is to be understood in particular as a scalar parameter which is correlated in particular with at least one fluid, in particular a combustion air flow, a fuel flow and / or a mixed flow, in particular consisting of a combustion air and the fuel, that is fed to a burner unit of the heating system . Advantageously, a volume flow and / or a mass flow of the at least one fluid can be inferred and / or the volume flow and / or the mass flow of the at least one fluid can be determined, in particular by a control and / or regulating unit of the heating system, at least on the basis of the fluid supply parameter. An example of a fluid supply parameter is the specification of an opening width of a fuel valve. A “temporary, temporal fluid supply change” is to be understood as a temporally limited variation of the fluid supply parameter so that it deviates from a largely constant value of the fluid supply parameter before the fluid supply change begins. The fluid supply parameter is preferably initially increased or decreased over the period of the fluid supply change and then to the largely constant value of Fluid supply parameter regulated before the beginning of the fluid supply change. The duration of the fluid supply change is preferably pulse-like and short compared to the time variations of the fluid supply parameter that occur during normal operation of the heating system.

A “pulse”, a “pulse-like change” or a “pulse-shaped signal” is to be understood as a characteristic of a characteristic which is brought from a first value to at least a second value different from the first value within a limited period of time. A "pulse" is sometimes referred to as an "impulse", especially in electrical engineering.

A “combustion parameter” is to be understood in particular as a scalar parameter which is correlated in particular with a combustion, in particular of the mixture, in particular of the combustion air and the fuel. An example of a combustion parameter is an ionization current, which is measured on a flame of the heating system. Advantageously, in particular by the control and / or regulating unit of the heating system, at least based on the combustion parameter, the presence and / or quality of the combustion can be inferred and / or the presence and / or the quality of the combustion can be determined. The combustion parameter advantageously corresponds to at least one or exactly one measured value that depicts and / or characterizes the combustion, or the combustion parameter can be clearly assigned to such a measured value. Examples of a measured value depicting and / or characterizing the combustion are a combustion signal, in particular a light intensity, a pollutant emission, a temperature and / or advantageously an ionization signal. A “relative signal maximum” is the maximum amplitude of the combustion parameter in a correlated with the change in the fluid supply parameter over time To understand period of time minus the largely constant amplitude of the combustion parameter before this period or the amplitude of the combustion parameter at the beginning of this period. The relative signal maximum is in particular a measure of the change in the combustion parameter due to the change in fluid supply.

By "faulty condition" is meant a condition of the heating system in which it cannot operate as intended. This includes defects and malfunctions as well as sub-optimal operation. Examples of malfunctions and defects are a not fully functional fan or suddenly occurring or slowly progressing blockages in the flow path of a fuel-air mixture. Causes of such blockages are, for example, wind, dirt, deposits or corrosion. Examples of non-optimal operation are over- or under-loading of the heating system or non-optimal combustion in a combustion chamber of the heating system, for example due to incorrectly set operating parameters and / or incorrectly set sensors for determining the fuel-air ratio.

An "attempt" to determine a relative signal maximum of a time change of at least one combustion parameter correlated with the change in fluid supply over time is to be understood as a method step in which a relative signal maximum of a time change of at least one combustion parameter that is correlated with the change in fluid supply over time is measured or is detected. Depending on the result or value of the relative signal maximum, different following steps can optionally be selected in the further course of the method, if this is necessary and / or desired.

"Calibrating the heating system" means the one-time or repeated, in particular periodic, setting of operating parameters of the heating system so that the heating system can always fully meet the specified and / or requested performance, especially under changing internal and external conditions, especially with wear processes and changing boundary and environmental conditions. "Operating parameters" are to be understood as parameters that are used by the control of the heating system to control and monitor processes taking place in the heating system. Examples of "operating parameters" are a fan speed or a fan speed characteristic or a flame ionization characteristic. Thus, “calibrating the heating system” is to be understood in particular as a calibration process in which the sensor system for measuring the fuel-air ratio is readjusted.

“Burner output parameter” should be understood to mean, in particular, a scalar parameter which is correlated with an output, in particular heating output, of the heating system. The power, in particular the heating power, of the heating system can advantageously be determined, in particular by the control and / or regulating unit of the heating system, at least on the basis of the burner power parameter. The burner output parameter advantageously corresponds to at least one or exactly one measured value representing the output or can be clearly assigned to such a measured value. Such a measured value can be, for example, a temperature, an air flow rate, a fan control signal or a fan speed.

The features listed in the subclaims allow advantageous developments of the method according to the main claim.

The fact that a fault condition is determined if no relative signal maximum has been determined makes the method particularly reliable. In this way, malfunctions and / or defects in particular are taken into account promptly in the detection of the change over time in at least one combustion parameter.

If the lower signal limit is selected as a function of the burner output parameter, then the correlation between the at least one combustion parameter and the fuel-air ratio is taken into account at a further point in the method. In this way, detection of a fault condition is further improved. Overall, this improves the reliability of the method.

If, in an additional step, a temporary additional fluid supply change is generated, this additional fluid supply change largely opposing the fluid supply change, this has the advantage that the minimal increase in burner output and pollutant emissions caused by the fluid supply change is compensated.

A “largely opposite additional fluid supply change” is to be understood as a fluid supply change in which the fluid supply parameter is varied over time in such a way that the change in an average fluid supply rate caused by the fluid supply change is compensated. In this way, an average amount of fluid supplied over the period of the fluid supply change and the additional fluid supply change largely corresponds to an average supplied fluid amount in an equally long period of time during the intended operation of the heating system without changes in the fluid supply rate shortly before the fluid supply changes are carried out. If, for example, the fluid supply change is implemented by a largely rectangular pulse with a certain positive relative signal level and a certain signal width, the additional Fluid supply change implemented by a largely rectangular pulse with largely the same signal width and a relative signal level, the magnitude of which largely corresponds to the relative signal level of the first largely rectangular pulse of the fluid supply change and is negative. Here, “largely rectangular shape of the fluid supply change” is to be understood as a time course of the fluid supply parameter in which the fluid supply parameter initially has a normal value. The fluid supply parameter is then quickly increased to a largely constant maximum supply value. The fluid supply parameter is then quickly reduced to the normal value. This time course of the fluid supply parameter has the shape of a rectangular function to a good approximation. Such a time course of the fluid supply parameter is usually referred to as a square-wave signal.

If the fluid supply parameter corresponds to a control signal for metering a fuel and / or the combustion air and / or a mixture of a fuel and combustion air, no measurement of the fuel and / or the combustion air and / or a mixture of a fuel and combustion air or a Flow rate of these fluids required. This simplifies the process and makes it robust against malfunctions.

If the at least one combustion parameter is determined by an ionization current measurement on a flame of the heating system, this is particularly advantageous since there is a functional relationship between the ionization current on a flame and the fuel-air ratio, which can be evaluated particularly favorably.

The method is further improved when the burner output parameter is a fan speed or depends on it and / or a mass flow of combustion air and / or a mixture of a fuel and Combustion air is or depends on it and / or a volume flow of a combustion air and / or a mixture of a fuel and combustion air is or depends on this and / or a running time of a combustion air and / or a mixture of a fuel and combustion air is or depends on this . The fan speed can be determined easily and reliably and provides a good estimate of the burner output. A mass flow and / or a volume flow of a combustion air and / or a mixture of a fuel and combustion air allow a particularly precise estimation of the burner output. A running time of a combustion air and / or a mixture of a fuel and combustion air can be determined particularly simply and inexpensively.

If the change in fluid supply over time has an at least largely rectangular shape, this has the advantage that the change over time in the at least one combustion parameter can be detected particularly easily. In this way the reliability of the method is further increased. Here, “largely rectangular shape of the fluid supply change” is to be understood as a time course of the fluid supply parameter in which the fluid supply parameter initially has a normal value. The fluid supply parameter is then quickly increased to a largely constant maximum supply value. The fluid supply parameter is then quickly reduced to the normal value. This time course of the fluid supply parameter has the shape of a rectangular function to a good approximation. Such a time course of the fluid supply parameter is usually referred to as a square-wave signal.

The use of a control unit for a heating system, the control unit being set up to carry out the method according to the invention for checking a fuel-air ratio in a heating system, has the advantage that, by largely preventing an incorrect Adjusting the fuel-air ratio increases the durability of the heating system, prevents malfunctions and thus increases safety. In addition, the avoidance of unnecessary calibration processes reduces the wear and tear on the heating system.

A heating system with a control unit according to the invention, with a metering device for a fuel and / or for combustion air and / or for a mixture of a fuel and combustion air, as well as with an ionization probe on a flame and with a fan with variable fan speed has the advantage that in Operation of the heating system an incorrect setting of the fuel-air ratio is largely prevented. In this way, unforeseen, heavy loads on the heating system due to, for example, too high burner temperatures and / or too high fan speeds and / or too high soot emissions and / or too strong vibrations are avoided. This enables the heating system to be manufactured inexpensively. In addition, the fuel consumption is reduced and the service life of the heating system is increased or the time interval between the required inspection intervals is reduced.

If the heating system has at least one metering device for a fuel and / or for combustion air and / or for a mixture of a fuel and combustion air, a change in a fluid supply parameter over time can thus be generated particularly easily.

In this context, a "metering device" is to be understood as meaning, in particular, an, in particular electrical and / or electronic, unit, in particular actuator unit, advantageously actuating unit, which is provided to feed the at least one fluid, in particular the combustion air flow, the fuel flow and / or the mixture flow in particular from the combustion air and the fuel. In particular, there is at least one doser for this provided to set, regulate and / or promote a volume flow and / or a mass flow, in particular the combustion air and / or the fuel. The metering device for combustion air can advantageously be designed as a fan, in particular with a variable speed, and / or preferably as a blower, in particular with a variable speed. The metering device for fuel can advantageously be designed as a, in particular throughput variable, fuel pump and / or preferably as, in particular throughput variable, fuel valve. In particular, the metering device for combustion air and / or the metering device for fuel are provided to modulate a heating output of the heater device.

If the heating system has an ionization probe on the flame of the heater, a particularly inexpensive and reliable sensor for measuring a combustion scan is implemented. Ionization detectors are usually used in heating devices for flame detection.

If the heating system has a fan with a variable fan speed, a simple and robust means for setting and determining the output of the heater is implemented in this way.

drawings

• Figur 1 eine schematische Darstellung des erfindungsgemäßen Heizsystems mit der erfindungsgemäßen Steuereinheit,
• Figur 2 das erfindungsgemäße Verfahren zum Regeln und/oder Kalibrieren eines Heizsystems,
• Figur 3 eine schematische Darstellung einer Fluidzufuhränderung und einer zeitlichen Änderung von einer Verbrennungskenngröße,
• Figur 4 schematische Darstellungen von zeitlichen Änderungen von einer Verbrennungskenngröße bei unterschiedlichen Messungen,
• Figur 5 eine schematische Darstellung einer Abhängigkeit des Ionisationsstroms vom Brennstoff-Luft-Verhältnis und
• Figur 6 eine Variante des Verfahrens zum Regeln und/oder Kalibrieren eines Heizsystems.

In the drawings, exemplary embodiments of the method according to the invention for controlling a fuel-air ratio in a heating system, the control unit according to the invention and the heating system according to the invention are shown and explained in more detail in the following description. Show it

• Figure 1 a schematic representation of the heating system according to the invention with the control unit according to the invention,
• Figure 2 the method according to the invention for regulating and / or calibrating a heating system,
• Figure 3 a schematic representation of a change in fluid supply and a change in a combustion parameter over time,
• Figure 4 schematic representations of changes in a combustion parameter over time with different measurements,
• Figure 5 a schematic representation of a dependence of the ionization current on the fuel-air ratio and
• Figure 6 a variant of the method for regulating and / or calibrating a heating system.

description

In the different embodiment variants, the same parts or steps are given the same reference numbers.

In Figure 1 a heater 10 is shown schematically, which is arranged on a memory 12 in the exemplary embodiment. The heater 10 has a housing 14 which accommodates different components depending on the level of equipment.

The essential components are a heat cell 16, a control unit 18, one or more pumps 20 as well as piping 22, cables or bus lines 24 and holding means 26 in the heater 10. The number and complexity of the individual components also depends on the degree of equipment of the heater 10.

The heat cell 16 has a burner 28, a heat exchanger 30, a fan 32, a metering device 34 and an air supply system 36, an exhaust system 38 and, when the heat cell 16 is in operation, a flame 40. Into the flame An ionization probe 42 protrudes 40. The metering device 34 is designed as a fuel valve 44. A fan speed 79 of the fan 32 is variably adjustable. The heating device 10 and the memory 12 together form a heating system 46. The control unit 18 has a data memory 48, a computing unit 50 and a communication interface 52. The components of the heating system 46 can be controlled via the communication interface 52. The communication interface 52 enables data to be exchanged with external devices. External devices are, for example, control devices, thermostats and / or devices with computer functionality, for example smartphones.

Figure 1 shows a heating system 46 with a control unit 18. In alternative embodiments, the control unit 18 is located outside the housing 14 of the heater 10. The external control unit 18 is designed in special variants as a room controller for the heating system 46. In preferred embodiments, the control unit 18 is mobile. The external control unit 18 has a communication link to the heating device 10 and / or other components of the heating system 46. The communication connection can be wired and / or wireless, preferably a radio connection, particularly preferably via WLAN, Z-Wave, Bluetooth and / or ZigBee. In further variants, the control unit 18 can consist of several components, in particular components that are not physically connected. In special variants, at least one or more components of the control unit 18 can be partially or entirely in the form of software that is executed on internal or external devices, in particular on mobile computing units, for example smartphones and tablets, or servers, in particular a cloud. The communication links are then corresponding software interfaces.

In Figure 2 the method 54 according to the invention for checking and regulating a fuel-air ratio 56 in a heating system 46 is shown. in the Embodiment, a fluid supply change 60 over time of a fluid supply parameter 62 is generated in a step 58. In the exemplary embodiment, the fluid supply parameter 62 is an intended opening width 64 of the metering device 34. The opening width 64 is a percentage, with an opening width 64 of 0% corresponding to a completely closed fuel valve 44 and an opening width 64 of 100% describing a completely open fuel valve 44. A relationship between the opening width 64 and a control signal required for this is stored in the control unit 18. The intended opening width 64 is implemented by selecting the control signal and transmitting this control signal to the fuel valve 44 by the control unit 18. The opening width 64 describes a request which is transmitted to the fuel valve 44.

The fluid supply change 60 is shown in FIG Figure 3 pictured. The first axis of abscissa 66 represents a time. On the axis of ordinate 68, the fluid supply parameter 62 is shown. The fluid supply change 60 runs in a largely rectangular pulse. Initially, the fluid supply parameter 62 has a normal supply value 70. Then the opening width 64 is increased to a maximum supply value 72 as quickly as possible. Thereafter, the opening width 64 is reduced to the normal supply value 70 as quickly as possible. One in Figure 3 shown pulse height 74 is 15%. One in Figure 3 the pulse width 76 shown is 120 ms.

In the exemplary embodiment, the fluid supply change 60 is dependent on a burner output parameter 77. The burner output parameter 77 is a fan speed 79. The fan speed 79 is a characteristic value determined by the control unit 18, which determines a fan control signal. The fan control signal is sent from the control unit 18 to the fan 32 and determines a speed of the fan 32. The pulse height 74 increases linearly with the fan speed 79. Between a minimal Fan speed and a maximum fan speed, the pulse height assumes 74 values in an interval between 10% and 20%. The pulse width 76 increases linearly with the fan speed 79. Between a minimum fan speed and a maximum fan speed, the pulse width assumes 76 values in an interval between 50 ms and 200 ms.

In a following step 75 (see Figure 2 ) an attempt is made to determine a relative signal maximum 80 of a change over time of a combustion parameter 78 that is correlated with the change in fluid supply over time. In the exemplary embodiment, the combustion parameter 78 is an ionization current 82. The ionization current 82 is determined by the ionization probe 42 on the flame 40 and transmitted to the control unit 18. After the fluid supply change 60, the time profile of the ionization current 82 has the relative signal maximum 80. The relative signal maximum 80 is determined from the difference between the absolute signal maximum 84 and the ionization current normal value 86 (see FIG Figure 3 ). The normal ionization current value 86 is determined in the exemplary embodiment in which the mean ionization current 82 measured over the pulse width 76 is determined. In the exemplary embodiment, the relative signal maximum 80 is determined in that the ionization current 82 is measured over a determination time. The largest value of the ionization current 82 occurring within the determination time is selected as the absolute signal maximum 84. The determination time has the length of a time threshold 88 stored in the control unit 18. The determination time begins at a first point in time 90 and ends at a second point in time 92 (see FIG Figure 3 ). In the exemplary embodiment, the time threshold 88 is 2 seconds. In variants, a time threshold 88 between 1 second and 5 seconds is selected.

If a relative signal maximum 80 could be determined in step 75, the method continues with path C (see FIG Figure 2 ). In a further step 94 a fault state 96 is determined if the relative signal maximum 80 falls below a signal lower limit 98. The lower signal limit 98 is a constant stored in the control unit 18. The control unit 18 compares the relative signal maximum 80 with the signal lower limit 98. If the relative signal maximum 80 is less than the signal lower limit 98, an error state 96 is determined in which an error variable is set to the value 1. The method 54 continues on path A (see FIG Figure 2 ). If the relative signal maximum 80 is greater than or equal to the lower signal limit 98, the error variable is set to the value 0 and the iteration of the method 54 is ended (path B in Figure 2 ).

If the method 54 is continued on path A, a step 100 is carried out. In step 100 the heating system 46 is calibrated. The heating system 46 is run in a special operating mode in which the sensors and analytics, in particular the ionization probe 42 and the characteristic curves stored in the control unit 18 based on the ionization current 82, are reset and adjusted. In this way, the determination of the fuel-air ratio 56 is made more precise. If necessary, when calibrating the heating system 46 in step 100, the heating system 46 or the processes and / or methods running on the heating system 46 are at least partially reinitialized or restarted.

If no relative signal maximum 80 is determined in step 75, the method is continued on path D (see FIG Figure 2 ). In a step 101 a fault condition 96 is determined. The error variable is set to the value 1. The method 54 continues with step 100 and the heating system 46 is calibrated.

The method 54 is repeated periodically in the exemplary embodiment. Figure 3 FIG. 9 shows a fluid supply change 60 following the change in the combustion parameter 78, which changes to the next iteration of the method 54 belongs. A time interval between the iterations of the method 54 is selected depending on the operating state of the heating system 46 and on the external conditions. In the exemplary embodiment, the time interval is between 1 second and 20 seconds, preferably 2 seconds. In the exemplary embodiment, a fault status counter is stored in the control unit 18. The deficiency counter is a variable which stores the number of defective conditions 96 detected in a specific time interval. If the defective state counter exceeds a critical defective state limit stored in the control unit 18, the heating system 46 is shut down for safety reasons. The defective state counter is decreased after the method 54 has been carried out without a defective state 96 being determined. In the exemplary embodiment, the heating system 46 is shut down after seven immediately successive determinations of a fault state 96.

The Figures 4 and 5 illustrate the principle of operation of the method 54. In Figure 4 a time is shown on a second abscissa axis 67. The ionization current 82 is plotted on the ordinate axis 68. The graphs of the ionization current 82 each show changes over time in the ionization current 82 which occur due to a change in fluid supply 60 over time in different measurements 102, 104, 106, 108 and 110. The measurements are carried out at a constant fan speed 79. Each of the measurements is carried out at a different fuel-air ratio 56 (marked in Figure 5 ). The fuel-air ratio 56 is calculated from an amount of air divided by an amount of fuel.

Figure 5 illustrates the relationship between the ionization current 82 and the fuel-air ratio 56 at a constant fan speed 79. The ionization current 82 is plotted on the ordinate axis 68. The fuel-air ratio 56 is shown on a third abscissa axis 69. The The course of the ionization current 82 has an ionization current maximum 112 at a fuel-air ratio 56 of 1 (measurement 104). With an increase or decrease in the fuel-air ratio 56 starting from the ionization current maximum 112, the ionization current 82 decreases, the magnitude of the slope increasing steadily. The heating system 46 is preferably operated with a fuel-air ratio 56 of 1.3 (measurement 108), that is to say with an excess of air. The method 54 ensures that the heating system 46 is operated with excess air. If the fuel-air ratio 56 is less than 1 or if the fuel-air ratio 56 is too close to the value 1, a fault condition 96 is determined.

Due to the change in fluid supply 60, the fuel-air ratio 56 is reduced for a short time. If the fuel-air ratio 56 has the value 0.85 (measurement 102), the change in fluid supply 60 causes the ionization current 82 to decrease (see FIG Figure 4 ). The relative signal maximum 80 is thus largely 0. The signal falls below the lower limit 98 and a fault state 96 is determined. If the fuel-air ratio 56 has the value one (measurement 104), the change in fluid supply 60 causes the ionization current 82 to decrease slightly, since in this area the slope of the graph of the ionization current 82 is approximately 0 and changes only slightly. In measurement 106, the fuel-air ratio 56 has the value 1.15. There is an excess of air which is not sufficiently large. The change in fluid supply 60 causes the ionization current 82 to rise. The relative signal maximum 80 is below the lower signal limit 98, since the amount of the slope of the graph of the ionization current 82 in the area of the fuel-air ratio 56 of the measurement 106 is too low. In measurements 108 and 110, the fuel-air ratio 56 is 1.3 and 1.45, respectively. The excess air is sufficient in each case. The magnitude of the slope of the graph of the ionization current 82 is sufficiently large in the areas of measurements 108 and 110. The fluid supply change 60 causes an increase in each case of the ionization current 82. The relative signal maximum 80 is in each case greater than the lower signal limit 98. In the measurements 108 and 110, no fault condition 96 is found.

In the exemplary embodiment, the relative signal maximum 80 is determined between the first point in time 90 and the second point in time 92. The time threshold 88 is selected with the aid of laboratory tests in such a way that under all operating states and boundary conditions, in particular at all fan speeds 79, the position of the maximum of the ionization current 82 is always between the first point in time 90 and the second point in time 92. In alternative variants with a smaller time threshold 88, the maximum of the ionization current 82 can occur after the second point in time 92. The relative signal maximum 80 determined in step 75 is then possibly smaller than the actual maximum of the ionization current 82, in particular with a low output of the heating system 46 or with low fan speeds 79. In preferred variants, this is achieved by a corresponding adaptation, in particular lowering the signal lower limit 98 , in particular as a function of the burner output parameter 77.

In alternative embodiments, the change over time of at least one combustion parameter 78 is determined by detecting the occurrence of a pulse in the course of the at least one combustion parameter 78 over time. The relative signal maximum 80 is determined as the maximum value of the detected pulse. To this end, in step 75 the control unit 18 checks whether, after the fluid supply change 60, the combustion parameter 78 increases beyond signal noise. The relative signal maximum 80 is the maximum combustion parameter 78 in the time range in which the combustion parameter 78 increases beyond signal noise. In preferred variants, the detection of the pulse in the time course of the at least one combustion parameter 78 is ended if the determination time the time threshold exceeds 88 and no pulse could be detected. Then no relative signal maximum 80 can be determined and the method 54 is continued on the path D.

In further embodiments, step 75 is ended as soon as the measured combustion parameter 78 exceeds the lower signal limit 98. Then the value of the relative signal maximum 80 is determined on the basis of the combustion parameter 78 which was last measured and which exceeded the lower signal limit 98. The process then continues with path C. If in step 75 the measured combustion parameter 78 does not reach the signal lower limit 98 within the time threshold 88, the method is continued with path C. These embodiments have the advantage that the required determination time is minimized.

In special embodiments, the time threshold 88 is a function of the burner output parameter 77. The time threshold 88 is preferably increased when the output of the heating system 46 is reduced.

In the exemplary embodiment, the lower signal limit 98 is selected as a function of the fan speed 79. For this purpose, the control unit 18 determines a relative lower signal limit 114 (see FIG Figure 3 ). The relative lower signal limit 114 is proportional to the negative fan speed 79. In this way, the higher signal noise of the ionization current 82 at low fan speeds 79 is taken into account. In the exemplary embodiment, the relative lower signal limit 114 is 1 µA for the maximum fan speed 79 and 10 µA for the minimum fan speed 79. Typically, a relative lower signal limit 114 between 3 µA and 7 µA is selected in control mode. The signal lower limit 98 is determined from the sum of the relative signal lower limit 114 and the ionization current normal value 86. The ionization current normal value 86 increases during regular operation of the heating system 46 Values between 10 µA and 100 µA, especially between 30 µA and 60 µA. In alternative embodiments, the choice of the dependence of the lower signal limit 98 on the fan speed 79 or the burner output parameter 77 is based on the technical properties of the heating system 46, in particular the dependence of the signal noise of the ionization current 82 or the combustion parameter 78 on the burner output parameter 77. In preferred variants the relative lower signal limit 114 is constant. In further variants, the relative lower signal limit is proportional to the burner power parameter 77. In particularly preferred variants, the functional dependence of the relative lower signal limit 114 on the burner power parameter 77 is largely proportional to the functional dependence of a strength of the signal noise of the ionization current 82 on the burner power parameter 77.

In alternative embodiments, an additional fluid supply change 118 is generated in an additional step 116. The additional fluid supply change 118 is largely opposite to the fluid supply change 60. In this way, the mean fluid supply parameter 62 over a period of the fluid supply change 60 and the additional fluid supply change 118 largely corresponds to the normal supply value 70. In preferred embodiments, the graph of the time course of the fluid supply parameter 62 of the additional fluid supply change 118 resembles the graph of the normal supply value 70 mirrored and shifted over time Temporal course of the fluid supply parameter 62 of the fluid supply change 60. In this case, step 116 can be carried out at any point in method 54. In Figure 6 a variant is shown in which step 116 is carried out after step 75 and before step 94 on path C or after step 75 and before step 101 on path D. In preferred embodiments, the step 116 is positioned such that the additional fluid supply change 118 corresponds to the fluid supply change 60 correlated change in the combustion parameter 78 is not influenced. Step 116 is preferably carried out after step 58, particularly preferably after step 75.

In the exemplary embodiment, the fluid supply parameter 62 is an opening width 64 of the fuel valve 44. Using the provided opening width 64, the control unit 18 determines and transmits a control signal to the fuel valve 44. In alternative embodiments, the fluid supply parameter 62 is a control signal to the fuel valve 44 or a scalar value that can be derived from the control signal. In further variants, the fluid supply parameter 62 corresponds to a control signal for metering a combustion air and / or a mixture of a fuel and a combustion air. The control signal sent by the control unit 18 is composed of at least one control command to at least one metering device 34. The at least one metering device 34 is at least one fuel valve 44 and / or at least one blower 32. In alternative embodiments, a metering value of the metering device 34 is measured and used as the fluid supply parameter 62. "Dosage value" is to be understood as a characteristic value which describes the state of the dosing device 34 and which allows conclusions to be drawn about the amount of substance supplied and / or allowed through by the dosing device 34. An example of a dosage value is a measured opening width of the fuel valve 44 and / or a measured fuel flow.

In the exemplary embodiment, the combustion parameter 78 is an ionization current 82. The ionization current 82 is determined by an ionization current measurement on a flame 40 of the heating system 46. The ionization current 82 is determined by the ionization probe 42 and transmitted to the control unit 18. In further embodiments, the combustion parameter 78 is one Light intensity, a lambda value, pollutant emissions and / or a temperature. The light intensity at the flame 40 is determined by a photodiode. The lambda value is measured with a lambda probe in an exhaust gas. The exhaust system 38 has the lambda probe. The pollutant emission is determined by a sensor device which is located on the flame 40 and / or in the exhaust system 38. The temperature is determined by a contact thermometer and / or a non-contact thermometer, in particular a pyrometer. The thermometer can be located in the exhaust system 38 and / or measure the flame 40.

In the exemplary embodiment, the burner output parameter 77 is the fan speed 79. The fan speed 79 is a characteristic value determined by the control unit 18, which determines a fan control signal. In alternative embodiments, the burner output parameter 77 is a measured fan speed and / or a temperature and / or an air flow rate and / or a flow rate of the air-fuel mixture. The air flow rate or the flow rate of the air-fuel mixture can be determined as a volume flow or as a mass flow. In further embodiments, the burner output parameter 77 is a running time of a combustion air and / or a mixture of a fuel and combustion air. In special variants, a running time is determined as the time difference between the fluid supply change 60 and the time change in the combustion parameter 78 correlated with the fluid supply change 60. The running time corresponds to the time which the mixture of fuel and combustion air needs to get from the fuel valve 44 to the ionization probe 42. The running time is a measure of the flow rate of the air-fuel mixture. These parameters can also be used in combination will. The temperature in the exhaust system 38 and / or by the flame 40 can be determined.

In the exemplary embodiment, the fluid supply change 60 has a largely rectangular shape. In alternative embodiments, the fluid supply change 60 has largely the shape of a ramp and / or largely a triangular shape and / or largely the shape of a sinus and / or largely a Gaussian shape. The change in a concentration of the fuel in the burner 28 resulting from the fluid supply change 60 generally has a different form than the fluid supply change 60. In the exemplary embodiment, the fluid supply change 60 depends on the burner output parameter 77. The pulse height 74 and pulse width 76 each depend linearly on the fan speed 79. In this way, it is ensured that the heating system 46 is not disturbed too much in its regular operation by the fluid supply change 60. In alternative embodiments, the fluid supply change 60 has a functional relationship with the burner output parameter 77. The functional relationship is selected so that good detection of the relative signal maximum 80 is possible, taking into account the technical properties of the heating system 46. If, for example, resonances occur at certain fan speeds 79, which increase the signal noise of the ionization stream 82, then at these fan speeds 79 the fluid supply change 60 is increased.

Claims (10)

1. Method (54) for controlling a fuel/air ratio (56) in a heating system (46), which method comprises the following steps:

   • generating a temporary, fluid supply change (60) in a fluid supply characteristic variable (62) with respect to time,

   • attempting to ascertain a relative signal maximum (80) of a change in at least one combustion characteristic variable (78) with respect to time which is correlated with the fluid supply change (60) with respect to time,

   • establishing a fault state (96) if the relative signal maximum (80) undershoots a signal lower limit (98) if a relative signal maximum (80) has been ascertained,

   • calibrating the heating system (46) if a fault state (96) is established,

   characterized in that the fluid supply change (60) is selected depending on a burner power parameter (77) and wherein the power of the heating system (46) can be ascertained on the basis of the burner power parameter (77) .
2. Method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to Claim 1, characterized in that a fault state (96) is established if no relative signal maximum (80) has been ascertained.
3. Method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to either of the preceding claims, characterized in that the signal lower limit (98) is selected depending on the burner power parameter (77).
4. Method (54) for controlling and a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims, characterized in that a temporary additional fluid supply change (118) with respect to time is generated in an additional step, wherein this additional fluid supply change (118) largely opposes the fluid supply change (60).
5. Method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims, characterized in that the fluid supply characteristic variable (62) corresponds to a control signal for metering a fuel and/or combustion air and/or a mixture of a fuel and combustion air.
6. Method (54) for controlling and a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims, characterized in that the at least one combustion characteristic variable (78) is determined by ionization current measurement at a flame (40) of the heating system (46).
7. Method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims, characterized in that the burner power parameter (77) is a fan rotation speed (79) or is dependent on the said fan rotation speed and/or is a mass flow of combustion air and/or a mixture of a fuel and combustion air or is dependent on the said mass flow and/or is a volume flow of combustion air and/or a mixture of a fuel and combustion air or is dependent on the said volume flow and/or is a propagation time of combustion air and/or a mixture of a fuel and combustion air or is dependent on the said propagation time.
8. Method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims, characterized in that that the fluid supply change (60) with respect to time has an at least largely rectangular form.
9. Control unit (18) for a heating system (46), wherein the control unit (18) is designed to be able to execute a method (54) for controlling a fuel/air ratio (56) in a heating system (46) according to one of the preceding claims.
10. Heating system (46) comprising a control unit (18) according to Claim 9, comprising at least one metering device (34) for a fuel and/or for combustion air and/or for a mixture of a fuel and combustion air, and also comprising an ionization probe (42) at a flame (40) and comprising a fan (32) with a variable fan rotation speed (79) .

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