DE102009029011B4 - Method and device for determining the composition of a fuel mixture for operating an internal combustion engine - Google Patents

Abstract

Method for determining the composition (30) of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their anti-knock properties, in their lean running limit (21) and/or in their exhaust gas recirculation compatibility, the composition of the fuel mixture is determined by means of an output signal of a knock sensor or from a knock control, characterized in that a first composition value (32) of the fuel mixture is determined from the output signal of a knock sensor or from the knock control of the internal combustion engine, that the lean running limit (21) of the internal combustion engine for the fuel mixture it is determined that a second composition value (33) is determined from the lean running limit (21) and that the composition (30) of the fuel mixture is a combination of the first composition value (32) and the second composition value (33 ) is determined, with weighting factors (36) for the composition values (32, 33) depending on the accuracy of the respective composition value (32, 33) being specified and the composition (30) of the fuel mixture from a weighted with the associated weighting factors (36). Combination of the composition values (32, 33) is determined.

Description

The invention relates to a method for determining the composition of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their anti-knock properties, their lean-running limit and/or their exhaust gas recirculation compatibility, the composition of the fuel mixture being determined by means an output signal of a knock sensor or a knock control is determined.

The invention also relates to a device for determining the composition of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their anti-knock properties, their lean-running limit and/or their exhaust gas recirculation compatibility, the internal combustion engine having a knock sensor and has a knock control and the composition of the fuel mixture is determined by means of an output signal of the knock sensor or the knock control, and wherein the internal combustion engine is assigned a control unit.

State of the art

A method and a device for determining the composition of a fuel mixture from a first and a second fuel are in DE 10 2007 034 189 A1 specified. In this known method and device for determining the composition of the fuel mixture from a first fuel and a second fuel, which have different octane numbers, the composition of the fuel mixture is determined by means of an output signal from a knock sensor under suitable operating conditions, in particular from a control process of the knock control for a mean adjustment of an ignition angle and/or an absolute ignition angle.

Internal combustion engines based on spark ignition engines are generally fueled with a hydrocarbon fuel from fossil fuels based on refined petroleum. Alcohol produced from renewable raw materials (plants), such as ethanol or methanol, is increasingly being added to this fuel in various mixing ratios. In the USA and Europe, a mixture of 75-85% ethanol and 15-25% gasoline is often used under the brand name E85. The internal combustion engines are designed in such a way that they can be operated both with pure petrol and with mixtures up to E85; this is referred to as "flex fuel operation". For economical operation with low pollutant emissions and high engine performance at the same time, the operating parameters in flex-fuel operation must be adapted to the respective fuel mixture. For example, a stoichiometric air/fuel ratio is 14.7 parts by weight of air per part of gasoline, but when using ethanol an air proportion of 9 parts by weight must be set.

In flex-fuel operation, due to the different temperature-dependent evaporation properties of ethanol and gasoline when the internal combustion engine is started, an adjusted enrichment factor must be specified depending on the mixing ratio. The ignition timing must also be adjusted depending on the fuel mixture. Knowledge of the present fuel mixture ratio is therefore of fundamental importance for the operation of the internal combustion engine.

Different types of fuel sensors, also referred to as “fuel composition sensors”, can be used to determine the composition of the fuel mixture. Fuel type sensors use the different properties of the fuels used, such as alcohol and petrol, to determine the fuel composition. For example, ethanol is a protic solvent that contains hydrogen ions and has a large dielectric constant that is dependent on the water content. Gasoline, on the other hand, is an aprotic solvent with a low dielectric constant. Based on this, there are fuel type sensors that determine the fuel composition based on the dielectric properties of the fuel mixture. Other types of fuel sensors use the different electrical conductivity or the different optical properties of the fuels, such as the different refractive indices. The disadvantage here is that the use of fuel type sensors increases the system costs. Another disadvantage is that, according to existing regulations, the correct functionality of the fuel type sensors must be monitored, which requires additional effort.

Therefore, software-based systems for determining the fuel composition were developed that do not use special fuel type sensors, but rather the signals from the combustion engine Evaluate the sensors present on the machine. These systems can be implemented more cheaply than fuel type sensor systems.

From the DE 3036107 C3 a control device is known for a fuel metering system in an internal combustion engine, consisting of a fuel supply device (fuel injection valve), a lambda probe, means (timer) for forming a basic metering signal which, after correcting the operating parameters, ultimately determines the control signal (ti) of the fuel supply device, a lambda controller, which is outgoing from a signal (λ) measured by the lambda probe, a correction factor is determined which, multiplicatively, influences the basic metering signal (tp) by the correction factor. It is provided that the lambda correction is dependent not only on the correction factor (KR λ) but also on an additive (KA λ) and/or a multiplicative (KL λ) correction variable, which is determined as a function of the correction factor and operating parameters.

The control device makes it possible to compensate for systematic deviations in the fuel metering specified by the basic metering signal, ie the so-called pilot control, from the value determined by the lambda control by means of an adaptation intervention with a corresponding long-term correction. Systematic deviations can be caused, for example, by the effects of aging or by manufacturing influences. On average, the amount of fuel defined by the corrected pilot control corresponds to the amount actually required. Short-term deviations can be compensated with the lambda controller, which now has the entire control range available again. The underlying process is also known as mixture adaptation.

The fuel mixture ratio can be determined without an additional fuel type sensor using a fuel adaptation. The fuel adaptation is activated after refueling detected by the tank gauge. A fuel composition that changes as a result of the refueling process leads to a changed stoichiometric air/fuel ratio. This change in the fuel properties is taken into account as part of the fuel adaptation by a corresponding intervention by the lambda control on the operating parameters of the internal combustion engine, in particular on the set air/fuel ratio and the ignition point. From the intervention of the lambda control or the fuel adaptation, conclusions can be drawn about the stoichiometric ratio and from this about the composition of the fuel mixture. Accordingly, the composition of the fuel mixture can be determined inexpensively using a pure software solution.

The disadvantage here is that after a refueling process, the mixture adaptation must be temporarily deactivated during the fuel adaptation, so that changed combustion properties of the fuel mixture are not incorrectly corrected by the long-term correction of the mixture adaptation. The deactivation of the mixture adaptation is undesirable and contradicts the requirements of the CARB in the USA in particular for continuous monitoring and diagnosis of the fuel supply system.

A further disadvantage of the method is that when filling up with small amounts of fuel, the tank filling level sensors do not always reliably detect this. If such small amounts of refueling are carried out repeatedly, the fuel composition may change significantly, particularly when the fuel level is low, without this being recognized and corrected by the fuel adaptation. The change in the combustion properties is then incorrectly corrected by the mixture adaptation.

The unpre-released DE 10 2009 028 875 A1 shows a method for determining the composition of a fuel mixture from a first and at least one second fuel, with parameters being determined which depend on the chemical and/or physical and/or combustion properties of the fuel mixture. The composition of the fuel mixture is determined from a combination of a first composition value and at least one second composition value.

The also not pre-released DE 10 2009 028 874 A1 shows a method and a device for determining the composition of a fuel mixture for operating an internal combustion engine. A lean running limit of the internal combustion engine is determined for the fuel mixture and a first composition value is determined from the lean running limit.

Furthermore, the unpublished shows DE 10 2009 028 877 A1 a method and a device for determining the composition of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their calorific value and in their anti-knock properties. A first composition value of the fuel mixture is determined from the consumption of fuel mixture when the internal combustion engine is idling, and a second composition value is determined from an output signal from a knock sensor or from a knock controller of the internal combustion engine. The composition of the fuel mixture is determined from a combination of the first composition value and the second composition value.

Another method for determining the fuel mixture ratio without using a special fuel type sensor is disclosed in the unpublished document DE 10 2009 028 878 A1 described by the applicant. The document describes a method for determining the composition of a fuel mixture from a first fuel and a second fuel for operating an internal combustion engine, the first and the second fuel having different octane numbers and the internal combustion engine having at least one knock sensor and a knock controller. The method is characterized in that the composition of the fuel mixture is determined using an output signal from the knock sensor.

Another method for determining the composition of a fuel mixture from a first and at least one second fuel for operating an internal combustion engine, fuel mixtures of different composition having different anti-knock properties, is characterized in that the composition of the fuel mixture is determined from the ignition angle at which no engine knock occurs. The procedure is in the DE 10 2007 042 718 A1 described by the applicant.

In the unpublished document DE 10 2009 028 329 A1 The applicant proposes a method for determining a fuel composition of a fuel mixture from a first and at least one second fuel for operating an internal combustion engine with a known air-fuel ratio or a lambda value with known fuel, in which the smooth running of the internal combustion engine is at least temporarily negatively influenced by targeted interventions is described. It is provided that the influencing is carried out by means of a variation of an ignition angle and/or by means of a variation of the air-fuel ratio, whereby a parameter for the composition of the currently present fuel mixture or a lambda value for the cylinders of the internal combustion engine can be derived.

The methods mentioned for determining the composition of a fuel mixture on the basis of various parameters and signals already present in the internal combustion engine or its control unit have the disadvantage that the uncertainty that can be achieved when determining the fuel composition is in some cases significantly greater than the accuracy required to control the internal combustion engine.

In the unpublished document DE 10 2008 000 603 A1 The applicant describes a method for controlling an internal combustion engine which is operated with a fuel mixture of a first and at least one second fuel, the internal combustion engine having a fuel metering device, a tank level gauge for determining the tank content and a change in the tank content, a sensor for detection the cylinder filling for determining an air mass supplied to the internal combustion engine and at least one exhaust gas probe for determining and regulating the oxygen content in the exhaust gas. The method is characterized in that in a first method step a first value for the composition of the fuel mixture is determined from the air mass supplied and the measured change in the tank contents, taking into account the oxygen content in the exhaust gas, that in a second method step a second value for the composition of the Fuel mixture is determined from the air mass supplied to the internal combustion engine when idling and the fuel quantity supplied by means of the fuel metering device, that the first and the second value are compared for agreement within a predetermined limit and that, if the values differ from one another, there is a fault in the fuel metering device , in the determination of the air mass supplied or the tank level gauge is closed. The method is therefore used to monitor the fuel metering device, to monitor the air mass supplied or to monitor the tank level gauge; the accuracy in determining the two values for the composition of the fuel mixture is limited to the accuracy of the individual underlying methods for determining the composition and is therefore subject to a great deal of uncertainty.

It is the object of the invention to provide a method which enables the composition of a fuel mixture of at least two fuels to be identified reliably and cost-effectively with a sufficiently low uncertainty of information.

Disclosure of Invention

The object of the invention relating to the method is achieved in that a first composition value of the fuel mixture is determined from the output signal of the knock sensor or from the knock control of the internal combustion engine, that the lean running limit of the internal combustion engine is determined for the fuel mixture, that a second composition value is determined from the lean running limit and that the composition of the fuel mixture is determined from a combination of the first composition value and the second composition value.

The composition values describe the composition of the fuel mixture as determined from the respective individual parameters.

Modern internal combustion engines often have knock sensors whose output signal is used for knock control. If fuels differ in their octane number or in their anti-knock properties, the octane number and anti-knock properties of a fuel mixture obtained from it are determined by the mixing ratio of the fuels involved. The composition of the fuel mixture can thus be inferred from the output signal of the knock sensor or a control process of the knock control, for example the ignition angle output, and the first composition value can be formed.

A leaner air-fuel mixture, i.e. an increase in the lambda value, leads to increasing rough-running in internal combustion engines. Different fuels and fuel mixtures made from them have a typical, so-called lean running limit. If the air-fuel mixture supplied to the internal combustion engine is made leaner beyond the lean running limit, then both the ignition delay is increased and the ignition is greatly slowed down, or the ignition does not start at all. This leads to extremely delayed combustion or misfires, which then result in significantly increased rough running. In the case of a fuel mixture of, for example, petrol and ethanol, the lean-burn limit shifts towards larger lambda values as the ethanol content increases. Knowledge of the lean running limit is therefore an indication of the mixing ratio of the fuel mixture used and can be used to determine the second composition value. It can easily be determined by correspondingly varying the air-fuel ratio supplied to the internal combustion engine, with the resulting change in combustion stochastics being able to be evaluated by evaluating the crankshaft speed, for example using corresponding smooth-running algorithms.

In order to control the internal combustion engine, the composition of the fuel mixture must be known with sufficient accuracy. Considered in isolation, this accuracy cannot be reliably achieved either by determining the composition from the output signal of the knock sensor or the knock control, or by determining the composition from the lean running limit. Only through the inventive combination of the composition values derived from the output signal of the knock sensor or the knock control and from the lean running limit can the composition of the fuel mixture be determined with sufficient accuracy for controlling the internal combustion engine. It is advantageous here that the composition values are determined from parameters that are independent of one another and are correspondingly stochastically distributed around the actual composition of the fuel mixture.

If a knock sensor or a knock regulator is already available to control the internal combustion engine, no additional components, such as additional fuel type sensors, need to be provided to carry out the method, which enables the method to be implemented cost-effectively as a pure software solution. The lean running limit and the second composition value derived from this can also be determined by software adaptation.

In order to carry out the method according to the invention, diagnostic functions required by law, such as mixture adaptation, do not have to be deactivated, which means that the acceptance of the method compared to existing software solutions is significantly increased.

Another advantage compared to existing solutions is that the determination of the fuel composition is not tied to the detection of a refueling process. If, with existing software-based methods for determining fuel compositions, which require the detection of a refueling process, small quantities that can no longer be detected by the tank level sensing system are often refueled, this can lead to a noticeable change in the composition of the fuel mixture - especially when the tank level is low - without that this is recognized by the procedure. This disadvantage is avoided by the method according to the invention.

Is it provided that a limit for the exhaust gas recirculation compatibility of the internal combustion engine for the fuel mixture is determined, that a third composition value is determined from the limit for the exhaust gas recirculation compatibility and that the composition of the fuel mixture is a combination of the first composition value and the third composition value or a combination the second composition value and the third composition value or from a combination of the first composition value, the second composition value and the third composition value is determined, the accuracy in determining the composition of the fuel mixture can be further increased.

Like a leaning of the air-fuel mixture supplied to the internal combustion engine, a change in the quantity of exhaust gas supplied to the internal combustion engine via the exhaust gas recirculation leads to an influence on the smooth running of the internal combustion engine depending on the composition of the fuel mixture. With a fuel mixture of gasoline and ethanol, the exhaust gas recirculation compatibility increases with increasing ethanol content. By varying the exhaust gas recirculation and evaluating the combustion stochastics, for example by evaluating the crankshaft speed using corresponding smooth-running algorithms, the limit for the exhaust gas recirculation compatibility can be determined and the composition of the fuel mixture can be deduced from this. Here, too, it is advantageous that the limit for exhaust gas recirculation compatibility can be determined by software adaptation without additional components.

The composition of the fuel mixture can be determined from the individual composition values using a simple arithmetic operation by combining the composition values by averaging the composition values.

The composition of the fuel mixture is determined precisely by specifying weighting factors for the composition values depending on the accuracy of the respective composition value and by determining the composition of the fuel mixture from a combination of the composition values weighted with the associated weighting factors. The weighting factors indicate how accurate the determination of the respective composition value is. The more accurate composition value is weighted more heavily than the less accurate composition value in accordance with the weighting factor when the composition values are combined to determine the composition of the fuel mixture.

The weighting factors can be determined from the reciprocal of the percentage accuracy of the composition value and using the Gaussian error propagation method the most probable composition of the fuel mixture and the mean deviation from the true value can be calculated. If the statement accuracy for a composition value is 20%, for example, the weighting factor 1/0.2 = 5 should be used. In the case of two composition values Z1 and Z2 and two weighting factors W1 and W2, the most probable composition is wZ $$\text{wZ}=\left(\text{W}{1}^2*\text{Z}1+\text{W}{2}^2*\text{Z}2\right)\text{/}\left(\text{W}{1}^2+\text{W}{2}^2\right).$$

The composition of the fuel mixture can accordingly be determined from the sum of the composition values multiplied by the squares of the associated weighting factors, divided by the sum of the squares of the weighting factors. After this calculation using the Gaussian error propagation method, the most probable value for the composition wZ is determined.

The accuracy m for the most probable composition wZ for a maximum ethanol content of 85% is calculated as follows $$\text{m}=85\%\text{/}\sqrt{\text{W}{1}^2+\text{W}{2}^2}$$

The accuracy m achieved in this way when determining the composition of the fuel mixture is significantly better than can be achieved using the individual composition values.

The accuracy of the determination of the composition values from the output signal of a knock sensor, from a knock control of the internal combustion engine, from the lean running limit and from the limit for the exhaust gas recirculation compatibility depends on the operating range in which the internal combustion engine is currently being operated. Provision can therefore be made for the weighting factors of the individual composition values to be specified as a function of the present operating range of the internal combustion engine in order to obtain high accuracy in determining the composition of the fuel mixture over all operating ranges.

The object of the invention relating to the device is achieved in that a program sequence for determining a first composite value from the output signal of the knock sensor or from the knock control of the internal combustion engine is provided in the control unit, that a program sequence is provided for determining the lean running limit, that a program sequence for Determination of a second composition value from the lean running limit is provided and that a program sequence is provided for determining the composition of the fuel mixture from a combination of the first composition value and the second composition value.

With an existing knock sensor and existing head control of the internal combustion engine, the composition of a fuel mixture can thus be determined by a pure software solution without additional components, without the diagnostic functions specified by the legislator having to be temporarily deactivated. The output signal of the knock sensor and thus the knock control as well as the lean running limit determined by corresponding evaluation of the running smoothness of the internal combustion engine with a changed air/fuel ratio supplied to the internal combustion engine correlate directly with the composition of the fuel mixture, so that the individual composition values are determined from this by corresponding program sequences can become. By combining the composition values, the composition of the fuel mixture can be determined much more precisely than is possible with the individual composition values.

According to a preferred embodiment variant of the invention, it can be provided that a program sequence for determining a limit for the exhaust gas recirculation compatibility of the internal combustion engine for the fuel mixture is provided in the control unit, that a program sequence is provided for determining a third composition value from the limit for the exhaust gas recirculation compatibility and that a program sequence for determining the composition of the fuel mixture from a combination of the first composition value and the third composition value or from a combination of the second composition value and the third composition value or from a combination of the first composition value, the second composition value and the third composition value is provided. The accuracy of the statement when determining the composition of the fuel mixture can be further improved by the third composition value additionally obtained from the limit for the exhaust gas recirculation.

According to the invention, it is provided that the program flow for determining the composition of the fuel mixture is a program routine for determining the mean value of the first composition value and the second composition value or the first composition value and the third composition value or the second composition value and the third composition value or the first composition value and the second composition value and the third composition value. The mean value of the composition values can be determined easily and without a large memory and computing requirement, and as a result leads to an accuracy in determining the composition of the fuel mixture that is significantly better than the accuracy in determining the individual underlying composition values.

According to a particularly preferred embodiment variant of the invention, it can be provided that weighting factors are specified depending on the accuracy of the composition values in the respective operating ranges of the internal combustion engine, that a memory area for storing the weighting factors of the composition values depending on the operating ranges of the internal combustion engine is provided in the control unit and that the program sequence for determining the composition of the fuel mixture contains a program routine for determining the composition of the fuel mixture from the composition values weighted with the respective weighting factors. The weighting factors make it possible when determining the composition composition of the fuel mixture, the different information accuracies of the composition values used must be taken into account. Since the information accuracy of the composition values depends on the respective operating range in which the internal combustion engine is operated, the weighting factors can be stored as a function of the operating range and taken into account accordingly when combining the composition values.

The method and the device can preferably be used to determine the composition of a fuel mixture of gasoline and alcohol, preferably a fuel mixture of gasoline and ethanol.

character list

• 1 ein Ablaufdiagramm zur Bestimmung einer Zusammensetzung eines Kraftstoffgemischs zum Betrieb einer Brennkraftmaschine
• 2 ein Diagramm zur Bestimmung einer Zusammensetzung eines Kraftstoffgemischs zum Betrieb einer Brennkraftmaschine.

The invention is explained below using an exemplary embodiment illustrated in the figures. It shows:

• 1 a flowchart for determining a composition of a fuel mixture for operating an internal combustion engine
• 2 a diagram for determining a composition of a fuel mixture for operating an internal combustion engine.

1 shows a flowchart for determining an in 2 shown composition 30 of a fuel mixture for operating an internal combustion engine using the example of a gasoline-ethanol fuel mixture.

A block ZW110 and a block WF 12 are provided in a first branch of the flowchart, and a block ZW2 11 and a block WF2 13 are provided in a second branch.

A signal 20 of a knock control is fed to block ZW110 as an input variable, and a lean running limit 21 of the internal combustion engine determined in a program routine (not shown) is fed to block ZW2 11 . The blocks WF112 and WF2 13 are supplied with the operating range 22 in which the internal combustion engine is currently being operated.

A block Z 14 follows the blocks WF1 and WF2.

Due to the higher octane number of ethanol compared to gasoline, an indication of the ethanol content in the fuel mixture can be obtained by evaluating the behavior of the knock control on different ignition angle outputs. In block ZW110, therefore, from signal 20 of the knock control, an in 2 shown first composition value 32 for the fuel mixture can be determined.

Internal combustion engines have a lean running limit 21 that is dependent on the combustion properties of the fuel used. If the air-fuel ratio supplied to the internal combustion engine is leaned beyond the lean running limit 21, this results in increased rough running of the internal combustion engine. In the case of a gasoline/ethanol fuel mixture, the lean running limit 21 shifts towards larger lambda as the ethanol content increases. The lean running limit 21 can be determined by evaluating the combustion stochastics, for example the standard deviation of the mean indicated pressure in at least one of the cylinders of the internal combustion engine. For example, the crankshaft speed can be evaluated using smooth-running algorithms. Due to the correlation of the lean running limit 21 determined in this way with the ethanol content of the fuel mixture, in the block ZW2 11 an in 2 second composition value 33 shown can be determined.

The first composition value 32 derived from the signal 20 of the knock control and the second composition value 33 derived from the lean running limit 21 are subject to an uncertainty of information that is greater than the maximum total deviation 34 required for controlling the internal combustion engine when determining the composition 30 of the fuel mixture. The accuracy that can be achieved in each case depends on the operating range 22 in which the internal combustion engine is operated. In block WF112, therefore, the first composition value 32 is assigned an in 2 shown first weighting factor 36a and in block WF2 13 the second composition value 33 is also assigned an in 2 shown second weighting factor 36b assigned depending on the operating range 22 of the internal combustion engine.

The composition 30 of the fuel mixture is then determined in block Z 14 by a combination of the first composition value 32 and the second composition value 33 weighted with the respective weighting factors 36a, 36b the associated weighting factor 36a, 36b is set and the more precise composition value 32, 33 is taken into account to a correspondingly greater extent in the combination for determining the composition 30 of the fuel mixture.

2 shows a diagram for determining a composition 30 of a fuel mixture for operating an internal combustion engine using the example of a gasoline-ethanol fuel mixture.

The percentage of ethanol in a gasoline-ethanol fuel mixture is plotted along a first axis 31 . In order to achieve a desired overall deviation 34 when determining the composition 30, a first composition value 32 is determined from the knock control signal 20 and a second composition value 33 is determined from the lean running limit 21. The composition values 32, 33 are shown as an expected value with standard deviation and are arranged next to one another along a second axis 17. The standard deviation of the composition values 32, 33 is greater than the desired maximum total deviation 34 for determining the composition 30 of the fuel mixture.

According to the invention, the composition 30 of the fuel mixture is determined by a combination of the composition values 32,33. The combination takes place by forming a weighted average over the composition values 32, 33. The composition values 32, 33 are assigned weighting factors 36, which are a measure of the information accuracy of the respective composition value 32, 33. For example, the first weighting factor 36a with the value 5 expresses an accuracy of 1/5=20%; the weighting factor 36b with the value 3 means an accuracy of 1/3=33.3%.

Since the composition values 32, 33 were determined independently of one another and a stochastic distribution of the composition values 32, 33 around the composition 30 can therefore be assumed, a suitable combination of the composition values 32, 33 can achieve the required accuracy represented by the total deviation 34 in the determination of the composition 30 of the fuel mixture can be achieved, even if the uncertainties of the underlying composition values 32, 33 are significantly greater.

The improvement in the accuracy of the statement by using the weighted averaging is explained below using a calculation example:

The first composition value 32 is 40%, the second composition value 33 is 20% ethanol content in a gasoline-ethanol fuel mixture. The weighting factors 36, which reflect the measurement deviation when determining the respective parameter, are 5 for the first weighting factor 36a and 3 for the second weighting factor 36b. According to the well-known method of considering Gaussian errors, the composition determined on the basis of the two pairs of values above is 30: $$\text{<figure-callout id="30" label="composition" filenames="DE102009029011B4\_0005.png" state="{{state}}">composition</figure-callout>}30=\left(40\%*5^2+20\%*3^2\right)\text{/}\left(5^2+3^2\right)=34.7\%$$

The accuracy in determining the composition 10 with the two weighting factors 18 with the values 5 and 3 and a maximum ethanol content of 85% can also be determined using the Gaussian error analysis method: $$85\%\text{/}\sqrt{5^2+3^2}=14.6\%$$

The combination of the two composition values 12, 13 with weighting factors 18 with the values 5 and 3, which correspond to accuracies of 85%/5 = 17% and 85%/3 = 28.3%, respectively, results in a statement with an improved accuracy of 14 .6%.

According to the invention, the weighting factors 36 can be defined as a function of the operating range 22 of the internal combustion engine. This improves the reliability of the method and its accuracy, since in some operating states parameters from which the composition values 32, 33 are determined, can be determined particularly precisely and the composition values 32, 33 obtained therefrom can therefore be weighted more highly than in other operating ranges.

Claims (9)

Method for determining the composition (30) of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their anti-knock properties, in their lean running limit (21) and/or in their exhaust gas recirculation compatibility, the composition of the fuel mixture is determined by means of an output signal of a knock sensor or from a knock control, characterized in that a first composition value (32) of the fuel mixture is determined from the output signal of a knock sensor or from the knock control of the internal combustion engine, that the lean running limit (21) of the internal combustion engine for the fuel mixture it is determined that a second composition value (33) is determined from the lean running limit (21) and that the composition (30) of the fuel mixture is a combination of the first composition value (32) and the second composition value ( 33) is determined, with weighting factors (36) for the composition values (32, 33) depending on the accuracy of the statement of the respective composition value (32, 33) being specified and the composition (30) of the fuel mixture from a weighting factor (36) weighted combination of the composition values (32, 33). procedure after claim 1 , characterized in that a limit for the exhaust gas recirculation compatibility of the internal combustion engine is determined for the fuel mixture, that a third composition value is determined from the limit for the exhaust gas recirculation compatibility and that the composition (30) of the fuel mixture consists of a combination of the first composition value (32) and the third composition value or from a combination of the second composition value (33) and the third composition value or from a combination of the first composition value (32), the second composition value (33) and the third composition value. procedure after claim 1 or 2 , characterized in that the composition values (32, 33) are combined by averaging over the composition values (32, 33). Method according to one of the preceding claims, characterized in that the weighting factors (36) of the individual composition values (32, 33) are specified as a function of the present operating range (22) of the internal combustion engine. Device for determining the composition (30) of a fuel mixture of a first fuel and at least one second fuel for operating an internal combustion engine, the fuels differing in their anti-knock properties, their lean running limit (21) and/or their exhaust gas recirculation compatibility, the internal combustion engine having a Has a knock sensor and a knock controller and the composition of the fuel mixture is determined by means of an output signal from the knock sensor or the knock controller and a control unit is assigned to the internal combustion engine, characterized in that a program sequence for determining a first composition value (32) from the output signal of the Knock sensor or from the knock control of the internal combustion engine, a program sequence for determining the lean running limit (21) is provided, that a program sequence for determining a second composition value (33). the lean running limit (21), that a program sequence is provided for determining the composition (30) of the fuel mixture from a combination of the first composition value (32) and the second composition value (33), that weighting factors (36) depending on the information accuracy of the Composition values (32, 33) are specified in the respective operating ranges (22) of the internal combustion engine, that a memory area for storing the weighting factors (36) of the composition values (32, 33) depending on the operating ranges (22) of the internal combustion engine is provided in the control unit and that the program sequence for determining the composition (30) of the fuel mixture contains a program routine for determining the composition (30) of the fuel mixture from the composition values (32, 33) weighted with the respective weighting factors (36). device after claim 5 , characterized in that a program sequence for determining a limit for the exhaust gas recirculation compatibility of the internal combustion engine for the fuel mixture is provided in the control unit, that a program sequence for determining a third composition value from the limit for the exhaust gas recirculation compatibility is provided and that a program sequence run to determine the composition (30) of the fuel mixture from a combination of the first composition value (32) and the third composition value or from a combination of the second composition value (33) and the third composition value or from a combination of the first composition value (32), the second composition value (33) and the third composition value. device after claim 5 or 6 , characterized in that the program sequence for determining the composition (30) of the fuel mixture contains a program routine for determining the mean value of the first composition value (32) and the second composition value (33). device after claim 6 , characterized in that the program flow for determining the composition of the fuel mixture includes a program routine for determining the mean value of the first composition value (32) and the third composition value or the second composition value (33) and the third composition value or the first composition value (32) and the second composition value (33) and the third composition value. Application of the method according to one of Claims 1 until 4 or the device according to any one of Claims 5 until 8th , for determining the composition (30) of a fuel mixture of gasoline and alcohol, preferably a fuel mixture of gasoline and ethanol.

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