Diagnosis of dew point detection in the exhaust system of an internal combustion engine

Abstract

Method for diagnosing dew point detection in the exhaust system of an internal combustion engine, comprising: - Multiplying a temperature difference ΔT(t) between the temperature T1(t) of an exhaust gas of the internal combustion engine at a dew point detection point and a fixed temperature T2 by a factor f(d) that depends on the rotational speed d of the internal combustion engine, - Integrating the resulting product f(d) x ΔT(t)T(t) over time t to obtain an integral I as a control value, and output an error if the integral I reaches or exceeds a threshold S before dew point detection has occurred, where the factor f(d) at an engine speed d greater than the idle speed dL is calculated according to the following formula: f(d) = Amin(d) / Amin(dL), where Amin(d) represents the minimum possible exhaust mass flow at engine speed d and Amin(dL) represents the minimum possible exhaust mass flow at idle speed dL.

Description

The present invention relates to a method for diagnosing dew point detection in the exhaust system of an internal combustion engine. It further relates to a computer program that executes all steps of the method according to the invention when run on a computing device. Finally, the invention relates to a computer program product with program code stored on a machine-readable medium. To carry out the method according to the invention, the program is executed on a computer or control unit. State of the art For ceramic sensors, especially lambda sensors, to function correctly, they must reach an operating temperature of approximately 700°C. Since the ceramic sensor cannot be heated to this temperature by the exhaust gas at all operating points of the internal combustion engine, these sensors have their own heating element to ensure the operating temperature is maintained across the entire range. A problem arises from the fact that water is produced during the combustion of air-fuel mixtures in internal combustion engines. At temperatures below approximately 60°C in the exhaust manifold and / or the catalytic converters installed there, water is in liquid form. In principle, water can even remain in liquid form in catalytic converters up to a temperature of 100°C. Furthermore, not only does the water produced during combustion condense, but also the humidity from the ambient air.If liquid water were to come into contact with the ceramic sensor, it could be destroyed. The immediate endothermic evaporation of the water would cause significant temporal and spatial temperature gradients, and ceramic probes are highly sensitive to temperature shocks. Therefore, for the use of such a ceramic probe, it is crucial that heating to operating temperature only begins once all liquid water is no longer present in the exhaust system—in other words, once the dew point has been reliably reached. This operating condition is not yet reached, particularly during the engine start-up process when the exhaust system is still cold.At this point, there is a risk that water from previous operating cycles of the internal combustion engine, i.e., from previous journeys of the vehicle, is still present in the exhaust system and has condensed. Simultaneously, condensation of newly formed water must be expected wherever temperatures in the exhaust system are below 60°C. On the other hand, it is essential to minimize emissions generated during the start-up phase and to precisely adjust the composition of the combustion gas so that optimal combustion minimizes pollutant emissions even in this operating state. This results in the problem that the probe heater should be activated as late as possible to ensure the probe's protection, while operational readiness to achieve the necessary emission limits should be reached as early as possible. Dew point detection is a physical calculation model used to activate the lambda sensor. It determines the point at which condensation, which has formed in the exhaust system of an internal combustion engine, has completely evaporated upstream at a specific location within the exhaust system. This location could, for example, be the position of a lambda sensor. Dew point detection is now used in every vehicle equipped with a lambda sensor. This function calculates how much heat the exhaust gas introduces into the exhaust system and integrates this heat flow over time. Once a certain amount of heat is reached, it is assumed that the condensation has evaporated and the lambda sensor heater is activated. New emissions legislation in the United States now requires a diagnostic test to verify that the Fuel Mass Observer (FMO), a model of the system being controlled, operates within an acceptable timeframe. The FMO uses the signal from the oxygen sensor (lambda sensor) as its input. This indirectly requires that the dew point detection function correctly and that the oxygen sensor is released in a timely manner. One diagnostic method is based on the principle of integrating a temperature difference instead of a quantity of heat. The integral of this temperature difference is compared to a threshold value. If this threshold is reached before the oxygen sensor is released, an error is triggered because the oxygen sensor was released too late or not at all. Otherwise, the system detects that the oxygen sensor was released in a timely manner. The threshold values ​​to be set for enabling the lambda sensor and for detecting a fault are determined while the internal combustion engine is idling. The lambda sensor enable threshold is chosen so that the value at which the condensation evaporates is increased by a safety factor. The diagnostic threshold is set so that the value at which the lambda sensor is enabled is increased by a further safety factor. The integrated heat quantity is calculated as the product of a temperature difference in the exhaust tract, the exhaust mass flow rate, and a constant factor. A temperature difference is integrated into the diagnostic dew point detection. This means that the information about the exhaust mass flow rate is ignored and not used in the diagnostic process.Because the threshold value is determined at idle, i.e., at the lowest possible exhaust gas mass flow, the diagnostic function automatically becomes more robust against false detection at all other operating points, because the release function is integrated faster than the diagnostic function. This additional safeguard against false detection, however, means that the diagnostic function runs more slowly than necessary. There are legal requirements that a diagnosis must be completed within a certain timeframe. This can lead to problems if a long time elapses before the dew point is reached. A method for diagnosing dew point failure in the exhaust system of an internal combustion engine is already known from DE 10 2009 002 037 A1. Disclosure of the invention The inventive method for diagnosing dew point detection in the exhaust system of an internal combustion engine comprises multiplying a temperature difference ΔT(t) in the exhaust system by a factor f(d) that depends on the rotational speed d of the internal combustion engine. The temperature difference ΔT(t) here denotes the difference between the temperature T1(t) of the exhaust gas from the internal combustion engine at a dew point detection point and a defined temperature T2. The dew point detection point is defined as the location in the exhaust system where all condensation has completely evaporated in the case of a positive dew point detection, including all condensation upstream of this point. This is typically the location where a lambda sensor is installed. The temperature T1(t) can be determined by measurement or modeling.The temperature T2 is specifically set to a value at which the evaporation of the condensate in the exhaust system of the internal combustion engine begins. The temperatures T1(t) and T2 converge over time until temperature T1(t) exceeds temperature T2, such that ΔT > 0. The resulting product f(d) x ΔT(t) is then integrated over time t to obtain an integral I as a control value. An error is reported if the integral I reaches or exceeds a threshold value S before dew point detection occurs. This method allows the influence of the exhaust mass flow rate to be indirectly considered in the dew point detection diagnosis via the factor f(d), without including the error-prone mass flow rate itself in the calculation. For the simple implementation of this method in an existing internal combustion engine, it is preferred according to the invention that the factor f(d) is stored as a characteristic curve in a control unit of the internal combustion engine. The most reliable fault detection occurs when the internal combustion engine is idling. Therefore, according to the invention, it is preferred that the factor f(d) at the idle speed dL of the internal combustion engine is 1. To ensure that fault detection is as reliable against false detection at all operating points as at idle, the invention provides that the factor f(d) is calculated according to formula 1 at the rotational speed d of the internal combustion engine, which is greater than the idle speed dList. In Formula 1, Amin(d) indicates the minimum possible exhaust mass flow rate at engine speed d. The value Amin(dL) indicates the minimum possible exhaust mass flow rate at idle speed dLan. At all other operating points of the internal combustion engine, except idle, this provides an otherwise unused additional safety margin. According to the invention, this margin is used for rapid integration, and the diagnostic function can be executed quickly. To facilitate the implementation of the inventive method on an internal combustion engine, the invention further relates to a computer program that executes all steps of the inventive method when run on a computing device. A computer program product with program code stored on a machine-readable medium serves to carry out the inventive method when the program is executed on a computer or control unit. Brief description of the drawing An embodiment of the invention is illustrated in the drawing and explained in more detail in the following description. Fig. 1 shows a process diagram of an embodiment of the method according to the invention. Embodiments of the invention Figure 1 schematically illustrates the process of a method according to one embodiment of the invention. The temperature difference ΔT(t) = T1(t) - T2 between the temperature T1(t) of the exhaust gas from the internal combustion engine at the point of dew point detection, where a lambda sensor is located, and a defined temperature T2 is continuously determined. Furthermore, the rotational speed d of the internal combustion engine is determined. From a characteristic curve K stored in the engine control unit, the factor f(d), which is assigned to the rotational speed d, is determined. The temperature difference ΔT(t) is multiplied by the factor f(d). Subsequently, the product thus obtained is integrated over time. This yields the integral I, which is calculated according to Formula 3. The integral I is compared to a threshold value S stored in the control unit. If the integral I reaches or exceeds the threshold value S before the dew point is detected, an error message is issued. All steps of the inventive method are preferably carried out by a computer program, which enables a simple implementation of the inventive method.

Claims

A method for diagnosing dew point failure in the exhaust system of an internal combustion engine, comprising: multiplying a temperature difference ΔT(t) between the temperature T1(t) of the exhaust gas of the internal combustion engine at a dew point failure point and a specified temperature T2 by a factor f(d) that depends on the rotational speed d of the internal combustion engine; integrating the resulting product f(d) x ΔT(t) over time t to obtain an integral I as a control value; and outputting an error if the integral I reaches or exceeds a threshold value S before dew point failure has occurred, wherein the factor f(d) is calculated at a rotational speed d of the internal combustion engine which is greater than the idle speed dList according to the following formula: f(d) = Amin(d) / Amin(dL) where A min (d) specifies the minimum possible exhaust mass flow at engine speed d and A min (d L ) the minimum possible exhaust gas mass flow at idle speed d L indicates. Method according to claim 1, characterized in that the specified temperature T2 corresponds to the temperature of the start of condensate evaporation in the exhaust tract of the internal combustion engine. Method according to claim 1 or 2, characterized in that the factor f(d) is stored as a characteristic curve K in a control unit of the internal combustion engine. Method according to one of claims 1 to 3, characterized in that the factor f(d) at the idle speed dL of the internal combustion engine is 1. Computer program that performs all steps of a method according to any one of claims 1 to 4 when run on a computing device. Computer program product comprising program code stored on a machine-readable medium for carrying out the method according to any one of claims 1 to 4 when the program is executed on a computer or control device.

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