The present invention relates to a method for monitoring at least one glow plug of an internal combustion engine, and a corresponding device.
Glow plugs are typically monitored in internal combustion engines in that the current flowing through the glow plugs is compared to a predefined fixed threshold value. If the power consumption by the glow plug is less than the threshold value, the glow plug is evaluated as faulty. Similar circuits use comparators or differential amplifiers for this purpose. Microcomputer-based glow time control units ascertain a digital value corresponding to the current through the glow plug via an analog/digital converter and compare this value to a stored digital threshold value.
It is problematic that the current flow of the glow plug after application of the supply voltage is strongly time-dependent. The monitoring of the glow plug on the basis of a fixed current value therefore only allows a very coarse evaluation.
It has proven to be disadvantageous that the current flow varies erratically with increasing reduction of the current value. It is shown in the figure that below a current value IB, the current flow is at least strongly wavy and has a discontinuity at a point in time t1. This current value IB is still above lower threshold value IU. In the time range of the current flow up to t1, above current value IB, a possible flow of the current is conventionally recognized. More precisely, the current flow may be predicted or modeled in this range. In the time range of the current flow from t1, below current value IB, in contrast, the possible flow may not be exactly inferred. This is disadvantageous because in this range, a drop below lower threshold value IU is hardly or not at all recognizable, or at least only with a delay. Fault recognition of the glow plug is thus unreliable, with the possible risk of fusing of the glow plug and total engine damage.
It is an object of the present invention to provide a method which allows reliable monitoring of at least one glow plug of an internal combustion engine. It is a further object of the present invention to provide a corresponding device.
This object may be achieved by an example method for monitoring at least one glow plug of an internal combustion engine, in which a time-dependent variable, which characterizes the current flowing through the at least one glow plug, is compared for fault recognition to at least one time-dependent minimum and/or maximum threshold value, and a fault is recognized if the time-dependent variable is greater and/or less than the minimum and/or maximum threshold variable. The example method is characterized in that the first derivative of the time-dependent variable is compared to the first derivative of the maximum threshold value and the second derivative of the time-dependent variable is compared to the second derivative of the maximum threshold value, and a fault is recognized if the first derivative of the time-dependent variable is less than the first derivative of the maximum threshold value and the second derivative of the time-dependent variable is less than the second derivative of the maximum threshold value.
In accordance with the example method of the present invention, a fault of the glow plug is reliably recognized.
It is therefore provided in an advantageous specific embodiment of the present invention that the time-dependent variable, which characterizes the current flowing through the at least one glow plug, is a resistance model of the glow plug. The circuitry complexity or the programming complexity in the control unit may thus be significantly reduced and a simple and cost-effective achievement of the object is available.
The time-dependent threshold value preferably contains the characteristic curve of the resistance of the corresponding glow plug. The circuitry complexity or the programming complexity in the control unit may also be significantly reduced here and a simple and cost-effective achievement of the object is available.
A glow plug circuit is preferably turned off in the event of fault recognition. Fusing of the glow plug, in particular of the heater in the case of metal cores, may thus be prevented.
Corresponding information is preferably entered in a fault memory in the event of fault recognition. An effective glow plug control may thus be ensured.
A diagnostic message is preferably transmitted in the event of fault recognition. An effective glow plug control may thus also be ensured.
A glow plug control is preferably changed in response to the diagnostic message. The glow plug control may thus always be updated to new parameters.
A notification about the status of the glow plug is preferably displayed to the driver in response to the diagnostic message. The driver is thus always informed about the status of the glow plug and may optionally adjust his driving behavior thereto.
In a preferred specific embodiment, a device for monitoring at least one glow plug of an internal combustion engine contains an evaluation unit, which contains a comparator arrangement, which compares a time-dependent variable, which characterizes the current flowing through the at least one glow plug, for error recognition to at least one time-dependent minimum and/or maximum threshold value, and the evaluation unit recognizes a fault if the time-dependent variable is greater and/or less than the minimum and/or maximum threshold value. The device is characterized in that the evaluation unit also contains a further comparison means, which compares the first derivative of the time-dependent variable to the first derivative of the maximum threshold value and the second derivative of the time-dependent variable to the second derivative of the maximum threshold value, and the evaluation unit recognizes a fault if the first derivative of the time-dependent variable is less than the first derivative of the maximum threshold value and the second derivative of the time-dependent variable is less than the second derivative of the maximum threshold value. The complexity in the application of the control unit may thus be significantly reduced.
The comparator arrangement preferably contain at least one comparator.
An example method according to the present invention and the corresponding device are explained in greater detail hereafter on the basis of exemplary embodiments. Identical or identically-functioning parts are provided with identical reference numerals.
Furthermore, a control unit 130 is provided, which includes an analyzer unit 133, an activation unit 135, and an error recognition unit 137, in addition to further elements (not shown). Activation unit 135 activates switch 110, to supply a desired energy to the glow plug. Analyzer unit 133 analyzes the voltage drop across ammeter 120, in order to ascertain the current which flows through the glow plug. Ammeter 120 is preferably implemented as an ohmic resistor. The voltage drop across ammeter 120 is supplied to an amplifier 140, which provides its output signal to analyzer unit 133. Furthermore, the output signal of measuring amplifier 140 reaches a comparator 150, to whose second input the output signal of a threshold value setpoint 160 is applied.
The glow plugs typically have a very low resistance at the beginning of energization. This has the result that a very large current flows at the beginning of energization. The resistance of the glow plug increases through their heating, which in turn has the result that the current drops. It has been shown to be disadvantageous that the current flow varies very erratically with increasing reduction of the current value.
In the specific embodiment, two comparators are provided, so that a threshold value query having a lower threshold value and an upper threshold value is possible. In a simplified specific embodiment, one of the two comparators and one of three resistors 202, 203, or 204 may be dispensed with. In this specific embodiment, only a comparison to a threshold value is possible. The same voltage which is applied to the glow plug to be monitored is applied to the voltage divider and the series circuit made up of capacitor 205 and resistor 201.
The voltage drop, which corresponds to the current which flows through the glow plug, is compared to the voltage drop across capacitor 205. The entire voltage is not analyzed for this purpose, but rather the voltage divided by the voltage divider, made up of resistors 202, 203, and 204. A signal is applied to each of the outputs of comparators 150a and 150b, which indicates a fault or indicates a faultless operation as a function of the comparison.
The circuit shown represents a simple simulation of the glow plug. The voltage at the capacitor is a function of the charge of the capacitor. The capacitor has an integrating effect and adds up the energy introduced into the glow plug. This is achieved in that a voltage proportional to the voltage drop across the glow plug is applied to capacitor 205. The charge state or the voltage at capacitor 205 is a measure of the temperature or the resistance of the glow plug. Through suitable selection of the values of the capacitor and the resistors, the behavior over time of the output voltage of the voltage divider, which is formed by resistors 202, 203, and 204, corresponds to the behavior over time of the faultless current through the glow plug. The lower and/or upper threshold values may be predefined by a corresponding allocation of the resistance values. It has been shown to be disadvantageous that the current flow behaves very erratically with increasing drop of the current value.
Resistance value Rplug is input into an evaluation unit 320. In evaluation unit 320, resistance value Rplug is input directly into a first comparison unit 330, which contains two comparators, for example. A minimum resistance threshold value Rmin and a maximum resistance threshold value Rmax of the glow plug are also input into first comparison unit 330. First comparison unit 330 compares resistance value Rplug to minimum resistance threshold value Rmin and maximum resistance threshold value Rmax of the glow plug, as already described in the explanation of
Evaluation unit 320 also contains a first derivation unit 340, which calculates a first derivative over time d/dt, and which is also connected to the signal circuit for supplying resistance value Rplug. First derivation unit 340 performs a first time derivation on resistance value Rplug for this purpose and feeds the result to a second comparison unit 350. Second comparison unit 350 compares the result of the first derivative from first derivation unit 340 to a value of a first derivative of maximum resistance threshold value {dot over (R)}max of the glow plug.
Evaluation unit 320 also contains a second derivation unit 360, which is connected to the output of first derivation unit 340. Second derivation unit 360 performs a derivation over time on the first derivative of resistance value Rplug and supplies the result, namely a second derivative of resistance value {umlaut over (R)}plug, to a third comparison unit 370. Third comparison unit 370 compares the result of the second derivative of resistance value Rplug from second derivation unit 360 to a value of a second derivative of maximum resistance threshold value {umlaut over (R)}max of the glow plug.
First comparison unit 330 of evaluation unit 320 is implemented in such a way that a signal is output which indicates a fault of the glow plug if resistance value Rplug is greater than minimum resistance threshold value Rmin, or resistance value Rplug is less than maximum resistance threshold value Rmax. In contrast, second comparison unit 350 of evaluation unit 320 is implemented in such a way that a signal which indicates a fault of the glow plug is output if the first derivative over time of the resistance value of the glow plug is less than the first derivative of maximum resistance threshold value {dot over (R)}max Furthermore, third comparison unit 370 of evaluation unit 320 is implemented in such a way that a signal which indicates a fault of the glow plug is output if the second derivative over time of the resistance value of the glow plug is less than the second derivative of maximum resistance threshold value {umlaut over (R)}max.
The output signals, which indicate a fault of the glow plug, are each input into a control unit 380, which turns off the glow plug circuit and/or enters appropriate information in a fault memory 382 and/or transmits a diagnostic message via a transmitter 384 in response to only a single input signal. Fault memory 382 and transmitter 384 may be connected for this purpose via an interface 386, which receives the input signals at control unit 380 and relays them to fault memory 382 and transmitter 384.
In response to the transmitted diagnostic message, a change in a glow plug control may optionally be caused in a glow plug unit 400, which is connected to transmitter 384. Furthermore, a notification about the status of the glow plug may be displayed to the driver via a display 410 in response to the transmitted diagnostic message.
Number | Date | Country | Kind |
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10 2008 007 397.0 | Feb 2008 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2008/065550 | 11/14/2008 | WO | 00 | 7/1/2010 |