Ignition electrode with heating device.

0001The invention relates to a method and a device for operating an electrical heating of ignition electrodes, the heating consisting of an electrical resistance material which is wound around the electrode insulator.

0002Ignition electrodes, in particular burners and regeneration burners for particle filters, are exposed to combustion or exhaust gases, through which the ceramic insulation of the ignition electrode is coated with soot particles. As a result, the high voltage connected between electrodes is dissipated due to the electrical conductivity of the soot and an arc is not formed or is not formed reliably.

0003In order to remedy this effect, the electrode insulator is known to be heated, which consists of an electrical resistance winding which is wound around the insulator and supplied with voltage for burning the soot coating.

0004The resistance of such heating is designed so that even under unfavorable conditions, such as. B. Too high voltage and high ambient temperature (in the case of a vehicle high vehicle electrical system voltage and high engine exhaust), the maximum permissible temperature at the electrode of approx. 800 - 1000 ° C is not exceeded in any case. As a result, a relatively long heating-up time of around 550 ° C is required when the vehicle electrical system voltage is low and the exhaust gas temperature is low.

0005The invention has for its object to provide a method of the type mentioned, in which a reliable heating of ignition electrodes is possible despite changing operating conditions.

0006The object is achieved with the measures of claim 1.

0007Due to the temperature-controlled operation of the heating device, it is no longer necessary to design the resistance value in accordance with the prior art, so that the focus can be placed on a short heating-up time, which also ensures a stable insulator temperature under changing operating conditions. This means that the soot burn-off temperature can be reached quickly during commissioning and kept constant during the entire operation, despite changing boundary conditions such as voltage and temperature.

0008According to one embodiment of the invention, the heating device is operated with periodic voltage pulses, while a temperature measurement is carried out between each voltage pulse. The temperature measurement can take place in every voltage break or in every second, third or nth voltage break.

0009The heating power is preferably regulated by temperature-dependent modulation of the voltage pulse widths.

0010In this way it is possible to implement effective control of the insulator temperature of ignition electrodes with simple manufacturing and control measures without the need to use additional temperature sensors.

0011The method according to the invention has the further advantage that an interruption or a short circuit in the electrode winding can also be diagnosed.

0012The invention extends to a device for carrying out the method, which is characterized by the features of claims 6 and 7.

0013The invention is described in more detail with reference to an embodiment shown schematically in the drawing.

00141 shows an exhaust system 10 of an internal combustion engine with a particle filter 11 through which the exhaust gas 12 flows. To raise the exhaust gas temperature, a pair of electrodes 13, 14 is arranged in the exhaust system 10, which is connected to a high-voltage source, not shown. The ignition electrodes 13, 14 each consist of an electrode 15 and a ceramic insulator 16 which, when exposed to the exhaust gases 12, can be coated with soot on its outer surface 17. To prevent this, a heater consisting of an electrical resistance wire 20 is provided in the insulator 16 and is supplied by a direct current source 21.

0015For a controlled heating of the insulators 16, a control device 26 is provided, which has a resistance temperature measuring device 28 and a processor 29. The temperature measuring device 28 consists of a resistance measuring bridge R₁ -R₄, where R₄ is the resistance of the heating wire 20 of the respective electrode 13. A temperature measuring switch 31 and a heating circuit 35 are controlled by the processor 29. The control unit 26 also contains a corresponding circuit for the second electrode 14. Both circuits are designed as parallel circuits and are controlled by a processor 29.

0016For temperature measurement recordings, the temperature measurement switch 31 is closed by means of a control pulse and thus the resistance bridge to the voltage U._B placed, the potentials U₃₄ and U₁₂ of the resistance bridge 28 recorded by the processor 29 and evaluated to calculate the instantaneous resistance value R₄ of the heating wire 20 according to the following formula:<maths id="math0001" num=""><img file="EP0505696A2_D0001.tif" /></maths> which can be derived from the ratio U₁₂ / U₃₄. The instantaneous temperature T at the insulator is finally calculated via the resistance value R₄ of the wire 20 and the resistance temperature function below.<maths id="math0002" num=""><img file="EP0505696A2_D0002.tif" /></maths>

0017In the formula, R₄₀ is the resistance value of the winding at 0⁰C, n is an integer dependent on the resistance material and a is a temperature coefficient.

0018Depending on the temperature measured in this way, the heating power for the insulator heater 20 is regulated by the processor 29. This is done by changing the pulse widths of a pulse-like voltage application for the resistance wire.

00192 shows the voltage U applied to the heating wire 20 as a function of the time t. During the measuring period, ie with the temperature measuring switch 31 closed, a voltage U₃ U is applied to the heating wire 20. For temperature measurements, measuring pulses 40 are of constant time t_T spent. For the load application, the switch 35 is closed, whereby the heating wire 20, the battery voltage U_B receives. This load application takes place in a pulsed manner with a time duration t (T) which depends on the measured temperature T. By means of a fixed specification of the maximum current value for the heating wire 20, a current limitation for the load circuit can thus be achieved by appropriate pulse width modulation. A temperature measurement can be carried out in each case in the time span Δt between two load pulses 41. In this case, it is advantageous if the temperature measurement and the load application are carried out alternately at a predetermined clock frequency. The period D of the cycle depends on the maximum required pulse width t (T)_Max of the load pulse 41.

0020Of course, other types of control are possible, such as, for example, that the resistance or temperature measurement is carried out regularly but not after each load pulse 41. Continuous voltage regulation for load operation is also possible, load operation being briefly interrupted for temperature measurements.

0021On the basis of the predetermined resistance values ​​of the heating wires 20 or windings, the state of the resistance wire 20 can also be monitored from the resistance measurement by interruptions and short-circuits can be detected.