Method and control unit for setting a temperature of a glow plug

Abstract

A method is described for setting a temperature of a glow plug, in particular for igniting a fuel/air mixture in an internal combustion engine in which the temperature of the glow plug is set as a function of a resistance of the glow plug with the aid of a control. To prevent temperature overshoots from occurring during the preheating phase of the glow plug, the temperature is controlled with the aid of a predictive model during a preheating phase during which an overvoltage is applied to the glow plug.

Claims

1. A method for controlling a temperature of a glow plug during a preheating phase of a glow process, the method comprising: measuring a resistance of the glow plug during the preheating phase, wherein a voltage is applied to the glow plug during the preheating phase that is greater than a voltage applied during a steady state operation of the glow plug; determining, using a physical model, a resistance quantity, representing a resistance difference between the measured resistance and a resistance of the glow plug at an end of the preheating phase, as a function of one or more inputs to the physical model, the determining including providing the one or more inputs to one or more corresponding characteristic curves implemented by the physical model, the one or more inputs including a temperature of the glow plug at a start of the glow process; adding the measured resistance to the determined resistance quantity to produce a sum; ascertaining an actual temperature value of the glow plug during the preheating phase based on the sum of the measured resistance and the determined resistance quantity, the ascertaining including providing the sum of the measured resistance and the determined resistance quantity as an input to a characteristic curve of a temperature of the glow plug as a function of a resistance of the glow plug during steady state operation; subtracting the actual temperature value from a temperature setpoint value to produce a temperature difference; supplying a voltage signal to the glow plug during the preheating phase, the voltage signal based on the temperature difference; and controlling and regulating a heater coil of the glow plug using the voltage signal, to heat the glow plug to the temperature setpoint value.

2. The method as recited in claim 1, wherein the method is for igniting a fuel and air mixture in an internal combustion engine.

3. The method as recited in claim 1, wherein: the resistance quantity is based on multiple partial resistance quantities, each partial resistance quantity being determined as a function of at least one operating parameter of the glow plug by providing the at least one operating parameter as at least one input to at least one corresponding characteristic curve implemented by the physical model.

4. The method as recited in claim 3, wherein the resistance quantity is a sum of the multiple partial resistance quantities.

5. The method as recited in claim 3, further comprising: determining a first partial resistance quantity as a function of an energy content of the glow plug at the start of the glow process by providing the energy content as an input to a corresponding characteristic curve implemented by the physical model.

6. The method as recited in claim 5, wherein the energy content of the glow plug includes one of an initial resistance, an initial amount of heat, and an initial performance of the glow plug.

7. The method as recited in claim 5, further comprising: determining a second partial resistance quantity as a function of the temperature setpoint value of the glow plug for the glow process by providing the temperature setpoint value as an input to a corresponding characteristic curve implemented by the physical model.

8. The method as recited in claim 7, further comprising: determining a third partial resistance quantity as a function of the temperature of the glow plug at the start of the glow process by providing the temperature of the glow plug at the start of the glow process as an input to a corresponding characteristic curve implemented by the physical model.

9. The method as recited in claim 8, wherein the temperature of the glow plug at the start of the glow process corresponds to an ambient temperature of the glow plug at the start of the glow process.

10. The method as recited in claim 8, further comprising: determining a fourth partial resistance quantity as a function of a preceding glow process of the glow plug that directly precedes the start of the glow process by providing information characterizing the preceding glow process as an input to a corresponding characteristic curve implemented by the physical model, wherein the characterizing information includes at least one of: a glow period, or a glow energy; and determining a factor that is multiplied against the fourth partial resistance quantity as a function of an initial resistance of the glow plug by providing the initial resistance of the glow plug as an input to a corresponding characteristic curve implemented by the physical model.

11. The method as recited in claim 1, further comprising determining the characteristic curve individually for the glow plug during the steady-state operation of the glow plug.

12. The method as recited in claim 1, wherein the determined resistance quantity is static during the preheating phase.

13. The method as recited in claim 1, further comprising determining the resistance quantity as a function of the temperature setpoint value by providing the temperature setpoint value as an input to a corresponding characteristic curve implemented by the physical model.

14. The method as recited in claim 1, the determining further including producing one or more corresponding outputs from the one or more corresponding characteristic curves, and then combining the one or more corresponding outputs to produce the resistance quantity.

15. The method as recited in claim 14, wherein the ascertaining further includes producing the actual temperature value of the glow plug during the preheating phase as an output from the characteristic curve of the temperature of the glow plug.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic diagram of the system of a glow plug in an internal combustion engine.

(2) FIG. 2 shows a schematic illustration regarding the modeling of the temperature of a glow plug during a rapid preheating phase.

(3) FIG. 3 shows a temperature/time diagram with and without predictive temperature modeling.

DETAILED DESCRIPTION

(4) Cold internal combustion engines, in particular diesel engines, require a starting aid for igniting the fuel/air mixture introduced into the diesel engine in the case of ambient temperatures of <40 C. As the starting aid, glow systems are then used which include glow plugs, a glow control unit, and a glow software which is stored in an engine control unit or in the glow control unit. Moreover, glow systems are also used to improve the emissions of the vehicle. Other areas of application for the glow system are the burner exhaust gas system, the engine block heater, when preheating the fuel (flex fuel) or when preheating the cooling water.

(5) FIG. 1 shows such a glow system 1. Here, a glow plug 2 protrudes into combustion chamber 3 of diesel engine 4. Glow plug 2 is on the one hand connected to glow control unit 5 and on the other hand leads to a battery 6 which activates glow plug 2 at the nominal voltage of 11 volts, for example. Glow control unit 5 is connected to engine control unit 7 which, in turn, leads to diesel engine 4.

(6) To ignite the fuel/air mixture, glow plug 2 is preheated by the application of an overvoltage during a preheating phase, also referred to as a push phase, which lasts for 1 to 2 seconds. The electric power which is thus supplied to glow plug 2 is converted into heat in a heater coil (not illustrated in greater detail), which is why the temperature rises rapidly at the tip of glow plug 2.

(7) The heating power of the heater coil is adapted via electronic glow control unit 5 to the requirement of particular diesel engine 4. The fuel/air mixture is conducted past the hot tip of glow plug 2 and heats up in the process. In conjunction with an intake air heating during the compressor stroke of diesel engine 4, the combustion temperature of the fuel/air mixture is reached.

(8) Glow plug 2 has different glow phases. As already explained above, an overvoltage, which is above the nominal voltage of glow plug 2, is supplied to cold glow plug 2 during a preheating phase which lasts for 1 to 2 seconds. During this short time period, the tip of the glow plug is heated to approximately 1000 C., while the rest of glow plug 2 is still below this temperature, whereby a non-steady-state temperature characteristic forms within glow plug 2. This preheating phase is followed by a heating phase of glow plug 2 during which the non-steady-state temperature distribution is balanced out to a steady-state temperature distribution over entire glow plug 2. Such a heating phase normally lasts for approximately 30 seconds. After the preheating phase of the glow plug, the resistance difference is dynamically adapted during the heating phase. The heating phase is followed by the glow phase during which a steady-state temperature distribution is ensured over the entire glow plug.

(9) FIG. 2 shows a schematic diagram for temperature modeling of glow plug 2 during the rapid preheating phase which is integrated as software into engine control unit 7 or glow control unit 5 and is taken into account there in the case of a temperature control of the glow plug. A temperature setpoint value T.sub.DES is provided as the control input variable by engine control unit 7 for the general temperature control of glow plug 2 in the course of the entire glow process. At the same time, a resistance Rm of the glow plug is measured which represents a value for the instantaneous temperature at glow plug 2. This measured resistance Rm is determined for each energization process which takes place in consistent time intervals. In a block 17, this measured resistance Rm is added to a resistance difference R which is determined with the aid of a predictive model 8. This predictive model 8 models the temperature of glow plug 2 during the rapid preheating phase. An initial resistance R01 of glow plug 2 is initially ascertained within predictive model 8. This initial resistance R01 is supplied to a characteristic curve 9 which was ascertained during the steady-state operation of the glow plug. A first partial resistance difference R1 is ascertained from this characteristic curve 9 based on measured initial resistance R01.

(10) Temperature setpoint value T.sub.DES, which identifies the end temperature of glow plug 2 to be reached, is provided as another input variable of predictive model 8. This temperature setpoint value T.sub.DES is provided on another characteristic curve 10 as an input variable which is also used to ascertain a second partial resistance difference R2. Partial resistance differences R1 and R2 thus ascertained are added in block 14.

(11) In addition to the already mentioned input variables in the form of initial resistance R01 and of temperature setpoint value T.sub.DES, operating temperature Tc of glow plug 2 is determined at the point in time of the start of the glow process, i.e., at point in time t=0. Third partial resistance difference R3 is determined from this temperature Tc with the aid of a third characteristic curve 11. In block 15, third partial resistance difference R3 is added to first and second partial resistance differences R1 and R2. These input variables in the form of initial resistance R01, temperature setpoint value T.sub.DES, and operating temperature Tc are determined once at point in time t=0 upon activation of glow plug 2 and may be stored in engine control unit 7 or glow control unit 5.

(12) To take into account that, shortly before the glow process to be carried out, glow plug 2 has already been subjected once to a glow process from which glow plug 2 has not yet sufficiently cooled down, a glow time/glow energy E (E=U*I*t) of the glow process of glow plug 2, which directly preceded the instantaneous glow process, is taken into account. A fourth partial resistance difference R4 is determined from glow time/glow energy E with the aid of a fourth characteristic curve 12. Since due to glow time/glow energy E of the directly preceding glow process the resistance of glow plug 2 changes if the heat, which has built up during the preceding glow process within glow plug 2, has not yet cooled down, resistance R01 is supplied to another characteristic curve 13 which supplies as a result a factor F which is multiplied by fourth partial resistance difference R4 in block 22. Factor F is selected here in such a way that it is equal to 1 if initial resistance R01, which was measured once, is greater than a predefined threshold value of resistance R01. Factor F moves towards the value zero if initial resistance R01 is lower than the predefined threshold value of resistance R01. This poses the precondition that the input variables of glow time/glow energy E having the modification of initial resistance R01, associated therewith, are only used to determine resistance difference R if glow plug 2 still has a sufficiently large resistance which is accompanied by a changed temperature of glow plug 2, due to a preceding glow process. In block 16, fourth partial resistance difference R4 is added to previously described partial resistance differences R1, R2, and R3, resulting in a resistance difference R which corresponds to a predetermined temperature which occurs at the end of the preheating process at glow plug 2.

(13) In block 17, resistance difference R, determined in predictive model 8, is added to measured resistance Rm. This sum of resistance difference R and measured resistance Rm is supplied to a characteristic curve 18 in which the resistance is plotted against the temperature. This characteristic curve 18 is a characteristic curve ascertained individually for each glow plug 2 in the case of a steady-state temperature distribution, since glow plugs have discrete transfer functions due to production tolerances. A basis temperature TBAS of glow plug 2 is ascertained from this resistance/temperature characteristic curve 18. In block 19, this basis temperature TBAS is aligned with a heat conduction model in which it is taken into account to what extent there is a temperature difference between the inside of the heater of glow plug 2 and the surface temperature of glow plug 2. In block 19, a temperature difference is supplied to basis temperature TBAS, the sum of which yields actual temperature T.sub.ACT of glow plug 2. This actual temperature T.sub.ACT is now used in the control cycle where it is subtracted from temperature setpoint value T.sub.DES in block 20. The difference between temperature setpoint value T.sub.DES and actual temperature T.sub.ACT is supplied to a controller 21 which determines a voltage U.sub.GOV which is supplied to glow plug 2, in particular to the heater of glow plug 2, for rapidly setting temperature setpoint value T.sub.DES.

(14) FIG. 3 shows two temperature-time diagrams in which measured temperature T.sub.m is illustrated without predictive modeling (FIG. 3a) and with predictive modeling (FIG. 3b). It is apparent from FIG. 3a that measured temperature T.sub.m, which is to be adjusted to temperature setpoint value T.sub.DES, has, shortly after the start of the glow process, a temperature overshoot which approaches temperature setpoint value T.sub.DES only after a period of approximately 30 seconds. For comparison purposes, temperature T.sub.mo is illustrated which is modeled mathematically according to FIG. 2 without model 8 and which reaches the level of temperature setpoint value T.sub.DES approximately after 5 seconds, and is controlled around this level.

(15) In contrast, FIG. 3b shows the characteristic of measured temperature T.sub.m taking into account resistance difference R anticipatorily determined with the aid of predictive temperature model 8. Measured temperature T.sub.m does not show a temperature overshoot, but approaches modeled temperature T.sub.m immediately after the preheating phase. With the aid of this control, temperature setpoint value T.sub.DES is reached already after approximately 4 seconds and is controlled around this level.

(16) Due to predictive model 8, it is possible that a temperature control of glow plug 2 may occur not only during the steady-state operation, during which fluctuations between the resistance and temperature no longer occur, but also during the non-steady-state operation, preferably during the rapid preheating phase at the start of the glow process and during the heating phase. During the temperature modeling of glow plug 2 in the rapid preheating phase, it is modeled how large resistance difference R will be at the end of the preheating process, this resistance difference R being supplied to the control process as an input variable.