Method for regulating or controlling the temperature of a sheathed-element glow plug

Abstract

A method for regulating or controlling the temperature of a sheathed-element glow plug in a heating phase of the sheathed-element glow plug, where a temperature value is determined as a function of a resistance of the sheathed-element glow plug. To render possible the regulation or control of the temperature of the sheathed-element glow plug also during a transient temperature distribution within the sheathed-element glow plug, the resistance used for determining the temperature value during a transient thermal response within the sheathed-element glow plug is calculated with the aid of a physical model.

Claims

1. A method for regulating or controlling a temperature of a sheathed-element glow plug in a heating phase of the sheathed-element glow plug, comprising: determining a temperature value as a function of a resistance of the sheathed element glow plug, the heating phase following upon a preheating phase; when the sheathed-element glow plug is cold, applying a heating voltage to the cold sheathed-element glow plug during the preheating phase, thereby producing a transient thermal response within the sheathed-element glow plug, wherein: the heating voltage is greater than an operating voltage provided for the sheathed-element glow plug, the temperature value is determined as a function of a measured resistance and a calculated resistance value, the calculated resistance used for determining the temperature value during the transient thermal response within the sheathed element glow plug is calculated with the aid of a physical model, the temperature value is determined in a plurality of time intervals, and the calculated resistance value changes as a function of preceding time intervals; initializing a first instance of the calculated resistance value by a start value, wherein the start value is determined from a difference between a resistance that is uniquely determined starting from a homogeneous temperature distribution in the sheathed-element glow plug and a measured resistance reached upon completion of the preheating phase; and determining the calculated resistance value as a function of a decreasing exponential function, wherein: exponents of the exponential function are formed by a thermal relaxation time and a time constant, and the time constant is uniquely determined for the sheathed element glow plug.

2. The method as recited in claim 1, wherein a resistance value computed for a preceding time interval forms a starting point for calculating a next resistance value in a following time interval.

3. The method as recited in claim 1, wherein, to form the temperature value, the measured resistance of the sheathed-element glow plug is measured upon completion of the preheating phase.

4. The method as recited in claim 1, wherein the measured resistance is determined from a voltage and a current that are ascertained by measuring a voltage being applied to the sheathed-element glow plug and a current flowing through the sheathed-element glow plug.

5. The method as recited in claim 1, wherein, once the heating phase of the sheathed-element glow plug has elapsed, in which a steady-state thermal response settles in the sheathed-element glow plug, the temperature is regulated as a function of the measured resistance value that represents the temperature of the sheathed-element glow plug.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention permits numerous specific embodiments. Examples of them will be clarified in greater detail with reference to the figures.

(2) FIG. 1 shows a schematic representation of the configuration of a sheathed-element glow plug in a combustion engine.

(3) FIG. 2 shows a schematic flow chart for calculating the temperature during the transient temperature distribution.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

(4) At ambient temperatures of <40° C., cold combustion engines, in particular diesel engines, require a starting aid for igniting the fuel-air mixture introduced into the diesel engine. As a starting aid, glow systems are then used that include sheathed-element glow plugs, a glow-time control unit and a glow function software that is stored in an engine control unit. Moreover, glow systems are also used for improving the emissions of the vehicle. Other fields of application of the glow system include a burner exhaust system, an engine-independent heating system, the preheating of fuel (flex fuel), or the preheating of coolant.

(5) FIG. 1 shows such a glow system 1. In this context, a sheathed-element glow plug 2 extends into combustion chamber 3 of diesel engine 4. Sheathed-element glow plug 2 is connected on one side to glow-time control unit 5 and, on the other side, leads to a vehicle system voltage 6 which drives sheathed-element glow plug 2 at the rated voltage of 11 V, for example. Glow-time 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, sheathed-element glow plug 2 is preheated by the application of an overvoltage in a push phase which lasts from one to two seconds. The electrical energy, which is thus supplied to sheathed-element glow plug 2, is converted in a heating coil (not shown in greater detail) into heat, which is why the temperature rises steeply at the tip of the sheathed-element glow plug. Glow-time control unit 5 adapts the heating power of the heating coil to the demands of the particular diesel engine 4. The fuel-air mixture is conducted past the hot tip of sheathed-type glow plug 2 and is heated in the process. The ignition temperature of the fuel-air mixture is reached in response to a heating of the intake air during the compressor stroke of diesel engine 4.

(7) Sheathed-element glow plug 2 has different glow phases. As already explained, a push voltage, which is higher than the rated voltage of sheathed-element glow plug 2, is supplied to cold sheathed-element glow plug 2 in a preheating phase, the push phase, which takes one to two seconds. During this brief period of time, the tip of the sheathed-element glow plug is heated to nearly 1000° C., while the remaining portion of sheathed-element glow plug 2 is still at a temperature far below this temperature, thereby producing a transient thermal response within sheathed-element glow plug 2. This preheating phase is followed by a heating phase of sheathed-element glow plug 2, during which the transient temperature distribution changes to a steady-state temperature distribution over entire sheathed-element glow plug 2. Such a heating phase normally lasts for approximately 30 s. During this time period, the temperature of sheathed-element glow plug 2 is not available to a control and/or regulation by engine control unit 7 containing the software for the glow function. Under known methods heretofore, the glow function cannot be regulated until after the steady-state thermal response of sheathed-element glow plug 2 has settled.

(8) FIG. 2 is a schematic flow chart for calculating the temperature during the heating phase that is integrated as software in engine control unit 7 or glow control unit 5 and is considered there in the case of a temperature regulation of the glow function of the sheathed-element glow plug.

(9) The energy of sheathed-element glow plug 2 is determined in block 100 by measuring the system voltage of the vehicle which is driven by diesel engine 4, and the current. The time duration of the push phase is determined as a function of this vehicle system voltage. Block 101 subsequently determines temperature T.sub.push reached by the tip of sheathed-element glow plug 2 due to the energy in the form of the push voltage that is made available to sheathed-element glow plug 2 during the push phase.

(10) In light of these preconditions, a resistance differential ΔR(t=0) is computed in block 102.
ΔR.sub.(t=0)=R.sub.(t=30)−R.sub.push(t=0)  (1).

(11) Alternatively, the resistance values may be directly converted into a temperature. It then holds that:
ΔT.sub.(t=0)=T.sub.(t=30)−T.sub.push(t=0)
T.sub.(t=30) being=f(R.sub.(t=30) and T.sub.push(t=0) being=f(R.sub.push(t=0).

(12) Provided that a steady-state temperature distribution is present, resistance value R.sub.(t=30) is uniquely measured following the installation of sheathed-element glow plug 2 in diesel engine 4, and is stored for further calculations. Alternatively, resistance value R.sub.(t=30) may be calculated from a resistance model that considers resistance value R.sub.(t=30) as a function of temperature T.sub.push of sheathed-element glow plug 2 reached in the push phase; as previously explained, temperature T.sub.push being a function of the energy made available in the push phase of sheathed-element glow plug 2.

(13) In block 103, the temperature equalization process, which takes place in the heating phase and follows the push phase, is modeled using an exponential formulation under consideration of thermal relaxation time t.
T.sub.mod=f(R.sub.meas)+ΔR(t.sub.K)  (2)

(14) In the case of a conversion into a temperature, it holds that:
T.sub.mod=T.sub.act+ΔT(t.sub.K), T.sub.act being=f(R.sub.meas)
ΔR(t.sub.K) being=f(exp(−dt.sub.K/τ)
respectively ΔT(t.sub.K) being=f(exp(−dt.sub.K/τ).

(15) A resistance R.sub.meas is determined which is present at a point in time t.sub.0 on glow filament of sheathed-element glow plug. To this end, the voltage being applied to the glow filament of sheathed-element glow plug 2 and the current flowing therethrough are measured, and resistance R.sub.meas is calculated therefrom.

(16) Point in time t.sub.o(t.sub.o=^t=0) represents the end of the push phase, but, at the same time, also the beginning of the temperature equalization process, thus of the heating phase.

(17) An initialization is performed in that resistance differential value ΔR.sub.(t=0), respectively temperature differential value ΔT.sub.(t=0) ascertained from equation (1) are multiplied by the exponential function.
ΔR(t.sub.0+1)=exp(−dt/τ).Math.ΔR.sub.(t=0)
or ΔT(t.sub.0+1)=exp(−dt/τ).Math.ΔT.sub.(t=0).  (3)

(18) In the process, time constant τ forms a quantity to be uniquely defined for each sheathed-element glow plug 2, prior to the use thereof, that is then stored in engine control unit 7. Parameter −dt indicates the time segment of the thermal relaxation (beginning with t(.sub.0), at which resistance value ΔR(t.sub.0+1) was ascertained). Thus, start value ΔR(t.sub.0+1) is obtained, which is inserted into function (2), and first modeled temperature value T.sub.mod is thus determined. This modeled temperature value is processed as an actual temperature value in the regulation of the glow characteristics of the sheathed-element glow plug (block 104).

(19) Resistance value ΔR(t.sub.K) is calculated k-times distributed over the entire heating phase, for example, every 100 ms during the heating phase, in that the most recently calculated resistance value is always multiplied by the exponential function in block 103. From this, the following is derived:
ΔR(t.sub.K)=exp(−dt.sub.K/τ).Math.ΔR(t.sub.K-1)  (4)
or in the case the resistance value is converted into a temperature
ΔT(t.sub.K)=exp(−dt.sub.K/τ).Math.ΔT(t.sub.K-1).

(20) Each resistance value ΔR(t.sub.K), respectively temperature value ΔT(t.sub.K) is subsequently used in block 104 to calculate temperature T.sub.mod for predefined time segment t.sub.K and to utilize the same as an actual temperature value in the regulation during the heating phase.

(21) The described model very effectively reproduces the transient thermal response, both in stationary air and also at the start of the diesel engine or during the idling thereof and may, therefore, be advantageously used for regulating the glow temperature of sheathed-element glow plug 2 in the heating phase.