WO2018091305A1

WO2018091305A1 - Process for feeding an engine glowplug

Info

Publication number: WO2018091305A1
Application number: PCT/EP2017/078392
Authority: WO
Inventors: Tomaz MRAK
Original assignee: Hidria Aet D.O.O.
Priority date: 2016-11-15
Filing date: 2017-11-07
Publication date: 2018-05-24
Also published as: EP3321500A1

Abstract

The invention concerns a process for electrical feeding of a heat-engine glowplug, including a step (27) of heating of the glowplug, in which the glowplug is fed by an electrical feed power, characterised in that the heating step (27) is followed by a final step of controlled cooling of the glowplug, in the course of which the mean electrical feed power of the glowplug is decreasing until a zero value has been attained in accordance with a temporal profile (34) of monotonic decrease, starting from a value, the so-called final nominal mean power, such that the theoretical straight line passing through the point (35) of the temporal profile (34) corresponding to the final nominal mean power and the point (36) of the temporal profile (34) corresponding to the zero value exhibits a slope amounting to between -150W/s and -0.1W/s.

Description

PROCESS FOR FEEDING AN ENGINE GLOWPLUG

The invention concerns a process for electrical feeding of a heat-engi ne glowplug. Glowplugs are devices for heating heat engines, notably diesel engines. Glowplugs enable the combustion chamber of the engine to be warmed up in order to trigger or facilitate the starting of the engine. In order to do this, the glowplug is mounted on the cylinder head of the engine and exhibits a heating finger extending into the combustion chamber and incorporating at least one electrically conductive resistive element, such as a metallic filament, fed with electrical energy for the heating thereof by virtue of the Joule effect.

Furthermore, in the course of a step, the so-called pre-heating step, the electrical feeding of the glowplug is caused by the activation of the starting changeover switch of the engine, and lasts up until the starting of the engine. The recent processes for electrical feeding of glowplugs also include, after the pre-heating step, a so-called post-heating step in which the feeding of the glowplug is maintained during a period which may persist for up to 1 80 seconds after the starting of the engine. The post-heating step enables the pollutant emissions and noise emissions of the engine to be minimised.

Numerous processes for electrical feeding of glowplugs are known. For example, EP 2 800 451 describes a device for controlling a glowplug, including a glowplug and a switch, which are connected in series between an electrical feed source and an earth. This control device also includes a control unit enabling the switch to be opened and closed. When the switch is closed, the glowplug is fed by a voltage defined by the electrical feed source, and when the switch is open the glowplug is no longer fed. The control unit generates a discrete signal of PWM type (pulse- width modulation), the so-called chopping signal, enabling the switch to be opened and closed in accordance with a duty cycle of this discrete signal. In this way. the control plug is fed by pulses, the duty cycle of the chopping signal then enabling the mean feed power of the glowplug, and consequently the temperature of the resistive element of the glowplug, to be controlled. In this way, the electrical feeding is controlled by the control unit during the pre-heating and post-heating steps. However, the inventor has noticed that such an electrical feed process has been the cause of deteriorations of the resistive element of the glowplug, such as cracks appearing in the material of the resistive element, or even a rupture of the resistive element. Now, such deteriorations may impede the warming-up of the resistive element, so that its temperature may not be sufficient to warm up the combustion chamber of the engine. This involves a necessary replacement cost of the damaged glowplug.

The invention therefore aims to propose a process for electrical feeding of a glowplug enabling the service life of the glowplug to be extended.

The i m ention also aims to propose such a process that is simple to implement.

In order to do this, the invention concerns a process for electrical feeding of a heat- engine glowplug. including a step of heating of the glowplug. in which the glowplug is fed in accordance with an electrical feed power that is suitable to bring about the heating, by virtue of the Joule effect, of an electrically conductive resistive element of the glowplug.

characterised in that the heating step is followed by a final step of controlled cooling of the glowplug, in the course of which the mean value, the so-called mean electrical feed power, of an instantaneous electrical feed power for feeding the glowplug over a period T_DC amounting to between 0. 1 millisecond and 0.5 second :

- is decreasing, over a length of time longer than 1 second, until a zero value has been attained in accordance with a temporal profile of monotonic decrease starting from a value, the so-called final nominal mean power. - such that the theoretical straight line passing through the point of the temporal profile corresponding to the final nominal mean power and the point of the temporal profile corresponding to the zero value exhibits a slope amounting to between - I 50W/S and -0. 1 W/s.

In this way, the mean feed power is reduced, starting from the final nominal mean power, until a zero value of the mean feed power and a final feed-halt temperature of the resistive heating element of the glowplug have been attained, in particular, the final feed-halt temperature of the resistive element of the glowplug is the minimum temperature of this resistive element that is capable of being attained, depending on the external temperature of this resistive element, when the glowplug is not being fed.

Moreover, a process for electrical feeding of a glowplug in accordance with the invention defines an operating cycle of the glowplug.

A process for electrical feeding of a glowplug in accordance with the invention enables the service life of a glowplug, and more particularly of its resistive element, which may be a metall ic filament, to be increased considerably. I ndeed. the inventor has ascertained that the degradations of the resistive element of the glowplug ( such as cracks appearing in the material of the resistive element) were due to an excessively rapid cooling of this resistive element after sudden cut-off of its electrical feed. In particular, the temperature of a resistive element may reach 1 100 °C in order to be able to warm up the combustion chamber. Under such conditions the resistive element is close to its liquid state. Now. when the cool ing of the resistive element is too rapid, the resistive element solidifies non- uniformly, so that deteriorations may appear on the resistive element. So a cooling retarded in controlled manner in accordance with the invention enables a uniform solidification of the resistive element to be ensured. In this way, in a process according to the i nvention the duration of cooling of the resistive element of the glowplug is longer in the course of the final cooling step than a duration of cooling of the resistive element of the glowplug that would be ascertained without said final cool ing step according to the invention. In particular, since the temporal profile of the mean feed power is decreasing and monotonic in the course of the final step of controlled cooling, this means that the mean feed power at a given instant is always less than or equal to a mean feed power at a preceding instant in relation to the given instant. A process for electrical feeding of a glowplug in accordance with the invention therefore enables the service life of the glowplug fed in accordance with such a process to be increased.

More particularly, in some embodiments of the invention, in the course of said final cooling step the mean electrical feed power is reduced linearly in continuous manner. In this way, the process is simple and easy to implement. Nevertheless, in some other embodiments of the invention, in the course of said final cooling step the mean electrical feed power is reduced in stages in discontinuous manner. In addition, in some embodiments of the invention, the final cooling step extends over a length of time amounting to between 1 second and 200 seconds.

The heating step preferably comprises a pre-heating step and a post-heating step in which the mean electrical feed power is at least substantially constant and less than or equal to that of the pre-heating step.

In particular, the pre-heating step consists in initiating the electrical feeding of the glowplug by activating a starting changeover switch of the engine, so as to heat the resistive element of the glowplug up to a temperature enabling the starting of the engine. Furthermore, the temperature of the resistive element in the course of the pre-heating step is greater than or equal to 900°C, e.g. of the order of 950°C. As regards the post-heating step, it consists in maintaining the feeding of the glowplug during a period Δί₂ that may persist for up to 1 80 seconds after the starting of the engine, in order to minimise the pollutant emissions and noise emissions of the engine. Furthermore, the temperature of the resistive element in the course of the post-heating step is between 600°C and 900°, e.g. of the order of 700°C.

Nevertheless, in some embodiments the heating step does not include a post-heating step. Thus the final step of controlled cooling follows the pre -heating step. The mean feed power at the end of the pre-heating step then defines the final nominal mean power, starting from which the mean feed power is reduced in the course of the final step of controlled cooling.

When the heating step includes a post-heating step, the latter is followed by the step of controlled cooling. The mean feed power at the end of the post-heating step then defines the final nominal mean power, starting from which the mean feed power is reduced in the course of the final step of controlled cooling. In this way. in the course of the final step of controlled cooling the temperature of the resistive element is reduced from the final nominal temperature - that is to say, the temperature of the resistive element at the end of the post-heating step.

In some embodi ments of the invention, the mean electrical feed power takes a value depending on a duty cycle of a discrete signal, the so-called chopping signal, supplied by a control unit and applied to a continuous feed voltage supplied by a feed source. In the course of the final cooling step, the mean electrical feed power is then modified by modification of the duty cycle of the chopping signal in accordance with a predetermined temporal profile of decrease of this duty cycle.

More particularly, the control unit may be a control unit of the engine or a control unit of the glowplug. The chopping signal is a signal constituted by pulses emitted at a given fixed frequency defining the period of the chopping signal, the period of the signal being shorter than or equal to the period J)_K . In addition, the duration is predetermined as a function of the desired mean electrical feed power of the glowplug. The duration of a pulse defines its instantaneous duty cycle - that is to say, the ratio of this pulse to the period of the chopping signal.

The chopping signal is supplied to a switch, in order to manipulate it. In particular, the switch is placed in series between the feed source and a feed input of the glowplug. When the switch is closed, it enables the feed voltage to be supplied to the feed input of the glowplug. When the switch is open, the glowplug is not fed.

Furthermore, in certain advantageous embodiments the rising edges of the pulses of the chopping signal are distributed in accordance with a frequency predefined by the control unit.

The use of a chopping signal enables the mean electrical feed power to be modified simply and efficiently. More particularly, in the course of the final cooling step the duty cycle of the chopping signal is modified, starting from a value, the so-called final nominal value of the duty cycle of the chopping signal, enabl ing a mean electrical feed power to be obtai ned that is equal to the final nominal mean power, having attained a maximum on a temporal profile of the mean feed power of the resistive element, as far as a zero value and the final feed-halt temperature of the resistive element of the feed plug in accordance with a temporal profile of decrease of the duty cycle of the chopping signal which is distinct from a transition of the duty cycle from its final nominal value to a zero value in accordance with a non-vertical gradient of decrease, the zero value being maintained until the final feed-halt temperature of the resistive element of the feed plug has been obtained. In some embodiments, the duty cycle of the chopping signal is reduced linearly in time in the course of the final cool ing step in accordance with a non-vertical gradient of decrease. Nevertheless, there is nothing to prevent modifying the duty cycle in non-linear manner in the course of the final cooling step.

In some embodiments of the invention, the duty cycle of the chopping signal in the course of the final cooling step is determined by the control unit in accordance with the formula: D_c = D₁ X k where D_c is the duty cycle duri ng the final cooling step. D₁ is the duty cycle at the end of the post-heating step and k is a coefficient which varies in time and depends of the temperature and the length of time of the final cooling step. More specifically, k is related to the desired speed of cooling of the final cooli ng step, k depends on the environment of use of the glow plug such as the glow plug appl ication, the engine and should be adjusted in accordance with this environment.

More particularly, the final cooling step comes to an end when the duty cycle calculated by the control unit is less than 5%.

In a preferred embodiment and according to the invention, the duty cycle of the chopping signal amounts to between 95% and 50% in the course of the pre-heating step. For example, the duty cycle of the chopping signal is of the order of 95% in the course of the pre-heating step.

In some advantageous embodiments, the duty cycle of the chopping signal in the course of the post-heating step is less than that of the pre-heating step, so that the mean feed power of the glowplug. and consequently the temperature of the resistive element, in the course of the post-heating step is less than that of the pre-heating step. More particularly, the duty cycle of the chopping signal amounts to between 5% and 80% in the course of the post-heating step. For example, the duty cycle of the chopping signal is of the order of 35% in the course of the post-heating step.

The invention also concerns a process for electrical feeding of a glowplug of an engine, characterised in combination by all or some of the characteristics mentioned heretofore or hereinafter.

Other objectives, characteristics and advantages of the invention will become apparent from the following description which is given as a non-limiting example and which refers to the appended Figures, in which:

- Figure 1 is a block diagram of a device that is capable of implementing a process for electrical feeding of a glowplug in accordance with the invention,

Figure 2 is a graph representing the temporal profile of the electrical feed power of a resistive element of a glowplug according to a process for electrical feeding of the glowplug in accordance with the state of the art,

Figure 3 is a graph representing the temporal profile of a chopping signal regulating the electrical feed power of a glowplug according to a process for electrical feeding of the glowplug in accordance with the state of the art,

Figure 4 is a graph representing the temporal profile of the temperature of a resistive element of a glowplug according to a process for electrical feeding of the glowplug in accordance with the state of the art,

- Figure 5 is a graph representing the temporal profile of the electrical feed power of a resistive element of a glowplug according to a process for electrical feeding of the glowplug in accordance with the invention,

----- Figure 6 is a graph representing the temporal profile of a chopping signal regulating the electrical feed power of a glowplug according to a process for electrical feeding of the glowplug in accordance with the invention,

Figure 7 is a graph representing the temporal profile of the temperature of a resistive element of a glowplug according to a process for electrical feeding of the glowplug in accordance with the invention. A feed device of a glowplug 21 enabling a process to be implemented for feeding a glowplug 2 1 in accordance with the invention is represented in Figure 1. The glowplug 21 is mounted on the cylinder head of an engine and exhibits a heating finger extending into a combustion chamber of a heat engine which is not represented.

The heating finger incorporates an electrically conductive resistive element, such as a metallic filament, which is capable of heating up by virtue of the Joule effect when it is fed electrically in accordance with an instantaneous electrical heating power greater than zero. The heating of the resistive element enables the combustion chamber to be heated up in order to facilitate the starting of the heat engine.

The feed device includes a feed source 22. More particularly, the negative pole of the feed source 22 is earthed, and the positive pole is connected to a glowplug 2 1 to be fed by virtue of an electrical coupling 41 .

The feed source 22 enables an electrical power, the so-called maximum feed power, to be supplied. For example, the maximum feed power amounts to between SOW and 500 W, more particularly is of the order of 1 00 W and a maximum voltage supply V, amounting to between 1 6 V and 8V, more particularly is of the order of 1 1 V.

The feed source 22 may be a battery or an alternator of a vehicle that includes the heat engine.

In particular, the resistive element of the glowplug 21 is fed by the feed source 22.

The feed device also includes a control unit 23 of the engine. The control unit 23 of the engine is fed by the feed source 22 by virtue of an electrical coupling 42. More particularly, the control unit 23 of the engine includes a microprocessor. The control unit 23 of the engine is able to receive several data items at the input, such as the quantity of fuel, the temperature of the cooling water etc. In some advantageous embodiments, the control unit 23 of the engine is suitable to communicate with a control unit 24 of the glowplug 21 . In order to do this, the control unit 23 of the engine is, for example, connected to the control unit 24 of the glowplug 21 by:

- a first electrical coupling 25 enabling the control un it 23 of the engine to transmit electrical signals to the control unit 24 of the glowplug 21 .

a second electrical coupling 26 enabling the control unit 23 of the engine to receive electrical signals emitted by the control unit 24 of the glowplug 2 ! .

The control unit 24 of the glowplug 2 1 is fed by the feed source 22. More particularly, the control unit 24 of the glowplug 2 1 includes a microprocessor.

Furthermore, the control unit 24 of the glowplug 2 1 includes a switch placed in series between the feed source 22 and the glowplug 2 1 , so that it prevents the feeding of the glowplug 21 with electric current when it is in an open state, and enables the feeding of the glowplug 2 1 with electric current when it is in a closed state. The switch may be a diode, a transistor or a thyristor. in order to be able to be driven by electrical signals supplied by one of the control units 23 or 24.

In some advantageous embodiments, the control unit 23 of the engine is capable of transmitting a discrete electrical signal, the so-called chopping signal, to the control unit 24 of the glowplug 21 . Nevertheless, there is nothing to prevent the control unit 24 of the glowplug 2 1 from emitting a chopping signal itself. In addition, the chopping signal is of pulse-width-modulation type. In this way, the chopping signal is a signal constituted by pulses 40 emitted at a given fixed frequency defining a period of the chopping signal. A temporal profile 39 of a chopping signal is represented in Figure 6 according to an embodiment of the invention. The chopping signal is transmitted to the switch of the control unit 24 of the glowplug 21 in order to drive its state (open state and closed state) at each pulse edge 40 of the chopping signal. In this way, for example, a pulse 40 on the chopping signal enables the switch to be opened on the rising edge of the pulse 40. and enables the switch to be closed on the falling edge of the pulse 40.

In addition, the duration t_t of a pulse 40 is predetermined as a function of a desired mean value, the so-called mean electrical feed power, of an instantaneous electrical feed power of the glowplug 2 1 over a period T_DC amounting to between approx. 3 1 2,5LI S and 0.5 second at 32 Hz. For example, the temporal profile 39 of the chopping signal represented in Figure 6 enables a temporal profile 38 of the mean feed power represented in Figure 7 to be obtained. Moreover, the temporal profile 39 of the chopping signal represented in Figure 3 enables a temporal profile 38 of the mean feed power represented in Figure 4.

The period of the chopping signal is less than or equal to period T_DC. The period of the chopping signal is preferably equal to period T_DC. In this way, the period of the chopping signal amounts to between 3 12.5,us and 0.5 second at 321 Iz, more particularly is of the order of 0.5 milliseconds. The duration t_t of a pulse 40 defines its instantaneous duty cycle - that is to say, the ratio of this pulse 40 to the period of the chopping signal. In addition, one of the control units 23 or 24 is capable of modifying the duty cycle of the chopping signal in order to obtain the desired mean feed power.

In this way. the chopping signal enables an electric feed current to be suppl ied to the glowplug 2 1 , said electric feed current exhibiting a profile similar to the chopping signal. The chopping signal enables the mean electrical power of the electric feed current to be controlled. In fact, the duty cycle of the chopping signal enables a mean electrical power of the electric feed current, the so-called mean feed power, to be defined. More particularly, when the duty cycle is equal to one. the mean feed power is equal to the maxi mum feed power, and when the duty cycle is zero, the mean feed power is zero.

In addition, the mean feed power enables the resistive element of the glowplug 2 1 to be set at a given temperature as a function of the value of the mean feed power. In this way, for example, the temporal profile 38 of the mean feed power represented in Figure 7 enables a temporal profile 37 of the temperature of the resistive element of the glowplug represented in Figure 5 to be obtained, and the temporal profile 38 of the mean feed power represented in Figure 4 enables a temporal profile 37 of the temperature of the resistive element of the glowplug represented in Figure 2.

In this way, the modification of the duty cycle of the chopping signal enables the temperature of the resistive element of the glowplug 2 1 to be varied.

A process for feeding a glowplug 2 1 includes a heating step 27 enabling the temperature of the resistive element to be increased. In particular, the heating step 27 includes a pre-heating step 28 consisting in heating the resistive element of the glowplug 2 1 sufficiently to be able to start the engine. So the pre-heating step 28 extends up during a period At ₁ until the starting of the engine. More particularly, in some advantageous embodiments, in the course of the pre-heating step 28 the mean feed power is close to the maximum feed power, so as to heat the resistive element of the glowplug 2 1 rapidly. The mean feed power is preferably constant in the course of the pre-heating step 28. For example, the mean feed power P, amounts to between 1 00 W and 300 W in the course of the pre-heating step 28, more particularly is of the order of 230 W. In this way, in the course of the pre-heating step 28 the duty cycle of the chopping signal amounts to between 50% and 95%. For example, the duty cycle of the chopping signal is of the order of 95% in the course of the pre-heating step 28. The duty cycle is preferably constant in the course of the post-heating step 29.

In the course of the pre-heating step 28 the mean feed power enables the temperature of the resistive element to be raised. More particularly, in the course of the post-heating step 29 the mean feed power enables the resistive element to be set at a temperature greater than 900°C, e.g. of the order of 950°C. Afterwards, in the advantageous embodiments in which the mean power is constant in the course of the pre-heating step 28 the resistive element is maintained at the same temperature.

In particular, the temporal profile 37 of the temperature of the resistive element in the series of the steps of the process exhibits an overall maximum Tj in the course of the pre-heating step 28. The heating step 27 includes a post-heating step 29 after the pre-heating step 28 ----- that is to say, after the starting of the engine. This post-heating step 29 consists in maintaining the feeding of the glowplug 2 1 , in order to minimise the pollutant emissions and noise emissions of the engine. More particularly, the post-heating step 29 may extend over a period which may persist for up to 1 80 seconds after the starting of the engine.

In the course of the post-heating step 29 the mean feed power is lower in comparison with that in the course of the pre-heating step 28, so that the temperature of the resistive element is lower in comparison with that in the course of the pre-heating step 28. The mean feed power is preferably constant in the course of the post-heating step 29 and less than or equal to the mean feed power in the course of the pre-heating step 28. For example, the mean feed power P₂ amounts to between 1 0 W and 1 50 W in the course of the post-heating step 29, more particularly is of the order of 35 W. In this way. in some advantageous embodiments the duty cycle of the chopping signal amounts to between 5% and 80%. For example, the duty cycle of the chopping signal is of the order of 35% in the course of the post-heating step 29. The duty cycle is preferably constant in the course of the post-heating step 29.

Furthermore, in the course of the post-heating step 29 the mean feed power enables the resistive element to be set at a temperature T₂ between 600°C and 900°C, e.g. of the order of 700°C. More particularly, when the mean feed power of the post- heating step 29 is less than that of the pre-heating step 28, at the start of the post- heating step 29 the temperature of the resistive element decreases from its temperature in the course of the pre-heating step 28. Afterwards, in the advantageous embodiments in which the mean power is constant in the course of the post-heating step 29 the resistive element is maintained at the same temperature.

As represented in Figures 5 to 7, the process for feeding a glowplug 21 includes a final step 30 of controlled cooling of the glowplug 2 1 before totally halting the feeding of the glowplug 21 . The final step 30 of controlled cooling occurs after the heating step 27. for example after the pre-heating step 28 or after the post-heating step 29. Thus, a process for electrical feeding of a glowplug in accordance with the invention defines an operating cycle of the glowplug. The final step 30 of controlled cooling consists in reducing the mean electrical feed power, over a length of time At_P longer than 0.5 second, until a zero value has been attained in accordance with a temporal profile 34 of monotonia decrease starting from a value, the so-called final nominal mean power, so that the theoretical straight line passing through point 35 of temporal profile 34, corresponding to the final nominal mean power, and point 36 of temporal profile 34, corresponding to the zero value, exhibits a slope amounting to between -150W/s and -0.1 W/s.

In this way, the mean feed power is reduced from the final nominal mean power until a zero value of the mean feed power and a final feed-halt temperature T_Q of the resistive element of the glowplug 21 have been attained. In particular, the final feed-halt temperature T₀ of the resistive element of the glowplug 21 is the minimum temperature of this resistive element that is capable of being attained, depending on the external temperature of this resistive element, when the glowplug 21 is not fed. In addition, in the course of the final step 30 of controlled cooling the mean feed power is reduced in accordance with a temporal profile 34 of decrease of electrical feed power which is distinct from a temporal profile 33 that is representative of an instantaneous cut-off of the electrical feeding of the glowplug 21 , for example represented in Figure 4, so that the length of time At_P that has elapsed in order that the mean feed power attains a zero value from the start of the final cooling step 30 is longer than the length of time that has elapsed in order that the mean feed power attains a zero value after a pre-heating step 28 or a post-heating step 29 at the time of an instantaneous cut-off. In particular, since the temporal profile 34 of the mean feed power is decreasing and monotonic in the course of the final step 30 of controlled cooling, this means that the mean feed power at a given instant is always less than or equal to a mean feed power at a preceding instant in relation to the given instant. In this way, in the course of the final step 30 of controlled cooling the temperature of the resistive element varies until the final feed-halt temperature T₀ of the resistive element has been attained in accordance with a temporal profile 32 which is distinct from a temporal profile 3 1 obtained at the time of an instantaneous cut-off of the feeding of the resistive element until the final feed-halt temperature T_Q of the resistive element has been obtained. Consequently, in the course of the final step 30 of controlled cooling the length of time Δ/_/ · for attaining the final feed-halt temperature T₀ of the resistive element is longer than the length of time At_T enabling the final feed-halt temperature T₀ of the resistive element to be attained at the time of an instantaneous cut-off of the feeding of the resistive element. In particular, in some embodiments the final nominal mean power is equal to the mean feed power at the starting of the engine when the final cooling step 30 directly follows the pre-heating step 28. In some other embodiments, the final nominal mean power is equal to the mean feed power at the end of the post-heating step 29 when the final cool ing step 30 follows the post-heati ng step 29.

In order to reduce the mean feed power, the duty cycle of the chopping signal is reduced in time by the control unit 23 or 24 driving the state of the switch. Furthermore, in the course of the final cooling step 30 the mean feed power may be reduced progressively in continuous manner, so that its temporal profile 34 of decrease is l inear. The mean feed power may be reduced progressively in discontinuous manner, so that its temporal profile 34 of decrease exhibits, for example, stages of duration that may extend over several periods T_DC. For example, with a frequency of the chopping signal of 32 Hz, the duration of a stage may amount to between 1 and 10 000 periods.

In order to reduce the mean feed power in the course of the final cooling step 30, the duty cycle of the chopping signal is reduced in time in accordance with a predetermined temporal profile 43 of decrease of this duty cycle. The duty cycle of the chopping signal is reduced linearly in time in the course of the final cooling step 30 in accordance with a non-vertical gradient of decrease.

In order to obtain a temporal profile 34 of the mean feed power exhibiting stages, the pulses 40 of successive periods of the chopping signal defining a stage of the temporal profile 34 of the mean feed power exhibit the same duty cycle.

For example, the duty cycle of the chopping signal for a given period in the course of the final cooling step 30 may be calculated by one of the control units 23 or 24 by the formula: D_c = D_t X k where D_c is the duty cycle during the final cooling step 30, D₁ is the duty cycle at the end of the post-heating step 29 and k is a coefficient which varies in time and depends of the temperature T₂ and the length of time At_T. More specifically, k is related to the desired speed of cooling of the final cooling step 30. k depends on the environment of use of the glow plug such as the glow plug application, the engine and should be adjusted in accordance with this environment.

In some advantageous embodiments, the final cooling step 30 comes to an end when the duty cycle calculated by the control unit 23 or 24 is less than 5%. The final step 30 of controlled cool ing may extend over a length of time amounting, for example, to between 1 second and 200 seconds, according to the desired speed of cooling of the resistive element of the glowplug 2 1 .

The final step 30 of controlled cooling enables the service life of a glowplug 2 1 to be increased considerably. Indeed, the fact of retarding the cooling of the resistive element of the glowplug 21 enables a uniform solidification of the resistive element to be ensured, so as to avoid deteriorations of the resistive element that may be due to an excessively rapid cooling of the resistive element. A process for electrical feeding of a glowplug 21 in accordance with the invention therefore enables the service life of the glowplug 2 1 fed in accordance with such a process to be increased.

The invention therefore concerns a process for electrical feeding of a glowplug 21 in which the mean feed power is reduced just before ceasing to feed the glowplug 2 1 , in order to cool the resistive element in controlled and retarded manner in comparison with a sudden cut-off of the feed after having heated the resistive element. A process according to the invention for electrical feeding of a heat-engine glowplug 21 may be employed to feed the glowplug 2 1 of a diesel engine of a vehicle such as an automobile, a lorry, a boat etc.

Claims

1. A process for electrical feeding of a heat-engine glowplug (21), including a step (27 ) of heating of the glowplug (21), in which the glowplug (21 ) is fed in accordance with an electrical feed power that is suitable to bring about the heating, by virtue of the Joule effect, of an electrically conductive resistive element of the glowplug ( 2 1 ),

characterised in that the heating step ( 27) is followed by a final step of controlled cool i ng of the glowplug (21 ), in the course of which the mean value, the so-called mean electrical feed power, of an instantaneous electrical power for feeding the glowplug (21 ) over a period T_DC amounting to between 0.1 millisecond and 0.5 second :

- is decreasing, over a length of time longer than 1 second, until a zero value has been attained in accordance with a temporal profile (34) of monotonic decrease starting from a value, the so-called final nominal mean power,

- such that the theoretical straight line passing through the point (35) of the temporal profile (34) corresponding to the final nominal mean power and the point (36) of the temporal profile (34) corresponding to the zero value exhibits a slope amounting to between - 1 50W/s and -0. 1 W/s.

2. The process according to Claim 1 . characterised in that in the course of said final cooling step the mean electrical feed power is reduced linearly in continuous manner.

3. The process according to Claim 1 . characterised in that in the course of said final cooling step the mean electrical feed power is reduced in stages in discontinuous manner.

4. The process according to any one of Claims 1 to 3, characterised in that the final cooling step extends over a length of time amounting to between 1 second and 200 seconds.

5. The process according to any one of Claims 1 to 4, characterised in that the heating step ( 27) comprises a pre-heating step (28) and a post-heating step (29) in which the mean electrical feed power is at least substantially constant and less than or equal to that of the pre-heating step ( 28).

6. The process according to any one of Claims 1 to 5, in which the mean electrical feed power takes a value depending on a duty cycle of a discrete signal, the so-called chopping signal, supplied by a control unit ( 23, 24) and appl ied to a continuous feed voltage supplied by a feed source (22),

characterised in that in the course of the final cooling step the mean electrical feed power is modified by modification of the duty cycle of the chopping signal in accordance with a predetermined temporal profile (43 ) of decrease of this duty cycle.

7. The process according to Claim 6, characterised in that the duty cycle of the chopping signal is reduced linearly in time in the course of the final cooling step in accordance with a non-vertical gradient of decrease.

8. The process according to either Claim 6 or Claim 7, characterised in that the final cooling step comes to an end when the duty cycle calculated by the control unit (23, 24) is less than 5%.

9. The process according to any one of Claims 6 to 8, characterised in that the duty cycle of the chopping signal amounts to between 95% and 50% in the course of the pre-heating step (28).

10. The process according to any one of Claims 6 to 9, characterised in that the duty cycle of the chopping signal amounts to between 5% and 80% in the course of the post-heating step (29).