EP1083389B1 - Managing pyrolysis time - Google Patents

Description

The invention relates to the field of oven pyrolysis management cooking, especially domestic, as well as pyrolysis ovens including such a pyrolysis time management system.

The principle of pyrolysis time management is to optimize the pyrolysis time depending on the degree of soiling in the oven cavity pyrolysis. The aim is to achieve minimum energy consumption while guaranteeing an optimal cleaning quality.

According to the prior art (US-A-4,493,976), the degree of soiling is estimated by the user who chooses the duration of pyrolysis. User rating remains necessarily approximate. This process often has the disadvantage either end up with a still dirty oven at the end of the pyrolysis, or continue the pyrolysis a long time after the oven has become clean thus a waste of energy.

The invention proposes a pyrolysis time management method which does not require user intervention to estimate the degree of dirt from the oven cavity. The proposed process automatically performs a fine estimate of the degree of soiling to which it then makes match a duration of pyrolysis. The invention also provides an oven with pyrolysis equipped with a pyrolysis time management system putting in implements a pyrolysis time management method according to the invention.

According to the invention, there is provided a method for managing the duration of pyrolysis of a baking oven, comprising at the start of pyrolysis, at least two successive heating phases, first especially from the upper part then especially from the lower part of the cavity oven characterized in that the method comprises, for each heating phase, the determination of a degree partial soiling of the cavity; association, at all degrees partial soiling, with a corresponding remaining pyrolysis time.

The determination of the partial degree of soiling of the cavity is preference made from temperature measurement at level of an exothermic cracking cell for dirt.

According to the invention, there is also provided a pyrolysis oven comprising a cooking cavity, at least one high heating element located at the level of the upper part of the cavity, at least one lower heating element located at the level of the lower part of the cavity, a time management system pyrolysis using during the start pyrolysis at least two heating phases, one heating phase top during which the top heating element is activated while the low heating element is not activated and a low heating phase during which the low heating element is activated the heating phase top being prior to the bottom heating phase, characterized in that the system comprises a cracking cell exothermic dirt from the cavity, the cell being located at near the upper part of the cavity, means for measuring temperature associated with the cell in that at each phase heating system, the system determines a partial degree of soiling performing a partial quantification of dirt in the cavity from temperature measurement at the cell level, and that at the set consisting of the partial degrees of soiling, the system then associates a corresponding remaining pyrolysis time by through a correspondence table.

• la figure 1 représente schématiquement un exemple de four à pyrolyse intégrant un système de gestion de pyrolyse selon l'invention ;
• la figure 2A représente schématiquement un détail de la figure 1, à savoir l'élément 6 ;
• la figure 2B représente schématiquement un détail de la figure 1, à savoir l'élément 7 ;
• la figure 3 représente schématiquement un exemple de relevé de courbe de température de la cellule de craquage avec une partie du traitement associé permettant d'estimer un degré partiel de salissures de la cavité de four ;
• la figure 4 représente schématiquement le même exemple de relevé de courbe de température de la cellule de craquage que la figure 3, avec une autre partie du traitement associé permettant d'estimer un autre degré partiel de salissures de la cavité de four.

The invention will be better understood and other features and advantages will become apparent from the following description and the attached drawings, given by way of examples, where:

• FIG. 1 schematically represents an example of a pyrolysis oven incorporating a pyrolysis management system according to the invention;
• FIG. 2A schematically represents a detail of FIG. 1, namely element 6;
• Figure 2B schematically shows a detail of Figure 1, namely the element 7;
• FIG. 3 schematically represents an example of reading of the temperature curve of the cracking cell with part of the associated treatment making it possible to estimate a partial degree of soiling of the oven cavity;
• FIG. 4 schematically represents the same example of reading of the temperature curve of the cracking cell as in FIG. 3, with another part of the associated treatment making it possible to estimate another partial degree of soiling of the oven cavity.

Figure 1 schematically shows an example of a pyrolysis integrating a pyrolysis management system according to the invention. The oven is shown in profile. The pyrolysis management system preferably constituted by a microprocessor is not shown in Figure 1. The oven has a cooking cavity 1 and a duct 2 evacuation connecting the cavity 1 to the external environment 9. The cavity 1 is delimited with respect to the external environment 9 by an enclosure 10 comprising a muffle and an insulator surrounding the muffle. The path of the air passing through the discharge pipe 2, from the cavity 1 to the external medium 9, is shown by arrows in solid lines.

The discharge conduit 2 comprises, at its entrance located on the side of cavity 1, a cell 4 for exothermic cracking of dirt from cavity 1. This inlet is preferably located in the upper part of the cavity 1, the cell 4 then being located in the vicinity of the upper part of the cavity 1. The cracking cell 4 is preferably a catalytic cell. Cell 4 is for example of the block type in ceramic pierced with channels through which air from the cavity passes 1. The dirt coming from the cavity 1 arrives in the duct 2 in the form of gaseous effluents which are broken down into smaller molecules by oxidation and cracking reactions. These reactions are exothermic and therefore contribute to the elevation of temperature of the cracking cell 4. The rise in temperature of the cracking cell 4 is therefore linked to the amount of dirt from the cavity 1 and crossing cell 4, amount of soiling which itself reflects the degree of soiling in the cavity 1. The exhaust duct 2 advantageously comprises a cell 5 heating element 4 located near it. Temperature measurement means 8 are associated with the cell 4. These means 8 consist for example of a platinum probe, but can also be for example a thermocouple. The duct 2 evacuation still has a tangential 3 expelling towards the middle outside 9 the air located in the exhaust duct 2, thus allowing the ventilation of the exhaust duct 2. The arrow in dotted lines represented on tangential 3 indicates the direction of expulsion of air.

The cavity 1 comprises at least one top heating element 6 located at the level of the upper part of the cavity 1 and at least one heating element 7 bottom located at the lower part of the cavity 1. These elements heating elements are for example resistors. The high heating element 6 is for example located in the upper part of the cavity 1 while the element 7 bottom heater is located under cavity 1, near the bottom part of cavity 1.

In the upper and lower parts of the cavity, dirt deposit during successive cooking. These soils are shown in Figure 1 by small oblique lines. The soiling of deposit on all the walls of the enclosure 10, but for reasons of clarity, only dirt deposited in the upper and lower parts is represented. Soiling is grease or other splashes generated during cooking. Part of this dirt is removed in the form of gaseous effluents during cooking. Another part is deposits on the walls of the enclosure 10, in particular at the level of the wall lower located above the lower heating element 7. The operation of the purpose of pyrolysis is to crack these deposited soiling, that is to say to transform into smaller, gaseous molecules which will be evacuated to the outside environment 9 through the discharge pipe 2, either solid in the form of ashes which will fall to the bottom of the lower part of the cavity 1 and which will only have to be picked up. The dirt deposited in the upper and lower parts of the cavity 1 are of a different nature. Indeed, the dirt deposited in the upper part is for the most part remains of upward liquid projections that have remained attached to enclosure 10 or the heating element 6 high in the upper part of the cavity 1. These soils are less important and less strongly attached than dirt deposited in the lower part of cavity 1 which represent all the soiling which may have fallen during cooking. Dirt deposited in the lower part of cavity 1 are therefore more difficult to detach only the dirt deposited in the upper part of the cavity 1.

FIG. 2A schematically represents a detail of embodiment preferential in which the top heating element 6 of FIG. 1 is consisting of two resistors 61 and 62. FIG. 2A represents a view of above the oven. Resistor 61 is the so-called grill resistor. Resistance 62 is the so-called vault resistance. The grill resistor 61 heats especially the center of the upper part of cavity 1 while the vault resistor 62 especially heats the periphery.

FIG. 2B schematically represents an embodiment detail preferential in which the bottom heating element 7 of FIG. 1 is consisting of two resistors 71 and 72. Figure 28 shows a view of above the oven. Resistor 71 is the so-called silk resistance. Resistance 72 is the so-called façade resistance. The sole 71 resistance heats above all the center of the lower part of cavity 1 while the façade resistance 72 especially heats the periphery.

In another exemplary embodiment, the oven has a single resistance of grill 61 and a single resistance 71 of sole.

Pyrolysis time management takes place at the start of pyrolysis and allows, after a certain time necessary for measurements and treatment, determine a duration of pyrolysis remaining during which the pyrolysis is completed. The temperature in cavity 1 in operation permanent, that is to say a certain time after all the resistances to be activated during pyrolysis have been activated, can reach 500 ° C. The pyrolysis temperature preferably remains constant whatever the degree of soiling estimated by the management system duration of pyrolysis.

Pyrolysis management has at least two phases consecutive heating. Each heating phase preferably lasts about 20 minutes. The upper part of the cavity is mainly heated in a first while the lower part of the cavity is mostly heated in a second time. Pyrolysis time management can include more than two phases, for example three, in particular when the cavity 1 has three groups of heating elements, for example one element high heater, a middle heater for example a resistance ventilator located in the vicinity of the cavity ventilator when the latter includes a, and a bottom heating element. In this case, for example, the pyrolysis time management may include three heating phases successively heating respectively the upper part, then especially the central part, and finally especially the lower part of the cavity 1. The number of heating phases can correspond to the number of geographic sites in the cavity 1 between which the different elements are distributed heaters of cavity 1. The upper part of cavity 1 must be heated before the lower part of the cavity 1, so as to allow dirt deposited in the upper part of cavity 1 to be evacuated for the most of them before most of the dirt deposited in the lower part of the cavity 1 does not begin to be evacuated. We heat first the upper part of the cavity 1 and then the lower part of the cavity 1, because the dirt deposited in the upper part is more easily detachable than dirt deposited in the lower part. Otherwise, the high heating element 6 is preferably chosen to be more powerful than the low heating element 7, which makes the successive evacuation of dirt deposited in the upper part of cavity 1 first and dirt deposited in the lower part of the cavity 1 then, more convenient to make and faster. So, the dirt deposited in the upper part of the cavity 1 arrive at the level of the cracking cell 4 first, and then we observe a first temperature peak on the temperature curve of cell 4 as a function of time, this first peak being essentially due to the cracking of the deposited dirt in the upper part of cavity 1 and reflecting the partial degree of soiling deposited in the upper part of the cavity 1. The dirt deposited in the lower part of the cavity 1 arrive at the level of the cracking cell 4 in a second step, and we then observe a second temperature peak on the temperature curve of cell 4 as a function of time, this second peak being due to cracking of dirt deposited in the part bottom of cavity 1 and reflecting the partial degree of dirt deposited in the lower part of cavity 1. If the lower part of cavity 1 was heated before the upper part of cavity 1, a significant part of dirt from the upper part of the cavity 1 would reach the level of the cell 4 before most of the dirt from the lower part of the cavity 1 has crossed cell 4, which would prevent a distinction really clear between the partial degree of soiling deposited in the part top of cavity 1 and the partial degree of dirt deposited in the part lower cavity 1. Or for the same amount of soiling, depending on whether these are located in the upper part or in the lower part of the cavity 1, the optimal pyrolysis time is not the same, since from on the other hand the soiling is of a different nature and since on the other hand the heating elements located in the different parts of the oven are generally of different power. The most partial degree of soiling penalizing in terms of duration of pyrolysis is the partial degree of soiling deposited in the lower part of the cavity 1.

For each heating phase, a partial degree of soiling is determined, preferably from the temperature measurement at the level of the cracking cell 4, by a treatment comprising one or more steps. A smoke detector could also be used. The different partial degrees of soiling can be determined either directly either indirectly using one or more parameters intermediaries representative of the partial degree of soiling.

Once the different partial degrees of soiling have been determined by treatment, the pyrolysis time management system associates with the whole partial degrees of soiling, remaining pyrolysis time, preferably through a correspondence table. The table correspondence has as many entries as types of partial degrees of soiling, for example two, namely one for the partial degree of dirt deposited in the upper part of the cavity 1 and one for the degree partial soiling deposited in the lower part of the cavity 1. The table of correspondence to an output for the remaining pyrolysis time. In the below, a preferred embodiment of the duration management is described. pyrolysis according to the invention.

At the start of pyrolysis, during the top heating phase of the cavity 1, the top heating element 6 is at least partially activated while that the bottom heating element 7 is not activated. When we say that an element heating is at least partially activated, this means either that the element is at least activated at a reduced power, or that some of its resistors at least, if it has several, are activated, at full power or reduced power. Preferably, the two resistances of grill 61 and vault 62 are started almost at their power Max. The high heating phase lasts until the temperature in the center of the oven has reached a first prefixed transition value, by example about 275 ° C, at which the cracking cell 4 is already initiated and operating in steady state. This temperature in the center of the oven is evaluated for example by another temperature probe placed in cavity 1 and not shown in Figure 1. Failing to reach this transition temperature, the high heating phase ends at the end of a first prefixed duration, worth for example approximately 24 minutes. At start of the high heating phase, the heating resistor 5 of the cell 4 is activated for a priming time allowing priming of cell 4 when the latter is a catalytic cell. The duration priming is determined so that cell 4 is primed before a substantial part of the dirt deposited in the upper part of the cavity 1 does not reach the level of the cracking cell 4. The priming time is worth for example about five minutes. The activation of resistance 5 of heating of cell 4 can be shortened or even made unnecessary and therefore suppressed in certain cases, such as when pyrolysis takes place immediately after cooking and cell 4 has already reached its priming temperature. Instead of using a heating resistor 5 of cell 4, another possibility to prime cell 4 may be to increase the hot air flow through cell 4 by increasing the speed of rotation of the tangential 3 located in the discharge duct 2.

FIG. 3 schematically represents an example of a statement of temperature curve of cracked cell 4 with part of the associated treatment to estimate the partial degree of soiling deposited in the upper part of the cavity 1. Curve C does not represent directly the temperature of the cracking cell 4 but the voltage V of the temperature probe 8 associated with the cracking cell 4. On the face 3, the voltage V, expressed in volts, is represented as a function of time t, expressed in minutes. The different heating phases are indicated by dashed lines. Voltage V is an intermediate parameter representative of the temperature of the cracking cell 4, temperature itself representative of the partial degree of soiling deposited in the upper part of the cavity 1. More precisely, it is the temperature of the cell 4 when most of the dirt deposited in the part high of the cavity 1 arrives at the level of the cell 4 which is representative of the partial degree of soiling deposited in the upper part of the cavity 1. This moment is represented on curve C by peak A. Indeed, if the general increase in curve C over time reflects the general increase in the temperature of the cracking cell 4 due to activation of the different heating elements, 5 for cell 4 of cracked, 6 for the upper part of the cavity 1 and 7 for the lower part of cavity 1, additional peaks like peak A reflect the amount of soiling causing exothermic cracking reactions, the cracked cell 4 is the seat. The importance of this peak A is representative of the partial degree of soiling deposited in the upper part of the cavity 1. The temperature measurement means of the cell 4 of cracking are for example either a thermocouple or a platinum probe. The thermocouple has the advantage of being more sensitive than the platinum probe. If the peak A is too fleeting, the platinum probe having a high inertia risks not not "see" the peak or "see it badly". With a platinum probe in particular, it is very useful to carry out a treatment on curve C to extract the information contained in peak A even if it is not sufficient important and therefore to estimate the partial degree of soiling deposited in the upper part of cavity 1 with sufficient precision despite a peak A which may be small and / or narrow. During operation normal to cracking cell 4, only one peak A is likely to be statement. The reading of several A peaks would indicate a saturation phenomenon at the level of the cracking cell 4, that is to say an operation abnormal cracking cell 4 due for example to phases of unsuitable heating resulting in too rapid temperature rise in the oven cavity and too brutal release of dirt which is then at the origin of this phenomenon of saturation of cell 4 of cracking.

The treatment applied to curve C, consisting of the determination the partial degree of soiling, here that from the upper part of the cavity 1, preferably comprises a bypass step making it possible to to highlight more precisely the information relating to the degree partial soiling deposited in the upper part of the cavity 1, information which is contained in peak A. The derivation step makes it possible to more to overcome variations in the furnace supply voltage. A method by direct reading of the temperature of the cracking cell 4 is also possible although less precise, provided you use then a temperature sensor which is itself very sensitive. By derivative of curve C, we obtain curve C 'on which a new peak A' corresponds to the old peak A. This peak A 'is representative of the partial degree of dirt deposited in the upper part of the cavity 1, and in particular the height Δ1 of peak A '. The ordinate axis for the curve C 'is staggered in arbitrary AU units.

Preferably, after the derivation step, the treatment includes also a peak height extraction step consisting in determining the ordinate of the top of the peak, this is the peak A 'for the heating phase top, as well as the ordinate of the base of this peak, then to realize the subtract these two ordinates from each other in order to obtain the height of the peak sought, here the height Δ1 of the peak A '.

Preferably, after the extraction step, the treatment of determination of partial degree of soiling preferably comprises a step of comparing the peak height extracted with one or more thresholds. In a particular example considered, the height Δ1 of the peak A 'will be compared with a threshold S1. The threshold S1, like all the other thresholds mentioned in the following text, is for example obtained by calibration. These thresholds are predefined thresholds.

After the top heating phase of cavity 1, i.e. when the center of the oven has reached a first prefixed transition value, by example 275 ° C, or failing this when a first duration prefixed, by example 24 minutes, has elapsed, then begins the low heating phase of the cavity 1. The bottom heating element 7 is at least partially activated. Preferably, the floor resistor 71 is started up practically at full power while the front resistance 72, when there is one, remains inactive. The grill resistor 61 stays on practically at full power while the vault resistor 62 is periodically switched between the "on" state and the "off" state so that the cavity 1 gradually rises in temperature, that is to say so that soiling from cavity 1 is gradually released so as not to cause saturation of the cracking cell 4. The low heating phase lasts until the temperature in the center of the oven has reached a second prefixed transition value, for example about 400 ° C, or failing to reach this temperature, after the flow a second prefixed duration worth for example 18 minutes. Advantageously, in the case where the temperature of 400 ° C. is reached, the resistors remain activated for the entire 18 minutes, although the period of time after the temperature of 400 ° C has been reached is either more taken into account for the determination of the partial degree of soiling deposited in the lower part of the cavity 1. After which, the tangential 3 advantageously changes gears to turn faster and accelerate the passage of air in the exhaust duct 2. Like to leave of 400 ° C, dirt begins to be cracked in cavity 1 and the are already when they arrive at the cracking cell 4, the increase in the air flow through the cracking cell 4 does not normally does not cause saturation at cell level 4.

Figure 4 shows schematically the same example of statement of the temperature curve of the cracking cell 4 as in FIG. 3, with another part of the associated treatment to estimate the partial degree dirt deposited in the lower part of the cavity 1. Curve C is the same as curve C shown in figure 3. The temperature peak B of cell 4 is observed on curve C when most of the dirt deposited in the lower part of cavity 1 arrives at cell 4. It is this temperature peak B which is representative of the degree partial soiling deposited in the lower part of the cavity 1. Indeed, if the general increase in curve C as a function reflects the increase general temperature of the cracking cell 4 due to the activation of different heating elements, 5 for the cracking cell 4, 6 for the upper part of cavity 1 and 7 for the lower part of cavity 1, the peaks additional as peak B reflects the amount of soiling originally exothermic cracking reactions of which the cracking cell 4 is headquarters. The importance of this peak B is representative of the partial degree of dirt deposited in the lower part of the cavity 1. If the peak B is too fugitive, the platinum probe having a high inertia risks not "seeing" the pic or ill see it. With a platinum probe in particular, it is very useful perform a processing on curve C to extract the information contained in peak B even if it is not large enough and therefore to estimate the partial degree of soiling deposited in the lower part of cavity 1 with sufficient precision despite a peak B which can be quite flat for example as in Figure 4.

The treatment applied to curve C, consisting of the determination of the partial degree of soiling, is substantially the same as that applied to curve C in Figure 3. The numerical parameters of step may be different, due to the different character presented by peaks A and B, that is to say that the curve C "obtained by derivation of curve C is on an arbitrary scale different from curve C ' shown in Figure 3. Except for this difference in scale concerning the curves C 'and C ", curve C is substantially subject to the same treatment for peak B than for peak A. Determination of the partial degree dirt deposited in the lower part of the cavity 1 includes the same preferential steps of derivation, peak height extraction and comparison of the peak height extracted with one or more thresholds which can be and are preferably different from the threshold (s) considered in Figure 3.

By derivative of curve C, we obtain curve C "on which a new peak B 'corresponds to the old peak B. This peak B' is representative of the degree of soiling deposited in the lower part of the cavity 1, and in particular the height Δ2 of the peak B '. In a particular example considered, the height Δ2 of the peak B 'will be compared with two thresholds S2 and S3 obtained as the threshold S1 of FIG. 3 by calibration. A number of thresholds different can be chosen, the number of thresholds corresponding to the finesse with which the partial degrees of soiling are estimated. The peak D of the curve C is not representative of the degree of soiling of the oven cavity, it simply translates the advantageous change of the tangential regime 3 which, as explained above, increases the air flow through the cracking cell 4, after the low heating phase is completed.

The association of a remaining pyrolysis time with all of the determined partial degrees of soiling can be effected by different means as through a function whose variables are partial degrees of soiling determined during the different phases of heater. Preferably, the remaining pyrolysis time comes from a correspondence table whose entries are these partial degrees of dirt. The total duration of pyrolysis is equal to the sum of the durations of the previously described high and low heating phases to which is added the remaining pyrolysis time. The duration of the heating phases always remaining essentially the same regardless of the degree of soiling of the oven cavity, only a fixed duration during which is carried out the estimate of the degree of soiling of the oven cavity separates the value of the remaining duration of pyrolysis and the value of the total duration of pyrolysis: it it is therefore equivalent to give one or the other.

For the sake of convenience, it is the value of the total duration of pyrolysis which is given in the following table representing, for the preferred numerical example, the correspondence table here associating the total duration of pyrolysis with a set of partial degrees of soiling: Δ2 <S2 S2 <Δ2 <S3 Δ2> S3 Δ1 <S1 1:30 2:15 3h Δ1> S1 1:45 2:15 3h

In this example, when the peak height Δ1 of peak A 'is below threshold S1 and when the height Δ2 of peak B 'is less than threshold S2, the oven is considered clean and the total duration of pyrolysis associated will be about an hour and a half, the remaining duration of pyrolysis worth about three quarters of an hour. When the peak height Δ1 of peak A ' is greater than the threshold S1 and when the height Δ2 of peak B 'is less at threshold S2, the oven is considered to be lightly soiled and the total duration of associated pyrolysis will be approximately one hour three quarters, the remaining duration of pyrolysis worth about an hour. When the height Δ2 of peak B 'is between threshold S2 and threshold S3, the oven is considered to be moderately dirty and the total duration of associated pyrolysis will be approximately two and a quarter hours, the remaining duration of pyrolysis being approximately one hour and a half. When the height Δ2 of peak B 'is greater than the threshold S2, the furnace is considered very dirty and the total duration of associated pyrolysis will be approximately three hours, the remaining duration of pyrolysis being approximately two quarter hours. The total duration of pyrolysis varies significantly from one hour and a half to three hours depending on the degree of soiling of the cavity, generally represented by the two partial degrees of soiling upper and lower parts of the cavity.

Claims (13)

1. A method of managing pyrolysis time in a cooking oven, comprising

   at the beginning of the pyrolysis process, at least two successive heating phases, taking place at first mostly in the upper part and then mostly in the lower part of the cavity (1) of the oven,

   characterised in that the method includes:

   for each heating phase, the determination of a partial amount of dirt in the cavity (1);

   the association of a corresponding remaining pyrolysis time with the partial amounts of dirt taken together.
2. A method of managing pyrolysis time in a cooking oven according to Claim 1, characterised in that the determination of the partial amount of dirt in the cavity (1) is made on the basis of measurement of the temperature at the level of a cell (4) for the exothermic cracking of the dirt.
3. A pyrolytic oven having a cooking cavity (1), at least one top heating element (6) situated level with the upper part of the cavity (1), at least one bottom heating element (7) situated level with the lower part of the cavity (1), a system for managing pyrolysis time which, at the beginning of the pyrolysis process, performs at least two heating phases, namely a top heating phase during which the top heating element (6) is activated while the bottom heating element (7) is not activated, and a bottom heating phase during which the bottom heating element (7) is activated, the top heating phase preceding the bottom heating phase, characterised in that the system includes a cell (4) for the exothermic cracking of dirt from the cavity (1) and temperature measuring means (8) associated with the cell (4), and in that, in each heating phase, the system determines a partial amount of dirt by partially quantifying dirt in the cavity (1) on the basis of measurement of temperature at the level of the cell (4), and in that the system subsequently associates a corresponding remaining pyrolysis time with the whole constituted by the partial amounts of dirt, by means of a correspondence table.
4. A pyrolysis oven according to Claim 3, characterised in that the cell (4) is a catalytic cell.
5. A pyrolysis oven according to Claim 4, characterised in that the oven includes a heating element (5) for the cell (4), and in that the said element (5) is activated at the start of the top heating phase so as to activate the catalytic cell (4).
6. A pyrolytic oven according to Claim 4, characterised in that the oven includes a fan (3) situated in the exhaust duct (2), and in that the speed of rotation of the fan (3) is increased at the start of the top heating phase whereby to activate the catalytic cell (4) .
7. A pyrolytic oven according to any one of Claims 3 to 6, characterised in that each of the heating phases has a duration of the order of twenty minutes.
8. A pyrolytic oven according to any one of Claims 3 to 7, characterised in that the cavity (1) includes two top heating elements (61 and 62), namely the oven roof element (62) and the grilling element (61), and in that, during the bottom heating phase, the grilling element (61) is continuously activated while the oven roof element (62) is switched between an activated state and a de-activated state in such a way that the cavity (1) rises progressively in temperature.
9. A pyrolytic oven according to any one of Claims 3 to 7, characterised in that the cavity (1) has a single top heating element (61), namely the grilling element (61), and a single bottom heating element (71), namely the oven floor element (71).
10. A pyrolytic oven according to any one of Claims 3 to 9, characterised in that the determination of a partial amount of dirt comprises a step of deriving, with respect to time, the curve (C) of temperature of the cell (4) as a function of time (t).
11. A pyrolytic oven according to Claim 10, characterised in that the determination of a partial amount of dirt comprises, after the derivation step, a step of extracting therefrom the height (Δ1, Δ2) of a peak (A', B').
12. A pyrolytic oven according to Claim 11, characterised in that the determination of apartial amount of dirt comprises, after the extraction step, a step of comparing the height (Δ1, Δ2) of the extracted peak (A', B') with one or more threshold values (S1, S2, S3).
13. A pyrolytic oven according to any one of Claims 3 to 12, characterised in that the total duration of pyrolysis varies substantially from an hour and a half to three hours, as a function of the amount of dirt in the cavity (1).

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