IT201800010501A1 - METHOD OF CONTROL OF A THERMOELECTRIC GENERATOR FOR AN INTERNAL COMBUSTION ENGINE - Google Patents

Description

DESCRIPTION

of the patent for industrial invention entitled:

"METHOD OF CONTROL OF A THERMOELECTRIC GENERATOR FOR AN INTERNAL COMBUSTION ENGINE"

TECHNIQUE SECTOR

The present invention relates to a control method of a thermoelectric generator (also called "TEG") for an internal combustion engine.

The present invention finds advantageous application to the control method of a thermoelectric generator for an exhaust system of an internal combustion engine, to which the following discussion will make explicit reference without thereby losing generality.

ANTERIOR ART

In the continuous search for an increase in the efficiency of internal combustion engines, it has recently been proposed to use part of the heat possessed by the exhaust gases (which would otherwise be completely dispersed into the atmosphere through the exhaust system) to generate electricity through the use of thermoelectric cells.

It has therefore been proposed to arrange along the exhaust system a thermoelectric generator provided with a plurality of solid state thermoelectric cells, each of which has a hot side which is exposed to the exhaust gases to be heated by the exhaust gases themselves (which can have a temperature of 250-750 ° C depending on the area of the exhaust system in which the thermoelectric generator is placed) and a cold side (opposite to the hot side) which is constantly cooled by a cooling fluid (which is strictly maintained isolated from the exhaust gases and generally consists of water that transfers heat to the external environment, also circulating through a radiator).

A solid-state thermoelectric cell is able to convert heat into electricity (through the Seebeck effect) when there is a temperature difference between its hot side and its cold side; to ensure the effectiveness of electricity generation it is necessary to ensure that the temperature of the cold side of the thermoelectric cell remains adequately lower than the temperature of the hot side and it is therefore necessary to provide for constant cooling of the cold side.

By way of example, patent applications WO2011107282 US2011083831A1, EP2765285A1, US2014305481A1, US2015128590A1, US2016155922A1, and EP3404227A1 describe thermoelectric generators for an exhaust system of an internal combustion engine.

A thermoelectric cell cannot withstand too high temperatures, i.e. the hot side of a thermoelectric cell must not exceed a critical temperature beyond which damage can occur due to overheating of the thermoelectric cell itself.

To avoid overheating of the thermoelectric cells when the exhaust gases are too hot (i.e. when the engine delivers high power for a relatively long time, for example during a high-speed motorway route), a bypass duct is generally provided which it is regulated by a special bypass valve and is arranged in parallel to the thermoelectric generator: normally the bypass duct is always kept closed to maximize the generation of electricity and only when the temperature of the hot side of the thermoelectric cells gets too close to the critical temperature the bypass duct is opened more or less completely to reduce the flow rate of the exhaust gases passing through the thermoelectric generator and therefore limit the temperature of the hot side of the thermoelectric cells.

The bypass duct is effective and efficient in preventing the temperature of the hot side of the thermoelectric cells from exceeding the critical temperature; however, the presence of the bypass duct (and above all the presence of the corresponding bypass valve) leads to an increase in the cost, weight and overall dimensions of the thermoelectric generator.

DESCRIPTION OF THE INVENTION

The purpose of the present invention is to provide a control method of a thermoelectric generator for an internal combustion engine, which control method allows to reduce the overall cost, weight and overall dimensions of the thermoelectric generator.

According to the present invention there is provided a control method of a thermoelectric generator for an internal combustion engine, according to what is claimed by the attached claims.

The claims describe preferred embodiments of the present invention forming an integral part of the present description.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the attached drawings, which illustrate a non-limiting example of embodiment, in which:

Figure 1 is a perspective view of a thermoelectric generator for an exhaust system of an internal combustion engine made in accordance with the present invention;

• figure 2 is a perspective view of the thermoelectric generator of figure 1 with the removal of an inlet pipe and an outlet pipe;

• figure 3 is a perspective view of the thermoelectric generator of figure 1 with the removal of parts for clarity;

Figure 4 is a schematic view of the electrical connection of the thermoelectric generator of figure 1;

Figure 5 is a graph illustrating the voltage / current characteristic and the voltage / power characteristic of the thermoelectric generator of Figure 1; And

Figure 6 is a schematic representation of a control logic of the thermoelectric generator of figure 1.

PREFERRED FORMS OF IMPLEMENTATION OF THE INVENTION

In figure 1, the number 1 indicates as a whole a thermoelectric generator (i.e. a device that is able to convert part of the heat possessed by the exhaust gases into electrical energy) for an exhaust system of an internal combustion engine.

The thermoelectric generator 1 can be arranged along the discharge system in different areas; for example, the thermoelectric generator 1 can be arranged immediately downstream of the exhaust manifold (and, if present, the compression turbine) of the internal combustion engine, it can be arranged between the catalyst and the anti-particulate filter, or it can be arranged downstream of the particulate filter.

The exhaust system of the internal combustion engine comprises an exhaust gas inlet pipe 2 through which the hot exhaust gases from the internal combustion engine are fed to the thermoelectric generator 1 (i.e. the inlet pipe 2 ends in the generator 1 thermoelectric) and an exhaust gas outlet pipe 3 through which the exhaust gases that leave the thermoelectric generator 1 are fed to the external environment (i.e. the outlet pipe 3 originates from the thermoelectric generator 1).

The thermoelectric generator 1 comprises a closed parallelepiped-shaped casing 4 inside which twelve solid-state thermoelectric cells 5 are housed (partially illustrated in Figure 3), each of which is capable of converting heat into electrical energy (through the effect Seebeck) when there is a difference in temperature between its own warm side and its own cold side; to ensure the effectiveness of electricity generation, it is necessary to ensure that the TCOLD temperature of the cold side of each thermoelectric cell 5 remains adequately lower than the THOT temperature of the hot side and it is therefore necessary to provide both a constant heating of the hot side and a constant cooling of the cold side.

According to what is illustrated in Figures 2 and 3, the thermoelectric generator 1 comprises two supply elements 6 superimposed on each other and each of which is provided with a tubular duct 7 which is able to be crossed by the exhaust gases. The tubular duct 7 of each supply element 6 has a parallelepiped shape (i.e. in cross section it has a rectangular shape) and extends along a supply direction (rectilinear in the illustrated embodiment) between an inlet opening 8 (through which the exhaust gases enter) and an outlet opening 9 (through which the exhaust gases exit). The tubular duct 7 of each feed element 6 has a pair of exchange walls 10 parallel to each other and opposite and arranged parallel to the feed direction; the hot side of the corresponding thermoelectric cells 5 rests on each heat exchange wall 10.

According to what is illustrated in Figures 2 and 3, the thermoelectric generator 1 comprises three cooling elements 11, each of which is adapted to subtract heat; the three cooling elements 11 are interleaved with the ducts 7 of the supply elements 6. In particular, each cooling element 11 has a parallelepiped shape and has a pair of heat exchange walls 12 parallel and opposite to each other and arranged parallel to the heat exchange walls 10 of the ducts 7 (i.e. parallel to the feeding direction of the ducts 7) ; the cold sides of the corresponding thermoelectric cells 5 rest on some heat exchange walls 12. In this way, in each thermoelectric cell 5 the hot side rests against the heat exchange wall 10 of a corresponding duct 7 and the cold side rests against the heat exchange wall 12 of a corresponding cooling element 11.

In other words, the ducts 7 of the two supply elements 6 are arranged interspersed with the three cooling elements 11 in such a way that each exchange wall 10 of a duct 7 faces a corresponding exchange wall 12 of a cooling element 11 ; between each exchange wall 10 of a duct 7 and the corresponding exchange wall 12 of a cooling element 11 there are interposed thermoelectric cells 5 (the hot side of each thermoelectric cell 5 rests against the heat exchange wall 10 of the duct 7 and the cold side of each thermoelectric cell 5 rests on the heat exchange wall 12 of the cooling element 11).

As shown in Figure 4, the twelve thermoelectric cells 5 of the thermoelectric generator 1 are electrically connected to each other in series and parallel to form a circuit that has two terminals 13 outside the thermoelectric generator 1; in use, a continuous electric voltage V is generated at the ends of the two terminals 13 and a continuous electric current I circulates through the terminals 13 and consequently the thermoelectric generator 1 supplies an electric power P equal to the product of the electric voltage V by the current I electric.

The two terminals 13 of the thermoelectric generator 1 are connected to an electrical system of the vehicle equipped with (at least) a battery 14 by means of the interposition of a control device 15 which performs two functions: on the output side (i.e. on the side connected to the vehicle electrical system equipped with the battery 14) always supplies an electrical voltage suitable for recharging the battery 14 (the battery 14 typically has a nominal voltage of 12 Volts, or the battery 14 could have a nominal voltage of 48 Volts in some vehicles more recent) while on the input side (ie on the side connected to the two terminals 13 of the thermoelectric generator 1) it “sees” a variable equivalent electrical resistance which allows to optimize (according to the methods described below) the operation of the thermoelectric generator 1. In other words, the control device 15 in the input side (i.e. in the side connected to the two terminals 13 of the thermoelectric generator 1) creates an equivalent electrical resistance such that the electrical voltage V between the two terminals 13 and the electrical current I which flows through two terminals 13 have values such as to optimize the operation of the thermoelectric generator 1.

The diagram of Figure 5 illustrates the straight line A which constitutes the locus of all possible work points (operation) of the thermoelectric generator 1 in the plane of electric voltage V and electric current I for a given temperature difference on the two faces of the thermoelectric cells 5 : the straight line A extends from a point IMAX to the extreme left (corresponding to the short-circuit current) in which the electric current I is maximum (that is, it is equal to a maximum value) and the electric voltage V is zero at a point VMAX to the extreme right (corresponding to the open circuit voltage) in which the electrical voltage V is maximum (that is, it is equal to a maximum value) and the electrical current I is zero. At the point IMAX at the extreme left of the line A, the electrical resistance is theoretically zero (since the electrical voltage V is zero) and the electrical power P supplied by the thermoelectric generator 1 is also theoretically zero (since the electrical voltage V is zero); in the point VMAX at the extreme right of the line A the electrical resistance is theoretically infinite (since the electrical current I is zero) and the electrical power P supplied by the thermoelectric generator 1 is theoretically zero (since the electrical current I is zero). The electric power P supplied by the thermoelectric generator 1 has an inverted parabola trend represented by curve B (trivially obtained with the product between the electric voltage V and the electric current I of all points of the straight line A).

By varying the temperature difference between the hot side and the cold side of the thermoelectric cells 5 of the thermoelectric generator 1, the curves A or B shown in figure 5 do not change their conformation but move upwards (in case of increase of the difference temperature which determines the generation of a greater electrical power P) or downwards (in the event of a decrease in the temperature difference which determines the generation of a lower electrical power P).

Figure 5 illustrates the (normal) operation of the thermoelectric generator 1 as a generator, i.e. as a device capable of generating and then deliver a positive electrical power P.

Figure 5 also illustrates the (atypical) operation of the thermoelectric generator 1 as a heat pump, i.e. as a device that it absorbs electrical power P (i.e. generates a negative electrical power P) to transfer heat from the hot side to the cold side of the thermoelectric cells 5.

According to other embodiments, the conformation of the thermoelectric generator 1 could be completely different (i.e. a different number of thermoelectric cells 5 can be provided, a different thermal connection of the thermoelectric cells 5 can be provided, a different electrical connection of the thermoelectric cells 5 can be provided 5 thermoelectric cells ...); the only fixed point is that the thermoelectric generator 1 must include at least one thermoelectric cell 5 which has a hot side and a cold side and can be electrically connected to a vehicle electrical system.

With reference to Figure 5, the operation of the control device 15 is described below in order to establish each time the (optimal) operating point of the thermoelectric generator 1 (the operating point of the thermoelectric generator 1 is defined by the torque constituted by the voltage Electric V generated by the thermoelectric generator 1 and by the electric current I which passes through the thermoelectric generator 1).

The thermoelectric cells 5 are not able to withstand too high temperatures, i.e. the THOT temperature of the hot side of the thermoelectric cells 5 must never exceed a critical TCRT temperature beyond which damage can occur due to overheating of the thermoelectric cells 5 themselves.

In use, the control device 15 cyclically determines the THOT temperature of the hot side of the thermoelectric cells 5 and compares the THOT temperature of the hot side of the thermoelectric cells 5 with a predetermined threshold TTRH value (suitably lower than the critical TCRT temperature). It is important to underline that the control device 15 can directly measure the THOT temperature of the hot side of the thermoelectric cells 5 by means of temperature sensors, or it can indirectly estimate (i.e. without a direct measurement) the THOT temperature of the hot side of the thermoelectric cells 5. for example as a function of the working point in the voltage / current plane of the thermoelectric generator 1 (ie as a function of the electrical voltage V generated by the thermoelectric generator 1 and the electrical current I that passes through the thermoelectric generator 1).

When the temperature THOT of the hot side of the thermoelectric cells 5 is lower than the threshold value TTRH, the control device 15 has the sole objective of maximizing the electric power P supplied by the thermoelectric generator 1 by operating according to known optimization methods and generally indicated with the acronym of “MPPT” (“Maximum Power Point Tracker”); in other words, the control device 15 modifies the equivalent electrical resistance seen by the terminals 13 of the control device 15 to try to always make the control device 15 work at point P1 (corresponding to the electrical current I1 and the electrical voltage V1) in where the generation of electrical power P is maximized. When the THOT temperature of the hot side of the thermoelectric cells 5 is higher than the threshold TTRH value, the control device 15 has as its main objective the decrease of the THOT temperature of the hot side of the thermoelectric cells 5 (i.e. to cool the hot side of the thermoelectric cells 5 ) by increasing the intensity of the electric current I which flows through the thermoelectric generator 1 (ie through the thermoelectric cells 5 of the thermoelectric generator 1); in other words, the control device 15 no longer pursues the generation of the maximum electrical power P, but aims to decrease the THOT temperature of the hot side of the thermoelectric cells 5 by increasing (at the cost of decreasing the electrical power P generated) the intensity of the electric current I flowing through the thermoelectric generator 1.

Preferably, a hysteresis is applied to the comparison between the THOT temperature of the hot side of the thermoelectric cells 5 and the threshold value TTRH (the hysteresis is a phenomenon whereby the value assumed by a quantity dependent on others is determined, in addition to instantaneous of the latter, even from the values they had previously assumed; or, in other words, the hysteresis is the characteristic of a system to react late to the applied stresses and depending on the previous state); in this way, the operating point control mode in the voltage / current plane of the thermoelectric generator 1 is not changed with too high a frequency when the temperature THOT of the hot side of the thermoelectric cells 5 is in a neighborhood of the threshold value TTRH.

What described above is well illustrated in Figure 5: when the temperature THOT of the hot side of the thermoelectric cells 5 is lower than the threshold value TTRH, the control device 15 has the sole objective of maximizing the electrical power P supplied by the thermoelectric generator 1 and therefore the thermoelectric generator 1 operates in a neighborhood of point P1 (corresponding to the maximum electrical power P that can be generated), on the other hand when the THOT temperature of the hot side of the thermoelectric cells 5 is higher than the threshold value TTRH the control device 15 tries to decrease the temperature THOT of the hot side of the thermoelectric cells 5 by increasing the intensity of the electric current I which flows through the thermoelectric generator 1 and therefore the thermoelectric generator 1 operates, for example, at the point P2 (corresponding to the electric current I2 and the electric voltage V2) in where the intensity of the electric current I is greater (but overall d decreases the electrical power P generated). Obviously, the greater the difference between the THOT temperature of the hot side of the thermoelectric cells 5 and the threshold TTRH value (i.e. the more the hot side of the thermoelectric cells 5 is "overheated"), the greater the intensity of the current must be Electrical I which flows through the thermoelectric generator 1 (to increase the cooling effect) and therefore the lower the electrical power P generated; in other words, the greater the difference between the THOT temperature of the hot side of the thermoelectric cells 5 and the threshold value TTRH, the more it moves to the left (i.e. towards the working point in the voltage / current plane at maximum electrical current I ) the working point in the voltage / current plane imposed on the thermoelectric generator 1 by the control device 15.

Each thermoelectric cell 5 is constituted by two doped semiconductor materials of type N and type P connected together by a lamella of metallic material; in case of temperature difference between the cold side and the hot side, the generation of a voltage difference (and therefore potentially the generation of an electric current) is established according to the Seebeck effect (the Seebeck effect is a thermoelectric effect for which, in a circuit consisting of metallic conductors or semiconductors, a temperature difference generates electricity). Furthermore, when each thermoelectric cell 5 is crossed by an electric current, a heat transfer is also established from the hot side to the cold side (i.e. a "pumping" of heat from the hot side to the cold side which tends to cool the hot side and heat the cold side) according to the Peltier effect (the Peltier effect is the thermoelectric phenomenon whereby an electric current flowing between two different metals or semiconductors placed in contact, or in a Peltier junction, produces a heat transfer; depending on the direction of the current, the junction emits or absorbs heat, making it possible to create a heating or cooling device).

The Peltier effect is generally a parasitic and unwanted condition that tends to reduce the temperature difference between the hot side and the cold side and therefore tends to reduce the electrical power P generated. The Peltier effect is all the more intense the greater the intensity of the electric current that passes through each thermoelectric cell 5 and therefore increasing the intensity of the electric current increases the heat transfer from the hot side to the cold side (i.e. increases the cooling of the heat of the thermoelectric cell 5).

According to what has been described above, when the temperature THOT of the hot side of the thermoelectric cells 5 is lower than the threshold value TTRH, the control device 15 has the sole objective of maximizing the electric power P supplied by the thermoelectric generator 1; consequently, the control device 15 is not interested in the Peltier effect which remains a parasitic (or undesirable) effect. On the other hand, when the THOT temperature of the hot side of the thermoelectric cells 5 is higher than the threshold value TTRH, the control device 15 tries to decrease the THOT temperature of the hot side of the thermoelectric cells 5 by increasing the intensity of the electric current I which flows through the thermoelectric generator 1; that is, the control device 15 exploits the Peltier effect (which is amplified by increasing the intensity of the electric current I flowing through the thermoelectric generator 1 to the detriment of the electrical power P generated) to decrease the THOT temperature of the hot side of the cells 5 thermoelectric.

According to a possible embodiment illustrated in Figure 6, when the temperature THOT of the hot side of the thermoelectric cells 5 is higher than the threshold value TTRH, the control device 15 uses a feedback control in which the feedback variable is the THOT temperature of the hot side of the thermoelectric cells 5 (therefore the control error ε is the difference between the THOT temperature of the hot side of the thermoelectric cells 5 and the threshold value TTRH) and the controlled variable (as a function of the control error ε) is the intensity of the electric current I flowing through the thermoelectric generator 1.

According to a possible embodiment, if by operating the thermoelectric generator 1 as an electric generator (i.e. by causing the thermoelectric generator 1 to generate a positive electric power P which is supplied to the outside by the thermoelectric generator 1) the THOT temperature of the hot side of the cells 5 thermoelectric power is not reduced sufficiently, it is also possible to use a special operating mode (emergency) in which the thermoelectric generator 1 is operated as a heat pump by absorbing an electrical power P from the outside (i.e. generating an electrical power P negative which is then absorbed by the thermoelectric generator 1). In other words, by operating the thermoelectric generator 1 as an electric generator, the maximum value of the electric current I is equal to the IMAX value; if this maximum value of the electric current I is not yet sufficient to determine (due to the Peltier effect of heat transfer from the hot side to the cold side of the thermoelectric cells 5) an adequate decrease in the THOT temperature of the hot side of the thermoelectric cells 5 is possible further increase the electric current I (in addition to the IMAX value) by making the thermoelectric generator 1 work as a heat pump by absorbing an electric power P from the outside. Obviously, the operating mode as heat pump of the thermoelectric generator 1 (ie absorbing an electric power P from the outside) is an exceptional (emergency) operating mode that is used as a "last resort" when it is not possible to keep in another way I control the THOT temperature of the hot side of the thermoelectric cells 5 to avoid damaging the thermoelectric cells 5 by overheating. In other words, to protect the thermoelectric cells 5 from possible damage due to overheating, in an emergency situation it is possible to operate (normally for a very limited time) the thermoelectric generator 1 as a heat pump (i.e. absorbing an electrical power P from the outside ) in order to maximize the heat transfer from the hot side to the cold side of the thermoelectric cells 5.

In the embodiment illustrated in the attached figures, the heat generator 1 is arranged in the exhaust system of the internal combustion engine (ie it exploits the heat possessed by the exhaust gases of the internal combustion engine). According to a different embodiment not shown, the heat generator 1 is arranged in the intake system of the internal combustion engine inside an intercooler (i.e. it exploits the heat possessed by the intake air leaving a turbocharger compressor ).

The embodiments described here can be combined with each other without departing from the scope of protection of the present invention.

The control method of the thermoelectric generator 1 described above has numerous advantages.

In the first place, the control method of the thermoelectric generator 1 described above allows to effectively and efficiently keep the maximum temperature of the hot side of the thermoelectric cells 5 under control, i.e. the control method of the thermoelectric generator 1 described above allows to avoid in effectively and efficiently that the hot side of the thermoelectric cells 5 exceeds the critical TCRT temperature.

Furthermore, the control method of the thermoelectric generator 1 described above is particularly economical to implement as it does not require the addition of any additional hardware component, but performs the cooling of the hot side of the thermoelectric cells 5 solely via software with a particular control mode. of the working point in the voltage / current plane of the thermoelectric generator 1. In particular, the control method of the thermoelectric generator 1 described above does not require any bypass duct arranged in parallel with the thermoelectric generator 1 and therefore allows to reduce the cost, weight and overall dimensions of the thermoelectric generator 1.

Finally, the control method of the thermoelectric generator 1 described above allows to increase the overall reliability since by not requiring any bypass duct (and above all any bypass valve) it reduces the number of parts that can be subject to a malfunction (i.e. that is not there cannot be broken).

LIST OF REFERENCE NUMBERS OF THE FIGURES

1 thermoelectric generator

2 inlet pipe

3 outlet tube

4 casing

5 thermoelectric cells

6 power element

7 duct

8 entrance opening

9 exit opening

10 exchange wall

11 cooling element

12 exchange wall

13 terminals

14 battery

15 control device V electric voltage

I electric current

P electric power

A straight line

B curve

P1 working point

P2 working point

THOT hot side temperature TTRH threshold value

ε control error

Claims (10)

R I V E N D I C A Z I O N I 1) Method of controlling a thermoelectric generator (1) for an internal combustion engine; the thermoelectric generator (1) comprises at least one thermoelectric cell (5) having a hot side and a cold side, and is able to generate, in the event of a temperature difference between the hot side and the cold side, an electrical voltage (V) which determines the circulation of an electric current (I); the control method comprises the steps of: cyclically determining a temperature (THOT) of the heat of the thermoelectric cell (5); And controlling a working point in a voltage / current plane of the thermoelectric generator (1) to maximize an electrical power (P) generated by the thermoelectric generator (1); the control method is characterized by the fact that it includes the further steps of: comparing the heat temperature (THOT) of the thermoelectric cell (5) with a threshold value (TTRH); check the working point in the voltage / current plane of the thermoelectric generator (1) to maximize the electrical power (P) generated by the thermoelectric generator (1) when the temperature (THOT) of the heat of the thermoelectric cell (5) is lower than the value ( TTRH) of threshold; And check the working point in the voltage / current plane of the thermoelectric generator (1) to reduce the temperature (THOT) of the heat of the thermoelectric cell (5) when the temperature (THOT) of the heat of the thermoelectric cell (5) is higher than the value ( TTRH) of threshold. 2) Control method according to claim 1, wherein, when the temperature (THOT) of the hot side of the thermoelectric cell (5) is higher than the threshold value (TTRH), the temperature (THOT) of the hot side of the cell (5 ) thermoelectric is reduced by increasing the intensity of the electric current (I) flowing through the thermoelectric generator (1). 3) Control method according to claim 2, wherein, when the temperature (THOT) of the hot side of the thermoelectric cell (5) is higher than the threshold value (TTRH), the temperature (THOT) of the hot side of the cell (5 ) is decreased by increasing, while decreasing the electrical power (P) generated, the intensity of the electrical current (I) flowing through the thermoelectric generator (1). 4) Control method according to claim 2 or 3, wherein, when the temperature (THOT) of the hot side of the thermoelectric cell (5) is higher than the threshold value (TTRH), the temperature (THOT) of the hot side of the cell (5) thermoelectric is decreased by increasing the intensity of the electric current (I) flowing through the thermoelectric generator (1) possibly also making the thermoelectric generator (1) work as a heat pump by absorbing an electric power (P) from the outside. 5) Control method according to claim 2, 3 or 4, wherein: when the thermoelectric generator 1 is operated as an electric generator by delivering an electric power (P) to the outside, the electric current (I) flowing through the thermoelectric generator (1) cannot exceed a maximum value (IMAX) depending on the difference in temperature between the temperature (THOT) of the hot side of the thermoelectric cell (5) and the temperature (TCOLD) of a cold side of the thermoelectric cell (5); when the thermoelectric generator (1) is operated as a heat pump by absorbing an electric power (P) from the outside, the electric current (I) flowing through the thermoelectric generator (1) exceeds the maximum (IMAX) value; And to obtain greater cooling of the hot side of the thermoelectric cell (5), the thermoelectric generator (1) can be operated as a heat pump by absorbing an electrical power (P) from the outside. 6) Control method according to one of claims 1 to 5, wherein, when the temperature (THOT) of the hot side of the thermoelectric cell (5) is higher than the threshold value (TTRH), a feedback control is used in which the feedback variable is the temperature (THOT) of the hot side of the thermoelectric cell (5), therefore the error (ε) of the control is the difference between the temperature (THOT) of the hot side of the thermoelectric cell (5) and the value (TTRH) threshold, and the controlled variable as a function of the control error ε is the intensity of the electric current (I) flowing through the thermoelectric generator (1). 7) Control method according to one of claims 1 to 6, wherein: when the THOT temperature of the hot side of the thermoelectric cell (5) is lower than the threshold value (TTRH) the only goal is to maximize the electrical power (P) generated by the thermoelectric generator (1), i.e. the only goal is to always operate the control device (15) at the point (P1) where the generation of electrical power (P) is maximized; and when the temperature (THOT) of the hot side of the thermoelectric cell (5) is higher than the threshold value (TTRH), the main objective is to decrease the temperature (THOT) of the hot side of the thermoelectric cell (5). 8) Control method according to one of claims 1 to 7, wherein: the temperature (THOT) of the hot side of the thermoelectric cell (5) must not exceed a critical temperature (TCRT) beyond which damage can occur due to overheating of the thermoelectric cell (5) itself; and the threshold value (TTRH) is lower than the critical temperature (TCRT). 9) Control method according to one of claims 1 to 8, wherein a control device (15) is connected to two terminals (13) of the thermoelectric generator (1) which is adapted to vary an equivalent electrical resistance seen from the terminals ( 13) themselves to modify the working point in the voltage / current plane of the thermoelectric generator (1). 10) Control unit of a thermoelectric generator (1) for an internal combustion engine; the control unit is adapted to implement the control method according to one of claims 1 to 9.