US20120090293A1

US20120090293A1 - Exhaust line for a motor vehicle with a closed recovery cycle for exhaust gas heat energy, and associated control method

Info

Publication number: US20120090293A1
Application number: US13/259,564
Authority: US
Inventors: Edouard Barrieu
Original assignee: Edouard Barrieu
Current assignee: Faurecia Systemes dEchappement SAS
Priority date: 2009-03-25
Filing date: 2010-03-25
Publication date: 2012-04-19
Also published as: JP2012522920A; WO2010109145A2; WO2010109145A3; DE112010001357T5; FR2943731A1

Abstract

An exhaust line of a motor vehicle includes a heat exchanger having a first side for circulating the exhaust gases and a second side for circulating a heat exchange fluid, and a closed recovery cycle for one portion of the heat energy of the exhaust gases, in which said heat exchange fluid circulates, wherein the second side of the heat exchanger is inserted inside the closed cycle. The heat exchanger includes an intermediate wall interposed between the first and second sides of the heat exchanger. The intermediate wall comprises at least one closed cavity including a phase change material. The intermediate wall has a first exchange surface in thermal contact with the exhaust gases and a second exchange surface in thermal contact with the heat exchange fluid.

Description

RELATED APPLICATION

This application is a United States National Phase Application of PCT Application No. PCT/FR2010/050545, filed on Mar. 25, 2010, which claims priority to French Application No. 09 51920, filed on Mar. 25, 2009.

TECHNICAL FIELD

The invention generally relates to the recovery of energy in motor vehicle exhaust lines.

BACKGROUND OF THE INVENTION

An exhaust line of a motor vehicle comprises: a heat exchanger having a first side for circulation of exhaust gases and a second side for circulation of a heat exchange fluid; and a closed cycle for recovering a portion of the heat energy of the exhaust gases, in which said heat exchange fluid circulates, the second side of the heat exchanger being inserted into the closed cycle.
Such an exhaust line is known from US A-2006/0201153. In such an exhaust line, the means for recovering the heat energy of exhaust gases have no, or very little, thermal inertia.
The energy recovered by the closed cycle is almost directly proportional to the energy available in the exhaust gases. Now, a motor vehicle operates with a variable load, the power of the engine requested by the driver may vary for example between: 0 kW, when the engine is idling; 10 kW, when the vehicle is running at 40 km/h; and up to 100 kW during aggressive acceleration.
When little energy is available in the exhaust, i.e. when the power of the engine requested by the driver is low, there is a risk of unpriming the closed cycle for recovering heat energy. Re-priming of the closed cycle is long, which causes a significant loss for the heat energy recovery yield from the exhaust gases.

SUMMARY OF THE INVENTION

The invention proposes an exhaust line in which recovery of the heat energy from the exhaust gases is carried out with a better yield.
An exhaust line of the aforementioned type comprises a heat exchanger that includes an intermediate wall interposed between the first and second sides of the heat exchanger, the intermediate wall comprising at least one closed cavity having a material with phase changes. The wall has a first exchange surface in thermal contact with the exhaust gases and a second exchange surface in thermal contact with the heat exchange fluid.
The exhaust line may also have one or more of the features below, considered individually, or according to all the technically possible combinations:

- the intermediate wall comprises a plurality of closed cavities each having an amount of a phase change material, the closed cavities being isolated from each other;
- the phase change material comprises one or more inorganic salts, selected from the group comprising NaOH, KOH, LiOH, NaNO₂, NaNO₃, KNO₃, Ca(NO₃)₂, LiNO₃, KCl, LiCl, NaCl, MgCl₂, CaCl₂, Na₂CO₃, K₂CO₃, Li₂CO₃, KF, LiF;
- the phase change material comprises one or more metals selected from the group comprising Sn, Pb and Zn;
- the phase change material has a melting temperature comprised between 100° C. and 500° C.;
- the phase change material has a latent heat of fusion comprised between 100 and 300 kJ/kg;
- the intermediate wall comprises a mass of phase change material selected in order to store heat energy comprised between 0.1 kWh and 10 kWh;
- the closed cycle is a Rankine cycle or a Hirn cycle;
- the closed cycle comprises a driving shaft, and a unit for driving into rotation the driving shaft by the heat exchange fluid;
- the heat exchange fluid essentially comprises water;
- the closed cycle is dimensioned so that the heat exchange fluid has at the outlet of the heat exchanger a reference temperature, the phase change material having a melting temperature comprised between the reference temperature and the reference temperature plus 100° C.

The exhaust line comprises:

- an upstream conduit for circulation of the exhaust gases, and fluidically connected to an inlet of the first side of the heat exchanger;
- a downstream conduit for circulation of exhaust gases, and fluidically connected to the outlet of the first side of the heat exchanger;
- a bypass conduit connecting the upstream conduit to the downstream conduit by bypassing the heat exchanger,
- a member for orientation of the exhaust gases capable of orienting a fraction of the exhaust gases towards the heat exchanger and another fraction of the exhaust gases towards the bypass conduit,
- a member for controlling the orientation member, and capable of selectively controlling said fraction of the exhaust gases oriented towards the heat exchanger and said other fraction of the exhaust gases oriented towards the bypass conduit.

According to a second aspect, a method for controlling an exhaust line having the above features comprises the following steps:

- evaluating a first quantity representative of an amount of heat energy provided by the exhaust gases exiting a heat engine of the vehicle;
- acquiring at least a second quantity representative of a temperature of the heat exchange fluid in the closed cycle;
- evaluating a third quantity representative of an amount of heat energy stored in the phase change material of the intermediate wall;
- determining the fraction of the exhaust gases oriented towards the main conduit, at least using the first, second and third quantities; controlling the orientation unit according to the determined fraction.

The method may further have the characteristics below:

- evaluating a first quantity representative of an amount of heat energy provided by the exhaust gases exiting a heat engine of the vehicle;
- acquiring at least a second quantity representative of a temperature of the heat exchange fluid in the closed cycle;
- evaluating a third quantity representative of an amount of heat energy stored in the phase change material of the intermediate wall;
- determining the fraction of the gases oriented towards the exchanger using at least the first, second and third quantities;
- controlling the orientation unit depending on the determined fraction.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent from the detailed description which is given thereof below as an indication and only as a limitation, with reference to the appended figures, wherein:

FIG. 1 is a schematic illustration of an exhaust line according to the invention;

FIG. 2 is a schematic illustration of the heat exchanger of the exhaust line of FIG. 1;

FIG. 3 is a diagram of steps indicating the main steps of the method for controlling the exhaust line of FIG. 1; and

FIGS. 4 and 5 are graphic and schematic illustrations of curves indicating the fraction of the exhaust gas flow oriented towards the heat exchanger as a function of the available energy at the exhaust.

DETAILED DESCRIPTION

The exhaust line 1 illustrated in a simplified way in FIG. 1 includes:

- a manifold 3 provided for collecting the exhaust gases exiting from the combustion chambers of a heat engine 5 of the motor vehicle;
- a heat exchanger 7, having a first side for circulation of the exhaust gases and a second side for circulation of a heat exchange fluid;
- an upstream conduit 9 for circulation of the exhaust gases, and which connects the manifold 3 to an inlet 10 on the first side of the heat exchanger 7;
- a downstream conduit 11 for circulation of the exhaust gases, and which is connected to an outlet 12 of the first side of the heat exchanger 7;
- a bypass conduit 13 that connects a point of the upstream conduit 9 to a point of the downstream conduit 11 by bypassing the heat exchanger 7;
- a member 15 for selectively orienting the exhaust gases towards the heat exchanger 7 and/or towards the bypass conduit 13;
- a member 17 for controlling the orientation member 15 and;
- a closed cycle 19 for recovering a portion of the heat energy of the exhaust gases.

The bypass conduit 13 connects a T intersection 21 made in the upstream conduit 9 to another T intersection 23 made in the downstream conduit 11.
Non-illustrated equipment such as a turbocharger, may be interposed between the manifold 3 and the T intersection 21.
The downstream conduit 11 is fluidically connected to the discharge end (not shown) for the release of purified exhaust gases into the atmosphere. Other members not illustrated, such as a silencer and members for purifying the exhaust gases, are interposed between the T intersection 23 and the discharge end.
The closed cycle 19 for recovering a portion of the heat energy of the exhaust gases is, for example, a Rankine cycle.
The Rankine cycle includes a turbine 25, a condenser 27, and a pump 29. Tubing 31 connects an outlet 33 of the second side of the heat exchanger 7 to a high pressure inlet of the turbine 25.
Tubing 35 connects a low pressure outlet of the turbine 25 to an inlet of the condenser 27. Tubing 37 connects an outlet of the condenser 27 to a suction inlet of the pump 29. Finally, tubing 39 connects a discharge outlet of the pump 29 to an inlet 41 of the second side of the heat exchanger 7.
The turbine 25 drives a driving shaft 43 into rotation, the latter for example being connected to an electric generator 45. Alternatively, the shaft 43 may drive a mechanical member of the vehicle. Moreover, the turbine 25 may be replaced with a steam engine coupled with the driving shaft 43.
Typically, the heat fluid which circulates in the closed cycle 19 essentially comprises water. The fluid may include various additives, for example with a view to limiting corrosion or avoiding frost. The heat fluid is vaporized in the heat exchanger 7 under the effect of the heat given up by the exhaust gases. The heat fluid may also be an organic fluid adapted to a cycle such as the Rankine cycle. The fluid may for example be Genetron® 245FA, marketed by Honeywell.
As visible in FIG. 2, the heat exchanger 7 includes an intermediate wall 47 interposed between the first and second sides 49 and 51 of the heat exchanger.
The intermediate wall 47 includes a plurality of cavities 53 each having an amount of the phase change material. Typically the intermediate wall 47 essentially is a material which is a good heat conductor, for example aluminum, an aluminum alloy, or steel.
This cavity 53 is entirely closed and isolated from the other cavities 53. Thus, the phase change material in the cavity 53 is entirely isolated from the medium outside the cavity.
The cavities 53 are preferably uniformly distributed on the largest portion of the surface of the wall 47. Preferably the cavities 53 are uniformly distributed over the whole surface of the wall 47.
The intermediate wall 47 has a first exchange surface 55 in thermal contact with the exhaust gases, and a second exchange surface 57 in thermal contact with the heat exchange fluid.
The surfaces 55 and 57 form the two large opposite faces of the intermediate wall.
Typically, the first exchange surface 55 is in direct contact with the exhaust gases circulating on the first side of the heat exchanger.
The surface 55 partly delimits the first side of the heat exchanger.
Also, the second heat exchange surface 57 is preferably in direct contact with the heat exchange fluid circulating on the second side. The surface 57 partly delimits the second side of the heat exchanger.
Thus, the heat exchanger 7 is built to put the exhaust gases into thermal contact with the first heat exchange fluid, the exhaust gases releasing through the intermediate wall a portion of their heat energy to the heat exchange fluid.
The phase change material typically comprises one or more inorganic salts. These inorganic salts are selected from the group comprising NaOH, KOH, LiOH, NaNO₂, NaNO₃, KNO₃, Ca(NO₃)₂, LiNO₃, KCl, LiCl, NaCl, MgCl₂, CaCl₂, Na₂CO₃, K₂CO₃, Li₂CO₃, KF, LiF.
For example, the phase change material consists of one of these inorganic salts or of a mixture of two or three of these inorganic salts.
The phase change material may also comprise one or more metals, selected from Sn, Pb and Zn. Advantageously, said material consists of one or more metals selected from Sn, Pb and Zn.
The phase change material has a melting temperature comprised between 100° C. and 500° C., preferably between 150 and 400° C., and further preferably between 200 and 350° C.
The phase change material typically has a latent heat of fusion comprised between 100 and 300 kJ/kg, for example comprised between 150 and 250 kJ/kg.
For example, the phase change material is a binary salt comprising about 60% of NaNO₃, and 40% of KNO₃. Alternatively, the phase change material may be the salt commercially sold under the name of HitecXL, which is a ternary salt comprising about 48% of Ca(NO₃)₂, 7% of NaNO₃, and 45% of KNO₃.
The phase change material contained in the intermediate wall is provided for forming a buffered capacity of heat energy.
Thus, when the heat energy provided by the exhaust gases on the first side of the exchanger is greater than the energy which may be absorbed by the closed recovery cycle, a portion of the excess energy is stored in the phase change material of the intermediate wall. As indicated above, these materials have a latent heat of fusion which is relatively high, the excess energy allowing said phase change material to be melted. Conversely, when the energy provided by the exhaust gases to the first side of the exchanger is less than the energy recovered by the closed cycle, the phase change material gives back the stored heat energy, by solidification of the material melted beforehand.
The phase change material mass incorporated into the intermediate wall is selected to allow storage of a total heat energy comprised between 0.1 and 10 kWh, preferably between 0.5 kWh and 5 kWh.
For example, the mass is selected to allow storage of an energy comprised between 1 and 2 kWh.
Moreover, the Rankine cycle 19 has a yield versus temperature having the shape of a bell curve. The relevant temperature here is the temperature of the heat exchange fluid at the outlet of the heat exchanger 7. This yield is zero below the temperature of onset of vaporization of the heat exchange fluid. It increases up to a reference temperature Tref, for which the cycle is dimensioned. The phase change material is selected so that its melting temperature substantially corresponds to the optimum operating temperature of the Rankine cycle. For example, said melting temperature is comprised between Tref and Tref+100° C., preferably between Tref and Tref+50° C.
The orientation member 15 is a 3-way valve mounted on the T intersection 21. It is controlled by a computer 17. The 3-way valve may selectively orientate the totality of the exhaust gases towards the heat exchanger 7, orientate the totality of the exhaust gases towards the bypass conduit 13, or orientate a determined fraction of the exhaust gases towards the exchanger 7, and the remainder of the exhaust gases towards the bypass conduit. Said fraction is determined by the computer 17, as described below.
The exhaust line is moreover equipped with a sensor 47 for measuring the temperature of the exhaust gases and a sensor 49 for measuring the flow rate of the exhaust gases, for example implanted in the upstream conduit 9. These sensors inform the computer 17.
The exhaust line further comprises a sensor 51 for measuring the temperature of the heat exchange fluid, and a sensor 53 for measuring the pressure of said heat exchange fluid, implanted on the conduit 31 connecting the outlet 33 of the second side of the heat exchanger 7, to the turbine 25. These sensors inform the computer 17.
As indicated above, the engine of a motor vehicle operates with a variable load. When the engine is idling, the power requested by the driver is about 0 kW. When the vehicle is running at a speed of 40 km/h, the power requested by the driver is of about 10 kW. In the case of aggressive acceleration, the power requested for the engine may rise up to 100 kW. The closed cycle 19 is dimensioned for recovering a heat power of about 40 kW in the exhaust gases.
Thus, in the phases when the heat engine operates under a strong load, the available heat power in the exhaust gases exiting the engine is for example 60 kW. In this case, the closed cycle recovers about 40 kW of heat power in the exhaust gases, and a portion of the excess 20 kW is taken and stored by the phase change material. Conversely, in the phases when the heat engine operates with a small load, the available heat power in the exhaust gases is for example only 20 kW. The buffer capacity formed by the phase change material is then emptied, a portion of the heat energy stored in the phase change material being transferred to the heat exchange fluid of the closed cycle.
The control of the exhaust line described above will now be detailed with reference to FIG. 3. In step S1, the computer acquires the temperature T and the flow rate Q of the exhaust gas flow at the outlet of the manifold 3 via the sensors 47 and 49. From these values, the computer evaluates in step S2 the available heat energy Edispo in the exhaust gases and which may be recovered by the closed cycle 19 in the heat exchanger 7.
In step S3, the computer acquires from the sensors 51 and 53 the pressure P and temperature T of the heat exchange fluid of the Rankine cycle. From the acquired pressure and temperature values, the computer evaluates in step S4 the heat energy Econso actually received by the Rankine cycle. This heat energy is either converted into mechanical energy by the turbine or lost.
In step S5, the computer evaluates the load of the phase change material. The term “load” refers to the amount of heat energy stored in the phase change material at the current instant. This load may be expressed as a percentage of the total capacity of heat energy storage of the phase change material. The load may also be directly expressed as a stored energy. The load of the phase change material is calculated, for example, by periodically determining energy balances for the phase change material. The load of the phase change material at instant t+1 is equal to the load of the phase change material at the instant t increased by the energy actually released by the exhaust gases in the exchanger, minus the energy Econso actually received by the closed cycle. The energy actually released by the exhaust gases in the exchanger is evaluated by the computer as determined from the heat energy available in the exhaust gases Edispo, from the fraction of the exhaust gas flow oriented towards the exchanger of the pressure and of the temperature of the heat fluid in the closed cycle, acquired in step S3.
In step S6, the computer determines the fraction of the exhaust gas flow which has to be oriented towards the heat exchanger 7, according to the heat energy Edispo provided by the exhaust gases, evaluated in step S2, to the fluid temperature T of the heat exchange fluid of the closed cycle, acquired via the sensor 51, and/or to the load of the phase change material evaluated in step S5.
The fraction of the exhaust gas flow which is not oriented towards the heat exchanger 7 is oriented towards the bypass conduit.
The computer then controls the displacement of the plate of the 3-way valve 15 up to a position in which the exhaust gas flow is distributed towards the exchanger and towards the bypass conduit as determined in step S6. The position of the plate is read by the computer in tables or on predetermined curves at least depending on the exhaust gas flow rate exiting the collector and on the fraction of the exhaust gas flow to be oriented towards the exchanger.
The fraction of the exhaust gas flow oriented towards the heat exchanger is determined by the computer, for example, by using the graph of FIG. 4. In this figure, a network of curves is illustrated, parameterized according to the load of the phase change material. Each curve corresponds to a different load condition of the phase change material. Only three curves have been materialized here, but the memory of the computer may contain a much larger number of curves. The curve in solid lines corresponds to a load of 0%, and the two curves in dashed lines to a load of 50% and 100%. Each curve indicates the fraction of the exhaust gas flow oriented towards the exchanger, depending on the heat energy provided by the exhaust gases exiting the engine.
It emerges from FIG. 4 that, when the energy provided by the exhaust gases is less than a reference energy Eref, 100% of the exhaust gases are oriented towards the heat exchanger 7. Eref for example corresponds to the heat energy for which the closed recovery cycle is dimensioned. For example, Eref has the value 40 kW.
When the energy provided by the exhaust gases is greater than Eref, the fraction of the flow of exhaust gases oriented towards the heat exchanger is gradually decreased. The rapidity with which this fraction decreases depends on the load of the phase change material. When the load is 0%, the fraction decreases relatively more slowly. When the load is 50%, this fraction decreases faster and when the load is 100%, the fraction decreases even faster.
The portions of the curve beyond Eref have been illustrated as linear. However, these portions may have another shape and be arched or include arched portions.
The curves of FIG. 4 are in practice determined by simulation and/or experimentally.
Alternatively, the fraction of the exhaust gas flow oriented towards the exchanger may be determined in step S6 by the computer by means of the graph of FIG. 5.
This graph includes a network of curves parameterized according to the temperature of the heat fluid at the outlet of the exchanger. Each curve corresponds to a different acquired temperature for the heat exchange fluid of the closed cycle 19. The curve in solid lines corresponds to the temperature of the heat exchange fluid for which the closed cycle has been dimensioned. If the heat exchange fluid is water, this temperature may for example be 250° C. This temperature may also be significantly different from 250° C.
The curve in dot-dash lines corresponds to an acquired temperature below the reference temperature. The curve in dashed lines corresponds to an acquired temperature above the reference temperature. Only three curves are illustrated in FIG. 5, but it is possible to integrate a larger number of them into the memory of the computer.
As in FIG. 4, 100% of the exhaust gas flow is oriented towards the heat exchanger when the energy provided by the exhaust gases is less than a reference energy.
When the energy provided is greater than the reference energy, the fraction of the exhaust gas flow oriented towards the exchanger decreases gradually. This fraction decreases at an average rate when the acquired temperature corresponds to the reference temperature for the dimensioning of the closed cycle. This fraction decreases less rapidly when the acquired temperature is below the reference temperature. This fraction decreases more rapidly when the acquired temperature is above the reference temperature.
In another alternative not shown, the computer may determine the fraction of the exhaust gas flow oriented towards the exchanger using a network of curves parameterized according to both the load of the phase change material and to the acquired temperature of the heat exchange fluid.
The exhaust line described above has multiple advantages.
Because the heat exchanger includes an intermediate wall interposed between the first and second sides of the heat exchanger, the intermediate wall comprising at least one closed cavity containing a phase change material, the wall having a first exchange surface in thermal contact with the exhaust gases and a second exchange surface in thermal contact with the heat exchange fluid, the closed recovery cycle has significant thermal inertia, with which it is possible to damp the variations in the amount of heat energy provided by the exhaust gases exiting the engine.
A certain amount of heat energy may actually be stored inside the heat exchanger.
When the energy available at the exhaust is greater than the energy that the closed cycle may recover and put to use, the phase change material is loaded. This loading is achieved by melting the phase change material, the latter being converted from the solid state to a liquid state. Such a phase change absorbs a significant amount of heat energy corresponding to the latent heat for melting said material.
On the contrary, when the energy amount available at the exhaust is less than the power which the closed cycle may recover and put to use, the phase change material releases a fraction of the stored heat energy transferring a surplus of energy to the heat exchange fluid of the closed cycle.
The closed recovery cycle may thus continue to operate for a certain time, even if the energy provided by the exhaust gases is reduced.
On the other hand, if the energy provided by the exhaust gases is maintained on a long term basis at a low level, the closed cycle will be unprimed once the phase change material has entirely released its heat energy. This unpriming occurs when the heat exchange fluid flows out of the heat exchanger 7 at a temperature below its vaporization temperature. With the invention it is possible to postpone the moment of the unpriming of the closed cycle.
Moreover, the use of the phase change material forming a buffer of heat energy in the exchanger allows operation of the closed cycle as close as possible to its reference temperature. Indeed, the melting temperature of the phase change material is selected close to the reference temperature of the heat exchange fluid. When the temperature of the heat exchange fluid at the outlet of the heat exchanger decreases and becomes lower than the reference temperature, a portion of the heat energy stored in the phase change material is transferred to the heat exchange fluid. Conversely, when the temperature of the heat exchange fluid at the outlet of the exchanger is greater than the reference temperature, the phase change material will increase its load by taking a portion of the heat energy transferred from the exhaust gases to the heat exchange fluid. This mechanism may contribute to maintaining the heat exchange fluid close to its reference temperature, i.e. at a temperature at which the yield of the closed cycle is optimum.
It should be noted that the orientation member may be positioned at the T intersection located downstream from the heat exchanger. Moreover, this member may not be a 3-way valve but may comprise two proportional 2-way valves, one on the bypass conduit and the other in series with the exchanger in order to modulate the exhaust gas flow through the exchanger.
The closed energy recovery cycle may not be a Rankine cycle, but a Hirn cycle or any other suitable cycle.
It should be noted that the third quantity representative of the amount of heat energy stored in the phase change material of the intermediate wall is evaluated by: evaluating a fourth quantity representative of a heat energy amount actually received by the closed cycle in the heat exchanger; and inferring the third quantity at least from the first quantity and the fourth quantity.
Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.

Claims

1. An exhaust line of a motor vehicle comprising:

a heat exchanger having a first side for circulation of exhaust gases and a second side for circulation of a heat exchange fluid;

a closed cycle for recovering a portion of heat energy of the exhaust gases, in which said heat exchange fluid circulates, the second side of the heat exchanger being inserted into the closed cycle; and

wherein the heat exchanger includes an intermediate wall interposed between the first and second sides of the heat exchanger, the intermediate wall comprising at least one closed cavity having a phase change material, the intermediate wall having a first exchange surface in thermal contact with the exhaust gases and a second exchange surface in thermal contact with the heat exchange fluid.

2. The exhaust line according to claim 1, wherein the intermediate wall comprises a plurality of closed cavities each including an amount of a phase change material, the closed cavities being isolated from each other.

3. The exhaust line according to claim 1, wherein the phase change material comprises one or more inorganic salts, selected from the group comprising NaOH, KOH, LiOH, NaNO₂, NaNO₃, KNO₃, Ca(NO₃)₂, LiNO₃, KCl, LiCl, NaCl, MgCl₂, CaCl₂, Na₂CO₃, K₂CO₃, Li₂CO₃, KF, LiF

4. The exhaust line according to claim 1, wherein the phase change material comprises one or more metals selected from the group comprising Sn, Pb and Zn.

5. The exhaust line according to claim 1, wherein the phase change material has a melting temperature comprised between 100° C. and 500° C.

6. The exhaust line according to claim 1, wherein the phase change material has a latent heat of fusion comprised between 100 and 300 kJ/kg.

7. The exhaust line according to claim 1, wherein the intermediate wall comprises a mass of phase change material selected for allowing storage of heat energy comprised between 0.1 kWh and 10 kWh.

8. The exhaust line according to claim 1, wherein the closed cycle is a Rankine cycle or a Hirn cycle.

9. The exhaust line according to claim 1, wherein the closed cycle comprises a driving shaft and a member for driving into rotation the driving shaft by the heat exchange fluid.

10. The exhaust line according to claim 1, wherein the heat exchange fluid essentially comprises water.

11. The exhaust line according to claim 1, wherein the closed cycle is dimensioned so that the heat exchange fluid has a reference temperature at the outlet of the heat exchanger, the phase change material having a melting temperature comprised between the reference temperature and the reference temperature plus 100° C.

12. The exhaust line according to claim 1, including:

an upstream conduit for circulation of the exhaust gases, the upstream conduit being fluidically connected to an inlet of the first side of the heat exchanger;

a downstream conduit for circulation of the exhaust gases, the downstream conduit being fluidically connected to an outlet of the first side of the heat exchanger;

a bypass conduit connecting the upstream conduit to the downstream conduit by bypassing the heat exchanger,

a first member for orienting the exhaust gases, the first member being capable of orienting a fraction of the exhaust gases towards the heat exchanger and another fraction of the exhaust gases towards the bypass conduit, and

a second member that controls the orientation of the first member, the second member being capable of selectively controlling said fraction of exhaust gases oriented towards the heat exchanger and said other fraction of the exhaust gases oriented towards the bypass conduit.

13. A method for controlling an exhaust line, including the following steps:

providing

a closed cycle for recovering a portion of heat energy of the exhaust gases, in which said heat exchange fluid circulates, the second side of the heat exchanger being inserted into the closed cycle;

wherein the heat exchanger includes an intermediate wall interposed between the first and second sides of the heat exchanger, the intermediate wall comprising at least one closed cavity having a phase change material, the intermediate wall having a first exchange surface in thermal contact with the exhaust gases and a second exchange surface in thermal contact with the heat exchange fluid;

evaluating a first quantity representative of an amount of heat energy provided by the exhaust gases exiting a heat engine of the vehicle;

acquiring at least one second quantity representative of a temperature of the heat exchange fluid in closed cycle;

evaluating a third quantity representative of an amount of heat energy stored in the phase change material of the intermediate wall;

determining the fraction of the exhaust gases oriented towards the exchanger using at least the first, second and third quantities;

controlling the first member according to the determined fraction.

14. The method according to claim 13, wherein the third quantity representative of the amount of heat energy stored in the phase change material of the intermediate wall is evaluated by:

evaluating a fourth quantity representative of an amount of heat energy actually received by the closed cycle in the heat exchanger; and

inferring the third quantity at least from the first quantity and the fourth quantity.