Field of the invention

The present invention relates to an engine arrangement comprising a waste heat recovery system, especially in a vehicle.

Technological background

For many years, attempts have been made to improve the efficiency of internal combustion engines, which has a direct impact on fuel consumption.

For this purpose, an engine can be equipped with a waste heat recovery system, i.e. a system making use of a heat source produced by the vehicle operation, such as the hot exhaust gases which contain a lot of thermal energy that would otherwise be lost.

One example of a waste heat recovery system is a circuit in which a working fluid undergoes the following successive processes:

- the working fluid is pumped from low to high pressure;

- the high pressure working fluid is heated in at least one heat exchanger by means of said heat source;

- the working fluid is expanded in an expander.

As a result, at least part of the thermal energy of the heat source used to heat - and possibly evaporate - the working fluid is recovered in the expander, for example into mechanical energy or electricity.

One weakness of the current engine arrangements comprising a waste heat recovery system is that it is not always possible to immediately use the mechanical energy or electricity so recovered, typically because it may exceed the vehicle needs at the time it is produced by the waste heat recovery system. As a consequence, the energy which could potentially be recovered from the heat source is not always fully used.

Besides, in some operating conditions, the working fluid temperature may greatly increase. With organic fluids (such as ethanol or a refrigerant), which are commonly used as working fluids in such systems, this could lead to thermal stability problems and ultimately to the deterioration of the working fluid.

Another problem may exist when the waste heat recovery system is a closed loop system, for example operating according to the Rankine cycle. In such a system, the working fluid flows in a closed loop and undergoes successive processes according to the Rankine thermodynamic cycle:

- the working fluid, which is a liquid at this stage, is pumped from low to high pressure;

- the high pressure liquid is evaporated into a gas by a hot fluid flowing in another circuit of the engine arrangement;

- the gas is expanded in an expander;

- finally, the gas is condensed in a condenser.

Said condenser is arranged at the front part of the vehicle, so that the working fluid can be condensed by means of the ambient air moving through the condenser due to the vehicle motion.

However, in some operating conditions, the cold provided by ambient air moving through the condenser is not enough to cool and condense the working fluid. This can typically happen at high loads, and/or when outside air temperature is fairly high. In other words, one of the key bottlenecks of the closed loop cycles such as the Rankine cycle is the increased need of cooling capacity to allow condensing of the working fluid. At high engine loads or relatively high external temperatures, the free cooling capacity of the vehicle cooling package is not sufficient to cool both the engine and to condensate the working fluid.

As a consequence, the activation of the Rankine circuit would lead to an activation of the vehicle fan in order to provide additional cold to the condenser. This would have negative consequences on the overall energy consumption of the vehicle. Alternatively, if the vehicle fan has already been activated to cool down the engine, its cooling capacity may not be sufficient for cooling down both the engine cooling circuit and the waste heat recovery system. This may result in a working fluid overheating.

Moreover, for high variations in load and engine speed - like for example in urban environments - the necessary changes of cycle conditions lead to variations in components working conditions far away from optimal working points. It therefore appears that engine arrangements comprising a waste heat recovery system are not fully satisfactory and could be improved.

Summary

It is an object of the present invention to provide an improved engine arrangement comprising a waste heat recovery system which can overcome the drawbacks of the prior art engine arrangements.

It is another object of the invention to provide such an engine arrangement which makes it possible to more efficiently use the thermal energy from the heat source without impairing the engine arrangement overall efficiency nor damaging the working fluid.

According to the invention such an engine arrangement comprises:

- an internal combustion engine;

- a waste heat recovery system comprising a main line in which a working fluid is successively pumped, heated in at least one heat exchanger by means of a heat source produced by the engine operation, and expanded in an expander;

the waste heat recovery system further comprising a derivation line in which the working fluid can flow, said derivation line having an inlet and an outlet both connected to the main line downstream from the heat exchanger and upstream from the expander, said derivation line including a heat storage device which is arranged so as to be capable of storing heat from the heat source and of releasing previously stored heat in order to heat the working fluid.

Thus, in an engine arrangement according to the invention, the thermal energy coming - directly or indirectly - from the heat source can be stored at times when it cannot be used, or when its use could have negative consequences on the overall engine arrangement performance, especially in terms of fuel consumption. Additionally, said stored energy can be later used, when the operating conditions of the engine arrangement, and more generally of the vehicle, are more favourable for it an efficient use of the energy previously stored in the waste heat recovery system.

Furthermore, the invention makes it possible to protect the working fluid from overheating, which increases the service life of the waste heat recovery system and avoids the need to change the working fluid, which can be fairly expensive.

Moreover, the fan activation due to the operation of the waste heat recovery system can be avoided, which makes it possible not to impair the engine arrangement efficiency.

Owing to the heat storage device, the invention enables to take better advantage of the heat source, insofar as its thermal energy can be used over a wider range of operating conditions, either immediately or later, when the operating conditions allow heat to be released by the heat storage device. As a result, the engine arrangement overall efficiency can be increased on average over a given duty cycle.

In practice, all or part of the working fluid can flow in the derivation line, through the heat storage device, depending on the operating conditions and on the current needs. Thus, the heat storage device is capable of storing heat indirectly from the heat source, by means of the working fluid heated by the heat source in the heat exchanger.

The specific location of the heat storage device within the waste heat recovery system, namely downstream from the heat exchanger and upstream from the expander, brings significant benefits.

On the one hand, by providing a heat storage device located outside from the heat exchanger, the invention allows using said device only when necessary, which further improves the system efficiency.

On the other hand, providing a heat storage device located downstream from the heat exchanger makes it possible to greatly improve the engine arrangement efficiency by taking maximum advantage of the available thermal energy. Indeed, during the heat storage phase, the fluid entering the heat storage device, having passed through the heat exchanger, potentially has a high energy which makes heat storage more efficient. Moreover, during the heat release phase, in case the heat exchanger temperature is fairly low, heat can however be transferred to the working fluid by means of the heat storage device which is appropriately located downstream from said heat exchanger, and, at the same time, some heat can nevertheless be recuperated by from the heat source, even if the heat source is at a lower temperature than the heat storage device.

These and other features and advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of an engine arrangement according to the invention.

Brief description of the drawings

The following detailed description of several embodiments of the invention is better understood when read in conjunction with the appended drawings, it being however understood that the invention is not limited to the specific embodiments disclosed.

Figure 1 is a schematic representation of an engine arrangement comprising a waste heat recovery system according to an embodiment the invention;

Figure 2 is a schematic representation of a heat storage device according to an embodiment of the invention;

Figure 3 is a schematic representation of a heat storage device according to another embodiment of the invention;

Figure 4a is a schematic and partial representation of a waste heat recovery system according to a another embodiment of the invention;

Figures 4b-4f show the waste heat recovery system of Figure 4a in different operating phases;

Figure 5 is a schematic and partial representation of a waste heat recovery system according to another embodiment of the invention.

Detailed description of the invention

The internal combustion engine arrangement 1 according to the invention comprises an internal combustion engine 2, which can be a diesel engine or a spark ignition engine. The invention relates in particular, but not exclusively, to industrial vehicles. The arrangement may comprise a turbocharger 40 including a turbine 41 and a compressor 42.

Intake air is carried by an air intake line 3, it is compressed by the compressor 42 of the turbocharger 40 and passes through a charge air cooler 4 before it enters the engine 2. An exhaust line 5 is provided for collecting exhaust gases from said engine 2 and for directing them through the turbine 41 of the turbocharger 40, then towards various exhaust gases after treatment devices 6 before they are released into the atmosphere. In order to cool the engine 2, there is further provided a coolant circuit 7 carrying an engine coolant and including a radiator 8.

A fan 9 can be mounted on the engine shaft and may create an air flow in a channel through which outside air can be sucked rearwards from the front of the vehicle, the charge air cooler 4 and radiator 8 being arranged in said channel.

The engine arrangement 1 also comprises a waste heat recovery system 10 carrying a working fluid in a main line 21 in which said working fluid is successively pumped to a high pressure, heated in a heat exchanger by means of a heat source produced by the engine 2 operation, and expanded in an expander to a low pressure. As a result, at least part of the thermal energy of the heat source used to heat or evaporate the working fluid is recovered in the expander in the form of mechanical work which may be used as such and/or may be for example transformed into electricity by a generator driven by the expander.

In the illustrated embodiment, the waste heat recovery system 10 is of the closed loop type, for example of the Rankine type or of the Kalinka type. However, other types of waste heat recovery systems are possible, such as, for example, systems of the Brayton, Stirling or Ericsson type.

Thus, in this embodiment, the working fluid flows in a main line 21 forming a loop and undergoes successive processes according to the Rankine thermodynamic cycle. In the shown embodiment, the working fluid is distinct from the engine fluids such as the fuel, oil, coolant, etc. In the case of a Rankine cycle, the working fluid can be water or an organic fluid such as ethanol or such as Chloro-Fluoro-Carbons (CFCs), Hydro-Chloro-Fluoro- Carbons (HCFCs), Hydro-Fluoro-Carbons (HFCs) or hydro-carbons (HCs).

The main line 21 of the waste heat recovery system 10 comprises a heat exchanger 1 1 in which the working fluid can be heated by means of the heat source. The heat source may be constituted by a hot fluid flowing in a pipe. In the embodiments shown in the figures, the heat source comprises the exhaust gases flowing from the engine 2 in the exhaust line 5; for example, the heat exchanger 1 1 can be located downstream from the exhaust gases after treatment devices 6. However, other implementations could be envisaged.

The system could comprise several heat exchangers, provided in series and/or in parallel, for heating the working fluid using the heat of a common heat source or of different heat sources. The heat source(s) can derive their heat from the internal combustion engine or from another heat generating device in the engine arrangement.

For example, the working fluid can be directly heated by said exhaust gases passing through the heat exchanger 1 1 . This heat exchanger 1 1 can be a boiler in which the working fluid flowing in the system 10 is at least partly but preferably totally evaporated by the hot exhaust gases.

Downstream from the heat exchanger 1 1 , the working fluid flows through an expander 12. The expander 12 is capable of recovering part of the energy of the hot working fluid and of transforming it into mechanical energy. It can be for example a turbine, or a screw expander or a piston expander.

In the case of a Rankine circuit, downstream from the expander 12, the working fluid, which has been expanded and thereby partially cooled, can flow towards a condenser 13 in which it becomes a liquid again. It has to be noted that the engine arrangement 1 is schematically depicted in Figure 1 and may not show the concrete implementation of the various components of the arrangement 1 . More precisely, the condenser 13 may be a direct air cooled condenser and would then preferably be located at the front part of the vehicle, in the air channel that can be created by the fan 9 and/or by the vehicle displacement. As a variant, the condenser 13 could be an indirectly cooled condenser where an intermediate fluid circuit is used to cool the condenser.

Downstream from the condenser 13, the working fluid - as a liquid

- will be pumped by a pump 14 before entering the heat exchanger 1 1 . In the pump 14, the working fluid is pumped from low to high pressure, and then directed towards the heat exchanger 1 1.

A reservoir 15 for the working fluid can further be provided, for example between the condenser 13 and the pump 14.

According to the invention, the waste heat recovery system 10 further comprises a derivation line 22 in which the working fluid can flow, and which includes a heat storage device 20.

As shown in figure 1 , the derivation line 22 has an inlet and an outlet both connected to the main line 21 downstream from the heat exchanger

1 1 and upstream from the expander 12.

A first three-way valve 23 and a second three-way valve 24 can be arranged at the connecting points between the main line 21 and the derivation line 22, i.e. at the inlet and outlet of the derivation line 22 respectively, so that the sub flow rate of working fluid flowing through the heat storage device 20 may be varied compared to the whole working fluid flow rate in the main line 21 , For example, the sub flow rate of working fluid flowing through the heat storage device 20 may be controlled between 0% and 100% of the whole working fluid flow rate in the main line 21 . Alternatively, a single valve arrangement could be provided for directing the flow either in the main line 15 and/or in the derivation line 22, for example with a three way valve at either of the connecting points or with a two way throttle valve in the main line between the connecting points.

Thus, the heat storage device 20 is arranged outside of the heat exchanger 1 , downstream from the heat exchanger 1 1 and upstream of the expander 12 relative to the working fluid flow direction.

The heat storage device 20 is arranged so as to be capable of storing heat from the heat source and of releasing previously stored heat in order to heat the working fluid. It therefore makes it possible to use more energy from the heat source than in the prior art, either immediately or later, depending on the needs and on the engine arrangement operating conditions.

Also, since the heat storage device is located in a derivation of the main line, it can be at least partially, preferably totally, by-passed by the working fluid under some operating conditions. Therefore, it is possible, for those operating conditions, to avoid at least partially, preferably totally, any interference of the heat storage device on the waste heat recovery cycle.

Moreover, when the waste heat recovery system 10 is activated, which in particular means that the working fluid is heated by the heat source in the heat exchanger 1 1 , the heat storage device 20 may be used or not, depending on the needs, insofar as it is located externally of the heat exchanger 1 1. The heat storage device 20 can therefore be used only when necessary, e.g. under highly transient engine work conditions, or when heat recuperation and mechanical energy recovery by the Rankine cycle is not possible due to high heat rejection demands of the engine, or when the vehicle cannot make use of the energy recovered by the Rankine cycle, for driveability or general control reasons.

In practice, since all or part of the working fluid can flow in the derivation line 22, through the heat storage device 20, the heat storage device 20 is capable of storing heat indirectly from the heat source, by means of the working fluid previously heated by the heat source in the heat exchanger 1 1. In other words, the heat storage device 20 can draw heat from the working fluid, with said working fluid flowing successively through the heat exchanger 1 1 , where it receives heat from the heat source, and then through the heat storage device 20. As a result, the heat storage device 20 through which flows the working fluid can, in a first operating period, store heat from the working fluid which has been heated by the heat source, and, during a second operating period, can directly release heat to the working fluid.

Additionally, as this is schematically illustrated in Figure 1 , the heat storage device 20 can be further arranged so as to be capable of storing heat directly from the heat source. In other words, the heat storage device 20 can be directly thermally connected to the heat source, without any intermediate heat transfer fluid. For example, when the heat source is constituted by a hot fluid, such as exhaust gases, said hot fluid can flow through the heat storage device 20.

In practice, several types of heat storage devices can be used, either alone or in combination.

In an embodiment, hereafter referred to as "physical heat storage", the heat storage device 20 can comprise a tank 20a in which part of the working fluid can be stored, as illustrated in figure 2. The tank 20a is preferably insulated, in order to limit heat exchanges with the environment, and to preferably keep the temperature level substantially constant during the loading process of the tank 20a. The tank 20a can have a fixed or variable volume, said volume setting the amount of heat that can be stored. This solution is advantageous in that it is very simple and relatively inexpensive. In particular, no additional heat exchanger is required. With such an implementation, the working fluid mass can be increased.

In another embodiment, as very schematically illustrated in figure 3, the heat storage device 20 can comprise a heat storage material 30 which is in thermal contact with the working fluid 31 through a partition wall 32.

In other words, the working fluid flowing in the circuit - for example the Rankine circuit - is not in physical contact with the heat storage material and cannot be mixed with it. Although the working fluid 31 and the heat storage material 30 could be the same material, they remain separate by means of the partition wall 32 which acts as a wall for transferring heat from the working fluid 31 to the heat storage material 30 and vice versa. In this implementation, the heat storage material cannot flow through the pump 14, heat exchanger 1 1 and expander 12. The partition wall 32 can be the wall of a pipe inside which the working fluid 31 can flow through the container, thereby being in thermal contact with the heat storage material 30. Preferably, the container 20b is insulated.

In an implementation of such heat storage device with a partition wall according to Figure 3, hereafter referred to as "sensible heat storage", the heat storage device 20 can comprise a container 20b containing a heat storage material which does not undergo a phase change when heat is transferred from the heat source to said heat storage material or when said heat storage material transfers heat to the working fluid. In other words, in the different operating conditions, said heat storage material can store heat by being heated, or release heat to the working fluid, without changing phase, by storing heat by an amount proportional to the material's heat capacity.

For example, said heat storage material 30 can be a liquid or a gas, and the working fluid 31 can flow through the container inside a pipe the peripheral wall of which forms said partition wall 32. Preferably, the container is also insulated. With this implementation, heat transfer rates can be high, leading to a high recovery efficiency.

In another implementation of a heat storage device with a partition wall according to Figure 3, hereafter referred to as "latent heat storage", the heat storage device 20 can comprise a container 20b containing a phase- change material, i.e. a heat storage material 30 which can undergo a phase change when heat is transferred from the heat source to said heat storage material 30 or when said heat storage material 30 transfers heat to the working fluid 31. In practice, said heat storage material 30 is chosen to ensure its melting or solidifying temperature is reached in the operative conditions of the engine arrangement 1.

Such a material stores or releases thermal energy when melting or solidifying. The corresponding amount of energy is large because the device makes use of the latent heat of fusion, which is generally much greater than the specific heat capacity of a material. The main advantages of this implementation are therefore the high thermal efficiency and also the high heat transfer rate during phase change.

Examples of phase-change materials contain paraffins, and/or inorganic salts like NaOH, KOH, LiOH, NaN0 ₂ and/or some metals like Sn or Pb. For example, the melting temperature can be around 200°C. The highest possible latent heat would make it possible to gain weight.

The partition wall 32 can be the wall of a pipe inside which the working fluid 31 can flow through the container, thereby being in thermal contact with the heat storage material 30. Preferably, the container 20b is insulated.

In still another implementation, hereafter referred to as "chemical heat storage", the heat storage device 20 can comprise a unit 20b including a sorbent and a gas or liquid which can be absorbed or adsorbed by said sorbent, the desorption being endothermic and the absorption or adsorption being exothermic. Here, the sorbent is the heat storage material 30, and the partition wall 30 can be formed by a wall of the unit or of a pipe inside which the working fluid 31 can flow through the unit.

The unit can typically comprise a first chamber including the sorbent and a second chamber, distinct from the first one, for storing the gas or liquid. The chambers can be connected or not, depending on the operating phase.

For example, the sorbent can be a salt, which is capable of absorbing or adsorbing water. Providing heat to the device leads to the desorption of water from the sorbent. The water steam can then be recovered in the second chamber, where it is separated from the sorbent. In this second chamber, water changes phase and pressure by condensation while condensation heat is rejected. When heat release is required, the condensed water is contacted with the salt again, which entails heat rejection.

Alternatively, the sorbent can be a zeolite, a silicon gel or a metal hydride, the material which can be absorbed or adsorbed by said sorbent then being H ₂ .

Such a device is advantageous in that it has a high heat storage capacity and further allows a longer storage period. Moreover, this device has a good performance under steady state conditions. Furthermore, no insulation of the unit is necessary.

The waste heat recovery system 10 may further comprise an expansion valve 33 arranged in parallel with the expander 12 in an expander by-pass line 34 which can branch from the main line 21 of the waste heat recovery system 10 downstream from the heat storage device 20. A valve 35 is arranged at the connecting point between the main line 21 and the expander by-pass line 34, to control the flow of working fluid in said expander by-pass line 34.

It has to be noted that the arrangement of the heat storage device 20 of Figures 1 to 3 is schematic and meant to illustrate the invention in a general way, and shall not be considered as limitative.

In practice, several implementations of the engine arrangement 1 - and more specifically of the waste heat recovery system 10 - could be envisaged, in particular the two embodiments which will now be described with reference to Figures 4a to 5. It has to be noted that, in the Figures, the heat source comprises the exhaust gases flowing from the engine 2 in the exhaust line 5. However, this implementation shall not be considered as limitative, and other heat sources produced by the engine operation could be used, alone or in combination, including EGR gases, engine oil, engine cooling fluid or any other heat carrying fluid of the engine arrangement. Besides, the waste heat recovery system 10 which is shown on Figures 4a to 5 can be a of the Rankine cycle, as illustrated schematically, or of any other appropriate cycle type.

Reference is first made to Figure 4a which shows a first embodiment of the invention.

In this embodiment, the heat exchanger 1 1 is arranged both on an exhaust derivation line 16 of the exhaust line 5 and on the main line 21 of the waste heat recovery system 10, meaning that it is designed to promote heat transfer between the fluids circulating respectively in the exhaust derivation line 16 and in the main line 21. A first valve 17 and a second valve 18 are arranged at the connecting points between the exhaust line 5 and the exhaust derivation line 16 so as to adjust the exhaust gases flow rate through the heat exchanger 1 1 according to the needs. A single valve arrangement could be provided, either at one of the connecting points or in the exhaust line 5

Besides, the heat storage device 20 is arranged in the derivation line 22 of the waste heat recovery system 10, downstream from the heat exchanger 1 1 and upstream from the expander 12, so as to be capable of storing heat indirectly from the heat source, by means of the working fluid heated by the heat source in the heat exchanger 1 1 .

Therefore, the working fluid can flow through the heat storage device 20 when valve 23 allows it. The flow rate in the derivation line 22 can be the full flow rate of working fluid, or part of it, or can be null (the heat storage device 20 then being by-passed), depending on the needs and on the engine operating conditions. The second valve 24 can also be used as the valve 35 at the connection point with the expander by-pass line 34.

In concrete terms, the working fluid can be heated - or even evaporated - by the exhaust gases in the heat exchanger 1 1.

Depending on the engine arrangement operating conditions, heat storage can be activated or not, and the waste heat recovery system 10 can work or not, with the meaning that the waste heat recovery system 10 can effectively deliver mechanical work from its expander 12, or not.

Different operating phases of the arrangement of Figure 4a will now be described with reference to figures 4b - 4f. A cross drawn on a line means that no fluid can flow in said line.

If the waste heat recovery system 0 can work, part of the exhaust gases flows in the derivation line 16 of the exhaust line 5 through the heat exchanger 1 1 , thereby evaporating the working fluid. The working fluid is subsequently expanded in the expander 12, thereby enabling energy recovery, before continuing the cycle until entering again the heat exchanger 1 1 . The valve 24 is preferably set so that no working fluid flows in the expander by-pass line 34.

In case the heat storage device 20 has not reached its maximum heat storage capacity and the working fluid temperature or enthalpy at the heat exchanger outlet is high enough, especially higher that within the heat storage device, the waste heat recovery system 10 can be operated as shown in figure 4b. In this operating phase, part of the working fluid may be directed to the derivation line 22 and may therefore flow towards or through the heat storage device 20. As a result, the heat storage device 20 can store heat from the working fluid which has been previously heated by the exhaust gases in the heat exchanger 1. Typically, the working fluid coming out of the heat storage device, if any, will have an enthalpy lower that the working fluid coming into the heat storage device. This operating mode can be used particularly when the cooling package does not allow rejecting all the available energy from the heat source. This would typically reduce the load on the condenser.

In case a physical heat storage is used, the heated or evaporated working fluid is stored in the heat storage device 20. During this filling phase of the tank 20a with working fluid in gaseous phase, the pressure level inside the tank 20a rises until it reaches the operational pressure of the working fluid. In case a sensible, latent or chemical heat storage is used, the working fluid flows through the heat storage device 20 and is thereby cooled down. Therefore, this operating phase can be used preferably if the working fluid temperature at the heat exchanger outlet is high enough, otherwise, the working fluid at the expander inlet may not have enough energy to ensure the expander 12 operates efficiently.

Alternatively, if the working fluid temperature at the heat exchanger outlet is too low to allow heat to be stored in the heat storage device 20, and if heat has been stored in the heat storage device 20, the waste heat recovery system 10 can be operated as shown in figure 4c. This mode of operation can be useful when the available enthalpy in the heat storage device is superior to that available in the working fluid and when the cooling package is able to reject the corresponding heat.

In this operating phase, in case a sensible, latent or chemical heat storage is used, all the working fluid flow may be directed towards the heat storage device 20. Thus, the working fluid recovers heat from both the heat exchanger 11 and, subsequently, the heat storage device 20, before it enters the expander 12.

If a physical heat storage is used, this operating phase is a discharge phase, during which the pressure level in the tank 20a is controlled by the working fluid flow through the tank 20a, by means of valves 23, 24, so that the pressure at the expander inlet is not too low. This requires that the working fluid going in the heat storage device has a higher pressure level than the pressure level inside the heat storage device. On the other hand, the temperature level of working fluid inside the heat storage device will be higher than that of the incoming fluid, and the mixture can form an expandable vapour. On the other hand it could be provided that at least part of the working fluid coming from the heat exchanger can flow in the main line, by-passing the heat storage device 20.

In some cases, the heat storage device 20 has to be by-passed, as shown in figure 4d. This operating phase may be used for example if the exhaust gases temperature is not high enough to both heat the working fluid in the heat exchanger 11 and allow heat to be stored in the heat storage device 20. It can also be used if the heat storage 20 device has reached its maximum capacity - or its maximum pressure level and the additional working fluid flow from the tank 20a cannot be condensed - or if the heat from the heat storage device 20 cannot be used, for example because it would lead to the working fluid being overheated. Then, the heat storage device 20 may be completely isolated from the main line 21 of the Rankine cycle.

If the waste heat recovery system 10 cannot work, for example for lack of available cooling power at the condenser 13, the expander 12 is preferably by-passed, all the working fluid, when circulating, flowing in the expander by-pass line 34 through the expansion valve 33.

If the heat storage 20 device has not reached its maximum capacity, the waste heat recovery system 10 may be operated as shown in figure 4e. In this operating phase, heat is recuperated from the exhaust gases by the working fluid in the heat exchanger 1 1 and, at least in part, transferred to the heat storage device 20.

In case a physical heat storage is used, the derivation line 22 may be closed at the outlet of the heat storage device, as shown in figure 4e, the tank 20a being loaded with working fluid.

On the other hand, in case a sensible, latent or chemical heat storage is used, the working fluid can be cooled down by flowing through the heat storage device 20 and may then condensate at high pressure. The working fluid may then be redirected to the reservoir 15 by passing the expansion valve 33 and the condenser 13 which can re-condensate the rest of the working fluid. In this implementation, some fluid may be allowed to flow in the derivation line 22.

When the heat storage device 20 cannot store any additional heat, or when the loading of the tank 20a has been completed, the waste heat recovery system 10 can be operated as shown in figure 4f: no exhaust gases flow through the heat exchanger 1 , in order to avoid a further increasing in the working fluid temperature; the heat storage device 20 is by-passed. Additionally, the pump may be turned off.

Reference is now made to Figure 5 which shows another embodiment of the invention.

The embodiment of Figure 5 is similar to the embodiment of Figure 4a, the exhaust line 5 being further provided with an additional derivation line 36 equipped with a first valve 37 and a second valve 38. In this second embodiment, the heat storage device 20 is arranged both on the derivation line 22 of the waste heat recovery system 10 and on said additional derivation line 36 of the exhaust line 5. The heat storage device 20 can therefore store heat (i) directly from the heat source and (ii) indirectly from the previously heated working fluid, as explained with respect to the first embodiment.

The operating phases of the arrangement of Figure 5 are similar to the one described with reference to figures 4b-4f.

Furthermore, in case the heat storage device 20 cannot store any additional heat, or the loading of the tank 20a has been completed, the system may be operated so that no exhaust gases flow through the heat exchanger 1 1 and, in addition, no exhaust gases flow through the heat storage device 20, in order to avoid a further increasing in the working fluid temperature.

Generally speaking, whatever the embodiment of the invention, when the waste heat recovery system 10 cannot be used, or can only be partially used, especially because this would lead to an activation of the fan 9 or the fan 9 would not be able to provide enough cooling effect for both the engine 2 and the condenser 13 of the waste heat recovery system 10, then heat storage is preferably activated.

The invention therefore makes it possible both to prevent the working fluid from overheating and to store heat even if the waste heat recovery system 0 is not working. The thermal energy of the heat source can thus be recovered even if it is not used immediately. Preferably, the invention provides a short or medium term energy storage device. Typically, it is contemplated that the heat storage device should be able to store an amount of energy enough to run the expander 12 during a period of time of at least 10 seconds, preferably of at least 30 seconds and most preferably of at least one minute.

In the case where a physical heat storage device is used, a 30 seconds capacity can be achieved by storing working fluid in a volume which may approximately range between:

Another advantage of the invention lies in the fact that the admission conditions of the expander 12 can be regulated even for very transient heat source conditions. Besides, the waste energy recovery can be more efficient and partly decoupled from the waste energy usage.

The invention is of course not limited to the embodiments described above as examples or depicted in the appended Figures, but encompasses all technical equivalents and alternatives of the means described as well as combinations thereof. For example, while many heat exchangers in the Figures are shown as parallel flow heat exchangers, some or all of the exchangers could be of the cross-flow or counter-flow type.