Abstract:
The present invention relates to a method of controlling a closed loop ( 10 ) performing a Rankine cycle for a motor vehicle, said loop comprising a circulation and compression pump ( 12 ) for a working fluid, a heat exchanger ( 14 ) swept by a hot source ( 16 ) for heating said working fluid, expansion means ( 22 ) for expanding the hot fluid and a cooling exchanger ( 26 ) swept by a cooling fluid ( 28 ) for cooling this working fluid. 
     According to the invention, the method consists, after detecting a vehicle accident situation, in communicating the inside of the loop with the ambient air.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of controlling a closed loop performing a Rankine cycle and to a loop using same. 
         [0002]    As it is well known, a Rankine cycle is a thermodynamic cycle through which heat coming from an external heat source is transmitted to a closed loop that contains a fluid. 
       BACKGROUND OF THE INVENTION 
       [0003]    There are many types of Rankine cycle loops and more particularly those involving a (liquid/vapour) phase change of a working fluid. 
         [0004]    This type of cycle generally consists of a stage wherein the working fluid used in liquid form is compressed in an isentropic manner, followed by a stage where this compressed liquid fluid is heated and vaporized on contact with a source of heat. 
         [0005]    This vapour is then expanded, in another stage, in an isentropic manner in an expansion machine, then, in a last stage, this expanded vapour is cooled and condensed on contact with a cold source. 
         [0006]    To carry out these various stages, the loop comprises a compressor pump for circulating and compressing the fluid in liquid form, an evaporator that is swept by a hot fluid for at least partial vaporization of the compressed fluid, an expansion machine for expanding the vapour, such as a turbine that converts the energy of this vapour to another energy such as a mechanical or electric energy, and a condenser by means of which the heat contained in the vapour is yielded to a cold source, generally outside air that sweeps this condenser so as to convert this vapour to a fluid in liquid form. 
         [0007]    In this type of loop, the fluid used is generally water but other types of fluid can also be used, for example organic fluids or organic fluid mixtures. 
         [0008]    By way of example, these organic fluids can be butane, ethanol, hydrofluorocarbons, ammonia, carbon dioxide, etc. 
         [0009]    It is also well known, notably through document FR-2,884,555, to use the calorific energy conveyed by the exhaust gas of internal-combustion engines, in particular those used for motor vehicles, as the hot source providing heating and vaporization of the fluid flowing through the evaporator. 
         [0010]    This allows to improve the energy efficiency of this engine by recovering a large part of the energy lost at the exhaust in order to convert it to an energy that can be used for the motor vehicle through the Rankine cycle loop. 
         [0011]    In some instances, the working fluid is caused to circulate at high pressures (up to about 40 bars or even 80 bars) and very high temperatures, close to 400° C. These fluid pressures and temperatures are maintained within the operating range for which the system is dimensioned by means of a control system that acts notably upon the pump, the expansion machine and on the actuators driving the various elements of the loop. 
         [0012]    Furthermore, when using certain working fluids, this fluid may also appear to be intrinsically dangerous, notably flammable. 
         [0013]    Thus, in case of an abnormal situation of the vehicle, an accident for example, the system controlling the Rankine cycle loop can become inoperative, either as a result of a malfunction of the control system itself or of a malfunction of the sensors or the driving actuators. 
         [0014]    Due to the thermal inertia of the hot source or after a vehicle fire that may occur during an accident, the pressure and the temperature of the working fluid can continue to increase. In this case, temperature or pressure levels of this fluid incompatible with the dimensioning of the Rankine system can be reached. These levels can lead to a sudden break in one or more elements of the loop (fluid circulation line, exchanger, etc.), thus causing yet another accident. 
         [0015]    The present invention aims to overcome the aforementioned drawbacks by means of a method allowing to take preventive measures so as to limit or even to prevent a closed loop break. 
       SUMMARY OF THE INVENTION 
       [0016]    The present invention therefore relates to a method of controlling a closed loop performing a Rankine cycle for a motor vehicle, said loop comprising a circulation and compression pump for a working fluid, a heat exchanger swept by a hot source for heating said working fluid, expansion means for expanding the hot fluid and a cooling exchanger swept by a cooling fluid for cooling this working fluid, characterized in that it consists, after detecting a vehicle accident situation, in communicating the inside of the loop with the ambient air. 
         [0017]    The method can consist in communicating the loop with the ambient air after detecting violent impacts on the vehicle. 
         [0018]    The method can consist in communicating the loop with the ambient air after detecting a car fire. 
         [0019]    The method can consist in controlling throttling means for an opening provided on the loop for communicating said loop with the ambient air. 
         [0020]    The invention also relates to a closed loop performing a Rankine cycle for a motor vehicle comprising a circulation and compression pump for a working fluid, a heat exchanger swept by a hot source for heating said working fluid, expansion means for expanding the hot fluid, a cooling exchanger swept by a cooling fluid for cooling this working fluid and circulation lines for said fluid, characterized in that it comprises a device for communicating the inside of the loop with the ambient air, said device being operative in a vehicle accident situation. 
         [0021]    The device can comprise an opening located on at least one element of the loop and throttling means controlling this opening. 
         [0022]    The opening can be provided on at least one of the exchangers and/or on at least one of the circulation lines. 
         [0023]    The throttling means can comprise a valve switching between an extreme position closing the opening and another extreme position of clearing the opening. 
         [0024]    The hot source can come from the exhaust gas of an internal-combustion engine, 
       BRIEF DESCRIPTION OF THE SOLE FIGURE 
       [0025]    Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non limitative example, with reference to the sole FIGURE showing a closed loop performing a Rankine cycle with its control system according to the invention. 
     
    
     DETAILED DESCRIPTION 
       [0026]    The present description relates more particularly to a closed loop with a phase change fluid, but any other Rankine cycle loops, such as those referred to as supercritical fluid loops (with CO 2  for example), can be used. 
         [0027]    In  FIG. 1 , Rankine cycle closed loop  10  comprises a circulation and compression means  12  for a working fluid, water here, circulating clockwise (arrows A) in this loop. This means, referred to as pump in the rest of the description, allows to compress this water between the pump inlet and its outlet where this water, still in liquid form, is at high pressure. 
         [0028]    This pump is advantageously driven in rotation by any known means such as an electric motor (not shown). 
         [0029]    This loop also comprises a heat exchanger  14 , referred to as evaporator, traversed by the compressed water coming from the pump that flows out of this evaporator in form of hot compressed vapour. 
         [0030]    This evaporator is swept by a hot source  16  coming from the exhaust gas circulating in exhaust line  18  of an internal-combustion engine  20 . 
         [0031]    Preferably, this engine is an internal-combustion engine of a motor vehicle. 
         [0032]    This loop also comprises a receiving expansion machine  22  receiving at the intake thereof the high-pressure compressed water vapour, from where the water vapour flows out of this expander in form of low-pressure expanded vapour. 
         [0033]    By way of example, this expansion machine is an expansion turbine whose rotor (not shown) is driven in rotation by the water vapour. This rotor is advantageously connected to any known device allowing to use the mechanical energy recovered, for example to a transmission system of a vehicle driving the wheels, or to convert the mechanical energy recovered to another energy, such as an electric generator  24  for example. 
         [0034]    The loop also comprises a cooling exchanger  26 , referred to as condenser in the rest of the description. This condenser allows to convert the expanded low-pressure vapour coming from the turbine to water in liquid form after passing through this condenser. 
         [0035]    By way of example, this condenser consists here of an assembly of cooling tubes and fins swept by a cooling fluid  28  that flows through the condenser between its inlet face and its outlet face while cooling and condensing the expanded vapour. 
         [0036]    This cooling fluid is here outside air at ambient temperature, but any other cooling fluid such as water can be used for condensing the vapour. 
         [0037]    The various elements of the loop are connected to one another by fluid circulation lines  30 ,  32 ,  34 ,  36  allowing to connect successively the pump to the evaporator (line  30 ), the evaporator to the turbine (line  32 ), the turbine to the condenser (line  34 ) and the condenser to the pump (line  36 ) so that the working fluid circulates, in liquid or vapour form, in the direction shown by arrows A. 
         [0038]    As it is widely known, this loop is connected to a control system  38  allowing management thereof. Notably, this system receives information on the operation of this loop through lines  40 . More particularly, some of these lines receive information from various detectors provided in this loop, such as the pressure or the temperature of the water (or of the water vapour). From the information received, system  38  controls elements of the loop through control lines  42  necessary to obtain the desired operating range. These control lines notably allow to act upon pump  12  and turbine  22 . 
         [0039]    The loop illustrated in the sole figure also comprises a closed loop venting device  44  allowing partial or total preventive discharge of the fluid contained in this loop. 
         [0040]    This device comprises an opening  46  provided on one of the circulation lines, here circulation line  30  provided between pump  12  and evaporator  14 . 
         [0041]    Without departing from the scope of the invention, this opening can also be provided on one or more elements of the loop such as the heating and/or cooling exchangers, the pump, the turbine. 
         [0042]    Throttling means are arranged on this opening so as to seal or clear it. 
         [0043]    Advantageously, these throttling means include a valve  48 , preferably bistable, which tilts around a tilt axis  50 . Tilting of this valve is controlled by any known means, by way of example here an electric motor (not shown). 
         [0044]    Tilting of this valve is advantageously controlled by control lines  42  of control system  38  acting upon the electric tilt motor. 
         [0045]    This control system  38  also receives, through a line  40 , information from a vehicle abnormal situation management unit. 
         [0046]    By way of example, this unit can be an accident controller  52 , commonly referred to as crash line, provided to reduce the consequences in case of a car crash, such as a collision, a fire, a vehicle overturn, etc. 
         [0047]    This controller thus allows to carry out many preventive actions intended to reduce the consequences of this accident situation and in particular in case of a violent impact against a stationary or moving obstacle. 
         [0048]    More precisely, this controller comprises information lines  54  some of which are connected to detectors  56  such as shock detectors and/or sudden vehicle deceleration detectors. 
         [0049]    After receiving the information relative to this car crash situation, the controller transmits, through one or more control lines  58 , one or more signals for triggering vehicle safety elements. These elements can be air bags and/or seat belt pre-tensioners. 
         [0050]    These signals can also trigger vehicle power cut, hood raising so as to provide a shock-absorbing device, notably in case of a collision with a pedestrian, retraction of the wiper blades so as to prevent a pedestrian from being hurt, etc. 
         [0051]    After receiving the signal coming from the controller that has detected an accident situation, control system  38  also controls loop venting. 
         [0052]    This system controls, through a line  42 , the motor of valve  48  so that it switches from an initial closed position of opening  46 , as shown in thick line in the figure, to an open position of this opening  46  (shown in dotted line in the figure). 
         [0053]    Thus, the inside of the loop is communicated with the ambient air by venting it. 
         [0054]    Opening  46  is thus no longer sealed and the working fluid contained in loop  10  can be discharged, totally or partly, through this opening, either through gravity or under the effect of the pressure prevailing in the loop. 
         [0055]    This working fluid can of course be discharged in liquid or gas form in the direction of circulation of arrows A. This fluid can also be discharged in an opposite direction of circulation (arrow A′) if the fluid contained in the loop cannot circulate in the conventional direction, for example due to the obstruction of one or more circulation lines. 
         [0056]    This preventive discharge can thus prevent a loop break that might occur long after detecting this abnormal vehicle situation, an accident as it happens here. 
         [0057]    This helps avoid worsening of the situation, notably during the intervention of a rescue team operating on the car. 
         [0058]    Alternatively, valve  48  can also be controlled by one of lines  58  coming directly from accident controller  52 . 
         [0059]    Of course, this vehicle accident situation can concern any other circumstance such as a fire, in which case communication with the ambient air allows preventive emptying of the loop so as to avoid any sudden loop break. 
         [0060]    This fire can be detected by any known means such as fume detectors or temperature detectors connected to accident controller  52 . 
         [0061]    Without departing from the scope of the invention, opening  46  can also be arranged on other circulation lines and/or on one of the loop elements: exchangers, pump, turbine, etc.