Abstract:
The present invention relates to a method for controlling the temperature of exhaust gas circulating in an exhaust line ( 20 ) of an internal-combustion engine ( 10 ), said line comprising means ( 30 ) of treating the pollutants contained in this gas and heat exchange means ( 32 ) for cooling or heating said exhaust gas flowing through these pollutant treatment means. 
     According to the invention, the method consists in using evaporator ( 38 ) of a fluid circulation closed circuit ( 34 ) as heat exchange means ( 32 ).

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method and to a device for controlling the temperature of exhaust gas from an internal-combustion engine, notably of Diesel type, flowing through a means of treating the pollutants contained in this gas. 
       BACKGROUND OF THE INVENTION 
       [0002]    In order to comply with environmental standards and to meet the severization of these standards, such as the standards known as EURO VI, these pollutants, in particular the nitrogen oxides (NO and NO 2 ), more commonly referred to as NOx, contained in this exhaust gas, have to be treated prior to discharging it into the atmosphere. 
         [0003]    As it is generally known, pollutant treatment devices are therefore installed on the exhaust line of vehicles. 
         [0004]    This exhaust line comprises, from the exhaust manifold and in the direction of circulation of the exhaust gas along the line, a three-way catalyst, referred to as triple-effect catalyst, whose purpose is to treat, through oxidation, the unburnt hydrocarbons (HC) and the carbon monoxide (CO) contained in the exhaust gas, and an SCR (Selective Catalytic Reduction) catalyst for treating the NOx. 
         [0005]    This SCR catalyst allows to selectively reduce the NOx to nitrogen through the action of a reducing agent. This agent, which is generally injected upstream from the catalyst, can be a hydrocarbon, hydrogen, carbon monoxide, ammonia or a compound generating ammonia through decomposition, such as urea. 
         [0006]    The problem that arises with such a device is that the SCR catalyst has an operating temperature ranging from around 300° C. to around 500° C. 
         [0007]    Now, it is not always possible to finely control the exhaust gas temperature because it greatly depends on the engine operating point. Thus, for high engine loads notably, the exhaust gas temperature can widely exceed 500° C. This high temperature, on the one hand, does not allow the catalyst to fulfil its NOx reduction function and, on the other hand, it can lead to a degradation of the constituent material of this catalyst and/or of the catalytic phases it comprises. 
         [0008]    It is already known to arrange heat exchange means on the exhaust line and upstream from the SCR catalyst in order to cool the exhaust gas before it enters this catalyst. 
         [0009]    A cooler through which the engine cooling fluid flows is therefore arranged on the exhaust line. This cooler allows to absorb the heat contained in the exhaust gas and thus to control the temperature of this gas so that it does not exceed the upper value of the operating temperature range of this catalyst. 
         [0010]    This however notably involves the drawback of generating an increase in the temperature of the cooling fluid that is heated by the exhaust gas. Since this fluid is also used for cooling the engine and other accessories of this engine through an intercooler, it is therefore necessary to increase the capacity of this intercooler. In case of limited space for the intercooler, it is necessary to provide additional intercoolers so as to bring this cooling fluid to a required temperature. This leads to a quite significant cost increase and to complexity of the engine general cooling circuit. 
         [0011]    Furthermore, when the temperature of the gas is lower than that required to provide smooth operation of the catalyst, notably when starting the engine or after stopping this engine, the exhaust gas containing NOx is not treated by the catalyst and it is discharged into the atmosphere with the pollutants it contains. 
         [0012]    The present invention aims to overcome the aforementioned drawbacks by means of a simple and economical device allowing to have exhaust gas at the required temperature to provide depollution treatment of this gas, whatever the operating conditions of the internal-combustion engine. 
       SUMMARY OF THE INVENTION 
       [0013]    The present invention therefore relates to a method for controlling the temperature of exhaust gas circulating in an exhaust line of an internal-combustion engine, said line comprising means of treating the pollutants contained in this gas and heat exchange means for cooling or heating said exhaust gas flowing through these pollutant treatment means. 
         [0014]    The method can consist in feeding a hot fluid into the evaporator so that it heats the exhaust gas. 
         [0015]    The method can consist in using a hot fluid contained in a fluid storage means connected to the circuit. 
         [0016]    The method can consist in connecting the storage means to the evaporator by means of a bypass line. 
         [0017]    The invention also relates to a device for controlling the temperature of exhaust gas circulating in an exhaust line of an internal-combustion engine, said line comprising means of treating the pollutants contained in this gas and reversible heat exchange means for cooling or heating said exhaust gas flowing through these pollutant treatment means, characterized in that the reversible heat exchange means are an evaporator of a fluid circulation closed circuit. 
         [0018]    The circuit can comprise a fluid storage tank. 
         [0019]    The tank can be a thermally insulated tank. 
         [0020]    The tank can comprise fluid heating means. 
         [0021]    The circuit can comprise a bypass line for allowing the fluid from the tank into the evaporator. 
         [0022]    The bypass line can comprise a throttling means. 
         [0023]    The reversible heat exchange means can comprise at least one thermopile. 
         [0024]    The thermopile can be connected to electric accumulators for supplying power thereto when heating the exhaust gas. 
         [0025]    The pollutant treatment means can comprise a selective catalytic reduction catalyst. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0026]    Other features and advantages of the invention will be clear from reading the description given hereafter by way of non limitative example, with reference to the accompanying figures wherein: 
           [0027]      FIG. 1  is a diagram showing a device for controlling the temperature of exhaust gas from an internal-combustion engine according to the invention, 
           [0028]      FIG. 2  is a more detailed view of a part of the device of  FIG. 1 , and 
           [0029]      FIG. 3  is another diagram illustrating a variant of the device as shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0030]    In connection with  FIG. 1 , internal-combustion engine  10 , notably of Diesel type, comprises at least one cylinder  12 , an intake manifold  14  and an exhaust manifold  16  allowing to collect the exhaust gas resulting from the combustion of a fuel mixture in the cylinders prior to sending it to inlet  18  of an exhaust line  20 . 
         [0031]    In  FIG. 1 , the exhaust line carries, in the direction of circulation of the exhaust gas from inlet  18  of this line to its outlet (symbolized by arrow  22 ), a three-way oxidation catalyst  24  arranged as close as possible to exhaust gas inlet  18 , followed by an injector  26  for a reducing agent, urea here, arranged opposite inlet face  28  of an SCR catalyst  30 . This line also carries an exhaust gas temperature detector  31  housed opposite and close to inlet face  28  of the SCR catalyst. This detector allows, in combination with the calculator any internal-combustion engine is usually provided with, to know the temperature of the exhaust gas entering this catalyst. 
         [0032]    As more visible in  FIG. 1 , a reversible heat exchanger  32  is provided between the two catalysts to control the temperature of the exhaust gas reaching inlet face  28  of the SCR catalyst. Advantageously, this exchanger is arranged downstream from the three-way catalyst and upstream from the temperature detector. 
         [0033]    This exchanger allows to cool the gas or to heat it so that its temperature is in the usual operating range of the SCR catalyst, generally between around 300° C. and around 500° C. 
         [0034]    More precisely, in connection with  FIG. 2 , this exchanger is part of a fluid circulation closed circuit  34 , more particularly of Rankine cycle type. 
         [0035]    This circuit comprises a means  36  intended for circulation and compression of a working fluid, water here, circulating clockwise in this circuit (arrows A). This means, referred to as compressor, allows to compress this water and it is advantageously driven in rotation by any known means such as an electric motor (not shown). 
         [0036]    This circuit comprises, after the compressor, a heat exchange means  38 , an evaporator here, traversed by the compressed water that leaves it in compressed steam form. 
         [0037]    This evaporator is arranged on portion  40  of exhaust line  20  contained between the two catalysts  24  and  30 , preferably upstream from injector  26  and detector  31 , so as to be able to cool the exhaust gas circulating in this portion or to heat it. 
         [0038]    The person skilled in the art may consider any possible configuration for arranging this evaporator in direct or indirect connection with this exhaust line portion so as to provide the best heat exchange possible with the exhaust gas. 
         [0039]    After this evaporator, the circuit also comprises an expander  42  receiving from this evaporator the high-pressure compressed steam that flows therefrom in form of low-pressure expanded steam. 
         [0040]    This expander can be, by way of example, an expansion turbine whose rotor is driven in rotation by the steam. This rotor is advantageously connected to a device for converting the mechanical energy recovered to another energy, such as an electric generator for example. 
         [0041]    The circuit also comprises a cooling exchanger  44  or condenser receiving the expanded low-pressure steam from the expander that is converted, at the outlet of this condenser, to water in liquid form. This condenser is, in the example of  FIG. 1 , swept by a cooling fluid that is advantageously outside air at ambient temperature. 
         [0042]    Fluid circulation lines allow to successively connect the various elements of this circuit so that the working fluid, in liquid or vapour form, circulates in the direction shown by the arrows. More precisely, this circuit comprises a line  46  between the compressor and the evaporator, a line  48  between the evaporator and the expander, a line  50  between the expander and the condenser, and a line  52  between the condenser and the compressor. 
         [0043]    Furthermore, this circuit comprises a bypass line  54  that starts on line  48  between the evaporator and the expander, and ends on line  52  between the condenser and the compressor. Besides, an advantageously thermally insulated tank  56  is connected to line  48  by a connecting line  58  that ends on the portion of line  48  contained between the evaporator and the starting point of bypass line  54 . This bypass line and connecting line  58  carry each a throttling means allowing to control the fluid circulation in these lines, such as a valve  60  and  62  respectively. 
         [0044]    Tank  56  associated with lines  54  and  58 , and with valves  60  and  62 , provides reversibility of the heat exchange from the evaporator so as to turn the cooler function of this evaporator into a heat generator function for heating the exhaust gas circulating in portion  40 . 
         [0045]    When the engine is running, the calculator (associated with detector  31 ) can evaluate that the exhaust gas temperature is either excessive (above about 500° C.) or insufficient (below about 300° C.) to provide smooth running of SCR catalyst  30 . 
         [0046]    In cases where this temperature is excessive, circuit  34  is started so as to cool the exhaust gas circulating in portion  40  of line  20  while providing heat exchange between the gas circulating in this portion and evaporator  48 . 
         [0047]    More precisely, valves  60 ,  62  are in closed position for lines  54 ,  58  and the water circulates in this circuit in a conventional clockwise direction with respect to the figure (arrows A) under the effect of compressor  36 . The compressed water leaving this compressor circulates in line  46  and ends into evaporator  38 . This compressed water then flows through the evaporator by collecting the heat carried by the exhaust gas, which is transmitted to this evaporator. Under the effect of this heat from the gas, the temperature thereof is lowered and the water is heated, thus leaving the evaporator in form of hot compressed steam. The steam then flows through expander  42  while transmitting thereto the energy it contains. The expanded steam leaving this expander through line  50  flows through condenser  44 , which it leaves in form of liquid water. This liquid water is finally brought through line  52  to compressor  36  in order to be compressed. 
         [0048]    Circuit  10  is thus kept in operation until the temperature of the exhaust gas reaching the inlet face of the SCR catalyst is the temperature required for operation of this catalyst. 
         [0049]    Just before the circuit is stopped, valve  60  of connecting line  58  is switched to an open position so that the steam leaving evaporator  38  is fed into thermally insulated storage tank  56  where it is kept at high temperature. 
         [0050]    This valve is then set to a closed position as soon as the tank is filled with this steam. 
         [0051]    Conversely, in case of an insufficient exhaust gas temperature for operation of SCR catalyst  30 , notably when starting the engine, valves  60  and  62  are set to an open position for lines  58 ,  54  and compressor  36  is started. 
         [0052]    In this configuration, the high-temperature steam contained in tank  56  is discharged therefrom through connecting line  58  and fed into line  48 . Under the effect of compressor  36 , this steam circulates clockwise with respect to arrows A′ in  FIG. 1 . 
         [0053]    The steam thus circulates in a portion of line  48 , then in bypass line  54  before it reaches the compressor inlet. This steam leaves the compressor and enters evaporator  38 . This evaporator thus inverts its initial function of collecting the heat contained in the gas. More precisely, the evaporator turns into a heat generator by yielding the heat contained in the steam to the exhaust gas circulating in portion  40  of the exhaust line. This allows the gas to be heated by thermal exchange. It is therefore possible to rapidly increase the gas temperature and to considerably reduce the time required to obtain the suitable exhaust gas temperature for operation of the SCR catalyst. 
         [0054]    Advantageously, an additional valve  64  (in dotted line in  FIG. 1 ) can be provided on line  50  or  52  so that, in closed position for these lines, the steam cannot flow through condenser  44 . 
         [0055]    Of course, this additional valve is in an open position for lines  50  or  52  in the configuration where this circuit is used for cooling the exhaust gas. 
         [0056]    Thus, by means of simple layouts of this closed circuit, it is possible to either cool the exhaust gas or to reversibly heat it using the same evaporator. 
         [0057]    Of course, without departing from the scope of the invention, it is possible to use, instead of the thermally insulated tank, a tank provided with means for heating the liquid it contains, such as heating resistors, a burner, etc. 
         [0058]      FIG. 3  shows a variant of the reversible heat exchange device  32  provided between the two catalysts  24 ,  30  to control the temperature of the exhaust gas reaching inlet face  28  of SCR catalyst  30 . 
         [0059]    This device comprises a thermopile or a succession of thermopiles  66  arranged on portion  40  of exhaust line  20  between three-way catalyst  24  and SCR catalyst  30 . 
         [0060]    Generally, this thermopile allows to recover the heat energy contained in the exhaust gas and to convert it, notably through Seebeck effect, to an electric energy that is then stored in electric accumulators  68  through conductors  70 . 
         [0061]    Of course, as in  FIG. 1 , the person skilled in the art can consider all the possible configurations for arranging this thermopile in direct or indirect connection with this exhaust line portion so as to provide the best possible heat exchange with the exhaust gas. 
         [0062]    Thus, in cases where the exhaust gas temperature is excessive (above about 500° C.), the thermopile is active to provide heat exchange between the gas circulating in portion  40  of line  20  and this thermopile, by collecting the heat contained in this gas. The exhaust gas is therefore cooled and the thermopile collects the heat energy contained in the exhaust gas stream so as to convert it to electric energy that is stored in accumulators  68 . 
         [0063]    Once the exhaust gas temperature is stabilized, operation of the thermopile can be stopped. 
         [0064]    In the opposite case where the exhaust gas temperature is below its minimum threshold value required to provide operation of SCR catalyst  30 , thermopile  66  is supplied with power by accumulators  68 . 
         [0065]    This power supply has the effect of heating the thermopile that can then transfer its heat to the exhaust gas circulating in portion  40  of line  20 . 
         [0066]    Similarly, once the exhaust gas temperature has reached the desired value, the thermopile supply is stopped so as to stop heating the exhaust gas, which has become unnecessary. 
         [0067]    Thanks to the reversibility of the thermopile heat exchange, the exhaust gas can be cooled or heated so that its temperature is in the usual operating range of the SCR catalyst.