Abstract:
The present invention relates to a method for reintroducing exhaust gas to the intake of an internal-combustion engine wherein, during the engine exhaust phase, part of the exhaust qas contained in combustion chamber ( 60 ) of cylinder ( 10 ) of the engine is fed, for an operating range of this engine, into a first fluid circulation circuit (C 1 ) and the other part of said gas is discharged into an exhaust circuit. According to the invention, the method consists: during the engine intake phase, in cooling the exhaust qas fed into fluid circulation circuit (C 1 ), in mixing the cooled recirculated exhaust qas with fresh supply air (AF) coming from engine supply circuit ( 44 ), in feeding the mixture of recirculated exhaust gas and of supply air into a second fluid circulation circuit (C 2 ), and in cooling said mixture fed into second circuit (C 2 ) prior to feeding it into combustion chamber ( 60 ).

Description:
FIELD OF THE INVENTION 
     The present invention relates to a method for reintroducing exhaust gas to the intake of an internal-combustion engine and to an engine using same. 
     In internal-combustion engines of this type, reintroduction of exhaust gas to the intake allows to significantly reduce the proportion of nitrogen oxides (NOx) discharged into the atmosphere. 
     BACKGROUND OF THE INVENTION 
     This recirculation is generally carried out by means of a circuit referred to as EGR (Exhaust Gas Recirculation) circuit comprising a pipe connecting the engine exhaust manifold to the intake distributor thereof and a valve, referred to as EGR valve, arranged in this pipe. 
     As it is widely known, in order to be able to minimize NOx discharge into the atmosphere, this recirculated exhaust gas is generally cooled prior to being fed to the engine intake. A heat exchanger is therefore arranged on the pipe and the exhaust gas is cooled while flowing therethrough. 
     This EGR circuit layout is well known to the person skilled in the art and it is better known as “external EGR”. 
     What is also known, as described in document WO-2006/018,699, is an “internal EGR”, commonly referred to as “IGR” (Internal Gas Recirculation or Internal Gas Residual), which allows to reintroduce exhaust gas to the engine intake without using a specific external exhaust gas recirculation circuit. 
     The internal-combustion engine therefore comprises a cylinder provided with a combustion chamber, an exhaust means with an exhaust valve, an exhaust manifold, an intake means with an intake valve, an intake distributor and a recirculated exhaust gas cooling exchanger, or cooler, arranged in the intake distributor. 
     In this type of engine, the exhaust gas is reintroduced to the intake in order to be mixed with the fresh air flowing in during the engine intake phase. 
     Thus, during the engine exhaust phase during which the exhaust valve is open, the intake valve is also open for a predetermined time. The exhaust gas contained in the combustion chamber is discharged simultaneously to the exhaust manifold and to the intake distributor. In the example described, the intake valve closes before the end of the exhaust phase and advantageously before the exhaust valve closes. In the example illustrated by this document, the exhaust gas contained in the intake distributor is cooled by the cooler and kept in this distributor at the desired temperature. 
     During the engine intake phase, the intake valve opens again, substantially as the exhaust valve closes. The cooled exhaust gas contained in the distributor and the fresh air flowing in are fed, through the intake valve, into the engine combustion chamber for the rest of the combustion cycle. 
     This layout, although satisfactory, however involves some quite significant drawbacks. 
     In fact, introduction of the recirculated exhaust gas and of the fresh air into the combustion chamber does not allow to obtain homogeneous mixing of the two fluids. This has the effect of disturbing the combustion of the mixture upon fuel injection in the neighborhood of the end of the engine compression phase. 
     Furthermore, during circulation of the exhaust gas in the cooler, the latter fouls up through deposition of the particles carried by the gas. The cooler therefore loses its exchange capacity, thus decreasing the performance thereof. 
     Moreover, as the exhaust gas flows through the cooler, condensation of the steam present in this gas may occur; this steam is then reintroduced into the cylinder during the intake phase and it is likely to disturb the combustion of the fuel mixture. 
     Similarly, it is difficult to control the amount of exhaust gas flowing through the cooler and a quite significant amount of this gas may not be cooled. 
     The present invention aims to overcome the aforementioned drawbacks by means of a method for reintroducing exhaust gas to the intake of an internal-combustion engine of simple design. 
     SUMMARY OF THE INVENTION 
     The present invention thus relates to a method for reintroducing exhaust gas to the intake of an internal-combustion engine wherein, during the engine exhaust phase, part of the exhaust gas contained in the combustion chamber of the engine cylinder is fed, for an operating range of this engine, into a first fluid circulation circuit and the other part of said gas is discharged into an exhaust circuit, characterized in that the method consists:
         during the engine intake phase, in cooling the exhaust gas fed into the fluid circulation circuit,   in mixing the cooled recirculated exhaust gas with fresh supply air coming from the engine supply circuit,   in feeding the mixture of recirculated exhaust gas and of supply air into a second fluid circulation circuit, and   in cooling said mixture fed into the second circuit prior to feeding it into the combustion chamber.       

     The method can consist in closing at least part of the fluid circulation circuit and in feeding part of the exhaust gas into said closed part of the circuit so as to pressurize said gas. 
     The method can consist in providing cooling of the exhaust gas and of the mixture through a heat exchanger common to the two fluid circulation circuits. 
     The invention also relates to an internal-combustion engine comprising at least one cylinder with a combustion chamber, characterized in that it comprises at least two intake means with intake valves, a distributor connected to each intake means and an outlet pipe connecting each distributor to a fresh air supply circuit. 
     The engine can comprise a heat exchanger arranged on the two outlet pipes. 
     Advantageously, the heat exchanger can be a double-flow cooler. 
     The two outlet pipes can join the fresh air supply circuit in a junction zone. 
     Preferably, one of the outlet pipes can carry a throttling means. 
     The throttling means can be arranged between the distributor and the heat exchanger. 
     The engine can comprise control means allowing the lift laws of at least one of the valves to be varied. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       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 accompanying figures wherein: 
         FIG. 1  is a diagram showing an internal-combustion engine using the method according to the invention, during the exhaust phase, 
         FIG. 2  is a diagram showing the engine of  FIG. 1  in another stage of the method according to the invention, and 
         FIG. 3  shows curves illustrating the various lift laws of the intake and exhaust valves of the engine using the method according to the invention. 
         FIG. 4  is a diagram showing the engine of  FIG. 1  in another stage of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In  FIG. 1 , an internal-combustion engine of direct fuel injection diesel type comprises at least one cylinder  10  provided with at least two intakes  12 ,  14  with an intake valve  16 ,  18  and an intake pipe  20 ,  22  for each intake. In the description hereafter, intake  12  with valve  16  and pipe  20  is referred to as the intake/exhaust so as to better distinguish it from the intake  14  with its valve  18  and its pipe  22 . The cylinder also comprises an exhaust circuit including at least one exhaust  24  with an exhaust valve  26 , an exhaust pipe  28  and an exhaust manifold that is not shown. 
     Closing and opening of valve  16  of the intake/exhaust  12  and possibly of intake valve  18  is controlled by the valve actuation apparatus  30 ,  32  allowing the lift laws of these valves to be varied, notably at the opening and closing times thereof, independently of one another, such as those known as VVA (Variable Valve Actuation). 
     Similarly, exhaust valve  26  can be controlled by the valve actuation apparatus  34  allowing the lift laws of this valve to be varied, such as VVA type. 
     the valve actuation apparatus  30 ,  32  and  34  are controlled by a control unit, or engine calculator (not shown), any engine is usually equipped with, which notably allows the lift laws of these valves to be controlled according to the engine operating conditions or ranges. 
     As can be better seen in  FIG. 1 , intake pipe  20  ends at a distributor  36 , referred to as intake/exhaust distributor, and pipe  22  ends at another intake distributor  38  distinct from distributor  36 . 
     Outlet pipes  40 ,  42  of these distributors end at a junction zone made up of a branch line  44  connected to a pipe  46  belonging to the fresh air supply circuit. This fresh air can be air at ambient pressure or supercharged air, i.e. compressed by any means such as a turbocompressor. 
     A double-flow heat exchanger  48 , or cooler, is arranged on the two outlet pipes  40 ,  42  between distributors  36 ,  38  and branch line  44  so as to be able to cool the fluid(s) circulating in these pipes. This cooler constantly carries, while the engine is running, a coolant (or heat carrier) that is introduced, at one end of the cooler, through an inlet  50  and flows out at the other end of this cooler through outlet  52 . This cooler, referred to as single-pass cooler, thus allows to cool any fluid circulating in pipe  40 , flowing therethrough in a first direction, then to cool this fluid or another fluid circulating in pipe  42 , by running therethrough in the same direction or in another direction, generally the opposite direction. 
     It can be observed that the coolant fed into the cooler through inlet  50  is heated by exchange with the fluid circulating in pipe  42  and it reaches, with a higher coolant temperature than the inlet temperature, the other part of cooler  48  for exchange with the fluid circulating in pipe  40 . 
     Preferably, the coolant used for the cooler comes from a bypass line of the engine cooling circuit or from a low-temperature water circuit. 
     Advantageously, but not obligatorily, a valve  54  is arranged on outlet pipe  40  between cooler  48  and branch line  44 . This valve can also be located between intake/exhaust distributor  36  and cooler  48 . 
     The valve can take all the positions contained between the fully open position (in dotted line in the figure) of outlet pipe  40  and its fully closed position (in axis line), or it can be a two-position valve with a fully open and a fully closed position. 
     Thus, this engine comprises a first fluid circulation circuit C 1  with the intake/exhaust  12 , intake/exhaust distributor  36 , outlet pipe  40  carrying valve  54  and part of cooler  48 . This engine also comprises a second fluid circulation circuit C 2  including the intake  14 , intake distributor  38  and outlet pipe  42  carrying the other part of cooler  48 . 
     Furthermore, as it is widely known, cylinder  10  comprises a piston  56  movable in a reciprocating translational motion between a lower position, referred to as bottom dead centre (BDC), and an upper position, referred to as top dead centre (TDC), by a rod  58  connected to a crankpin of a crankshaft (not shown) such an engine is usually provided with. 
     This cylinder also comprises a combustion chamber  60  delimited by the top of the piston, the cylinder wall and the wall of cylinder head  62  opposite the piston. Intakes  12 ,  14  and the exhaust  24  open into this combustion chamber, as well as any fuel injection parts (not shown) allowing a fuel mixture to be obtained in the chamber. 
     In the example described in connection with the present invention, one considers that the engine runs according to a four-stroke mode, with an intake, a compression, an expansion and an exhaust phase. 
     Thus, when the engine runs under conditions that require recirculation of part of the exhaust gas to the intake, for example conditions corresponding to operation at low or medium loads, the engine calculator controls this engine in such a way that it runs with such an exhaust gas recirculation to the intake thereof. 
     From the configuration illustrated in  FIG. 1  and in connection with  FIG. 3  showing the lift laws (in full line) of the corresponding intake valves  16 ,  18  and exhaust valve  26 , piston  56  is in the neighborhood of its exhaust bottom dead centre (PMBe in  FIG. 3 ), at the beginning of the engine exhaust phase. The valve actuation apparatus  30  and  34  are then controlled by the engine calculator so as to open intake valve  16  of the intake/exhaust  12  and exhaust valve  26 . The hot exhaust gas contained in combustion chamber  60  is discharged, under the impulse of the motion of piston  56  towards the TDC as shown by arrow E, both into at least part of circulation circuit C 1 , i.e. at least into intake line  20  through valve  16  and intake/exhaust distributor  36 , and through exhaust valve  26  into an exhaust manifold the engine is usually equipped with. 
     Intake valve  16  then closes under the impulse of control means  30  at a crank angle V 1 , whereas exhaust valve  26  remains open until it closes in the neighborhood of the intake top dead centre (PMHa) of the piston, as illustrated in  FIG. 3 . 
     Of course, the engine calculator controls means  30  controlling intake valve  16  in such a way that closing of this valve occurs at a crank angle V 1  before the intake top dead centre PMHa, so that the exhaust gas cannot flow beyond intake/exhaust distributor  36  or, worse, branch line  44 . 
     Of course, during this exhaust phase, valve  54  is in one of its open positions that can reach the fully open position. 
     Thus, after closing of intake valve  16  and of exhaust valve  26 , the piston is in the neighborhood of the intake top dead centre PMHa and hot exhaust gas is present in intake pipe  20  and in intake/exhaust distributor  36 , and possibly in pipe  40  and cooler  48 . 
     During the next engine intake phase, as illustrated in  FIG. 2  and still in connection with  FIG. 3 , piston  56  has a descending motion, shown by arrow A, from its intake top dead centre PMHa to its compression bottom dead centre PMBc. At the beginning of this intake phase where the piston is in the neighborhood of PMHa, the engine calculator controls the valve control means in such a way that intake valve  16  and exhaust valve  26  remain closed and intake valve  18  opens. 
     Under the effect of the descending motion of the piston, a depression is created through intake valve  18 , and the exhaust gas contained in intake/exhaust pipe  20  and in intake/exhaust distributor  36  are led to circulate in outlet pipe  40 . During this circulation, the exhaust gas flows through cooler  48  (arrow R 1 ) and ends, in a cooled state, at branch line  44 , valve  54  being in fully open position. 
     The fresh air that is allowed into pipe  46  (arrow AF) reaches branch line  44  where it mixes with the cooled exhaust gas. This mixture circulates then in second fluid circulation circuit C 2 , more precisely in outlet pipe  42  by flowing through cooler  48  in an opposite direction (arrow R 2 ) to the direction of flow of the exhaust gas in pipe  40 , and ends at intake distributor  38 . 
     This distributor thus contains a mixture of fresh air and of exhaust gas that has been cooled by passage through the cooler. 
     This is particularly advantageous when using a fresh air that is supercharged by compression and that undergoes a temperature rise during this compression. This supercharged air must thereafter be subjected to cooling to recover all the characteristics sought for a compressed air, such as its density. 
     The mixture of fresh air and of exhaust gas is then sent through intake pipe  22  to intake valve  18 , in open position, to be fed into combustion chamber  60 . This introduction is continued until this valve closes in the neighborhood of the position of the piston corresponding to the compression bottom dead centre PMBc. 
     When the intake valve closes, the mixture present in the combustion chamber is quasi-homogeneous and at the desired temperature for the engine operating cycle to continue with the phase of compression of this mixture. 
     Advantageously, valve  54 , as mentioned above, can be used for pressurizing the exhaust gas contained in the intake pipe, intake/exhaust distributor  36  and part of pipe  40 . At the beginning of the engine exhaust phase, intake valve  16  of the intake/exhaust  12  and exhaust valve  26  are therefore open, whereas valve  54  is in a closed position (illustrated by an axis line in  FIG. 1 ) of outlet pipe  40 , thus isolating this pipe from branch line  44 . Under the combined effect of the pressure of the exhaust gas leaving combustion chamber  60  and of the motion of piston  56  as shown by arrow E, the volume delimited by intake pipe  20 , intake/exhaust distributor  36  and part of outlet pipe  40  between this distributor and valve  54  is at a higher pressure than the pressure of the supply air in pipe  46 . The exhaust gas under pressure is thus contained in at least a closed part of circuit C 1  between valve  16  and valve  54 . 
     Thus, during the next intake phase illustrated in  FIG. 2 , exhaust valve  26  and intake valve  16  are in a closed position, intake valve  18  opens and valve  54  is actuated into the open position (dotted line in  FIG. 2 ). Under the effect of the pressure prevailing upstream from this valve (considering the direction of circulation from intake/exhaust valve  16  to branch line  44 ) combined with the depression, downstream from this valve, generated by piston  56  as it moves in the direction shown by arrow A, the exhaust gas circulates in the direction of branch line  44  by flowing through cooler  48 . 
     This cooled exhaust gas then mixes with the fresh air fed into pipe  46  and the process continues as described above. 
     During engine operation without exhaust gas recirculation, notably at high or full loads, it is not necessary to send the exhaust gas to the engine intake and the engine runs under conventional conditions. 
     In this case, as illustrated in  FIG. 3  for the lift laws of intake valves  16 ,  18  and of exhaust valve  26  shown in dotted lines, only exhaust valve  26  is actuated during the exhaust phase through the agency of control means  34 . This valve opens in the neighborhood of the exhaust bottom dead centre PMBe and it closes in the neighborhood of the intake top dead centre PMHa, thus allowing discharge of the exhaust gas contained in the combustion chamber to the exhaust manifold. 
     During the intake phase, one or the other of intake valves  16 ,  18 , preferably both, are actuated between an open position in the neighborhood of PMHa and a closed position in the neighborhood of the compression bottom dead centre PMBc. During opening of these two intake valves, as illustrated in  FIG. 4 , the fresh air fed into pipe  46  reaches branch line  44  where it divides into two streams, a stream circulating through circuit C 1  in pipe  40  in the direction of intake valve  16  and another stream circulating through second circuit C 2  in pipe  42  in the direction of valve  18 . These two streams flow through cooler  48  in the same direction and they are cooled prior to reaching distributors  36 ,  38 . From these distributors, the cooled air streams circulate in intake pipes  20 ,  22 , then they are fed into combustion chamber  60  through intake valves  16 ,  18 . 
     By means of this layout, in cases where the fresh air is supercharged air, it is cooled prior to being fed into the combustion chamber. Furthermore, circulation of a supercharged air under pressure in an opposite direction to the direction of circulation of the exhaust gas (direction R 1 ) in outlet pipe  40  allows to remove a large part of the impurities present in the exhaust gas and adhering to the constituent elements of the cooler in contact with this gas, such as soots. 
     The invention furthermore allows to use a single exchanger providing, from the same place, both cooling of the recirculated exhaust gas and of the supercharged fresh air. 
     In the example described above for exhaust gas recirculation to the engine intake, exhaust valve  26  and intake valve  16  open substantially simultaneously at the beginning of the exhaust phase, but the lift laws of these exhaust and intake valves  26  and  16  can be different from those described above. The main thing is that the interval between the opening and the closing of intake/exhaust valve  16  occurs during the engine exhaust phase. 
     Thus, as shown by dashed lines in  FIG. 3 , exhaust valve  26  opens at the exhaust bottom dead centre PMBe, then it closes at crank angle V 2  and discharges part of the exhaust gas contained in the combustion chamber towards the exhaust manifold. In the neighborhood of this angle V 2 , intake/exhaust valve  16  opens, then it closes in the neighborhood of the intake top dead centre PMHa with discharge, to intake/exhaust distributor  36 , of the exhaust gas still contained in chamber  60 . For the next engine intake phase, intake valve  18  behaves as described above. 
     It can be noted that, for some engine operating ranges, such as engine cold operation, it may be necessary to reintroduce to the intake exhaust gas that has not been cooled. 
     During the intake phase that follows the exhaust phase of  FIG. 1 , valves  16  and  18  are therefore open throughout the phase. The hot exhaust gas contained in pipe  20  and in intake/exhaust distributor  36  is fed into combustion chamber  60  through intake valve  16 , and the fresh air circulating in circuit C 2 , from fresh air supply pipe  46 , is fed into this chamber through intake valve  18 . 
     Thus, at the end of the intake phase, a stratified mixture of fresh air and of recirculated exhaust gas is present in the chamber. 
     By means of the invention, it is possible to do without or at least to simplify the conventional EGR circuit. 
     Furthermore, it is no longer necessary to bypass the cooler if warm exhaust gas is to be reintroduced to the intake, notably for the cold operating range of the engine. 
     Furthermore, when the engine runs without exhaust gas recirculation and with supply air circulation both in pipe  40  and in pipe  42 , valve  54  can be used as a swirl control valve so as to feed into the combustion chamber the fresh air in such a way that it is subjected to a rotating motion (or swirl) around the vertical general axis of the cylinder. 
     Moreover, the double-flow cooler allows to avoid or to limit condensation of the steam present in the exhaust gas when the gas flows through this cooler. The heat-carrying fluid used for cooling the exhaust gas reaches the part of this cooler concerned by this cooling at a temperature that is higher than its inflow temperature. Cooling of the recirculated exhaust gas and of the air/recirculated exhaust gas mixture thus takes place progressively, which avoids or limits condensation of the steam in this gas and in this mixture: first the recirculated exhaust gas with moderate cooling, then the air/recirculated exhaust gas mixture with more intense cooling. 
     In the case of supercharged fresh air, the invention also allows to use a turbocompressor that can operate at a lower exhaust pressure upstream from the turbine than the intake pressure downstream from the compressor. 
     The present invention is not limited to the example described and it encompasses any variant and equivalent. 
     Notably, it is possible to use control means comprising no camshaft instead of VVA type control means. In this case, an actuating means dedicated to each valve, such as an electromagnetic, hydraulic, electrohydraulic, pneumatic or electropneumatic actuator, allows to directly or indirectly act on the valve rod of the intake and exhaust valves.