Abstract:
The present invention relates to a method of scavenging residual burnt gas of a direct-injection internal-combustion engine, notably of diesel type, comprising at least one cylinder ( 10 ) including a combustion chamber ( 12 ), at least one exhaust means ( 14 ) with an exhaust valve ( 18 ) controlled by exhaust control means ( 42 ), at least one intake means ( 24 ) with an intake valve ( 28 ) controlled by intake control means ( 40 ) and a processing unit ( 44 ) receiving the values relative to the intake pressure (Pa) and to the exhaust pressure (Pe) of the engine. 
     According to the invention, the method consists, when the engine runs under low speed and high load conditions, in:
       carrying out a sequence of opening/closing of exhaust valves ( 18 ) during the exhaust phase of the engine,   during this exhaust valve opening/closing sequence, in carrying out at least one sequence of opening/closing of intake valves ( 28 ) so as to achieve scavenging of the residual burnt gas.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a method of scavenging residual burnt gas of a direct-injection supercharged internal-combustion engine, notably of diesel type. 
       BACKGROUND OF THE INVENTION 
       [0002]    Generally, the power delivered by an internal-combustion engine depends on the amount of air fed into the combustion chamber, this amount of air being itself proportional to the density of this air. 
         [0003]    As it is well known, if high power is required, an increase in this amount of air is provided by compression of the air before it is fed into this combustion chamber. This operation, referred to as supercharging, can be carried out using any means such as a turbocompressor or a driven compressor such as a screw compressor. 
         [0004]    Furthermore, in order to increase even further this amount of air in the cylinder, the residual burnt gas initially contained in the dead volume of the combustion chamber is discharged before the end of the engine exhaust phase and it is replaced by supercharged air. This stage is commonly referred to as burnt gas scavenging. 
         [0005]    As described in document FR-A-2,886,342, this scavenging can consist in carrying out, at the end of the engine exhaust phase and at the start of the intake phase, overlapping of the exhaust and intake valves of a cylinder. This overlap is obtained by opening simultaneously these exhaust and intake valves for some degrees to some ten degrees of crank rotation angle. 
         [0006]    The intake air is thus fed into the combustion chamber before the end of the exhaust phase by expelling the exhaust gas contained therein. This gas is thus discharged through the exhaust valve and replaced by intake air. 
         [0007]    Although this type of engine gives satisfaction, it however involves drawbacks that are by no means insignificant. 
         [0008]    In fact, such scavenging requires recesses of great depth in the piston, which consequently degrades the shape of the combustion chamber and the progress of the fuel mixture combustion. Furthermore, this type of engine concurrently requires modifying the opening angles of the exhaust valves and the closing angles of the intake valves. 
         [0009]    The present invention aims to overcome the aforementioned drawbacks by means of a scavenging method of simple design allowing to improve the limit fuel/air ratio of the fuel mixture and to increase supercharging and filling of the combustion chamber without degrading the shape thereof. Furthermore, particle emissions are limited and the engine power is increased. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention therefore relates to a method of scavenging residual burnt gas of a direct-injection internal-combustion engine, notably of diesel type, comprising at least one cylinder including a combustion chamber, at least one exhaust means with an exhaust valve controlled by exhaust control means, at least one intake means with an intake valve controlled by intake control means and a processing unit receiving the values relative to the intake pressure and to the exhaust pressure of the engine, characterized in that it consists, when the engine runs under low speed and high load conditions, in:
       carrying out a sequence of opening/closing of the exhaust valve during the exhaust phase of the engine,   during this exhaust valve opening/closing sequence, in carrying out at least one sequence of opening/closing of the intake valve so as to achieve scavenging of the residual burnt gas.       
 
         [0013]    The method can consist in starting the intake valve opening/closing sequence when the intake pressure is higher than the exhaust pressure. 
         [0014]    The method can consist in performing closing of the intake valve at least before the end of the exhaust phase. 
         [0015]    The method can consist in performing closing of the intake valve at the end of the exhaust phase. 
         [0016]    The method can consist in performing at least one intake valve opening/closing sequence in a zone of the exhaust phase where the pressure differential between the intake pressure and the exhaust pressure is the highest. 
         [0017]    The method can consist in decreasing the height of the intake valve lift during the exhaust phase so that it is lower than the height of the exhaust valve lift. 
         [0018]    The method can consist in decreasing the intake valve lift spread during the exhaust phase so that it is lower than the exhaust valve lift spread. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0019]    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: 
           [0020]      FIG. 1  shows an internal-combustion supercharged engine using the method according to the invention, 
           [0021]      FIG. 2  shows curves illustrating the various lift laws (L) of the intake and exhaust valves as a function of the crank rotation (in crank degrees ° V) of the engine using the method according to the invention, and 
           [0022]      FIG. 3  is a graph with curves illustrating the pressure (P in bar) at the intake (Pa) and at the exhaust (Pe) of a cylinder in the burnt gas scavenging phase as a function of the crank rotation (in degrees). 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    In  FIG. 1 , the internal-combustion engine illustrated is a supercharged internal-combustion engine of self-ignition type, notably a diesel engine, working in four-stroke mode with an intake phase A, a compression phase C, an expansion phase D and an exhaust phase E. 
         [0024]    This engine comprises at least one cylinder  10 , four cylinders here, in which a piston (not shown) slides in a rectilinear reciprocating motion between a top dead centre (PMH in the figure) and a bottom dead centre (PMB) and delimits a combustion chamber  12  in which combustion of a fuel mixture takes place. 
         [0025]    As it is widely known, this fuel mixture can be either a mixture of supercharged air mixed with recirculated exhaust gas (or EGR) with a fuel, or a mixture of supercharged air with a fuel. 
         [0026]    The cylinder comprises at least one burnt gas exhaust means  14 , two here, including an exhaust pipe  16  associated with a shutoff means such as an exhaust valve  18 . 
         [0027]    Exhaust pipes  16  end in an exhaust manifold  20  allowing the burnt gas from the combustion chambers to be discharged, this manifold being connected to an exhaust line  22 . 
         [0028]    This cylinder also comprises at least one intake means  24 , two here, including an intake pipe  26  controlled by a shutoff means such as an intake valve  28 . 
         [0029]    Usually, an intake manifold  30  is connected to intake pipes  26  and it allows fresh air (generally supercharged, mixed or not with recirculated exhaust gas) to be distributed in combustion chambers  12 , this manifold being also connected to a supply line  32 . 
         [0030]    The intake manifold is connected by line  32  to the outlet of compression section  34  of a turbocompressor  36 , whereas exhaust manifold  20  is connected by line  22  to the inlet of turbine  38  of this turbocompressor. 
         [0031]    Opening and closing of intake valves  28  is controlled by any known means allowing to achieve a double lift of these valves during running conditions requiring high engine power, notably at low engine speeds, or a single lift under conventional engine running conditions at medium and high engine speeds. 
         [0032]    Control means  40  of VVA (Variable Valve Actuation) camshaft type allowing the two lift laws of these valves to be achieved are therefore used. A first law allows to perform at least one sequence of opening/closing of intake valves  28  during the exhaust phase E of the engine, followed by a conventional sequence of opening/closing of these valves during the intake phase A. The other lift law allows to perform only a sequence of opening/closing of the intake valves during the intake phase A of the engine. 
         [0033]    By way of non limitative example, this camshaft comprises a cam associated with a second cam allowing to provide the double lift law for these intake valves during the exhaust phase E and the intake phase A of the engine, as well as a disengaging device making one of the cams, the second cam for example, inoperative, to achieve the single lift of the intake valves during the engine intake phase. 
         [0034]    Of course, without departing from the scope of the invention, these control means can be specific control means for each valve, such as an electromagnetic, electropneumatic actuator or other, directly acting on the valve rod. 
         [0035]    It can be noted that the term “lift” corresponds to the graphical representation (along two axes) of the motion of a valve from the moment it starts opening the orifice of the pipe to the orifice full open position to the moment it ends closing this orifice. 
         [0036]    Opening and closing of exhaust valves  18  is controlled by any known means such as a conventional camshaft  42  whose rotation is controlled by a driving means connected to the crankshaft of this engine, such as a toothed belt. 
         [0037]    This engine also comprises a processing unit  44  referred to as engine calculator that contains mappings or data charts allowing, according to the values of the engine parameters transmitted by data lines  464 , such as the intake pressures in intake manifold  30  and the exhaust pressures in exhaust manifold  20 , the engine speed or the load, to evaluate the power to be generated by this engine to meet the driver&#39;s request. 
         [0038]    More precisely, this engine calculator allows, according to these values, to control more particularly the lift laws of intake valves  28  through a control line  48  acting upon means  40  so as to allow a single or a double lift of these valves. 
         [0039]    Thus, when the engine has to run under conditions corresponding to a high power request, in particular for low engine speeds, the engine calculator controls this engine so that it works with scavenging of the residual burnt gas present in the combustion chamber when the pressure Pa recorded in the intake manifold is higher than the pressure Pe prevailing in the exhaust manifold. 
         [0040]    In connection with  FIG. 2  showing the various lift laws for the intake  28  and exhaust  18  valves between an open (O) and a closed (F) position as a function of the crank rotation angle (° V), associated with  FIG. 3  showing these crank rotation angles, calculator  44  controls more particularly control means  40  through line  48  to achieve a double lift of intake valves  28  in order to meet the power requirement. 
         [0041]    More precisely, as better illustrated in  FIG. 2 , during the exhaust phase E of the engine, exhaust valves  18  conventionally follow an opening/closing sequence between the exhaust bottom dead centre (PMBe) and the intake top dead centre (PMHa) of the piston so as to discharge the exhaust gas contained in the combustion chamber towards exhaust manifold  20 . 
         [0042]    Together with this exhaust valve opening/closing sequence, control means  40  are controlled by the engine calculator so as to achieve at least one sequence of opening/closing of intake valves  28  during this exhaust phase and during the exhaust valve opening/closing sequence. This sequence is more particularly carried out when the calculator receives information according to which the pressure Pa considered at the level of intake valves  28  is higher than the pressure Pe recorded at the level of exhaust valves  18 . 
         [0043]    More particularly, these intake valves open at the exhaust bottom dead centre PMBe or after this PMBe and they close at the intake top dead centre PMHa. Considering the pressure differential between intake pressure Pa and exhaust pressure Pe (see  FIG. 3 ), which is globally positive for the intake pressure, the exhaust gas contained in combustion chamber  12  is discharged through exhaust valves  18  towards exhaust manifold  20  prior to being sent to exhaust line  22 . This exhaust gas is thus replaced by supercharged air that allows to globally increase the amount of air present in the combustion chamber at the end of the intake phase A of the engine. 
         [0044]    Preferably, as illustrated in thick line in  FIG. 2 , spreading of the lift of intake valves  28  can be lower than that of exhaust valves  18 . These intake valves thus open at a crank angle Va 1  after PMBe and they close at an angle Va 2  in the vicinity of PMHa. The lift of the intake valves between angles Va 1  and Va 2  corresponds to a zone of the exhaust phase wherein the pressure differential between intake pressure Pa and exhaust pressure Pe is globally the highest (see  FIG. 3 ) for the exhaust phase considered while being positive for the intake pressure. 
         [0045]    Advantageously, the lift height of the intake valves is substantially equal to the lift height of the exhaust valves but, as illustrated in dotted line in  FIG. 2 , the lift height of these intake valves may be varied, for example between a full open position O and a one-third open position O/3, so as to be able to control discharge of the residual burnt gas. 
         [0046]    Similarly, the spread of the intake valves lift can be variable so as to start the opening/closing sequence at angle Va 3  after angle Va 1  and to end it at angle Va 4  before angle Va 2 . 
         [0047]    Preferably, the maximum lift and the maximum spread of these intake valves during the exhaust phase are lower than those of the exhaust valves. 
         [0048]    During the intake phase A of this engine that follows this exhaust phase, calculator  44  controls means  40  controlling these intake valves  28  so that they open again conventionally in the vicinity of PMHa and close in the vicinity of compression bottom dead centre PMBc. 
         [0049]    Thus, during this second lift of the intake valves, supercharged air adds further to the supercharged air already present in combustion chamber  12  after the scavenging operation so as to obtain a larger amount of air at the end of the intake phase A. 
         [0050]    Under conventional engine running conditions without burnt gas scavenging, engine calculator  44  then controls, through line  48 , control means  40  so as not to achieve, during exhaust phase E, a lift of intake valves  28  by making inoperative the second cam of the camshaft as mentioned above, which allows these intake valves to be maintained in a closed position. During this exhaust phase, only exhaust valves  18  conventionally follow an opening/closing sequence between PMBe and PMHa. 
         [0051]    This exhaust phase is followed by an intake phase A during which intake valves  28  follow a conventional opening/closing sequence between PMHa and PMBc. 
         [0052]    The invention therefore readily allows to change from engine running conditions with possibility of scavenging parameters adjustment (amount of burnt gas discharged, time of burnt gas discharge, etc.) by acting on the lift law of the intake valves during the exhaust phase to conventional engine running conditions, and vice versa. Furthermore, the modularity of the intake law allows to control scavenging of the residual burnt gas according to the pressure difference between the intake pressure and the exhaust pressure. 
         [0053]    The present invention is not limited to the example described and it encompasses any variant or equivalent.