Abstract:
A method facilitates vaporization of a fuel in a direct-injection four-stroke engine including at least one cylinder with a combustion chamber, at least one fluid intake with an intake pipe and a valve, at least one burnt gas exhaust with an exhaust pipe and a valve, a valve controller and fuel injector. The method includes, in the vicinity of the intake phase start, in opening the intake valve so as to allow the intake fluid into the combustion chamber, in opening exhaust valve so as to feed again into this chamber at least part of the burnt gas contained in exhaust pipe; prior to the end of the intake phase, in closing the exhaust valve; in the vicinity of the intake phase end, in closing the intake valve; and during this intake phase, in carrying out at least one fuel injection into the combustion chamber.

Description:
[0001]    The present invention relates to a method for facilitating vaporization of a fuel in a direct-injection four-stroke engine comprising at least one cylinder ( 10 ) with a combustion chamber ( 18 ), at least one fluid intake means with an intake pipe ( 20 ) and a valve ( 22 ), at least one burnt gas exhaust means with an exhaust pipe ( 24 ) and a valve ( 26 ), valve control means ( 28 ,  30 ) and fuel injection means ( 32 ). 
         [0002]    According to the invention, this method consists: 
         [0003]    in the vicinity of the intake phase start, in opening intake valve ( 22 ) so as to allow the intake fluid into the combustion chamber, in opening exhaust valve ( 26 ) so as to feed again into this chamber at least part of the burnt gas contained in exhaust pipe ( 20 ), 
         [0004]    prior to the end of the intake phase, in closing the exhaust valve, 
         [0005]    in the vicinity of the intake phase end, in closing the intake valve, 
         [0006]    during this intake phase, in carrying out at least one fuel injection into said combustion chamber. 
       FIELD OF THE INVENTION 
       [0007]    The present invention relates to a method for facilitating vaporization of a fuel in a direct-injection internal-combustion engine with compression and self-ignition of a fuel mixture, in particular in a diesel type engine. 
         [0008]    Advantageously, it relates to an engine that can work according to two combustion modes, a homogeneous combustion mode, in particular HCCI (Homogeneous Combustion Compressed Ignition), and a conventional combustion mode. 
       BACKGROUND OF THE INVENTION 
       [0009]    The homogeneous operating mode, preferably used for low and medium engine loads, consists in injecting fuel into the combustion chamber very early (for example during the engine intake phase) so as to obtain homogeneous mixing of the fuel with a fluid such as air or a mixture of air and recirculated exhaust gas (EGR). 
         [0010]    For the conventional combustion operating mode, preferably used at high engine loads, a fuel injection is performed around the piston compression top dead center and a conventional combustion by self-ignition, then by diffusion occurs. 
         [0011]    With this combustion mode, it is also possible to carry out an early fuel injection referred to as “pilot” injection, at the intake phase start for example. 
         [0012]    In the homogeneous combustion operating mode, it is advantageous for the fuel injection to occur very early in the engine running cycle in order to obtain a homogeneous mixture, but there are risks of cylinder wall wetting by the fuel injected. 
         [0013]    Thus, a fuel injection at the start of the intake phase has the advantage of confining the fuel injected in the bowl the piston of such an engine is usually equipped with, while limiting contact of this fuel with the cylinder wall, but the temperature of the fluid contained in the combustion chamber is not high enough. It is therefore difficult for the fuel injected in this bowl, then in the combustion chamber, to vaporize. 
         [0014]    In the conventional operating mode, the pilot injection involves the same drawbacks as regards the injected fuel vaporization difficulties as those mentioned above. 
         [0015]    This fuel vaporization difficulty can lead to disruptions in the progress of the fuel mixture combustion and to an increase in the discharge of pollutants into the atmosphere, as well as a fuel overconsumption by the engine. 
         [0016]    For gasoline type indirect-injection engines, it is already known to perform vaporization by injecting this fuel in liquid form or in form of fine droplets into the exhaust gas contained in the intake pipe. Upon contact with this hot gas, the fuel vaporizes to mist and mixes not only with this gas, but also with the fluid fed thereafter into the combustion chamber of the engine. 
         [0017]    Such a transposition to direct-injection engines is impossible, essentially because fuel injection cannot take place in the intake pipe. 
         [0018]    However, better fuel vaporization essentially has the advantage of reducing carbon oxides (CO) and unburnt hydrocarbons (HC) emissions upon combustion of the fuel mixture. 
         [0019]    The present invention aims to overcome the aforementioned drawbacks by means of a method using the components usually present in a direct-injection engine. 
       SUMMARY OF THE INVENTION 
       [0020]    The invention thus relates to a method for facilitating vaporization of a fuel in a direct-injection four-stroke internal-combustion engine that can run according to a homogeneous operating mode and to a conventional operating mode, said engine comprising at least one cylinder with a combustion chamber, at least one fluid intake means with an intake pipe and an intake valve, at least one burnt gas exhaust means with an exhaust pipe and an exhaust valve, means for controlling opening/closing of the valves and fuel injection means, characterized in that it consists: 
         [0021]    in the vicinity of the intake phase start, in opening the intake valve so as to allow the intake fluid into the combustion chamber, in opening the exhaust valve so as to feed again into this chamber at least part of the burnt gas contained in the exhaust pipe, 
         [0022]    prior to the end of the intake phase, in closing the exhaust valve, 
         [0023]    in the vicinity of the intake phase end, in closing the intake valve, 
         [0024]    during this intake phase, in carrying out at least one fuel injection into said combustion chamber. 
         [0025]    The method can consist in carrying out at least one fuel injection before opening the exhaust valve. 
         [0026]    The method can consist in carrying out at least one fuel injection after opening the exhaust valve. 
         [0027]    The method can consist in carrying out at least one fuel injection after closing the exhaust valve. 
         [0028]    The method can consist in carrying out successive exhaust valve opening and closing cycles as the exhaust gas is re-introduced into the combustion chamber. 
         [0029]    The method can consist, in the vicinity of the start of the engine intake phase, in opening the exhaust valve before the intake valve. 
         [0030]    The method can consist, in the vicinity of the start of the engine intake phase, in opening the exhaust valve simultaneously with the intake valve. 
         [0031]    The means controlling at least the exhaust valve can allow to vary the lift law of said valve. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0032]    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: 
           [0033]      FIGS. 1 to 3  are diagrams of a direct-injection engine illustrating the progress of the various stages of the method according to the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0034]    In  FIGS. 1 to 3 , the internal-combustion engine shown is a direct-injection four-stroke (or four-cycle) internal-combustion engine that can run according to two combustion modes, a homogeneous combustion mode, in particular HCCI (Homogeneous Combustion Compressed Ignition), and a conventional combustion mode. 
         [0035]    The homogeneous operating mode, used for low and medium engine loads, consists in injecting fuel into the combustion chamber very early (during the engine intake phase for example) so as to obtain homogeneous mixing of the fuel with a fluid such as air or a mixture of air and recirculated exhaust gas (EGR). 
         [0036]    For the conventional combustion mode, preferably used at high engine loads, a fuel injection is carried out around the compression top dead center of the piston and a conventional combustion by self-ignition, then by diffusion occurs. It is also possible to perform an early fuel injection referred to as “pilot” injection, for example at the intake phase start. 
         [0037]    This engine comprises at least one cylinder  10  with a cylinder body  12  within which a piston  14  slides in a rectilinear reciprocating motion under the effect of a rod controlled by a crankshaft (not shown). The cylinder body is closed, in the upper part, by a cylinder head  16  thus delimiting a combustion chamber  18  consisting of the lateral wall of the cylinder body, the cylinder head and the upper part of the piston. 
         [0038]    What is referred to as the combustion chamber is the volume defined above plus the volume made up of a possible hollow bowl housed at the top of the piston. 
         [0039]    The cylinder head carries at least one intake means with an intake pipe  20  whose opening into the combustion chamber is controlled by an intake valve  22 . The cylinder head also carries at least one exhaust means with an exhaust pipe  24  whose communication with the combustion chamber is controlled by an exhaust valve  26 . 
         [0040]    Opening/closing of the intake and/or exhaust valves is controlled by means  28  and  30  allowing to vary the lift laws of these valves, as regards their opening/closing time as well as their lifts, independently of one another or in combination. These means are better known as VVT (Variable Valve Timing), VVL (Variable Valve Lift) or VVA (Variable Valve Actuation). 
         [0041]    It is also possible to control this exhaust valve and possibly the intake valve with control means referred to as “camless”, i.e. with no camshafts. In this case, the engine comprises an actuating means dedicated to each valve, such as an electromagnetic or hydraulic or electrohydraulic or pneumatic or electropneumatic control actuator that directly or indirectly acts upon the valve rod. 
         [0042]    In the example described in connection with  FIGS. 1 to 3 , opening/closing of intake valve  22  is controlled by conventional means  28  such as a camshaft, whereas exhaust valve  26  is controlled by means  30  allowing to vary its lift law, in particular with VVA type means. 
         [0043]    The engine also comprises fuel injection means  32 , preferably in form of a multijet injection nozzle (shown in the figure by an axis line), which spray fuel into combustion chamber  18  so as to obtain a fuel mixture with the fluid contained therein. 
         [0044]    Exhaust valve control means  30  and fuel injection means  32  are controlled by a calculating unit (not shown) commonly referred to as engine calculator, which an engine is usually equipped with. Of course, if intake valve control means  30  are camless type means or of the type allowing the lift laws of these valves to be varied, the calculator also controls these control means. 
         [0045]    The purpose of this calculator is notably to control opening/closing of the valves, as well as the nozzle injection parameters, such as the injection time during the engine cycle, the fuel injection duration, . . . . 
         [0046]    The description of the method hereafter is given in connection with the engine of  FIGS. 1 to 3 . 
         [0047]      FIG. 1  shows the configuration of the direct-injection engine after the end of the exhaust phase of this engine and at the start of its intake phase. In this configuration, piston  14  is at its intake top dead center (PMHa), combustion chamber  18  contains burnt gas  34  (or exhaust gas), the exhaust pipe only contains burnt gas and the intake pipe contains intake fluid. 
         [0048]    From this PMHa position, piston  14  is driven under the effect of the rod and the crankshaft for an intake phase wherein it follows a vertical downward motion (arrow F in  FIGS. 1 and 2 ) starting from its intake top dead center PMHa towards its compression bottom dead center (PMBc in  FIG. 3 ). 
         [0049]    In the vicinity of PMHa, the calculator controls control means  30  in such a way that exhaust valve  26  is in an open position while intake valve  22  is conventionally opened under the action of camshaft  28 . In this position and under the effect of the motion of piston  14 , burnt gas  34  contained in exhaust pipe  24  and the fluid contained in the intake pipe are fed into combustion chamber  18 . 
         [0050]    The calculator also controls, during this intake phase, at least one fuel injection into the combustion chamber by means of injection nozzle  32 . 
         [0051]    Advantageously, according to the example described, at least one fuel injection is carried out in the mixture of fluid admitted and of burnt gas re-introduced into the combustion chamber before closing of the exhaust valve. This fuel injection in the hot fluid/burnt gas mixture has the effect of vaporizing this fuel upon contact with this mixture while significantly reducing contact between the liquid fuel and the cylinder wall. 
         [0052]    Of course, without departing from the scope of the invention, this injection can occur before opening of the exhaust valve so as to perform vaporization of this fuel with the residual burnt gas usually present in the dead volume of the combustion chamber, then to continue this vaporization during introduction of the burnt gas contained in the exhaust pipe. 
         [0053]    Also, the fuel injected can be contacted with the mixture of fluid admitted and of burnt gas re-introduced into the combustion chamber after closing of the exhaust valve to provide vaporization of this fuel. 
         [0054]    Advantageously, at the start of the intake phase, exhaust valve  26  opens before opening of intake valve  22  as soon as allowed by the piston/cylinder head distance, more precisely the distance between the bottom of the piston recesses and the corresponding surface on the valve. This exhaust valve then closes at point V°. Opening of the intake valve can be considered very shortly after opening of the exhaust valve or quasi-simultaneously with the closing of the exhaust valve at point V°. Of course, any other configuration between these two valves can be considered, such as simultaneous opening thereof at the intake phase start. 
         [0055]    After a limited displacement of piston  14  that corresponds to some ten degrees of crank rotation angle (V° in  FIG. 2 ) from PMHa, the calculator controls closing of exhaust valve  26 , intake valve  22  conventionally remains in its open position and the fluid continues to be fed into combustion chamber  18  to be mixed with the carburetted feed. The injection nozzle can continue its fuel injection(s) into the combustion chamber during all or part of the descending motion of this piston. Upon contact with this feed mixed with the intake fluid, the injected fuel can thus vaporize better under the effect of the temperature of the feed contained in the chamber, and the homogeneity of the fuel mixture obtained is greatly improved. 
         [0056]    At the end of this intake phase, piston  14  is in the vicinity of the compression bottom dead center PMBc ( FIG. 3 ), intake valve  22  is in the closed position under the effect of camshaft  28 . 
         [0057]    The mixture obtained in chamber  18  upon closing of intake valve  22  in the vicinity of the end of the intake phase therefore is a fluid/vaporized fuel mixture that contains a certain amount of burnt gas. 
         [0058]    As it is known, the engine operating method continues with a fuel mixture compression phase and the possibility of continuing the fuel injections and combustion. 
         [0059]    During the intake phase with opening of the exhaust valve as described above, it is possible to inject the fuel as the burnt gas is fed into the combustion chamber. 
         [0060]    It is also possible to carry out a succession of exhaust valve opening/closing cycles associated with a fuel injection until the programmed closing at V° of this valve. This has the effect of minimizing the local temperature drop of the burnt gas when the calories contained therein are used for vaporizing the fuel injected into the combustion chamber. 
         [0061]    Of course, the person skilled in the art will control closing of the exhaust valve at V° in the engine intake phase in such a way that the amount of burnt gas present in the combustion chamber at the end of this phase does not alter the fuel mixture combustion by compression and self-ignition. 
         [0062]    The present invention is not limited to the embodiment examples described above and it encompasses any variant and equivalent. 
         [0063]    Notably, the method described can be used with then engine as described in French patents No. 2,818,324 and 2,818,325 filed by the applicant, comprising at least a cylinder, a piston sliding in this cylinder, a combustion chamber delimited on one side by the upper face of the piston comprising a teat arranged at the centre of a concave bowl with a bottom wall connecting the base of the teat and the peripheral wall of this bowl. The engine also comprises an injection nozzle for injecting fuel with a nappe angle smaller than or equal to 
         [0000]    
       
         
           
             
               2 
                
               Arctg 
                
               
                   
               
                
               
                 CD 
                 
                   2 
                    
                   F 
                 
               
             
             , 
           
         
       
     
         [0000]    where CD is the diameter of the cylinder and F the distance between the point of origin of the fuel jets from the injection nozzle and the piston position corresponding to a crank angle of 50° with respect to the top dead center.