FR2884284A1 - Piston for direct injection diesel engine, has piston head terminated by base in which bowl is made, reinforcement arranged at surface of boss in base of bowl, and another reinforcement made in base of head and outside bowl - Google Patents

Abstract

The piston has a piston head (10) terminated by a base (12) in which a bowl (14) is made. The bowl is delimited by a lateral wall (16) and a base (18) presenting a central boss (20). A reinforcement (26) is arranged at the boss surface. Another reinforcement (28) is made in the base (12) and outside the bowl. The reinforcements are made under the form of circular grooves coaxial with the piston head.

Description

Air reserve piston

  The present invention relates to a piston, in particular of the type used in diesel engines.

  An evolution of diesel engines consists in reducing emissions of pollutants, especially nitrogen oxides (NOx). However, it is difficult to greatly reduce the formation of nitrogen oxides while keeping a fairly low level of particles, including soot.

  To achieve this goal, several solutions are being implemented.

  On diesel direct injection engines, a particular injection strategy consisting of a multiple injection (multi injection) is implemented. However, this multi injection more frequently solicits the injector and thus reduces its life.

  Another known solution is to achieve a richer combustion by decreasing the volume of the combustion chamber defined by the cylinder head, the cylinder and a bowl formed in a base of the piston head. This solution however has the disadvantage that a rich combustion increases the formation of soot particles.

  In addition, to reduce the amount of soot emitted, a proposed solution lies in the use of particulate filters. However, this equipment is expensive both for installation on the vehicle and for use since it generates overconsumption. In addition, this particle filter reduces the performance of the vehicle due to the increase of the exhaust back pressure.

  To reduce the amount of soot, patent EP 0 969 191 also discloses a piston comprising a generally cylindrical piston head terminated by a base in which a bowl is dug. This bowl is delimited by a side wall and a bottom having a boss. This bowl is surrounded by a groove formed on the base of the piston head, which forms a reserve of air at the moment when the piston is at its top dead center. With such a device, it is possible to obtain a richer air + fuel mixture before combustion, for example by producing a bowl of reduced volume, thus making it possible to achieve a rich combustion. This makes it possible to keep a low level of nitrogen oxides in the gases formed by the combustion of the mixture. In return, a large amount of soot forms inside the cylinder. During the expansion of the combustion gases, that is to say during the time of descent of the piston from its top dead center position to its bottom dead position, the air, which was trapped in the groove when the air + fuel mixture burned, can provide oxygen to improve the oxidation of soot. Thus, the soot is post oxidized before it reaches the oil film on the inner surface of the cylinder and does not soil this film of oil.

  However, this piston has the disadvantage that only a limited fraction of the soot is post oxidized when the piston goes down.

  The object of the invention is to provide a solution for limiting the formation of nitrogen oxides while limiting the amount of soot formed.

  This object of the invention is achieved by means of a piston intended for an internal combustion engine comprising a head of generally cylindrical shape terminating in a base in which a bowl is hollowed out, the bowl being delimited by a lateral wall and a bottom. having a boss, remarkable in that a first recess is formed on the surface of the boss.

  During the expansion of the combustion gases, the soot particles are thus advantageously post-oxidized by means of the air reserve formed by the recess provided on the surface of the boss, where the air, which has not been mixed with fuel, did not react at the time of ignition in the bowl. Thus, there is obtained a device for reducing the level of NOx nitrogen oxides in the exhaust gas while limiting the amount of soot formed.

  Preferably, said first recess is in the form of a first circular groove, preferably coaxial with the piston head.

  Thus, the distribution of the air reserve is more homogeneous in the bowl and thus the distribution of oxidized soot is more homogeneous in the bowl and the cylinder. In addition, this gives a better balance of the piston head and the piston.

  Preferably, a second recess is formed in said base of the piston head, outside the bowl.

  It is thus possible to postoxidize a larger part of the combustion gases in the cylinder during the relaxation succeeding the combustion of the mixture.

  Preferably, said second recess is in the form of a second circular groove, preferably coaxial with the piston head.

  Thus, the distribution of the air reserve is more homogeneous and therefore the distribution of post-oxidized soot in the bowl and the cylinder is also more homogeneous. Moreover, a better balancing of the piston head and the piston is thus obtained.

  Preferably, the volume and the shape of this first recess are adapted as a function of the position of the recess and the circulation of the fuel flow injected into the bowl.

  Thus, preferably, the volume of the first recess is less than 10% of the total volume of said bowl and the second recess.

  Preferably, the volume of said second recess is greater than 10% of the total volume of said bowl and the second recess, and / or less than 50% of the total volume of said bowl and second recess.

  A large volume of this second recess allows to oxidize a larger portion of the soot.

  Preferably, said boss is of revolution and coaxial with said piston head.

  Thus, advantageously, the piston head is well balanced, as well as the piston.

  The invention also relates to a remarkable internal combustion engine in that it comprises at least one piston according to the invention.

  Other advantages and characteristics of the invention will appear on examining the description of the preferred embodiments which follow, presented solely by way of non-limiting examples, with reference to the appended figures in which: FIGS 1 and 2 show schematically in section the head of a piston according to the invention, according to a first embodiment; - Figure 3 shows schematically in section the piston head of Figures 1 and 2, the piston cooperating with a cylinder and a cylinder head; and - Figure 4 shows schematically in section a half of the head of a piston according to the invention, according to a second embodiment.

  In these figures, the same alphanumeric references have been used to designate the identical elements.

  Figure 1 shows schematically in section the piston head 10 of a piston according to the invention for an internal combustion engine.

  This piston head 10 of axis A is generally cylindrical in shape, delimited by an external lateral wall 11 and terminated by a base 12. In this base 12 is hollowed out a bowl 14 with symmetry of revolution and delimited by a lateral wall 16 and a bottom 18. The bottom 18 of the bowl 14 has a central boss 20 and, in this case, symmetrical of revolution with respect to the axis A. The base 12 thus includes the side wall 16 and the bottom 18 of the bowl, as well as the surface 22 outside the bowl, delimited by the edge 24 of the outer side wall 11 of the piston head 10 on the one hand and by the upper edge 25 of the side wall 16 of the bowl 14 of the other go.

  Furthermore, a first recess 26 is formed on the surface of the boss 20, this first recess 26 being made, in this case, in the form of a first circular groove having a flared U-shaped section. In the example presented, this circular groove 26 and the piston head 10 are coaxial. Of course, this embodiment of the first recess 26 is not limiting.

  In particular, the recess 26 may take other forms such as, for example, the shape of several grooves, possibly discontinuous, extending circumferentially to the surface of the boss 22. The recess 26 may also, according to other embodiments of the invention, not be coaxial with the piston head 10. However, the realization of this first recess 26 in the form of a circular groove coaxial with the piston head 10 allows a homogeneous distribution of the air reserve as well as a better balancing of the piston head 10 and the piston.

  In the embodiment shown in Figure 1, the piston head 10 further comprises a second recess 28 formed in the base 12 of the piston head 10, this second recess 28 being formed on the surface 22 outside the piston. bowl 14. In this case and without limitation, this second recess 28 is formed as a circular groove, coaxial with the piston head 10 and substantially U-shaped section flare. However, this recess may also take other forms and may for example be made by several grooves, optionally discontinuous, extending circumferentially. In addition, this recess 28 is not necessarily coaxial with the piston head 10. However, the realization of this second recess 28 in the form of a circular groove coaxial with the piston head 10 allows a homogeneous distribution of the reserve air, and a better balance of the piston head 10 and the piston.

  With reference to FIG. 2, the following volumes are defined: the total volume V of the bowl 14 comprises the partial volume V 'of the bowl 14 without the first recess 26 and the volume V' of the first recess 26 and is therefore defined by the cylindrical envelope 30 (represented by a dashed line) of the piston head 10, the side wall 16 of the bowl 14 as well as by the bottom 18 of the bowl 14, the volume V2 is the volume of the second recess 28 and is therefore delimited on the one hand by the cylindrical envelope 30 of the piston head 10 and on the other by the second recess 28 itself - the partial volume V 'corresponds to the volume V of the bowl 14 without the recess 26, - the volume V, "is the volume of the first recess 26 and is delimited by the bottom 18 of the bowl 14 if it had a boss 20 without the first recess 26 (dashed line in Figure 2) and, other on the other hand, by the recess 26 itself; and the total volume V representing the sum of the two volumes V, and V2: V = V, + V2 (E1). Thereafter, the invention will be described, in a nonlimiting manner, in the context of its implementation in a diesel engine, with reference to FIG. 3 which shows in section the cooperation of the piston head 10 with a cylinder 32 and a cylinder head 34, the piston being in its top dead center position.

  In a conventional manner, the piston slides in the cylinder 32 closed at its upper end by the yoke 34.

  Initially at its top dead center position, the piston slides in the cylinder 32 down to its bottom dead position, the cylinder 32 filling with air. With the piston in its low dead position, the cylinder 32 and the bowl 14 are filled with air. The piston then rises to its top dead position and the air present in the cylinder 32 is compressed. Fuel is then injected by means of an injection device 36 into the partial volume V 'of the bowl 14 while the piston is between its bottom dead position and its top dead center position.

  Due to the shape of the bowl 14 and in particular the presence of the boss 20, the fuel spreads substantially homogeneously in the partial volume V, 'of the bowl 14. However, the first and second recesses 26, 28 are arranged and shaped so as to remain substantially filled with air after fuel injection, shown in Figure 3 by the arrows 38, 40, so the fuel does not propagate in the volumes V, "and V2 of the first and second recesses 26, 28. Shortly before the piston is in its top dead position, the air-fuel mixture formed in the partial volume V, of the bowl 14 burns, however, the air in volumes V, V and V2 of the first and second recesses 26, 28 do not react since it is not loaded with fuel. With a bowl 14 of relatively small size, it is possible to achieve a rich combustion, that is to say that the fuel is in excess of the air available in the partial volume V, 'of the bowl 14. Such a rich combustion makes it possible to reduce the production of NOx nitrogen oxides. However, this rich combustion generates an important formation of soot.

  These soot are then oxidized when the piston slides towards its bottom dead position while the gaseous mixture present in the bowl 14 expands: the soot present in the peripheral zone of the bowl 14, that is to say close to the side wall 16 of the bowl 14, are oxidized at the beginning of the expansion with the oxygen available in the second recess 28; the soot present in the central zone of the bowl 14 is oxidized by means of the air available in the first recess 26 and possibly the remainder of the air still available in the second recess 28.

  With such a piston head 10, it is thus possible to post-oxidize a large part of the soot formed during the ignition of the mixture, richer air + fuel.

  In order to be able to oxidize the soot during the expansion phase, the oxygen reserve in the second recess 28 is preferably large, the volume V2 defined by this second recess 28 being preferably greater than 10% of the total volume V However, the volume V2 is preferably less than 50% of the total volume V. Finally, the volume V, defined by the first recess 26, is preferably reduced. preferred, less than 10% of the total volume V. Figure 4 shows schematically in section a half of the head of a piston according to the invention, according to a second embodiment. In this FIG. 4, the piston head 10 is substantially identical to that shown in FIGS. 1 to 3. However, the piston head 10 of FIG. 4 has an oil circulation duct 42, substantially made to the right of the second recess 28, which, in a manner known per se, makes it possible to circulate oil in the piston head 10, in order to improve the cooling of this piston head 10. Thus, it is possible to use the piston while that the motor load is greater. The bottom 44 of the piston is also shown in this FIG. 4. As shown in FIG. 4, three main dimensions determine the configuration of the first and second recesses 26, 28 as well as the shape of the bowl 14. Thus, the distance di represents the minimum distance between the side wall 16 of the bowl 14 and the duct 42.

  Advantageously, d is at least equal to a minimum value d, min to ensure a suitable thermomechanical behavior and therefore the desired lifetime for the piston. Preferably, d, min is between 6 and 7 mm.

  Similarly, the distance d2, representing the distance between the duct 42 and the second recess 28 is at least equal to a minimum value d2 min, between 6 and 7 mm, which advantageously increases the service life of the piston head 10.

  Finally, the distance d3, representing the distance between the bottom of the first recess 26 and the bottom 44 of the piston, is preferably at least equal to the minimum value d32min making it possible to ensure a satisfactory thermomechanical behavior of the piston head 10. preferably, this value d3 min is of the order of 10 mm.

  Of course, the present invention is not limited to the embodiments presented above. In particular, the bowl as well as the first and second recesses may not be coaxial with the piston head. Furthermore, the first and second recesses can be made in a less regular shape than circular grooves, in particular in the form of several grooves, optionally discontinuous, extending circumferentially.

  Finally, the invention has been described in preferred embodiments where two recesses are provided in the piston head. However, the piston head may have only a recess, made at the boss in the bottom of the bowl, the second recess, made outside the bowl, being only optional.

Claims (10)

  1. Piston for an internal combustion engine having a piston head (10) of generally cylindrical shape terminated by a base (12) in which is hollowed a bowl (14), the bowl (14) being delimited by a side wall (16) and a bottom (18) having a boss (20), characterized in that a first recess (26) is provided on the surface of the boss (20).   2. Piston according to claim 1, characterized in that said first recess (26) is in the form of a first circular groove.   3. Piston according to claim 2, characterized in that said first circular groove (26) and said piston head (10) are coaxial.   4. Piston according to one of the preceding claims, characterized in that a second recess (28) is formed in said base (12) of the piston head (10), outside the bowl (14).   5. Piston according to claim 4, characterized in that said second recess (28) is in the form of a second circular groove.   6. Piston according to claim 5, characterized in that said second circular groove and said piston head (10) are coaxial.   7. Piston according to any one of the preceding claims, characterized in that the volume (V, ") of the first recess (26) represents less than 10% of the total volume (V) integrating the total volume (V,) of said bowl (14) and, if appropriate, the volume (V2) of the second recess (28).   8. Piston according to any one of claims 4 to 7, characterized in that the volume (V2) of said second recess (28) represents more than 10% of the total volume (V) and / or less than 50% of the total volume (V).   9. Piston according to any one of the preceding claims, characterized in that said boss (20) is of revolution and coaxial with said piston head (10).   10. Internal combustion engine, characterized in that it comprises at least one piston according to any one of the preceding claims.