FR2734316A1 - IC engine with weak and normal mixture options - Google Patents

Abstract

Each cylinder's spark-plug(19) is located on the inter-section of two inclined planes comprising the combustion chamber roof, one containing two inlet(5a,5b), the other two exhaust(4a,4b) valve seats. An engine speed sensor(17) controls the larger inlet(3b,5b) via a butterfly(16) - opt., by modifying the valve action (5b) directly: the smaller inlet(3a,5a) contains the fuel injector(13). The exhaust outlets (4a,4b) coalesce(2a,2b). The angle and curvature of the smaller inlets produce intense combustion chamber turbulence, giving a homogeneous distribution. As load and speed increase (e.g. over 3500 r/m) the larger inlet opens (17,16), augmenting the air supply. The combined inlets produce stratified mixture and air layers. Exhaust gas recycling may be combined with either operating mode.

Description

 The invention relates to an internal combustion engine with positive ignition capable of operating with a mixture of air and fuel which is low in fuel or an air-fuel mixture diluted by exhaust gases.

 Reducing the consumption of spark-ignition engines is an important area of research in the automotive industry. The use of a fuel-poor air-fuel mixture is a known means to achieve this objective. However, if the operation with a lean mixture does indeed make it possible to obtain significant consumption gains and low nitrogen oxide (NOx) emission levels at the outlet of the combustion chamber, this operation has the drawback that the exhaust gases are very strongly oxidizing. As a result, the nitrogen oxides emitted by the engine cannot be reduced in the exhaust line by a conventional catalyst.

 To reduce the emissions of nitrogen oxides in the exhaust gases of lean-mixture engines, it is possible to use a specific catalyst called the De-NOx catalyst which uses unburnt hydrocarbons to carry out the reduction of oxides of nitrogen.

De-NOx catalysts do not, in their current form, have significant nitrogen oxide conversion rates or tend to poison themselves quickly. It is therefore necessary to reach emission levels of nitrogen oxides sufficiently low to meet anti-pollution standards, during engine operation with a lean mixture. This result must also be obtained without degrading the engine operating characteristics too much, when a lean mixture is not used.

 On the other hand, to meet increasingly demanding emission standards, it becomes necessary to operate the engines with an exhaust gas recircu lation (EGR) at intake, at least in part of the cycles of standardized operation. The use of EGR which consists in reintroducing part of the exhaust gases to the intake of the engines makes it possible to reduce the emissions of nitrogen oxides to a very low level, since the emission levels of NOx at the engine outlet are very low (of the same order as the levels observed with a lean mixture) and that on the other hand, it is then possible to use a three-way catalyst, this type of catalyst now being perfectly developed and with high efficiency. Operation with EGR significantly reduces fuel consumption compared to operation without EGR; however, this reduction is significantly less than that obtained by using a lean mixture.

 The operation of lean-mix internal combustion engines is a technique which has become commonplace and which requires circulating the air loaded with fuel or the gases inside the cylinders of the engine in a suitable manner so that combustion can initiate and spread in good conditions. These gas circulation or aerodynamic conditions also make it possible to obtain a gain in fuel consumption at very low speed due to the increase in the combustion speed, without even using a lean mixture.

 The operating mode of lean mixture engines results in the presence inside the cylinder chamber either of lean mixture and air in stratified form or of a lean mixture in homogeneous form.

 In the case of the first mode of operation, the combustion chamber of the cylinder has a zone rich in fuel, in particular around the spark plug, and one or more zones very poor in fuel. In this way, it is therefore possible to initiate combustion under good conditions, but the presence of fuel-rich zones requires the use of a very lean mixture to obtain acceptable NOx emissions.

 There is known an engine with four valves per cylinder comprising a spark plug in a central arrangement in the chamber of the cylinder which implements this first operating mode. Such an engine is described for example in European patent applications EP-A0,558,073, EP-A-0,558,080 and EP-A-0,558,081 filed by the company Mitsubishi Jidosha Kogyo Kabushiki Kaisha; this engine is designated by the name MVV. The fuel-rich zone around the spark plug is obtained by separating each of the intake ducts opening into a cylinder into two parts and by performing a high intensity vortex movement around an axis perpendicular to the axis of the cylinder. (called a tumble movement), in the combustion chamber of the cylinder, by means of a particular shape of the upper surface of the piston and of the intake ducts. The internal part of the intake ducts, i.e. say the part closest to a plane passing through the axis of the cylinder and perpendicular to the camshaft of the engine, receives a mixture of air and fuel whereas the external part receives only air. This results in the creation of a stratification at the time of intake, with a fuel-rich zone in the center of the combustion chamber.

This stratification is maintained during compression, thanks to the intensity of the tumble movement which also makes it possible to create, during its final dissipation, the turbulence necessary for good propagation of the flame.

 This type of engine therefore has certain qualities and in particular, it keeps the spark plug in a central position in the combustion chamber, guarantees an optimal path of the flame during combustion and has good knock resistance at full load.

 However, this engine has a significant defect, since it cannot adapt perfectly to all operating conditions. In particular, the shape of the conduits necessary for obtaining a significant level of tumble making it possible to operate the engine with a lean mixture is not at all favorable for obtaining a satisfactory filling at full load and at high engine speed. . This type of engine therefore operates at the expense of maximum power and torque.

 A second type of engine which can use a lean mixture is designed to obtain the most homogeneous air-fuel mixture possible at the entry into the cylinder head of the engine.

 Multipoint petrol injection, that is to say independently in each of the engine intake ducts, does not allow perfect homogeneity to be achieved. One can use one injection per phase and take advantage of the heterogeneities of the resulting mixture by trying to have a richer area around the spark plug at the time of lighting.

 In this type of engine, the richness differences between the different zones of the combustion chamber are however much smaller than in the case of engines producing a stratification of the mixture. Therefore, it is easier to limit the emission level of nitrogen oxides.

 An engine is known, for example, produced by the Toyota Motor Corporation and designated by the name 4AFE with four valves per cylinder which has, for each of the cylinders, an injector forming two jets of fuel and a throttle valve in one of the conduits. intake opening into the cylinder which is closed during lean mixture operation, so as to increase the intensity of the tumble movement in the combustion chamber. Because relatively good homogeneity of the mixture is obtained in the combustion chamber, low emission levels of nitrogen oxides can be achieved without having to use very poor mixtures.

The intake duct of each of the cylinders which does not have the closing butterfly has a deflector to increase the level of vortex flow in the combustion chamber. A high level of vortex flow in the cylinder limits the possibilities of filling the cylinder, especially at high speed.

 Also known is an engine manufactured by the Honda company and designated under the name VTEC-E which has a variable distribution system (VTEC) making it possible to close almost entirely one of the two intake valves and thus to increase the intensity. low-speed vortex flow, especially when operating with a lean mixture.

 In the case of the Toyota company engine, as in the case of the Honda company engine, it is necessary to introduce quantities of fuel which may be significant in a partially closed conduit in which circulates a low air flow. The richness of the mixture in this conduit is therefore very much higher than the average richness of the mixture, which is a factor of heterogeneity.

 In the case of certain engines which can use lean mixtures and in particular in the case of the engine of the company Mitsubishi (type MVV), the upper surface of the cylinders machined in the cylinder head of the engine is a roof-shaped surface with two slopes having an upper edge along a median plane of the cylinder. In this case, the spark plug is placed in the cylinder head, so as to open into the cylinder chamber in the vicinity of the edge of the roof-shaped surface. The vortex movement, inside the cylinder chamber, around an axis perpendicular to the cylinder axis, which is produced by the shape and arrangement of a duct and possibly an inlet valve, must be such that a sweep of the area of the roof ridge into which the spark plug opens.

 The recycling of exhaust gases, which makes it possible to reduce the fuel consumption of engines and to limit the rejection of polluting substances, consists in introducing into the air sent into the engine cylinders through the intake ducts, a certain fraction of the gases. exhaust and to achieve a homogeneous mixture of exhaust gases, air and fuel before entering the combustion chamber of the engine. The admission of the homogeneous mixture obtained must be carried out so as to obtain aerodynamic conditions in the cylinder chamber similar to those which are sought in the case of a homogeneous lean mixture.

 The introduction of exhaust gases into the intake portion of the engine which uses a valve called an EGR valve can be carried out in two different ways. The exhaust gases can be introduced collectively for all the cylinders, upstream of the intake manifold, this introduction being called single point or, on the contrary, individually for each of the cylinders in each of the branches of the intake manifold (multipoint admission).

 In the case of the Honda VTEC-E engine, the recycled exhaust gases are introduced into each of the intake ducts opening into the engine cylinders. Such multipoint recycling which can be implemented in engines using lean mixtures has the advantage of providing a very short response time during engine operations in transient mode but risks producing poor distribution of the recycled gases between the cylinders.

 So far, there has been no known internal combustion engine with positive ignition capable of operating with a lean mixture or with exhaust gas recycling, in a completely satisfactory manner and having good operating characteristics outside of use of the engine in lean mixture.

 The object of the invention is therefore to propose an internal combustion engine with positive ignition comprising at least one cylinder in which a piston is mounted mobile, having a combustion chamber defined at one of its ends inside a cylinder head of the engine, by a roof-shaped surface with two slopes having an edge in the vicinity of which opens a spark plug inside the cylinder chamber and two intake ducts formed by the cylinder head and opening into the cylinder, each of the intake ducts being equipped with an intake valve for its opening and closing and at least one of the intake ducts and the corresponding valve being produced so as to ensure an intense vortex flow in the cylinder, at least during certain engine operating phases, to ensure the sweeping of the open part of the spark plug, this engine being able to operate satisfactorily comfortable with a lean mixture or with exhaust gas recycling, so as to limit fuel consumption and nitrogen oxide emissions from the engine, while having satisfactory performance outside of use in a lean mixture or with recycled exhaust gases.

For this reason,
- the intake ducts have different diameters from each other, so as to constitute a first duct called large duct having a diameter greater than the diameter of the second duct called small duct, the corresponding intake valves being called large valve and small valve, respectively,
- The large intake duct includes a means for controlling the admission of gas into the cylinder to keep the large duct closed below a fixed value of the engine speed and to ensure opening and maintenance at open condition of the large intake duct, at the set value of the engine speed and above,
a fuel injector opens into the small intake duct, so as to inject fuel into a gas flowing in the small intake duct, and
- The small duct and the small intake valve have a shape and an arrangement relative to the cylinder such that they can produce an intense vortex flow of gas introduced into the cylinder through the small intake duct.

 In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the appended figures, an internal combustion engine according to the invention and its operation at low speed and at high speed. .

 Figure 1 is a schematic top view of a cylinder of an engine with four valves per cylinder according to the invention, in a configuration allowing it to operate with a lean mixture.

 FIG. 2 is a view in axial section along 2-2 of FIG. 1 or of FIG. 3.

 Figure 3 is a schematic top view of an engine cylinder shown in Figures 1 and 2, in a configuration allowing its operation at high speed.

 In FIG. 1, there is seen a cylinder 1 of an internal combustion engine according to the invention comprising two exhaust valves 4a and 4b and two intake valves 5a and 5b capable of closing or opening two conduits exhaust 2a and 2b and two intake ducts 3a and 3b respectively.

 As can be seen in Figure 2, the cylinder 1 is arranged inside a cylinder block 7 on which rests a cylinder head 8 in which are machined the intake ducts such as 3a and the exhaust ducts such than 2b.

 A piston 9 is movably mounted inside the cylinder 1, in the axial direction 10 of the cylinder.

 The combustion chamber 6 of the cylinder 1 is delimited, at its lower part, by the upper surface of the piston 9 and, at its upper part, by a surface 11 machined inside the cylinder head 8. The surface 11 is a surface in the form of a roof with two slopes, the two parts of the roof having in common a straight edge 12 constituting the upper edge of the combustion chamber 6.

 The exhaust conduits 2a and 2b and the intake conduits 3a and 3b are machined in the cylinder head and connected on the lateral faces of the cylinder head to exhaust or intake pipes.

 The exhaust valves such as 4b and the intake valves such as 5a which are shown in FIG. 2 have a valve stem mounted movably in the cylinder head 8 and a valve head intended to come to bear in the position of closing the valve on a valve seat arranged around the part of the corresponding exhaust or intake duct opening into the combustion chamber 6 of the cylinder.

 The exhaust valves and the intake valves are controlled by a camshaft which ensures their opening and closing at precise moments in the engine operating cycle.

 According to the invention, the two intake ducts 3a and 3b opening into the combustion chamber 6 of the cylinder 1 have different diameters, the diameter of the intake duct 3b being substantially greater than the diameter of the intake duct 3a.

 The heads of the valves 5a and 5b ensuring the closing of the respective intake conduits 3a and 3b themselves have different diameters, the valve 5b comprising a head whose diameter is substantially greater than the diameter of the head of the valve 5a.

 Thereafter, the intake duct 3b will be designated as the large duct, the valve 5b as the large valve, the intake duct 3a as the small duct and the valve 5a as the small valve.

 The circular section of the small duct 3a and of the head of the valve 5a have a diameter which is at most equal to one third of the bore A of the cylinder 1.

 On the other hand, the section of the large conduit 3b and of the head of the large valve 5b generally have a diameter greater than one third of the bore A of the cylinder 1.

 Inside the small duct 3a opens a fuel injector 13 capable of injecting, at well-defined instants of the engine operating cycle, into a stream of air or gas circulating in the small duct 3a, as indicated by schematically by the arrow 15, a jet of fuel 14. The quantities of fuel injected into the air stream or gas 15 flowing in the small intake duct 3a are carefully dosed, so as to adjust the richness of the mixture which is introduced into the combustion chamber 6 of the engine, through the intake duct 3a.

 The mixture is introduced into the chamber 6 of the cylinder 1, when the small intake valve 5a of the conduit 3a is in its raised position, as shown in FIG. 2.

 Inside the large intake duct 3b is disposed a throttle valve 16 connected to a control unit 17 making it possible to open or close the throttle valve 16 according to the engine speed expressed in revolutions / minute.

 The engine speed is measured by a sensor which is connected to the control unit 17 of the throttle valve 16.

 As long as the engine speed remains below a certain fixed value, for example 3500 rpm, the control unit 17 maintains the throttle valve 16 in its closed position as shown in FIG. 1.

 Air or gas mixed with fuel enters the combustion chamber 6 of the cylinder 1, at defined times, through the small duct 3a, when the small intake valve opens. 5a.

 The tubing connected to the large intake duct 3b is supplied with air by the engine intake manifold. This air cannot reach the large duct 3b, because the butterfly 16 is in the closed position.

 The combustion chamber 6 of the cylinder 1 is supplied only with a mixture of air and fuel or air, gas and fuel, through the intake duct 3a.

 As can be seen in FIG. 2, when the valve 5a is in the raised position, the head of the valve which has moved towards the interior of the combustion chamber 6 is at a distance 1 from the valve seat constituting the opening end of the small conduit 3a, this distance 1 constituting the value of the lifting of the valve 5a.

 The small intake valve 5a is designed so that, in the fully open position of the valve (maximum lift l shown in FIG. 2), the distance p from the edge of the valve head 5a to the cylindrical surface lateral of cylinder 1 is at least equal to the lift l.

 This arrangement allows an introduction of the mixture of air and / or gas and fuel into the combustion chamber 6 of the cylinder with very high aerodynamic intensity. There is a vortex movement of gas around an axis perpendicular to the axis 10 of the cylinder 1, as shown by the arrows 18.

 This vortex movement which is a tumble movement is inclined so as to carry out an efficient aerodynamic scanning of the edge 12 of the roof-shaped surface 11 and of the end of the spark plug 19 of the cylinder which is disposed at the in the vicinity of the edge 12 and in a substantially central position in the chamber 6 of the cylinder 1.

 This aerodynamic scanning of the zone into which the spark plug 19 opens makes it possible to move the flame away from the walls of the cylinder, as soon as it is formed and to favor the initiation of the ignition of the mixture in chamber 6 of the cylinder.

 In addition, the small intake duct 3a itself is produced so as to contribute to the formation of a high energy tumble ensuring the scanning of the area of the edge 12. The small duct 3a which alone provides the admission of the mixture into the cylinder chamber, in the operating mode shown in Figure 1 must have good permeability, so as to promote the circulation of gases and the filling of the cylinder chamber.

 These results are obtained by providing a small intake duct 3a as shown in FIG. 2.

 The conduit 3a comprises a rectilinear part having an axis 20 and a slightly curved part, following the rectilinear part, machined in the cylinder head 8 and opening into the cylinder chamber. The axis 20 of the rectilinear part of the small conduit 3a is concurrent with the stem of the valve 5a and made with the stem of the valve 5a, an angle a.

 The curved part of the small intake duct 3a machined in the cylinder head 8 has a maximum radius of curvature R.

 In order to obtain the desired aerodynamic conditions inside the chamber 6 of the cylinder and good permeability of the conduit 3a, conditions relating to the position of the head of the valve 5a in the lifted position are imposed in addition to the angle a is at most equal to 45 "and the maximum radius of curvature R of the curved part of the intake duct 3a is at most equal to approximately 1.3 times the diameter d of the duct 3a.

 When the conditions mentioned above are met, one can obtain a completely satisfactory operation of the engine comprising the cylinder 1 which has just been described, at low speed (for example at less than 3500 rpm) as well with a richness mixture 1, that is to say a mixture of air and fuel of stoichiometric composition, without recycling of exhaust gases, with a richness mixture 1 with EGR recycling of exhaust gases or again with a lean mixture.

 In all cases, the geometry of the small intake duct and of the small intake valve 5a and the maintenance in the closed position of the large duct 3b make it possible to obtain engine operation with low fuel consumption and low production of nitrogen oxides.

 Preferably, inside the small intake duct 3a, an injector of the air jacket type is used to obtain good spraying of the fuel.

 When the engine reaches a high speed, the threshold value being fixed beforehand, the control unit 17 opens the shutter 16, as shown in FIG. 3.

 Air circulated inside the large duct 3b, as shown diagrammatically by the arrow 21, is introduced into the part 6b of the combustion chamber 6 of the cylinder 1.

 The small intake duct 3a in which the injector 13 forming the fuel jet 14 is arranged ensures the introduction into the part 6a of the combustion chamber 6 of the cylinder of a mixture of air and fuel.

 The opening of the large duct 3b makes it possible to achieve good conditions for filling the cylinder, when the engine is running at high speed.

 The shutter 16 control optimizes the filling of the cylinder during engine operation. When the engine is running at full load, the shutter 16 remains closed until the engine speed reaches a value such that the filling is improved by opening the large duct 3b.

 It is thus possible to optimize the value of the engine torque for all of the operating speeds.

 In the case of the operation represented in FIG. 1, the mixture filling the combustion chamber of the cylinder is substantially homogeneous. In the case of the operation shown in Figure 3, the arrangement of the mixture and air layers is laminated.

 As a variant, the engine according to the invention can be produced without using a butterfly for closing the large intake duct 3b, the opening and closing of the duct 3b depending on the engine speed being effected by the large valve d 'admission 5b. In this case, the large intake valve 5b is controlled by a variable distribution allowing it to be placed either in the closed position or in the open position, depending on whether the engine is at low or high speed.

 The spark plug 19 can be shifted slightly towards the small injection valve 5a ensuring the introduction of the fuel mixture. However, it is necessary to maintain the spark plug in a position close to the center of the combustion chamber 6.

This position provides better knock resistance and a significant flame path without contact with the walls of the chamber.

 It is preferable to provide an ignition energy having a high level, for example an ignition energy at least equal to 100 mJ, so as to ensure good ignition conditions when operating with a lean mixture or with consistent recycling of exhaust gases. A high ignition energy makes it possible to improve the operation of the engine at the highest revs of the operating range in which the large intake duct 3b remains closed.

 In the case where an EGR recycling of the exhaust gases is carried out in a homogeneous manner, the exhaust gases can be introduced at the intake, either upstream of the manifold (single point EGR), or by a supply ramp allowing individual introduction into each of the small cylinder intake ducts (multipoint EGR).

 In both cases, the flow can be controlled by a single EGR valve.

 Ultimately, the engine according to the invention can operate in a completely satisfactory manner, both at low speed and at high speed, with a lean mixture or with a richer mixture with or without recycling of exhaust gases. . The engine according to the invention therefore has satisfactory performance both when it is used with a lean mixture and in a different use. This engine also makes it possible to obtain gains in terms of fuel consumption, for operations at very low load or at idle, the use of a lean mixture not generally being suitable for operations of this type.

 The engine according to the invention also makes it possible to obtain an increase in the torque at low speed while maintaining a satisfactory torque at high speed.

 The engine according to the invention makes it possible to limit the emissions of nitrogen oxide NOx to a low level, these emissions being able to be further reduced by carrying out EGR recycling of exhaust gases and by using a three-way catalyst.

 The invention is not limited to the embodiment which has been described.

 This is how one can envisage a geometry of the intake ducts different from those which have been described.

 It is also possible to control the opening and closing of the large intake duct in a manner different from those which have been described.

 The invention applies to any internal combustion engine with positive ignition comprising two intake valves per cylinder.

Claims (9)

 1.- Internal combustion engine with positive ignition comprising at least one cylinder (1) in which a piston (9) is movably mounted, a combustion chamber (6) of the cylinder (1) delimited at one of its ends at the interior of a cylinder head (8) of the engine by a surface (11) in the form of a roof with two slopes having an edge (12) in the vicinity of which a spark plug (19) opens inside the chamber (6) of the cylinder (1) and two intake ducts (3a, 3b) formed in the cylinder head (8) and opening into the chamber (6) of the cylinder (1), each of the intake ducts (3a, 3b) being equipped with an inlet valve (5a, 5b) for its opening and closing and at least one of the inlet ducts (3a) and the corresponding valve (5a) ensuring an intense vortex flow (18 ) in the chamber (6) of the cylinder (1), at least during certain operating phases of the engine to ensure sweeping of the open part of the spark plug d 'ignition (19), characterized in that  - the intake ducts (3a, 3b) have different diameters from one another, so as to constitute a first duct (3b) called large duct having a diameter greater than the diameter of the second duct (3a) called small duct, the corresponding intake valves (5a, 5b) being called large valve and small valve, respectively  - that the large intake duct (3b) includes a gas intake control means (16, 5b) in the chamber (6) of the cylinder (1) to keep the large duct (3b) closed below d '' a fixed value of the engine speed and to ensure the opening and maintenance in the open state of the large intake duct (3b) at the fixed value of the engine speed and above,  - a fuel injector (13) opens into the small intake duct (3a) so as to inject fuel (14) into a gas flowing in the small intake duct (3a), and  - that the small duct (3a) and the small intake valve (5a) have a shape and an arrangement relative to the chamber (6) of the cylinder (1) such that they can produce an intense vortex current of gas introduced into the chamber (6) of the cylinder (1) through the small intake duct (3a).  2.- Motor according to claim 1, characterized in that the small valve (5a) comprises a valve head for closing a valve seat at the open end of the small conduit (3a) whose minimum distance to the inner wall of the chamber (6) of the cylinder (1) in the fully open position of the small valve (5a) is at least equal to the distance (1) of maximum lift of the valve, i.e. - tell the distance between the valve head and the valve seat in the fully open position of the small valve (5a).  3.- Motor according to claim 2, characterized in that the small intake duct (3a) comprises a rectilinear part having an axis (20) concurring with the rod of the small valve (5a) and a curved part of radius maximum curvature (R) opening into the chamber (6) of the cylinder (1), that the rod of the small valve (5a) makes an angle a at most equal to 45C with the axis (20) of the rectilinear part of the small intake duct (3a) and that the maximum radius of curvature (R) of the curved part of the small intake duct (3a) is at most equal to 1.3 times the diameter (d) of the small intake duct admission (3a).  4.- Engine according to any one of claims 1 to 3, characterized in that the opening part of the spark plug (19) is arranged in the vicinity of the axis (10) of the cylinder chamber (1 ), in a position slightly offset towards the small intake valve (5a).  5.- Engine according to any one of claims 1 to 4, characterized in that the spark plug (19) has an ignition energy at least equal to 100 mJ.  6.- Motor according to any one of claims 1 to 5, characterized in that the gas intake control means of the large intake duct (3b) is constituted by a shutter of the butterfly type disposed at the inside the large duct (3b) and a control unit (17) of the shutter (16), according to a set value of the engine speed.  7.- Engine according to any one of claims 1 to 5, characterized in that the gas intake control means of the large intake duct (3b) is constituted by the large intake valve (5b) and a variable distribution system for ensuring the opening or closing of the large intake valve (5b), at the set value of the engine speed.  8.- Engine according to any one of claims 6 and 7, characterized in that the setpoint of the engine speed is close to 3500 revolutions / minute.  9.- Motor according to any one of claims 1 to 8, characterized in that the small valve (5a) comprises a valve head whose diameter (D) is at most equal to one third of the bore (A) of the cylinder (1).