WO2001051786A1

WO2001051786A1 - Method and device for improving the operation of supercharged internal combustion engines at low engine revs

Info

Publication number: WO2001051786A1
Application number: PCT/FR2001/000053
Authority: WO
Inventors: Daniel Drecq
Original assignee: Daniel Drecq
Priority date: 2000-01-11
Filing date: 2001-01-09
Publication date: 2001-07-19
Also published as: AR026829A1; WO2001051786B1; FR2803628A1

Abstract

The supercharged 4-stroke internal combustion engine (M) comprises: a first cylinder (1) provided with an intake valve and an exhaust valve; means for controlling the opening of said exhaust valve after the opening of the intake valve during part of the intake phase of the above-mentioned cylinder; supercharging means generating boost pressure in an intake manifold (56, 1a) so that the boost pressure is greater than the pressure inside the first cylinder and the pressure inside the first cylinder is greater than the pressure in an exhaust manifold (57, 1e), said supercharging means including a main turbocharger (5); in addition to at least one second cylinder (4) connected to the same exhaust manifold (57) as the first cylinder, whereby the supercharging means include an auxiliary turbocompressor (15) and the inertia thereof is considerably lower than the inertia of the main turbocompressor (5).

Description

METHOD AND DEVICE FOR IMPROVING LOW RPM OPERATION OF SUPERCHARGED THERMAL ENGINES.

The invention relates to a method for improving the operation at low speed of a four-stroke supercharged engine and to an engine arranged for the implementation of this method. More specifically, the invention relates to heat engines in which an exhaust valve of a cylinder is open during part of a suction phase of said cylinder to store an intake mixture in an exhaust manifold in order to post-fill the cylinder.

Currently, combustion engines supercharged by means of a turbocharger increasing the air pressure at the intake have a response time to acceleration affecting driving comfort. In particular, in the case of a diesel engine, a temporary lack of charge air occurs during the acceleration phase; this temporary lack of air results in the emission of puffs of black smoke from the exhaust, which are indicative of poor combustion resulting from this lack of air. The supercharging of intake air of a heat engine is carried out by means of a turbocharger comprising a turbine driven by the exhaust gases, said turbine driving a compressor compressing the intake air to supercharge the heat engine. The increase in the intake air pressure makes it possible to increase, at constant volume, the mass of air introduced into the cylinder of the heat engine during the intake phase. An additional increase in this mass of intake air can be obtained by cooling the air circulating between the compressor outlet and the air intake manifold. For this purpose, there is generally an air cooler between the outlet of the compressor and the intake of the heat engine.

The presence of the acceleration response time is explained by the fact that the energy recoverable in the exhaust gases to actuate the turbine of the turbocharger is not instantly available. When the driver accelerates, the quantity of fuel admitted or injected is increased, while the intake air is not yet compressed to supercharge the engine with an amount of air corresponding to good combustion of the amount admitted fuel. The enthalpy of exhaust gas is then not sufficient for the turbocharger turbine to recover enough energy to compress the intake air. This transient acceleration phase corresponds to combustion in very rich mode, that is to say with an excess of fuel which thus generates, in the case of diesel engine, puffs of black smoke at the exhaust.

Document WO 95/14853 discloses a method for improving the operation of a four-stroke thermal engine, supercharged by means of at least one main turbocharger or another main compression means, in which the steps are carried out following:

- open an exhaust valve at the same time as an intake valve of a first engine cylinder during at least part of an intake phase of said first cylinder, in order to transfer intake air from an intake manifold through said first cylinder to an exhaust manifold, a boost pressure being generated in said intake manifold so that, during said transfer, said boost pressure is greater than the pressure in said first cylinder and the pressure in said first cylinder is greater than the pressure in said exhaust manifold, said boost pressure being generated essentially by means of said main turbocharger when the engine is operating at medium or high speed, - temporarily store in said exhaust manifold said transferred intake air,

- generate an overpressure in said exhaust manifold after closing the intake valve and before closing the exhaust valve of the first cylinder by means of an exhaust puff from a second cylinder connected to the same manifold the first cylinder, in order to post-fill said first cylinder with said intake air previously stored in said exhaust manifold, possibly mixed with exhaust gases. Also known, from document WO 95/14853, is a four-stroke supercharged thermal engine arranged for the implementation of the above method, in which are provided:

- a first cylinder provided with an intake valve to alternatively open or close a communication between said first cylinder and an intake manifold and a valve exhaust to alternatively open or close a communication between said first cylinder and an exhaust manifold,

- valve control means for controlling an opening of said exhaust valve after an opening of said intake valve during part of an intake phase of said first cylinder, in order to transfer intake air from said intake manifold through said first cylinder to said exhaust manifold and temporarily storing in said exhaust manifold said transferred intake air,

- supercharging means for generating a supercharging pressure in said intake manifold so that, during said transfer, said supercharging pressure is greater than the pressure in said first cylinder and that the pressure in said first cylinder is greater than pressure in said exhaust manifold, said supercharging means comprising a main turbocharger or other main compression means to essentially contribute to generating said supercharging pressure when the engine is running at medium or high speed,

- At least one second cylinder connected to the same exhaust manifold as the first cylinder, the opening of an exhaust valve of said second cylinder being controlled so as to generate an overpressure in said exhaust manifold after closing of said inlet valve of said first cylinder and before closing of said exhaust valve of said first cylinder, said exhaust valve of the first cylinder remaining open only during the start of an exhaust phase of the second cylinder.

These systems prove effective in operating zones where it is possible to store intake air beforehand, and make it possible to remedy the inadequacy of the compressor fields with the air requirement of the reciprocating engines. However, the implementation of these methods to improve the operation of a supercharged heat engine requires imperatively having a positive pressure difference between an intake manifold and a cylinder on the one hand, and between the cylinder and a exhaust manifold on the other hand. Thus, contrary to what one might have believed from the teaching of document WO 95/14853, the method which it describes is impossible to implement with a conventional turbocharger in which the required positive pressure difference is nonexistent in low speed areas. This will be better understood with reference to Figures 7, 8 and 9 attached.

These figures show, for a diesel engine of the above type, with indirect injection and 2.11 cubic capacity, supercharged by a conventional turbocharger, the evolution of the instantaneous pressures P2 in the first cylinder, PI in its intake manifold and P3 in its exhaust manifold, as a function of an angle α identifying a rotation of an engine connecting rod, during a complete four-stroke cycle of said cylinder.

In FIG. 7, the engine operates at a speed of 1250 rpm and at full load, that is to say that the accelerator is depressed to the maximum. The instant A corresponds to the opening of the exhaust valve, the intake valve being already open. The instant B corresponds to the closing of the intake valve and the instant B 'to the closing of the exhaust valve. On the common opening range of the two valves of the first cylinder, between A and B, we see that the pressures are staggered in the right direction to allow the transfer of the intake air, ie P1> P2> P3. On the range at the end of opening of the exhaust valve, between B and B ', it can be seen that the pressures P2 and P3 are also staggered in the right direction to allow post-filling of the cylinder with air from admission previously transferred, i.e. P3> P2.

In FIG. 8, the engine operates at a speed of 1000 rpm and at 10% load, that is to say that the accelerator is slightly depressed. On the common opening range of the two valves of the first cylinder, between A and B, it can then be seen that the pressure P3 in the exhaust manifold is greater than the pressure P2 in the cylinder, which prevents any transfer of the intake air into the exhaust manifold. On the contrary, it is burnt gases which risk being injected into the cylinder from the exhaust manifold between times A and B 'because the pressure P3 is also higher than the pressure PI in the intake manifold.

In FIG. 9, the engine operates at a speed of 1000 rpm and at full load. On the common opening range of the two valves of the first cylinder, between A and B, it can also be seen that the pressure P3 is higher than the pressure P2, which prevents any transfer of intake air into the exhaust manifold. However, the pressure PI in the intake manifold is greater than the pressure P3 in the exhaust manifold between times A and B, which hinders the injection of burnt gases into the cylinder from the exhaust manifold on this interval.

In conclusion, the operation of the process with the motor of the example above is possible at 1250 rpm, but not at 1000 rpm. The use of such a method is not only ineffective at low speed in the case of a conventional turbocharger, but also the acceleration delay is increased and the thrust effect when the engine torque appears is also increased.

This drawback is illustrated in the attached FIG. 6, which represents, as a function of a rotation speed Ω of the engine, the engine torque F supplied by two different versions of a diesel engine with direct injection of 1.91 of supercharged displacement at using a variable geometry turbocharger, the inlet section of the turbine being controlled by the boost pressure. Curve 50 represents the torque T supplied by such a standard engine for a vehicle, while curve 51 represents the torque F supplied by such a modified engine to implement the known improvement process. Engine modifications relate to the exhaust manifold, valve opening and closing profiles and angles, and injection specifications.

As can be seen in FIG. 6, the engine torque F supplied by the modified engine is effectively increased compared to the standard engine over a wide range of revolutions going from Ω = 1,000 rpm to approximately 3800 rpm. However, for the modified engine, on the one hand, the engine torque at low speed, for example between 1000 and 1250 rpm, is not yet high enough and the growth rate of the engine torque between 1000 and 1500 rpm min is much too high, which results in very brutal acceleration, qualified as a “kick” effect, which impairs the driver's control of the vehicle. On the other hand, the maximum torque reached at around 2000 rpm is unnecessarily high. In addition, the modified engine shows a slight loss of torque at speeds above 3800 rpm, but the maximum vehicle speed is not an important criterion in this case.

The delay in acceleration followed by a significant thrust during the effective activation of the turbocharger has an effect unwanted driving discomfort. To remedy this, it is possible to use a turbocharger turbine having a smaller section and having a shorter response time. However, if the turbine section of the turbocharger is reduced, the turbocharger is then unable to ensure operation at full load of the heat engine and the loss of torque at high revs becomes unacceptable.

One could also reduce the pressure P3 at the exhaust by increasing the cross section of the turbine. However, this would cause a reduction in the speed of rotation of the turbine and therefore in the boost pressure. Consequently, the maximum torque would be reduced as much as the torque at low speed.

An object of the invention is to remedy the drawbacks of the known technique, in order to improve operation at low speed and / or during an acceleration phase from substantially an idle speed of supercharged thermal engines of the above type.

Another object of the invention is to obtain a diagram of acceleration of supercharged engine analogous to the diagram of relatively linear acceleration of the atmospheric engines.

Another object of the invention is to provide an increased amount of air to the exhaust to reduce pollution of the engines, in particular to properly burn the injected fuel, by reducing the puffs of black smoke at the exhaust in the case of diesel engines, or by burning the exhaust gases in the exhaust pipe in the case of spark ignition engines.

For this, the subject of the invention is a method of the aforementioned type characterized in that it consists, at low speed and / or during at least one acceleration phase from substantially an engine idling speed, of generating the pressure supercharging in the intake manifold essentially using an auxiliary turbocharger with inertia significantly lower than the inertia of the main turbocharger, in order to improve engine operation at low speed.

The invention also relates to an engine of the aforementioned type, characterized in that said supercharging means comprise an auxiliary turbocharger with inertia significantly less than the inertia of the main turbocharger, the compressor of said auxiliary turbocharger being connected to said intake manifold to essentially contribute to generating said boost pressure in the intake manifold at least when the engine is running at low speed and / or during an acceleration phase from substantially an engine idle speed.

According to other advantageous features of the invention:

- Said exhaust manifold is connected to a branch between an inlet of the turbine of the main turbocharger and an inlet of the turbine of the auxiliary turbocharger, a means for distributing or calibrating flow being provided at said branch to orient part of the gases of exhaust to the turbine inlet of the auxiliary turbocharger at least when the engine is running at low speed and / or during said acceleration phase.

- Said exhaust manifold has substantially the same length and substantially the same volume between said branch on the one hand, and respectively the first and second cylinders on the other. - Said exhaust manifold has two openings opening respectively into said first and second cylinders, the section of said openings being substantially greater than an effective section of the main turbine.

- A first bypass line provided with a first bypass valve is provided between the inlet of the turbine of the main turbocharger and the exhaust pipe, to limit an exhaust gas flow in said main turbine.

- A second bypass line provided with a second bypass valve is provided between the inlet of the auxiliary turbine and an exhaust pipe, to limit the load of said auxiliary turbine to a predetermined maximum value.

- Said first and second bypass valves are servocontrolled as a function of the boost pressure so as to maintain the pressures at the outlet of the main compressor and of the auxiliary compressor at substantially equal values.

- An air intake manifold connects the compressor of the auxiliary turbocharger to said intake manifold, said intake manifold being optionally provided with a refrigerant. - A means of orienting a flow of air sucked in by the compressor of the auxiliary turbocharger in the direction of the exhaust is provided to reduce pollution in the exhaust.

- Said orientation means is a duct provided with a first servo-controlled valve, said intake manifold being provided with a second servo-controlled valve, said first and second valves being servo-controlled as a function of the boost pressure so as to close the first valve and opening the second valve when said pressure at the outlet of the main compressor is below a predetermined threshold, and opening the first valve and closing the second valve when said pressure at the outlet of the main compressor exceeds said predetermined threshold.

- the main turbocharger and the auxiliary turbocharger suck in the supply air through a common inlet, preferably through a filter.

- the engine comprises an even number of cylinders, preferably 4, 6 or 8 cylinders, said cylinders being matched by common exhaust manifolds, the respective operating cycles of the cylinders of each pair being out of phase so that one of said cylinders produces a burst of exhaust gas in the exhaust manifold common to the pair of cylinders so as to generate said overpressure while the other of said cylinders is substantially at the end of a suction phase. - the intake manifold is common to all the cylinders.

- The low operating speed is less than 1300 rpm, preferably less than 1200 rpm, and for example of the order of 1000 rpm or less. - the engine is a diesel engine with direct or indirect injection or petrol with direct injection.

The invention will be better understood from the description which follows, given by way of nonlimiting example with reference to the accompanying drawings in which: - Figure 1 schematically shows a first embodiment of the engine according to the invention;

- Figure 2 schematically shows a second embodiment of the engine according to the invention;

- Figure 3A shows the evolution of the pressure in a cylinder, in an intake manifold and in a engine exhaust manifold according to the invention during a complete 4-stroke cycle of the cylinder; FIG. 3B represents the evolution of the positions of the intake and exhaust valves of the cylinder of FIG. 3A and of the cylinder connected to the same branch of the exhaust manifold, during the complete 4-stroke cycle; FIGS. 4A, 4B and 4C represent three successive stages of operation of two cylinders of the engine according to the invention matched by a common exhaust manifold; FIG. 5 represents a cam for controlling the intake and exhaust valves of the engine according to the invention; - Figure 6 shows the engine torque as a function of the operating regime for a standard version and a modified version, both according to the prior art, of a diesel engine with direct injection of 1.91 cubic capacity and supercharged at using a variable geometry turbocharger, the inlet section of the turbine being controlled by the boost pressure; Figures 7 to 9 are representations similar to that of FIG. 3A for a four-stroke diesel engine according to the prior art, with indirect injection and of 2.11 cubic capacity, supercharged by a conventional turbocharger, in three different modes of operation; Figure 10 is a partial view in partial section of an exhaust manifold of the engine according to the invention, - Figure 11 is a partial view in partial section of an alternative embodiment of the exhaust manifold of Figure 10 .

With reference to FIG. 1, a four-stroke heat engine, for example a diesel heat engine, is represented diagrammatically by its engine block M, comprising four cylinders 1, 2, 3, 4, in line represented diagrammatically. Each cylinder 1 to 4 comprises at least one intake duct or manifold 1a to 4a and at least one exhaust duct or manifold 1a to 4a.

A main turbocharger 5 comprising a turbine 6 and a compressor 7 is provided for compressing the intake air arriving via a pipe 8 through an atmospheric filter 55. Advantageously, a refrigerant 9 is provided for cooling the compressed air by the compressor 7 before it is admitted to the intake manifolds 1a to 4a. The intake manifolds 1a to 4a are formed by branches connecting an intake conduit 56 common to all the cylinders to each cylinder. i The motor M according to the invention is designed to implement the method described in document WO 95/14853, as will be described below. Thus, the exhaust manifolds 1e to 4e actually form two manifolds, each being common to two cylinders. For example, the manifolds 1e and 4e are formed by two end sections of a first common manifold 57 opening out through a middle section 58 into a chamber 10 at the inlet of the main turbine 6. The chamber 10 is shown in the form elongated in FIGS. 1 and 2 only for reasons of readability, but it in fact occupies a generally cylindrical volume of diameter substantially equal to one or two diameters of the sections 1 to 4 and of comparable length, as best seen in FIGS. 10 and 11 Likewise, the collectors 2e and 3e are formed by two end sections of a second common manifold 59 opening out by a median section 60 in the chamber 10.

The operation of the motor M according to the method described in document WO 95/14853 will now be recalled with reference to FIGS. 3A to 4C. ! In FIG. 4A, two of the cylinders of the engine M are shown paired by a common exhaust manifold. These are for example the cylinders 1 and 4 and the manifold 57. The cylinders 2 and 3 matched by the manifold 59 are designed and operate in the same way. The sections 1e and 4e of the common manifold 57 respectively connecting the cylinders 1 and 4 to the middle branch 58 are of the same length and of the same volume.

A partially represented piston 25 moves alternately in cylinder 1 between a bottom dead center 26 and a top dead center 27. A partially shown piston 28 moves alternately in cylinder 4 between a bottom dead center 29 and a top dead center 30. The ends of the intake manifold 1a and of the exhaust manifold opening into the cylinder 1 can be opened or closed respectively by an intake valve 31 and an exhaust valve 32. The ends of the intake manifold 4a and of the 4th exhaust manifold opening into the cylinder 4 can be opened or closed respectively by an intake valve 35 and an exhaust valve 36.

FIGS. 3A, 3B and 6 to 9 bear on the abscissa a crankshaft angle α which makes it possible to identify the position of the various pistons of the engine M, which are each phase shifted by 180 ° relative to that which precedes it in the ignition sequence 1-3-4-2, as is known for a four-cylinder, four-stroke engine. Thus, the pistons 25 and 28 are phase shifted by substantially 360 °. For piston 25, the angles -360 °, 0 ° and + 360 ° correspond to top dead center 27 and the angles -180 ° and + 180 ° correspond to bottom dead center 26.

In FIG. 3A, the instantaneous pressures PI in the intake manifold 1a, P2 in the cylinder 1 and P3 in the exhaust manifold 1a have been shown on a complete four-stroke cycle of the piston 25. In FIG. 3B, the curve 37 represents the successive positions of the valve 31 during the cycle, between a closed position F and a maximum open position O. The curve 38 represents the positions of the valve 32 and the curve 38 ′ those of the valve 35.

With reference to FIG. 4A, which corresponds to instant A of FIGS. 3A and 3B, during the intake phase of the cylinder 1, the valve 31 being open, the piston 25 being near its bottom dead center 26 and driven at a low speed, an opening of the exhaust valve 32 is caused. During this time, the cylinder 4 is in the combustion / expansion phase and the valve 35 is closed. The pressure PI being substantially equal to the pressure P2 and greater than the pressure P3, the intake air present in the intake manifold la is injected into the cylinder 1, as represented by the arrow 40 and part of this air crosses cylinder 1 to the exhaust manifold 1c to be temporarily stored there, as shown by arrow 41.

The intake air transferred to the exhaust manifold cannot be a gasoline-based precarburized mixture, since the high temperature in the exhaust manifold would cause such a mixture to self-ignite. The process is therefore not applicable to a petrol engine with indirect injection.

With reference to FIG. 4B, which corresponds to instant B of FIGS. 3A and 3B, at the end of the intake phase of the cylinder 1, the valve 31 is closed. The cylinder 4 being at the start of the exhaust phase, the valve 35 opens and sends a puff of burnt gas into the manifold 57, as shown by the arrow 42, thus generating an overpressure 49 greater than the pressure P2 which causes the intake mixture previously stored in the end section 1c, possibly mixed with burnt gases, to flow back to the cylinder 1, as shown by arrow 43. This thus gives post-filling the cylinder 1 with the intake mixture previously stored.

To promote the flow of the exhaust puff 42 towards the section Ia, and not towards the turbine 16, the section 19 of the section Ia open on the cylinder 1 is designed to be larger than the effective section of the turbine 16. The opening of the valve 32 lasts only during the start of the exhaust phase of the cylinder 4, until time B '.

With reference to FIG. 4C, which corresponds to time C. of Figures 3 A and 3B, after closing the valve 32, the cylinder 4 continues its exhaust phase in a conventional manner. The open valve 35 causes the flue gases to flow in the manifold 57 and in the middle section 58 towards the turbine (s), as shown by the arrows 44, to drive the turbocharger (s).

Referring to Figure 5, there is shown in section a camshaft 45 of the engine M carrying cams intended to cooperate in known manner with cam followers to control the movements of the valves 31 and 32 in accordance with Figure 3B. The arrow 46 indicates the direction of rotation of the shaft 45. The exhaust pipe 47 has a first boss 47a intended to cause a conventional lifting 38a of the exhaust valve 32, as visible in FIG. 3B, and a second boss 47b intended to cause an additional lift 38b of the exhaust valve during the intake phase. The intake cam has a single boss 48 intended to cause the lift 37 of the intake valve 31.

The maximum of the lift 38a is marked by the angular mark D. It is located approximately 51 ° before the top dead center of the piston 25, marked by the angular mark E. The maximum of the lift 37, marked by the angular mark I, is located approximately 45 ° after top dead center E. The start of the additional lift 38b, identified by the angular reference G, is located approximately 8 ° after the maximum intake I. The maximum of the additional lift 38b, identified by the angular coordinate system H, is located approximately 85 ° after top dead center E. According to the invention, there is a second turbocharger 15, called an auxiliary turbocharger, bypassed on the intake and exhaust circuits of the heat engine. The chamber 10 has a first branch 11 which directs part of the exhaust gases into the turbine 6 of the turbocharger 5. The exhaust gases expanded through the turbine 6 are then directed through the outlet 12 of the turbine towards the pot of exhaust 13., comprising a silencer 61 and an outlet 14 to the atmosphere. For this, the chamber 10 has a second branch 21 which directs the other part of the exhaust gases in the turbine 16 of the turbocharger 15. The exhaust gases expanded through the turbine 16 are then oriented by the outlet 63 of the turbine. to the exhaust pipe 13.

The auxiliary compressor 17 draws in the intake air through an air inlet 66 common with the main compressor 7 and through the atmospheric filter 55. There is also advantageously provided on the supercharging pipe 18 at the outlet of the compressor 17, a refrigerant, distinct or not from the refrigerant 9, for cooling the supply air compressed by the turbine 17.

The additional turbocharger 15 is much smaller in size than the turbocharger 5 and comprises a turbine 16 capable of being driven by exhaust gases and a compressor 17 capable of compressing the supply air. The size of the turbocharger 15 is such that it is already effective at the engine idling speed M. As an example, the turbocharger 15 could be a turbocharger adapted under normal conditions for an engine of 0.5 liters of displacement.

The turbocharger 5 is a turbocharger suitable for the medium and high rpm of the engine M which has, for example, two liters of displacement. Advantageously, provision is made to provide the main turbocharger 5 with a first exhaust gas bypass valve 67, known as "waste spoiler", according to the known technique. The bypass valve 67 is controlled to open or close a bypass duct 68 between the branch 11 upstream of the turbine 6 and the outlet 12 of the turbine 6. The bypass valve 67 is controlled by means of a control line 69 as a function of a boost pressure measured at the outlet of the main compressor 7. In high speeds and at high load, the bypass valve 67 is open and limits the boost pressure and therefore the maximum combustion pressure in the engine cylinders M.

To avoid any risk of centrifugation of the auxiliary turbocharger 15, it is advantageous to provide this small turbocharger 15 with a second exhaust gas bypass valve 64. The bypass valve 64 is controlled to open or close a bypass duct 65 between the branch 21 upstream of the turbine 16 and the outlet 63 of the turbine 16.

The distribution of the exhaust flow rates leaving the chamber 10 is carried out between the duct 11 for inlet of the turbine 6, and the duct 21 for the inlet of the turbine 16 by a flow calibration member 34, for example a nozzle , a calibrated orifice or any other means comprising a calibrated section allowing permanent passage of a limited flow of exhaust gas. The flow section of the flow calibration means 34 is predetermined to obtain, from the engine idling speed M and / or from the start of the acceleration phase, a sufficient passage of exhaust gas through the duct 21 to ensure the proper functioning of the "small" turbocharger 15. This predetermined flow rate to start, with no significant response time, the turbine 16 of the small turbocharger 15 is of the order of a quarter of the flow rate passing through the inlet duct 11 of the turbine 6 of the "normal" turbocharger 5. Indeed, when the "normal" turbocharger 5 is not within its effective operating range, the turbine 6 causes an additional pressure drop which results in sending a more large amount of exhaust gas through the conduit 21 than the "normal" amount passing through the conduit 21 when the turbocharger 5 is operating at full speed.

As a variant, the means 34 for calibrating the flow rate is replaced by a means for distributing the flow rates mounted at the inlet to the duct 21 in the chamber 10, as shown in FIGS. 10 and 11.

With reference to FIG. 10, a chamber 10 at the end of an exhaust manifold presented in partial section comprises a duct 11 channeling the exhaust gases towards the inlet of a turbine 6, of a turbocharger 5 " normal "and also comprises a pipe 21 for channeling the exhaust gases towards the inlet of a turbine 16 of a" small "turbocharger 15. The distribution of the exhaust gas flows between the conduits 11 and 21 is carried out by a flow modulation valve 22, provided with a control means 25. The valve 22 is analogous to a exhaust gas recirculation control of known type, in particular from document FR 2 744 491.

In this document FR 2 744 491, an exhaust gas recycling system for an internal combustion engine comprises a first exhaust gas turbocharger and an additional exhaust gas turbocharger. However, this arrangement differs entirely from the present invention, since the additional turbocharger of document FR 2 744 491 is intended to compress a quantity of exhaust gas for recycling, and is not used to compress the supply air , as provided in the present invention to decrease the acceleration response time.

With reference to FIG. 11, another embodiment of the exhaust manifold end chamber 10 is shown in partial section. The valve 22 is eliminated and the distribution of the flow rates between the conduits 11 and 21 is carried out by a flow modulation valve 33 controlled by a control member not shown. The valve 33 is a valve similar to the valve of known type, integrated in the turbine casing of the turbochargers to avoid exceeding a limit pressure of air supercharging. These valves of the type known under the name “waste-gate” have the function in turbochargers of the known type of reducing the flow rate of exhaust gas driving the turbine of the turbocharger, when the boost pressure has reached its maximum value.

In Figure 11, the valve 33 is shown in its extreme positions: a closed position of the conduit 21 shown in solid lines and a fully open position of the conduit 21 shown in dotted lines. In a first embodiment of the engine, the manifold 18 comprises, downstream of the compressor 17, a servo-controlled valve 71 for opening or closing the manifold 18 in the direction of the intake manifold 56. The intake manifold 18 also comprises, at upstream of the valve 71, a branch 70 towards a discharge pipe 24 for directing the air or part of the air sucked by the compressor 17 directly towards the exhaust 13. The discharge pipe 24 is provided with another servo-controlled valve 23 intended to open or close it. The two valves 71 and 23 are servo-controlled respectively by means of control lines 72 and 73 as a function of the boost pressure measured at the outlet of the compressor 7.

The operation of the device is as follows:

At start-up, and in the acceleration phase, the air sucked in through inlet 66 is essentially compressed by compressor 17 of turbocharger 15 with very low response time, is cooled by refrigerant 9 and supplies cylinders 1 to 4 of the engine M. Due to the low inertia of the turbocharger 15 and the energy available in the exhaust gas from idling at low speeds, no notable response time to acceleration is noticeable by the driver.

For this, at low speed and / or in an acceleration phase from substantially the idle speed of the engine M, which is for example substantially 1000 rpm, the valve 71 is open and the valve 23 is closed, so that the compressor 17 blows compressed air towards the manifold 56, generating a boost pressure P1 sufficient to transfer, according to the known method, an intake mixture into an exhaust manifold at the time of the joint opening of the valve intake and exhaust valve of a cylinder, that is to say greater than the pressure P2 in the cylinder, itself greater than the pressure P3 in the corresponding exhaust manifold.

In the areas of higher load, the enthalpy of the exhaust gases is sufficient to drive not only the small turbine 16 of the small turbocharger 15, but also the turbine 6 of the "normal" turbocharger 5. As soon as the normal operating speed turbocharger 5 is reached, this essentially contributes to generating the satisfactory boost pressure PI and it is possible to send, by acting on the distribution valve 23, part of the intake air through the duct 24 directly in the exhaust 13.

Thus, when the engine rises, for example from 1200 or 1300 rpm, the turbocharger 15 becomes efficient and the increase in the boost pressure closes the valve 71 and opens the valve 23 to send a quantity progressively larger air sucked by the compressor 17 through the pipe 24 in the exhaust 13, in order to reduce pollution at the exhaust.

At high speed, for example above 1500 rpm, the small turbocharger 15 thus acts as an air generator sent directly to the exhaust, thereby making it possible to reduce the exhaust pollution. In fact, for spark-ignition engines, the addition of air from the duct 24 makes it possible to burn the unburnt hydrocarbons in the exhaust 13, in a manner known per se, which reduces polluting emissions. For diesel thermal engines, or equivalent compression-ignition engines, the addition of additional air allows dilution of the exhaust gases, in a manner also known, which also contributes to reducing pollution.

With reference to FIG. 2, a second embodiment of the invention comprises elements identical or functionally equivalent to the elements in FIG. 1 and identified by reference figures identical to the figures in FIG. 1.

In the second embodiment, the two servo-controlled valves 71 and 23 are eliminated, as well as the duct 24. The auxiliary compressor 17 blows compressed air only towards the intake manifold 56, through the refrigerant 9. In this case, the bypass valve 64 is obligatorily provided. The bypass valves 64 and 67 are controlled as a function of the boost pressure at the outlet of the main compressor 7 by means of the control lines 53 and 69 respectively. Automatic control means 54 make it possible to control the opening and closing of the bypass valves so that the pressure at the outlet of the main compressor 7 and of the auxiliary compressor 17 remain substantially equal, which avoids any risk of air reflux at wrong in one of the compressors. The operation of the engine in the second embodiment is as follows: at low speed or during acceleration from substantially the idle speed, the valve 64 is closed, so that the auxiliary turbocharger 15 with low response time essentially contributes to generating the boost pressure in the manifold 56. During the engine revving, for example from 1200 or 1300 rpm, the main compressor becoming effective in generating the boost pressure satisfactorily, the valves 64 and 67 open to limit it to the maximum values allowed by the engine.

In all cases, the adaptation of the main turbocharger 5 and the auxiliary turbocharger 15 is such that the pressure differences PI-P2 and P2-P3 are positive during the intervals where this is required by the known improvement process, whatever or the engine operating speed. Adaptation of main turbocharger 5 at high speeds is facilitated. The auxiliary turbocharger 15 essentially serves to create the necessary pressure differences as soon as the engine idles and, for example, up to 1200 or 1300 rpm. With reference to FIG. 6, the curve 52 represents the engine torque F supplied by an engine according to the invention as a function of the operating speed Ω of the engine. It can be seen that the torque r is improved from the idling speed compared to that of the modified engine of the prior art (curve 51), that the growth of the torque between appreciably the idling speed and the maximum torque speed is less abrupt that in the modified engine of the prior art, that the maximum torque is lower than in the modified engine of the prior art and that the torque at high speed is not deteriorated compared to the modified engine of the prior art . Thus, the use according to the invention of the small turbocharger 15 combined with the method for improving the operation of a supercharged and air-swept heat engine, described in document WO 95/14853, makes it possible to create, in the idle zones at low loads and revs, the pressure difference between the intake manifold and the exhaust manifold, necessary for the transfer of fresh air into the exhaust manifold to then post-fill the cylinder using the energy from the exhaust puff of another cylinder. In this way, the advantage of increasing torque at low revs described in document WO 95/14853 can be real as soon as idling and erase the "hole" during acceleration followed by a "kick" effect then . The general response time of the supercharged combustion engine is therefore reduced. The engine's response to acceleration from substantially the idle speed is linearized. The invention therefore covers also all combinations of the device described in FIG. 1 with the arrangements described in document WO 95/14853, the content of which is considered to be incorporated by reference to the present application.

The invention applies to supercharged engines having a number of cylinders such that the specific arrangements of the exhaust manifold make it possible to use the energy of a puff of a cylinder at the start of the exhaust phase to post-fill. another cylinder connected to the same exhaust manifold branch with an intake mixture previously stored in the exhaust manifold. The engine M could be with 6 cylinders in lines or 8 cylinders in V. On the other hand, for the engines with 3 or 5 cylinders or 6 cylinders in V, the regrouping of the cylinders by common manifolds would be ineffective due to too great length of said manifolds, so that the application of the invention to these engines is, if not impossible, at least difficult and not very advantageous.

The invention described with reference to several particular embodiments is in no way limited thereto, and covers, on the contrary, any modification of form and any variant of embodiment, within the scope and spirit of the invention, the essential being to use a "small" turbocharger to overcome the shortcomings of the process described in document WO 95/14853.

Claims

1. Method for improving the low-speed operation of a four-stroke thermal engine (M), supercharged by means of at least one main turbocharger (5) or another main compression means, said method comprising the steps consists in :

- open an exhaust valve (32) at the same time as an intake valve (31) of a first cylinder (1) of the engine during at least part of an intake phase of said first cylinder, so transferring intake air (40) from an intake manifold (56, la) through said first cylinder to an exhaust manifold (le, 57), a boost pressure (PI) being generated in said intake manifold so that, during said transfer, said boost pressure is greater than the pressure (P2) in said first cylinder and the pressure in said first cylinder is greater than the pressure (P3) in said manifold exhaust, said boost pressure being generated essentially using said main turbocharger (5) when the engine (M) is running at medium or high speed (Ω),

- temporarily store in said exhaust manifold (le, 57) said transferred intake air (41),

- generate an overpressure (49) in said exhaust manifold after closing (B) of the intake valve and before closing (B ') of the exhaust valve of the first cylinder by means of a puff of exhaust (42) of a second cylinder (4) connected to the same exhaust manifold (57) as the first cylinder, in order to post-fill said first cylinder with said intake air (43) previously stored in said manifold d exhaust, possibly mixed with exhaust gases, characterized in that it consists, at low speed and / or during at least one acceleration phase from substantially an engine idling speed (M), in generating said boost pressure (PI) in the intake manifold (56, la) essentially using an auxiliary turbocharger (15) of inertia significantly lower than the inertia of the main turbocharger (5).

2. Supercharged four-stroke engine (M) comprising:

- a first cylinder (1) provided with an intake valve (31) for alternately opening or closing a communication between said first cylinder and an intake manifold (56, la) and with an exhaust valve (32 ) alternatively to open or close a communication between said first cylinder and an exhaust manifold (le, 57),

- valve control means (45,47,48) for controlling an opening of said exhaust valve after an opening of said intake valve during part of an intake phase (37) of said first cylinder, in order to transfer intake air (40, 41) from said intake manifold through said first cylinder to said exhaust manifold and to temporarily store said transferred intake air in said exhaust manifold,

- boost means for generating a boost pressure (PI) in said intake manifold (56) so that, during said transfer, said boost pressure is greater than a pressure (P2) in said first cylinder and that the pressure in said first cylinder is greater than a pressure (P3) in said exhaust manifold, said boost means comprising a main turbocharger (5) or other main compression means to essentially contribute to generating said boost pressure when the engine (M) operates at medium or high speed (Ω),

- at least one second cylinder (4) connected to the same exhaust manifold (57) as the first cylinder, the opening of an exhaust valve (35) of said second cylinder being controlled so as to generate an overpressure (49 ) in said exhaust manifold (57) after closing (B) of said intake valve (31) of said first cylinder and before closing of said exhaust valve (32) of said first cylinder, said exhaust valve of the first cylinder remaining open only during the start of an exhaust phase (38 ') of the second cylinder, characterized in that said supercharging means comprise an auxiliary turbocharger (15) of inertia significantly less than the inertia of the main turbocharger (5), the compressor (17) of said auxiliary turbocharger being connected to said intake manifold (56) to essentially contribute to generating said boost pressure (PI) in the intake manifold at least when the engine (M) is running at low speed and / or during an acceleration phase from substantially an engine idle speed (M) .

3. A heat engine (M) according to claim 2, characterized in that said exhaust manifold (57) is connected to a branch (10) between an inlet (11) of a turbine (6) of the main turbocharger and an inlet (21) of a turbine (16) of the auxiliary turbocharger, a means of distribution (22, 33) or of calibration (34) of flow being provided at said branch to direct a part of the exhaust gases towards the inlet of the turbine (16) of the auxiliary turbocharger (15) at least when the engine (M) is running at low speed and / or during said acceleration phase.

4. A heat engine (M) according to claim 3, characterized in that said exhaust manifold (57) has substantially the same length and substantially the same volume between said branch (10) on the one hand, and respectively the first (1) and second (4) cylinders on the other hand.

5. Heat engine (M) according to claim 4, characterized in that said exhaust manifold (57) has two openings opening respectively into said first and second cylinders, the section (19) of said openings being substantially greater than a section main turbine efficiency

(6).

6. Heat engine (M) according to one of claims 3 to 5, characterized in that a first bypass line (68) provided with a first bypass valve (67) is provided between the inlet (11 ) of the turbine (6) of the main turbocharger and an exhaust pipe (12, 13), to limit a flow of exhaust gas into said main turbine.

7. Heat engine (M) according to claim 6, characterized in that a second bypass line (65) provided with a second bypass valve (64) is provided between the inlet (21) of the auxiliary turbine (16) and the exhaust pipe, to limit the load of said auxiliary turbine to a predetermined maximum value.

8. Heat engine (M) according to claim 7, characterized in that said first and second bypass valves are servo-controlled as a function of the boost pressure (PI) so as to maintain the pressures at the outlet of the main compressor (7) and the auxiliary compressor (17) at substantially equal values.

9. Heat engine (M) according to one of claims 6 to 8, characterized in that it comprises an air intake manifold (18) connecting the compressor (17) of the auxiliary turbocharger (15) to said manifold intake (56), said intake manifold being optionally provided with a coolant (9).

10. Heat engine (M) according to claim 9, characterized in that a means of orientation (23, 24) of an air flow sucked by the compressor (17) of the auxiliary turbocharger (15) in the direction of the exhaust (13) is provided to reduce pollution of the exhaust (14).

11. Heat engine (M) according to claim 10, characterized in that said orientation means is a conduit (24) provided with a first servo-controlled valve (23), said intake manifold (18) being provided with a second servo-controlled valve (71), said first and second valves being servo-controlled as a function of the boost pressure (PI) so as to close the first valve (23) and to open the second valve (71) when said pressure at the outlet of the main compressor (7) is less than a predetermined threshold, and to open the first valve and to close the second valve when said pressure at the outlet of the main compressor (7) exceeds said predetermined threshold.

12. Thermal engine (M) according to any one of claims 2 to 11, characterized in that the turbocharger main (5) and the auxiliary turbocharger (15) draw the supply air through a common inlet (66), preferably through a filter (55).

13. A heat engine (M) according to any one of claims 2 to 12, characterized in that it comprises an even number of cylinders, preferably 4, 6 or 8 cylinders, said cylinders (1-4) being matched by common exhaust manifolds (57,59), the respective operating cycles of the cylinders of each pair being phase shifted so that one (4) of said cylinders produces a burst of exhaust gas (42) in the manifold exhaust (57) common to the pair of cylinders (1,4) so as to generate said overpressure (49) while the other (1) of said cylinders is substantially at the end of a suction phase.

14. A heat engine (M) according to any one of claims 2 to 13, characterized in that said intake manifold (56) is common to all the cylinders (1-4).

15. Heat engine (M) according to any one of claims 2 to 14, characterized in that said low operating speed is less than 1300 rpm, preferably less than 1200 rpm, and for example l '' order of 1000 rpm or less.

16. A heat engine (M) according to any one of claims 2 to 15, characterized in that it is a diesel engine with direct or indirect injection or gasoline with direct injection.