WO2009074845A1

WO2009074845A1 - Operating method for an internal combustion engine in compression braking mode, internal combustion engine capable of operating in braking mode and automotive vehicle equipped with such an engine

Info

Publication number: WO2009074845A1
Application number: PCT/IB2007/004436
Authority: WO
Inventors: Nicolas Auffret; Romain Le Forestier
Original assignee: Renault Trucks
Priority date: 2007-12-11
Filing date: 2007-12-11
Publication date: 2009-06-18

Abstract

This method operates an internal combustion engine (1) of an automotive vehicle (T) in compression braking mode and includes the steps of recovering at least a part of the exhaust gases of the engine (1) via EGR means (2), mixing the recovered gases with fresh air (F1) and fuel in order to form a gas-to-fuel mixture and injecting this gas-to-fuel mixture within at least a cylinder (12) of the engine (1) when a piston of this cylinder moves from its bottom dead centre position (BDC) to its top dead centre position (TDC). The engine (1) comprises two turbo-chargers (3, 4) whose turbines (31, 41) are located in two different exhaust lines (61, 62) which extend in parallel, downstream of an exhaust gas manifold (13).

Description

OPERATING METHOD FOR AN INTERNAL COMBUSTION ENGINE IN

COMPRESSION BRAKING MODE, INTERNAL COMBUSTION ENGINE CAPABLE

OF OPERATING IN BRAKING MODE AND AUTOMOTIVE VEHICLE EQUIPPED

WITH SUCH AN ENGINE

TECHNICAL FIELD OF THE INVENTION

This invention concerns a method for operating an internal combustion engine of an automotive vehicle in compression braking mode. The invention also concerns an internal combustion engine capable of operating either in positive mode or in compression braking mode. Finally, the invention concerns an automotive vehicle equipped with such an engine.

BACKGROUND ART OF THE INVENTION

It is known, e.g. from WO-A-2007/019879, to control the intake and exhaust valves of an internal combustion engine in order to make it run either in four-stroke positive mode or in two-stroke compression braking mode. In two-stroke compression braking mode, the engine delivers a negative torque because the gas which are compressed during the motion of a piston from its bottom dead centre position, or BDC, to its top dead centre position, or TDC resists the upward motion of the piston. In order to increase the gas pressure inside the cylinder during the upward motion of the piston and therefore further enhance this braking effect, it is known from US-B-6 336 447 to inject some fuel within the cylinders of an internal combustion engine running in compression braking mode, which induces combustion during the upward motion of the piston, from BDC to TDC. This increases the resulting pollution since it cannot be guaranteed that combustion is finished before the piston reaches TDC where the exhaust valves of the cylinder are opened. Moreover, combustion starts within the cylinder when the piston moves upwardly as soon as the air-to-fuel mixture is achieved with appropriate pressure and temperature conditions in the internal volume of the cylinder, for direct or indirect fuel injection, which means that the combustion is not evenly distributed in the cylinder internal volume. This uneven distribution of the burning mixture within the internal volume of this cylinder leads to incomplete or rather slow combustion and increases pollution.

On the other hand, it is known from EP-A-1 387 058 to use two turbo- chargers with an internal combustion engine adapted to work in positive mode and in compression braking mode. The turbines of these two turbo-chargers are installed in series, in an exhaust line of the engine. This induces that the exhaust line is bulky and must have one or several bends in order to fit into the engine compartment of a vehicle. This increases the pressure drop within this exhaust line and reduces the overall efficiency of the turbo-chargers. This problem of space required by the exhaust line is even more complicated when the engine is also equipped with an exhaust gas recirculation system, or EGR system.

SUMMARY OF THE INVENTION

A first aim of the invention is to solve the problems listed here-above and to provide a method which enhances the braking effect obtained with an internal combustion engine running in compression braking mode, without substantially increasing the induced pollution.

To this aim, the invention concerns a method for operating an internal combustion engine of an automotive vehicle in compression braking mode, this method including at least the following steps : a - recovery of at least a part of the exhaust gases of the engine via EGR means b - mixing of the recovered gases with fresh air in order to form a gas mixture c - intake of said gas mixture within at least a cylinder of the engine d - intake or injection of fuel within the cylinder and e - combustion of the fuel in presence of the gas mixture within the cylinder when a piston of this cylinder moves from BDC to TDC.

In this description, the following initials are used: TDC for "top dead centre position", BDC for "bottom dead centre position" and EGR for "exhaust gas recirculation".

Thanks to the method of the invention, the gas-to-fuel mixture present in the cylinder during the upward movement of the piston is loaded with soot, particulate materials and gases, e.g. carbon monoxide or dioxide, resulting from previous combustion, which induces that combustion within the internal volume of the cylinder starts after a rather long ignition delay. This allows the gas-to-fuel mixture to be homogenised within the internal volume of the cylinder before combustion of the fuel starts. Because of this homogenised state of the gas-to-fuel mixture prior to the beginning of the combustion, combustion is fast, so that the fuel is fully burnt before the piston reaches TDC. Therefore, the exhaust gases are not loaded with unburnt particles which would otherwise increase the pollution resulting from the operation of the engine. In other words, thanks to the use of exhaust gases recovered via the EGR means, combustion of the gas-to-fuel mixture within the internal volume of the cylinder is better controlled, since it takes place once the gas-to-fuel mixture has been homogenised and since it is complete. The invention can be used when the compression braking mode of the engine is a two-stroke mode, a four-stroke mode or more.

According to further aspects of the invention, an operating method for an engine might incorporate one or several of the following features:

- Step c starts before step d, which takes place before step e

- As an alternative, the method includes, prior to steps c and d, a step f of mixing of the gas mixture and the fuel into a gas-to-fuel mixture, whereas steps c and d occur together by the intake of the gas-to-fuel mixture within the cylinder - Intake or injection of the fuel starts when a crankshaft of the engine, which drives the piston, has an angular position between 70° and 30° before the piston reaches TDC.

- Intake or injection of the fuel ends when a crankshaft of the engine, which drives the piston, has an angular position between 50° and 10° before the piston reaches TDC.

- Intake or injection of the fuel takes place during a period of time which corresponds to an angular movement of the crankshaft with an amplitude lying between 10 and 30°.

- In case the compression mode is a four stroke compression mode, intake of the gas mixture starts when a crankshaft of the engine driving the piston has an angular position between 380° and 340° before the piston reaches its high pressure top dead center position.

- In case the compression mode is a four stroke compression mode, intake of the gas mixture ends when a crankshaft of the engine driving the piston has an angular position between 190° and 150° before a piston reaches its high pressure top dead center position.

- In case the compression mode is a two stroke compression mode, intake of the gas mixture starts when a crankshaft of the engine driving the piston has an angular position between 320° and 200° before the piston reaches its top dead center position.

- In case the compression mode is a two stroke compression mode, intake of the gas mixture ends when a crankshaft of the engine driving the piston has an angular position between 180° and 140° before the piston reaches its top dead center position.

- Intake of the gas mixture and intake or injection of the gas-to-fuel mixture takes place during a time interval such that combustion ends before the piston reaches TDC. - The method comprises a step of utilising two compressors to load the cylinder with the gas-to-fuel mixture, each compressor being powered by a turbine located in a dedicated exhaust line of the engine. In such a case, one of the turbo- chargers is preferably used only when the engine works in compression braking mode. - The engine comprises several cylinders and intake or injection of the fuel takes place in each cylinder when its respective piston moves from BDC to TDC.

The invention also concerns an internal combustion engine which allows to efficiently load one of its cylinders with a gas-to-fuel mixture, whereas its exhaust line does not have to be bulky and include several bends. According to this aspect, the invention concerns an internal combustion engine capable of operating either in positive mode or in compression braking mode, this engine comprising several cylinders, each of which is provided with a piston and means for the intake of a gas mixture within each cylinder, this engine comprising also two turbo-chargers adapted to provide the intake means with gas under pressure. This engine is characterized in that the respective turbines of the turbo- chargers are located in two different exhaust lines which extend in parallel, downstream of an exhaust gas manifold of the engine.

Thanks to this aspect of the invention, the two parallel exhaust lines are each relatively small in diameter and can be easily installed within an engine compartment of an automotive vehicle.

According to further aspects of the invention, an internal combustion engine might incorporate one or several of the following features:

- The engine includes EGR means adapted to provide the intake means with exhaust gases of the engine. In such a case, an outlet of the EGR means is advantageously connected to an air inlet line which feeds the intake means. Preferably, the EGR means advantageously include a gas mixer adapted to mix exhaust gases with fresh air loaded by at least a compressor of one of the turbo- chargers. - Control means are adapted to actuate a first turbo-charger at least when the engine is in positive mode and to actuate a second turbo-charger only when the engine is in braking mode. These control means can also actuate the first turbo charger in braking mode.

Finally, the invention concerns an automotive vehicle equipped with an engine as mentioned here-above. Such an automotive vehicle, which is advantageously an industrial vehicle like a truck or a bus, has efficient engine braking capabilities.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood on the basis of the following description, which is given in correspondence with the annexed figures and as an illustrative example, without restricting the object of the invention. In the annexed figures:

- Figure 1 is a scheme of a truck equipped with an internal combustion engine according to the invention.

- Figure 2 is a schematic view of a cylinder of the engine of figure 1.

- Figure 3 is a theoretical diagram showing the negative work obtained by the movement of the piston when an engine operates in four-stroke braking mode according to the prior art. - Figure 4 is a theoretical diagram similar to figure 3 for an engine operating according to the method of the invention in four-stroke braking mode.

- Figure 5 is a diagram showing the injection and combustion phases within the cylinder of figure 2, as a function of the angular position of a crankshaft of the engine, for an engine working in four-stroke braking mode. - Figure 6 is a diagram similar to figure 3 for an engine operating in two-stroke braking mode.

- Figure 7 is a diagram similar to figure 4 for an engine operating in two-stroke braking mode, and - Figure 8 is a diagram similar to figure 5 for an engine working in two-stroke braking mode.

DETAILED DESCRIPTION OF SOME EMBODIMENTS The Diesel engine 1 represented on figure 1 is mounted onto a truck T and equipped with an exhaust gas recirculation system or EGR system 2. Engine 1 is also equipped with a first turbo-charger 3 and a second turbo-charger 4. EGR system 2 and turbo-charger 3 feed, via a main inlet or intake line 5, an air inlet manifold 11 of engine 1 with gases to be used as comburant during fuel combustion in the cylinders 12 of the engine when the engine runs in four-stroke positive mode where it delivers positive power to drive the transmission train and the wheels W of truck T.

An exhaust gas manifold 13 of engine 1 is connected to a main exhaust line 6 in order to evacuate exhaust gases from engine 1. Exhaust gases flow is represented by arrows E on figure 1. EGR system 2 includes an inlet line 21 branching off exhaust line 6 and feeding an EGR gas cooler 22 provided with a liquid cooling medium, as shown by arrow l_i, this medium being evacuated from cooler 22 as shown by arrow L₂. According to a further embodiment of the invention, cooler 22 can be a gas/gas heat exchanger. In such a case, the cooling medium is a gas which may come directly from the intake line 5 or exhaust line 6.

An outlet line 23 of cooler 22 connects exhaust gas cooler 22 to a gas mixer 24 which also belongs to EGR system 2. An EGR valve 25 controls exhaust gases flow between lines 6 and 21 and also belongs to EGR system 2.

Gas mixer 24 and EGR valve 25 are piloted by an electronic control unit 8 via electronic control signals S₁ and SV

After the derivation corresponding to inlet line 21 , exhaust line 6 reaches a flow divider 9 formed by a control valve 9 which is piloted by unit 8 via an electronic signal S₉. Two secondary exhaust lines 61 and 62 run, in parallel from each other, from valve 9 to a gas mixer 10 piloted by unit 8 via another electronic control signal S₂. Valve 9 selectively connects line 6 to lines 61 or 62, or to both of these lines, depending on the order received from unit 8 via signal S₉. Gas mixer 10 is helping recovering the totality of the exhaust mass flow of the engine 1 , the mass flow coming via lines 61 and/or 62 at possibly different pressures. Line 61 feeds a turbine 31 which belongs to turbo-charger 3. This turbine is connected by a shaft 32 to a compressor 33. Line 62 feeds a turbine 41 which belongs to turbo-charger 4 and is connected by a shaft 42 to another compressor 43.

Operation of turbo-chargers 3 and 4 is controlled by unit 8 via respective electronic control signals S₃ and S₄.

Compressors 33 and 43 are installed within a first part 51 of inlet line 5, upstream of gas mixer 24. Part 51 is fed with fresh air, as shown by arrow F-|. When they are operating, compressors 33 and 43 increase the pressure of fresh air in inlet line 5, in part 51 upstream of mixer 24. A by-pass line 46 belongs to turbo-charger 4 and by-passes compressor 43.

A solenoid valve 48 controls air flow within by-pass line 46 and is piloted by unit 8 via an electronic control signal S₅.

A heat exchanger 47, which is used to improve the global compression efficiency when both compressors are running in serial, is located downstream of the compressor of turbocharger 4 and upstream the connection between line 46 and the pipe which leads to compressor 33 of turbocharger 3. When compressor 43 is bypassed, heat exchanger 47 is not used at all.

A heat exchanger 26 is also installed in the intake line 5, downstream of EGR mixer 24 and upstream of the intake manifold 11. When engine 1 runs in four-stroke positive mode, valve 9 connects line 6 to line 61 , but not to line 62. Unit 8 actuates turbo-charger 3 via signal S₃, in a conventional manner, in order to increase air pressure within line 5. Turbo-charger 4 is not active in this working mode of engine 1. In other words, signal S₄ does not actuate turbo-charger 4 which does not rotate because no gas flows in line 62. Solenoid valve 48 is actuated by unit 8 in order to permit air flow within line 46. EGR system 2 is active and mixer 24 mixes fresh air loaded by compressor 31 with exhaust gases coming from cooler 22. A gas mixture, which includes exhaust gases, flows toward manifold 11 in a second part 52 of line 5, downstream of mixer 24. This is shown by arrow F₂ on figure 1. In compression braking mode of engine 1 , which can be two-stroke or four- stroke, valve 9 connects line 6 at least to line 62. Turbo-charger 4 is actuated by unit 8 and valve 48 is closed. In practice, valve 9 is controlled to direct the flow in line 6 towards lines 61 and 62. Line 62 has a smaller diameter than line 61 and turbine 41 is smaller than turbine 21 , which induces smaller flow area at turbine 41 than at turbine 31. Due to the smaller flow area at turbine 41 , pressure within line 62 upstream of gas mixer 10 is higher than in positive mode. Engine back pressure created by this low critical area of turbine 41 is then really high, which is needed to get high engine brake power. Mixer 10 is then used to balance the flows E in lines 61 and 62, in particular to avoid a backward re-circulation in one of these lines, if pressure at the downstream end of the other line is higher.

In compression braking mode, EGR system 2 is active. In other words, as shown by arrow F₂, a part of the exhaust gases of engine 1 is redirected to collector 1 1 after being mixed with high pressure fresh air in gas mixer 24 The fact that the two turbines 31 and 41 of turbo-chargers 3 and 4 are located in two independent and parallel secondary exhaust lines 61 and 62 induces that line 61 is easy to design and to install within the engine compartment of truck T. Exhaust lines 61 and 62 do not have to follow a complicated path within the engine compartment, which induces that pressure drop, before the exhaust gases reach the turbines 31 or 41 , can be kept low.

As shown on figure 2, each cylinder 12 of engine 1 comprises a cylinder head 121 , a cylinder wall 122, a piston 123, a fuel injector 124, an exhaust valve 125 and an air intake valve 126.

Turning now to figure 3 which corresponds to the prior art, one considers an engine running in four-stroke braking mode. The negative work WN obtained on this occasion is as represented on figure 3, where the abscises correspond to the variable volume Vi₂ defined in cylinder 12 between the upper surface 127 of piston 123 and cylinder head 121 and the ordinates represent the pressure P within this volume. Starting from a point A where a mass of gas is trapped within the cylinder in volume V₁₂ when the piston is close to a first BDC, the air inlet valve is closed and a quasi-isentropic compression B takes place where P x V^γ equals a constant, when

the piston moves toward a first TDC, where γ is defined as the real gas polytropic coefficient (between 1.3 and 1.4) A first portion of the braking power is achieved by compressing the gases within the cylinder. The peak cylinder pressure PPEAK is reached at a point C which corresponds roughly to the point where the piston reaches the first TDC. At this stage, the outlet valve is partially opened and the pressure within the cylinder decreases, as shown by curve D, up to a point E which corresponds to a second BDC portion of the piston. The upward movement of the piston, from second BDC to second TDC starts at point E and, as shown by curve F₁ goes to a point G which corresponds to TDC and where the air inlet valve opens whereas the exhaust valve closes. From there, the pressure within the cylinder remains low, up to when the piston reaches BDC in point A. The shape of curve F results from the fact that exhaust gases must be pushed out of the cylinder by the piston against a back pressure which prevails in the exhaust manifold 13.

The negative work W_N obtained can be considered as represented by the hatched surface defined by points A, C, E and G and by curves B, D and F on figure 3.

On Figure 4, the Pressure-Volume diagram of a four stroke compression braking process according to the embodiment described earlier is presented. The specificity is that fuel injection and combustion are occurring in the compression stroke so that maximum cylinder pressure reached at the end of the compression stroke described on Figure 4 is higher than the pressure reached on Figure 3.

One considers here an angle θ representative of the angular position of the crankshaft of engine 1 . This angle is the value represented by the abscises on figure 5.

In a four stroke braking mode of engine 1 , one can define four specific positions of the piston, namely:

- a first top dead center position which is reached by the piston at the end of the compression stroke (shortly after combustion when combustion braking operations are considered) and where θ is arbitrarily set to the value 0. This position will be referred to as combustion TDC or CTDC hereafter and corresponds to a high pressure top dead center position.

- a first bottom dead center position reached after CTDC and where θ equals 180°. This position will be referred to as exhaust BDC or EBDC hereafter, since exhaust gases are evacuated from volume Vi₂ in this position.

- a second top dead center position reached after EBDC and where θ equals 360° or - 360°. This position will be referred to as breathing TDC or BTDC hereafter, since some valves are opened in this configuration and cylinder 12 can "breath".

- a second bottom dead center position reached after BTDC and before CTDC and where θ equals - 180°. This position will be referred to as pre-combustion BDC or PBDC hereafter, since it is reached before combustion takes place. On figure 5, curve M represents gas mixture intake, whereas curve I represents fuel injection, curve C represents fuel combustion and curve E represents burnt gases exhaust.

With the method of the invention, when engine 1 runs in four stroke braking mode, one injects a gas mixture GM in the cylinder when piston 123 moves from

BTDC to PBDC,. This gas mixture GM is obtained from part 52 of line 5, and includes fresh air and exhaust gases recovered by EGR system 2. Intake M of gas mixture

GM within cylinder 12 takes place by opening of valve 126.

At a later stage of the braking mode cycle, fuel F is injected within volume V₁₂ by injector 124 when piston 123 moves from PBDC to CTDC. This is shown by the injection arrow I on figure 2. The fuel is represented by the grey zone on figure 2.

Intake M of the gas mixture GM, starts at a point Mi near BTDC, where θ equals - 370°.

Actually, the value of θ at point M₁ can lie in the range of - 380° to - 340°. Intake of the gas mixture M ends at a point M₂ near PBDC, where θ equals - 160°. Actually, the value of θ at point M₂ can lie in the range of - 190° to - 150°.

Once, fuel F and gas mixture GM are present in volume V₁₂, fuel F burns within volume 12, in presence of gas mixture GM and this combustion resists the upward motion of piston 123 towards CTDC. Therefore, the peak pressure is reached at a point C on figure 4 which corresponds to the combustion of the gas-to-fuel mixture within the internal volume of the cylinder. As shown on figure 4, this substantially increases the negative work W_N obtained, which is shown by the hatched zone on figure 4.

As shown on figure 5, injection I of the fuel F starts at a point I₁ for a value of θ equal to -40°, that is 40° before piston 123 reaches CTDC. Injection stops when piston 123 is at a point I₂ 20° before CTDC. Actually, fuel injection can start when θ has a value between -70° and -30°, that is when the crankshaft has an angular position between 70° and 30° before piston 123 reaches CTDC. Fuel injection I can end when θ is between -50° and -10°, that is when the crankshaft has a position between 50° and 10° before piston 123 reaches CTDC. In other words, the values of θ at points I₁ and I₂ lie respectively in the ranges of - 70° to - 30° and - 50° to - 10°.

In the example represented on figure 5, the amplitude AG₁ of the angular movement of the crankshaft between the beginning I₁ and the end I₂ of the fuel injection I is 20° and injection stops when θ equals -20°. Actually, this amplitude Δθi can lie between 10° and 30°.

Due to the use of EGR system 2 in braking mode, the air mixed to fuel to form the gas-to-fuel mixture GM + F after the fuel injection in volume V₁₂ is partly loaded with soot resulting from the combustion which already took place in engine 1 during the previous cycles. This air also includes molecules of gases resulting from the prior combustion, that is CO, CO₂, NO, NO₂, NO_x, ... The presence of prior combustion particulate materials and gases in the gas-to-fuel mixture GM + F avoids that the fuel F injected in volume V12 by injector 124, starts to burn as soon as it is present in volume Vi₂. On the contrary, gas mixture GM spreads itself within volume V12, during a time interval which corresponds to the angular value difference Δθ₂ on figure 5 and fuel F spreads itself within volume Vi₂ during a time interval which corresponds to the angular value Δθ₃ on figure 5. In other words, since a part of the gas used to constitute the gas-to-fuel mixture injected in the cylinder comes from prior combustion, ignition of this mixture GM + F within the internal volume V₁₂ of cylinder 12 is delayed. This allows time for this mixture to be homogenised within this volume before combustion. This enhances the combustion which can be considered as a "homogeneous combustion" occurring at relatively low temperature. This means that this combustion is more complete and takes place at a lower temperature than the combustion that would take place if the mixture were to start burning as soon as fuel F exits injector 124. This low temperature combustion will not create as many pollutants as the standard combustion usually met in diesel engine development.

Due to the homogenization of mixture GM + F within volume V₁₂ prior to combustion, combustion is faster, which corresponds to the fact that curve C on figure 5 ends before angle θ reaches value 0, that is before piston 123 reaches CTDC.

In other words, the negative work W_N in braking mode is high, whereas combustion C takes place during a short time period, prior to TDC. This induces that all the fuel mixture F is burnt before the exhaust valve 125 is opened just after piston 123 reaches CTDC, as shown by curve E. Therefore, no pollution results from the combustion used in four-stroke compression braking mode of engine 1.

The invention can also be used with an engine working in two-stroke braking combustion mode, as represented on figures 6 to 8. The cycle of an engine according to the prior art, which is represented on figure 6, happens on a rotation of 360° of the crankshaft. Figure 7 corresponds to the same rotation of 360° for an engine embodying the invention, where the pick pressure PPEAK is substantially increased with respect to the configuration of figure 6.

As in the first embodiment, a gas mixture GM is formed with fresh air and gases coming from the EGR system 2.

In a two-stroke braking mode of engine 1 , one can define two specific positions of the piston, namely :

- a top dead center position TDC where θ is arbitrarily set to the value 0 and

- a bottom dead center position BDC where the value of θ is 180° or - 180°. As shown on figure 8, intake of the gas mixture starts at a point Mi where θ equals about - 300° and stops at a point M₂ where θ equals about - 160°. The value of θ at point M₁ can lie within a range of - 320° to - 200° whereas the value of θ at point M₂ can lie within a range of - 180° to - 140°.

Injection I of fuel F starts at a point U where θ equals about -40° and ends at a point I₂ where θ equals - 20°. The value of θ at points Ii and I₂ can lie respectively in a range of - 70° to - 30° and in a range of - 50° to - 10°. Combustion takes place after injection of fuel F.

As shown on figure 8, exhaust of the burnt gases E starts before a piston reaches TDC The invention has been represented for an engine 1 having a short route

EGR system 2. However, it can also be used with a long route EGR system (EGR circuit from turbine outlet to compressor inlet) or with an intermediate route EGR system for multiple-stage turbocharger system.

The invention can also be used with a sixth stroke compression braking mode, provided that gas mixture intake M, fuel injection I and exhaust E are adapted accordingly.

Thanks to the use of both turbo-chargers 3 and 4 in four stroke, two- stroke or six stroke compression mode, the gas mixture created by mixer 24 has a high pressure, which allows fast injection of the gas-to-fuel mixture within volume Vi₂. This allows the injection period Δθi to be short and to correspond to an angular rotation of the crankshaft lying under 30°, preferably in the order of 20°.

According to another embodiment of the invention, creation of the gas-to-fuel mixture GM + F can occur outside the cylinder 12, in a premixing chamber. Intake of this gas-to-fuel mixture can then take place, as explained for gas mixture M here above.

The invention has been described with the intake of gas-to-fuel mixture within one cylinder 12. Actually, since engine 1 has several cylinders, this mixture is advantageously injected in all cylinders when their respective pistons move from BDC to TDC.

Claims

1 - A method for operating an internal combustion engine (1 ) of an automotive vehicle (T) in compression braking mode, characterized in that said method includes at least the following steps of: a) - recovery of at least a part of the exhaust gases of said engine via EGR means (2) b) - mixing of said recovered gases with fresh air (F₁) in order to form a gas mixture (GM) c) - intake of said gas mixture (GM) within at least a cylinder (12) of said engine d) - intake or injection of fuel (F) within said cylinder and e) - combustion of said fuel (F) in presence of said gas mixture (GM) within said cylinder (12) when a piston (123) of said cylinder moves from its bottom dead centre position (BDC) to its top dead centre position (TDC).

2 - Method according to claim 1 , characterised step c) starts before steps d) which takes place before step e).

3 - Method according to claim 1 , characterised it includes, prior to step c) and d), a step of : f) - mixing of said gas mixture (GM) and said fuel (F) into a gas-to-fuel mixture (GM + F) and in that steps c) and d) occur together by the intake of said gas-to-fuel mixture within said cylinder (12).

4 - Method according to one of the previous claims 1 characterised in that intake or injection (I) of fuel (F) starts when a crankshaft of said engine driving said piston (123) has an angular position (θ) between 70° and 30° before said piston reaches its top dead centre position (TDC).

5 - Method according to one of the previous claims characterized in that intake or injection (I) of fuel (F) ends when a crankshaft of said engine driving said piston (123) has an angular position (θ) between 50° and 10° before said piston reaches its top dead centre position (TDC). 6 - Method according to one of claims 4 or 5 characterized in that intake or injection (I) of fuel (F) takes place during a period of time which corresponds to an angular movement of said crankshaft with an amplitude (Δθ-i) lying between 10° and 30°.

7 - Method according to one of the claims 1 , 2 and 4 to 6, characterised in that said compression mode is a four stroke compression mode and in that intake (M) of said gas mixture (GM) starts when a crankshaft of said engine driving said piston (123) has an angular position (θ) between - 380° and 340° before said piston reaches its high pressure top dead center position (CTDC).

8 - Method according to one of claims 1 , 2 and 4 to 7, characterised in that said compression mode is a four stroke compression mode and in that intake (M) of said gas mixture (GM) ends when a crankshaft of said engine driving said piston (123) has an angular position (θ) between - 190° and 150° before said piston reaches its high pressure top dead center position (CTDC).

9 - Method according to one of claims 1 , 2 and 4 to 6 characterized in that said compression mode is a two stroke compression mode and in that intake (M) of said gas mixture starts when a crankshaft of said engine driving said piston (123) has an angular position (θ) between 320° and 200° before said piston reaches its top dead center position (TDC).

10 - Method according to one of claims 1 , 2, 4 to 7 or 9 characterized in that said compression mode is a two stroke compression mode and in that intake (M) of said gas mixture (GM) ends when a crankshaft of said engine driving said piston (123) has an angular position (θ) between 180° and 140° before said piston reaches its top dead center position (TDC).

1 1 - Method according to one of the previous claims characterized in that intake (M) of the gas mixture (GM) intake or injection (I) of fuel (F) takes place during a time interval such that all combustion (C) ends before said piston (123) reaches its top dead centre position (TDC). 12 - Method according to one of the previous claims characterized in that it comprises a step of: d) utilizing two compressors (33, 43) to load said cylinder (12) with said gas- to-fuel mixture, each compressor being powered by a turbine (31 , 41 ) located in a dedicated exhaust line (61 , 62) of said engine (1).

13 - Method according to claim 12 characterized in that one (4) of the turbo- chargers is used only when said engine (1 ) works in compression braking mode.

14 - Method according to any one of the previous claims characterized in that said engine (1 ) comprises several cylinders (12) and intake or injection (I) of said fuel (F) takes place in each cylinder of said engine, when its respective piston (123) moves from its bottom dead centre position (BDC) to its top dead centre position (TDC).

15 - An internal combustion engine (1 ) capable of operating either in positive mode or in compression braking mode, said engine comprising several cylinders (12), each of which is provided with a piston (123) and means of (126) a gas mixture within each cylinder, said engine comprising two turbo-chargers (3, 4) adapted to provide said intake means with gas under pressure, characterised in that the respective turbines (31 , 41 ) of said turbo-chargers are located in two different exhaust lines (61 , 62) which extend in parallel, downstream of an exhaust gas manifold (13) of said engine.

16 - Engine according to claim 15 characterized in that it includes EGR means (2) adapted to provide said intake means (126) with exhaust gases of said engine.

17 - Engine according to claim 16, characterised in that an outlet (23) of said

EGR means (2) is connected to an air inlet line (5) feeding said intake means (126). 18 - Engine according to one of claim 16 or 17 characterized in that said EGR means (2) include a gas mixer (24) adapted to mix exhaust gases with fresh air (F-i) loaded by at least a compressor (33, 43) of one of said turbo-chargers (3, 4).

19 - Engine according to one of claim 15 to 18 characterized in that it includes control means (8, 9) adapted to actuate a first turbo-charger (3) when at least said engine (1) is in positive mode and to actuate a second turbo-charger (4) only when said engine is in braking mode.

20 - Engine according to claim 19, characterised said control means (8, 9) actuate the first turbo charger (3) also when said engine is in braking mode.

21 - An automotive vehicle (T) equipped with an engine (1 ) according to one of claims 15 to 20.