EP4735741A1

EP4735741A1 - Internal combustion engine with variable intake valve actuation, boot profile cams, and engine control method

Info

Publication number: EP4735741A1
Application number: EP24734133.2A
Authority: EP
Inventors: Raffaele Ricco; Marcello Gargano
Original assignee: Centro Ricerche Fiat SCpA
Current assignee: Centro Ricerche Fiat SCpA
Priority date: 2023-06-27
Filing date: 2024-06-06
Publication date: 2026-05-06
Also published as: WO2025003805A1; IT202300013263A1

Abstract

An internal combustion engine has at least one intake valve (V1, V2) for each cylinder actuated by a cam (14) shaped to create a lift profile having a boot conformation. The intake valve is actuated by a respective cam by means a hydraulic circuit which can be pressurized or discharged by means of an electrically actuated control valve (24), governed by an electronic controller. The electronic controller is configured to actuate the intake valve (V1, V2), under given engine operating conditions, so that during a final part of the descent of the respective piston towards the BDC at least one intake valve (V1, V2) is kept completely closed or is kept open to an extent sufficiently small to generate a pressure drop of at least 0.4 bar (40,000 Pa) in the cylinder due to the simultaneous descent of the piston towards the BDC. During a subsequent ascent phase of the respective piston towards the TDC, substantially from the BDC, the intake valve is opened, to generate a jet of air entering the cylinder, due to the depression created in the cylinder during the previous descent phase of the piston towards the BDC.

Description

“Internal combustion engine with variable intake valve actuation, boot profile cams, and engine control method”

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Field of the invention

The present invention refers to internal combustion engines of the type indicated in the preamble of claim 1 .

Engines of this type are described for example in documents EP 0 803 642 B1 , EP 1 555 398, EP 1 508 676 B1 , EP 1 674 673 B1 and EP 2 261 471 A1 of the same Applicant.

Prior art

The Applicant has long developed internal combustion engines including a variable intake valve drive system of the type indicated above, marketed under the “MULTIAIR” brand. The same Applicant is the owner of various patents and patent applications relating to engines equipped with a system of the type specified above.

Figure 1 of the attached drawings shows a sectional view of an engine equipped with the “MULTIAIR” system, as described in the European patent EP 0 803642 B1.

With reference to this figure 1 , the engine illustrated therein is a multicylinder engine, for example a four-cylinder in-line engine, comprising a cylinder head 1. The head 1 includes, for each cylinder, a cavity 2 formed by the base surface 3 of the head 1 , defining the combustion chamber, into which two intake ducts 4, and two exhaust ducts 6 lead. The connection of the two intake ducts 4 with the combustion chamber 2 is controlled by two intake valves 7, of the traditional mushroom type, each comprising a stem 8 mounted for sliding in the body of the head 1 .

Each valve 7 is returned to the closed position by springs 9 placed between an internal surface of the head 1 and an end cup 10 of the valve. The connection of the two exhaust ducts 6 with the combustion chamber is controlled by two valves 70, also of the traditional type, which are associated with return springs 9 for returning to the closed position.

The opening of each intake valve 7 is controlled, in the way that will be described below, by a camshaft 11 mounted rotatable around an axis 12 within supports of the head 1 , and comprising a plurality of cams 14 for actuating the intake valves 7.

Each cam 14 which controls an intake valve 7 cooperates with the plate 15 of a tappet 16 mounted to slide along an axis 17 which, in the case of the example illustrated in the cited previous document, is directed substantially at 90° with respect to the valve axis 7. The plate 15 is returned against the cam 14 by a spring associated with it. The tappet 16 constitutes a pumping plunger slidably mounted within a bushing 18 carried by a body 19 of a pre-assembled group 20, incorporating all the electrical and hydraulic devices associated with the actuation of the intake valves, as described in detail below.

The pumping plunger 16 is able to transmit a thrust to the stem 8 of the valve 7, so as to cause the opening of the latter against the action of the elastic means 9, by means of pressure fluid (preferably oil coming from the lubrication circuit of the engine) present in a pressure chamber C which the pumping plunger 16 faces, and by means of a piston 21 mounted to slide in a cylindrical body consisting of a bushing 22 which is also carried by the body 19 of the pre-assembled group 20.

Still in the known solution illustrated in figure 1 , the pressure fluid chamber C associated with each intake valve 7 can be connected with an exhaust channel 23 by means of a solenoid valve 24. The solenoid valve 24, which can be of any known type, suitable for the function illustrated here, is controlled by electronic control means, indicated schematically with 25, as a function of signals S indicative of engine operating parameters, such as the accelerator position and the number of engine revolutions.

When the solenoid valve 24 is opened, the chamber C is connected with the channel 23, whereby the pressure fluid present in the chamber C flows into this channel and a decoupling of the cam 14 and the respective tappet 16 from the intake valve is obtained 7, which then quickly returns to its closed position by the action of the return springs 9. By controlling the connection between the chamber C and the discharge channel 23, it is therefore possible to vary, as desired, the opening time and stroke of each intake valve 7.

The exhaust channels 23 of the various solenoid valves 24 all flow into the same longitudinal channel 26 connected with pressure accumulators 27, only one of which is visible in figure 1 . All the tappets 16 with the associated bushings 18, the pistons 21 with the associated bushings 22, the solenoid valves 24 and the relative channels 23, 26 are carried and obtained from the aforementioned body 19 of the pre-assembled group 20, to the advantage of speed and ease of assembly of the engine.

The exhaust valves 70 associated with each cylinder are controlled, in the embodiment illustrated in figure 1 , in a traditional way, by a respective camshaft 28, by means of respective tappets 29, although in principle it is not excluded, in the case of the document mentioned above, an application of the hydraulic drive system also to the control of the exhaust valves.

Again with reference to figure 1 , the variable volume chamber defined inside the bushing 22 and facing the piston 21 (which in figure 1 is illustrated in its minimum volume condition, the piston 21 being in its stroke end upper position) is connected with the pressure fluid chamber C by means of an opening 30 obtained in an end wall of the bushing 22. This opening 30 is engaged by an end nose 31 of the piston 21 in such a way as to achieve hydraulic braking of the movement of the valve 7 in the closing phase, when the valve is near the closed position, as the oil present in the variable volume chamber is forced to flow into the pressure fluid chamber C passing through the gap existing between the end nose 31 and the wall of the opening 30 engaged by it. In addition to the connection constituted by the opening 30, the pressure fluid chamber C and the variable volume chamber of the piston 21 are connected with each other by means of internal passages obtained in the body of the piston 21 and controlled by a non-return valve 32 which allows the passage of fluid only from the pressure chamber C to the variable volume chamber of the piston 21 .

During the normal operation of the known engine illustrated in figure 1 , when the solenoid valve 24 excludes the connection of the pressure fluid chamber C with the discharge channel 23, the oil present in this chamber transmits the movement of the pumping plunger 16, imparted by the cam 14, to the piston 21 which controls the opening of the valve 7. In the initial phase of the valve opening movement, the fluid coming from the chamber C reaches the variable volume chamber of the piston 21 passing through the non-return valve 32 and further passages which connect the internal cavity of the piston 21 , which has a tubular shape, with the variable volume chamber. After an initial movement of the piston 21 , the nose 31 comes out of the opening 30, so that the fluid coming from the chamber C can pass directly into the variable volume chamber through the opening 30, now free.

In the reverse closing movement of the valve, as already mentioned, during the final phase the nose 31 enters the opening 30 causing the hydraulic braking of the valve, so as to avoid impacts of the valve body against its seat, for example following an opening of the solenoid valve 24 which causes the immediate return of the valve 7 to the closed position.

In the system described, when the solenoid valve 24 is activated, the engine valve follows the movement of the cam (full lift). An early closing of the valve can be achieved by deactivating (opening) the solenoid valve 24, so as to empty the hydraulic chamber and obtain the closing of the engine valve by the action of the respective return springs. Similarly, a delayed opening of the valve can be achieved by delaying the actuation of the solenoid valve, while the combination of a delayed opening with an early closing of the valve can be achieved by activating and deactivating the solenoid valve while pushing the relevant cam. According to an alternative strategy, according to the teachings of the patent application EP 1 726 790 A1 of the same applicant, each intake valve can be controlled in “multi-lift” mode, i.e. according to two or more repeated “sub-cycles” of opening and closing. In each sub-cycle, the intake valve opens and then closes completely. The electronic control unit is therefore able to obtain a variation of the opening time and/or the closing time and/or the lift of the intake valve, depending on one or more engine operating parameters. This allows maximum engine efficiency and the lowest fuel consumption to be achieved in all operating conditions.

In the known system described above it can be provided that the two intake valves 7 associated with the same engine cylinder are controlled by a single pumping plunger 16 in turn controlled by a single cam on the engine camshaft.

In this case, if it is desired to actuate the two intake valves of the same cylinder in a differentiated manner, the solution known from document EP 2 693 007 A1 of the same Applicant can be provided, in which the electrically actuated control valve is a three-way and three-position solenoid valve, with an inlet connected to both the pressure chamber and the hydraulic actuator of one of the two intake valves, an outlet connected to the fluid accumulator and a further outlet connected to the hydraulic actuator of the other intake valve (see figure 20 attached here, corresponding to figure 4 of the document cited above).

Alternatively, the further solution known from document EP 3832078 A1 , also from the same Applicant, can be provided, which provides two solenoid valves arranged in series in the connection between the pressure chamber and the hydraulic accumulator and with the two hydraulic actuators of the two intake valves connecting, one with the hydraulic line between the two solenoid valves and the other with the pressure chamber (see figure 20 attached here, corresponding to figure 11 of EP 3 832 078 A1).

However, for the purposes of the present invention, it can also be provided that each intake valve of each engine cylinder is controlled by a respective cam of the camshaft and by a respective hydraulic circuit including a respective pumping plunger, in which case it can have total flexibility in differentiating the openings of the two intake valves of each cylinder.

In the Italian patent applications IT 102022000025410 filed on December 13, 2022 and IT 102023000003450 filed on February 27, 2023, both still classified at the first filing date of the present invention, the Applicant disclosed progressions of the known system described above, specifically dedicated to an engine with two intake valves per cylinder and with a single intake valve per cylinder. The present invention is based on the need to simplify this solution from a construction point of view and to facilitate its application on an already existing engine.

An engine having the features of the preamble of claim 1 is known from document EP 3 032 054 A1 of the same Applicant. A further solution is known from document WO 20212/087421 A1 .

Purpose of the invention

The main object of the invention is to provide an internal combustion engine of the type indicated at the beginning of this description which is characterized by high combustion efficiency in all engine operating conditions.

In particular, an object of the invention is to provide an internal combustion engine in which the intake valves of each cylinder can be controlled with alternative strategies which, combined together, achieve the maximum advantages in terms of combustion efficiency in each engine operating condition.

Finally, an important aim of the invention is to achieve said objectives with a solution that is simple in construction and easy to apply on an already existing engine.

Summary of the invention

In order to achieve the aforementioned purposes, the invention has as its object an internal combustion engine having the features of claim 1 and a control method according to claim 9.

Preferred and advantageous features of the invention are indicated in the dependent claims.

Detailed description of the invention

Further features and advantages of the invention will emerge from the following description with reference to the attached drawings, provided purely by way of non-limiting example, in which:

- figure 1 is a sectional view of the cylinder head of an internal combustion engine equipped with an electronically controlled hydraulic system for operating the engine intake valves, according to the prior art illustrated in document EP 0 803 642 B1 and discussed above,

- figure 2 is a schematic view of the variable actuation system of the engine intake valves, according to a first embodiment of the present invention, in which the two intake valves of each cylinder are controlled by two distinct cams, by means of respective tappets, respective pumping plungers and respective hydraulic circuits,

- figure 3 illustrates, with a solid line, the lift profile having a boot conformation of each intake valve, determined by the profile of each cam of the system illustrated in figure 2, in the hypothesis that the hydraulic control circuit of the intake valve is always kept under pressure, while the dotted line illustrates the lift profile that can be obtained by unloading the hydraulic circuit in an intermediate period between two distinct opening periods of the intake valve, - figure 4 indicates a further example of intake valve lift of the type indicated with the dotted line in figure 3,

- figure 5 illustrates a further example of a lift profile which refers to a different embodiment, in which the cam which controls each intake valve is configured to generate a first opening period at a substantially constant, very low level, for example equal to 1/5-1/6 of the level corresponding to the maximum point of the second part of the lift profile, and

- figure 6 illustrates the variation in pressure in each engine cylinder as the crank angle varies, respectively in the case of the solution in figure 4 (line IV) and figure 5 (line V).

Starting from the known solution described above with reference to figure 1 , the invention provides a plurality of new opening strategies of the two intake valves of each engine cylinder for the purpose of increasing combustion efficiency, with consequent advantages in terms of the fuel consumption, and therefore a reduction in CO2 emissions, and the reduction of harmful exhaust gases, in all engine operating conditions.

Figure 2 shows a diagram of the variable actuation system of the intake valves of each engine cylinder in a first embodiment of the invention.

According to the invention, each engine cylinder has two intake valves V1 , V2.

In this first embodiment, each of the two intake valves V1 , V2 is controlled by a respective cam 14 with a respective hydraulic circuit, including a respective pumping plunger 16, a respective pressure chamber C, a respective hydraulic actuator 21 of the valve intake, a respective two- position solenoid valve 24 capable of controlling the connection between the pressure chamber C and a pressure accumulator 270 which is also in connection with a low pressure circuit of the engine lubricating oil.

In an alternative embodiment, the two intake valves of each cylinder can be controlled by a single pumping plunger 16, and a single hydraulic circuit including a pressure chamber C suitable for communicating with both hydraulic actuators 21 of the two intake valves of each cylinder, and with a single solenoid valve 24 capable of controlling the communication between the pressure chamber C and the pressure accumulator 270 which is in communication with the low-pressure circuit of the engine lubricating oil.

Whatever the solution chosen, the invention provides that each cam 14 is so shaped as to generate, when the pressure chamber is always kept under pressure, a lift profile of each intake valve, as a function of the crank angle, having the boot conformation illustrated in figure 3 with a solid line. As visible in figure 3, the cam 14 is so shaped as to generate, when the pressure chamber C is always kept under pressure, a lift profile of a corresponding intake valve, as a function of the crank angle, having a first part defining a substantially constant first level of lift, followed by a second bell-shaped part, which substantially starts from the BDC and has a maximum point M defining a second level of lift equal to at least 2.5 - 3 times the lift level of the first part.

In the case of the solution of figure 3, the electronic controller 25 is configured to govern the solenoid valve 24 associated with said hydraulic circuit of each cylinder, under given engine operating conditions, so that in the intake stage, in each cycle operating of each cylinder, the lift profile of each intake valve is that indicated with a dotted line in figure 3. In this example, during a final part of the descent of the respective piston towards the BDC (which corresponds to a crank angle of 540° in the diagram of figure 3) the hydraulic circuit is kept in exhaust mode, so each intake valve remains completely closed, being insensitive to the action of the cam. Instead, before and after this period, the hydraulic circuit is kept under pressure, so each intake valve has a first opening period, substantially from TDC to an intermediate point between the TDC and the BDC, and then a second opening period, substantially from the BDC.

In this solution, since during the final part of the descent of the piston towards the BDC each intake valve is kept closed, a depression is created in the cylinder which gives rise to the entry of a jet of air into the cylinder when, during the subsequent ascent phase of the respective piston towards the TDC, substantially from BDC, each intake valve is opened.

In a variant, at a crank angle of about 420°- 440° (adopting the convention of TDC at 360° and BDC at 540°), the boot profile drops by approximately 30% of the height of the first level of lift to facilitate the carrying out of the cam profile part, and to be able to start the second opening period slightly before the BDC.

Preferably, the first opening period of an intake valve of a given cylinder is always of the same type: said first opening period substantially begins when the respective piston is in its TDC and ends when the piston is substantially midway between the PMS and the BDC.

Again preferably, the second opening period of an intake valve of a given cylinder can be of two types: a first type, referred to below for brevity as “type 2A” and a second type, referred to below for brevity as “type 2B”.

In the case of type 2A, the second opening period of an intake valve of a given cylinder begins when the piston has passed 4/5 of its path from the TDC to the BDC and has not yet reached its BDC, preferably not more than 30° of crank angle before the BDC, and at not less than 20° of crank angle before the BDC. Again in the case of type 2A, the second opening period ends when the piston has passed the BDC and is rising towards the TDC, preferably at least 20° of crank angle after the BDC, and at not more than 30° of crank angle after the BDC.

In the case of type 2B, the second opening period of an intake valve of a given cylinder begins when the piston has passed BDC and is rising towards the TDC, preferably at least 20° of crank angle after the BDC, and at not more than 30° of crank angle after the BDC, said second opening period ending at least 80° of crank angle after the BDC, and at not more than 100° of crank angle after the BDC.

In the same engine, the two different types of the second opening period described above can be obtained in various ways.

For example, it is possible to modulate the actuation of the electrically actuated control valve, to obtain a different lift profile, at the beginning and/or at the end of the second opening period of the intake valve, than that which would be determined by the respective driving cam. In this case the second part of the boot profile is configured with sufficient width to allow both types of lift in the second opening period. Figure 6 of the attached drawings shows the lift profiles C1 , C2 which would be determined by the first part and the second part of the boot profile, corresponding to the first opening period and the second opening period. Thanks to the intervention of the electrically actuated control valve, which can unload the respective hydraulic circuits, in the first opening period the lift profile can become the profile 1 B, while in the second opening period the lift profile can become the profile 2A of the first type or profile 2B of the second type. Alternatively, to obtain one or the other type of second opening period in the same engine, a timing shifting device can be provided associated with the camshaft which controls the intake valves, capable of delaying the entire lift profile of approximately 40° of crank angle (obviously in this case the first opening would also be translated). This solution is schematized in figure 7, where C is the lift profile that would be determined by a conventional cam (with a single lobe), 1A, 1 B are the lift profiles in the first opening period before and after an intervention of the timing shifting device, and 2A, 2B are the two lift profiles of the second opening period before and after the intervention of the timing shifting device.

Still alternatively, to obtain one or the other type of the second opening period in the same engine, it is possible to use actuation devices of the type known per se in which different cam profiles can be activated selectively. In this way it is possible to provide both types 2A, 2B of the second opening period and possibly also the conventional profile C with a single opening (see figure 8)

In the event that the engine does not require the use of both types of the second opening period, it is obviously possible to optimize the design of the cam in order to limit the consumption absorbed for pressurizing the oil and therefore the cam profile can follow the desired opening profile of the intake valves.

Preferably, the maximum lift of the first opening period is between 1/8 and 1 Z10 of the diameter of the circular head of the respective intake valve, and the maximum lift of the second opening period, of the first type (2A) and second type (2B), is between 1 /4 and 1 /5 of the diameter of the circular head of the respective intake valve.

Figure 4 shows a further example of a lift profile of each intake valve of the type illustrated with a dotted line in figure 3, i.e. of the type in which each intake valve has two opening periods spaced in time, with a opening period where each intake valve is fully closed.

Figure 5 illustrates a further example of a lift profile which refers to a different embodiment, in which the cam which controls each intake valve is configured to generate a first opening period L1 at a substantially constant, very low level, for example equal to 1/5-1/6 of the level corresponding to the maximum point M of the second part L2 of the lift profile. In the case of this embodiment, the electronic controller 25 is configured to maintain the pressure chamber C always under pressure, so that each intake valve V1 or V2 remains constantly open during the intake stage in each operating cycle of each cylinder. However, in this case, the first level L1 of lift is sufficiently low to ensure that, during the descent of the respective piston towards its BDC inside the cylinder, the pressure drops by at least 0.4 bar (40,000 Pa). Starting from when the piston reaches the BDC and begins to ascend towards TDC, each intake valve opens further, reaching point M, which causes a jet of air to enter the cylinder, due to the depression previously created in the cylinder.

This effect is evident in figure 6, which illustrates the variation in pressure in each engine cylinder as the crank angle varies, respectively in the case of the solution in figure 4 (line IV) and figure 5 (line V). In both cases a depression is obtained in the cylinder in the final descent phase of the piston towards the BDC. The depression is greater in the case of the solution in figure 4. In the case of the solution in figure 5 the pressure drop is smaller (in the order of 0.4 bar), but still sufficient to give rise to the jet of air in the cylinder when the intake valve is opened more, in the phase in which the piston begins to ascend towards TDC.

Preferably, in the case in which the two intake valves V1 , V2 of each cylinder are actuated by a single cam and a single pumping plunger 16 through a pressure chamber C in communication with both hydraulic actuators 21 of the two intake valves, the ratio of the surface area of the pumping plunger facing the pressure chamber to the sum of the corresponding areas of the two hydraulic actuators is greater than 2 and even more preferably is not less than 2.4

In a further variant, said boot profile has three parts: a first part with a substantially constant lift starting from a crank angle of about 240°, an intermediate part with a substantially constant lift, at a level greater than the level of the first part, and a final bell-shaped part.

Of course, notwithstanding the principle of the invention, the construction details and the embodiments may vary widely with respect to what has been described and illustrated purely by way of example, without thereby departing from the scope of the present invention, as defined by the attached claims.

Claims

1. An internal combustion engine, comprising:

- one or more cylinders and a piston movable in each cylinder and operatively associated with a crankshaft, wherein each engine cylinder has respective operating cycles including an intake stage, a compression stage, an expansion stage and an exhaust stage,

- at least one intake valve (V1 , V2) associated with each engine cylinder, to control a flow of intake air from a respective intake duct (5) during the intake stage into the cylinder in each cylinder operating cycle,

- a camshaft (11 ) driven by the crankshaft, carrying a cam, to drive said at least one intake valve of each engine cylinder, via a tappet (15),

- wherein said at least one intake valve (V) of each cylinder is actuated by said tappet (15), against the action of a return spring (9), by interposition of a hydraulic circuit including:

- a pumping plunger (16) actuated by the tappet (15) and configured to transfer pressurized fluid, through a pressure chamber (C), to a hydraulic actuator (21 ) associated with said at least one intake valve (V) of each engine cylinder,

- an electrically actuated control valve (24) adapted to connect said pressurized fluid chamber (C) with a low-pressure drain channel (23) communicating with a pressurized fluid accumulator (270), so that when said control valve (24) is open, pressurized fluid drains from the pressure chamber (C) into said low-pressure drain channel and said at least one intake valve (V) closes by the effect of the respective return spring (9), regardless of the action of the respective cam, said engine further comprising an electronic controller (25) for controlling the electrically actuated control valve (24) associated with said hydraulic circuit, depending on a plurality of engine operating parameters, including engine load and engine rotational speed, the engine being characterized in that:

- said cam (14) associated with said at least one intake valve (V1 , V2) of each engine cylinder is so shaped as to generate, when the pressure chamber (C) is always kept under pressure, a lift profile of said at least one intake valve, as a function of the crank angle, having a boot conformation, with a first part defining a substantially constant first level of lift, followed by a second bell-shaped part, which substantially starts from the BDC and has a maximum point defining a second level of lift equal to at least 2.5 times said first level of lift, said electronic controller is configured to govern the electrically actuated control valve (24) associated with said hydraulic circuit of each cylinder, under given engine operating conditions, so that:

- during the intake stage, in each operating cycle of each cylinder, during a final part of the descent of the respective piston towards the BDC said at least one intake valve (V1 , V2) is kept completely closed or is kept open to an extent sufficiently small to generate in the cylinder, due to the simultaneous descent of the piston towards the BDC, a pressure drop of at least 0.4 bar (40,000 Pa),

- during a subsequent ascent phase of the respective piston towards the TDC, substantially from the BDC, said at least one intake valve is opened, to generate a jet of air entering the cylinder, due to the depression created in the cylinder during the previous descent phase of the piston towards the BDC.

2. The engine according to claim 1 , characterized in that said electronic controller (25) is configured to govern the electrically actuated control valve (24) so that said at least one intake valve is actuated with a first opening period and a second opening period, respectively at said first part and said second bell-shaped part of said boot profile, with an intermediate phase in which said at least one intake valve is kept closed, while the respective piston is descending towards its BDC, so as to create said depression in the respective cylinder generating said jet of air entering the cylinder during the subsequent second opening period of said at least one intake valve, wherein said first opening period of said at least one intake valve of each cylinder begins substantially when the respective piston is at its TDC and ends when the piston is substantially midway between its TDC and its BDC, and wherein said second opening period of said at least one intake valve of each cylinder is of a first type (2A) or a second type (2B), wherein said second opening period of the first type (2A) begins when the piston has passed 4/5 of its path from the TDC to the BDC and has not yet reached its BDC, preferably at not more than 30° crank angle before the BDC, and at not less than 20° crank angle before the BDC, wherein said second opening period of the first type (2A) ends when the piston has passed the BDC and is in the process of rising to the TDC, preferably at least 20° crank angle after the BDC, and at not more than 30° crank angle after the BDC, wherein the second opening period of the second type (2B) begins when the piston has passed the BDC and is in the process of rising to the TDC, preferably at least 20° crank angle after the BDC, and at not more than 30° crank angle after the BDC, and wherein said second opening period of the second type (2B) ends at least 80° crank angle after the BDC, and at not more than 100° crank angle after the BDC.

3. The engine according to claim 1 , characterized in that said electronic controller (25) is configured to maintain the pressure chamber (C) always under pressure, so that said at least one intake valve (V1 , V2) remains constantly open during the intake stage in each operating cycle of each cylinder, and in that the boot profile of the cam has said first level that is low enough so that during the descent of the respective piston towards its BDC inside the cylinder the pressure drops by at least 0.4 bar (40,000 Pa), while, at the second bell-shaped part of the boot profile, starting when the piston reaches the BDC and begins to ascend towards the TDC, said at least one intake valve opens further, allowing said jet of air to enter the cylinder.

4. The engine according to claim 3, characterized in that the cam is configured to generate a lift profile having the maximum point of the second bell-shaped part corresponding to a lift equal to about 5-6 times the first lift level defined from said first part at substantially constant lift.

5. The engine according to claim 1 , characterized in that it comprises a first intake valve (V1 ) and a second intake valve (V2) for each cylinder, and in that the two intake valves (V1 , V2) of each cylinder are driven by a single cam and a single pumping plunger (16) through a pressure chamber (C) in communication with both hydraulic actuators (21 ) of the two intake valves, the ration of the surface area of the pumping plunger facing the pressure chamber to the sum of the corresponding areas of the two hydraulic actuators is greater than 2 and preferably is not less than 2.4

6. The engine according to claim 5, characterized in that said boot profile has a first part at substantially constant lift from a crank angle of about 240°, where a crank angle of 360° corresponds to the BDC, and an intermediate part at substantially constant lift between said first part and said second bell-shaped part, said intermediate part defining a lift level greater than the lift level of said first part.

7. The engine according to claim 2, characterized in that said camshaft (11 ) is associated with a timing shifting device configured to vary the angular position of said camshaft with respect to said crankshaft.

8. The engine according to claim 2, characterized in that the cam (14) associated with each intake valve (V1 ; V2) of each engine cylinder is of a type with multiple profiles that can be rendered active selectively.

9. A method for controlling an internal combustion engine, wherein the engine comprises:

- an electrically actuated control valve (24) adapted to connect said pressurized fluid chamber (C) with a low-pressure drain channel (23) communicating with a pressurized fluid accumulator (270), so that when said control valve (24) is open, pressurized fluid drains from the pressure chamber (C) into said low-pressure drain channel and said at least one intake valve (V) closes by the effect of the respective return spring (9), regardless of the action of the respective cam, said engine further comprising an electronic controller (25) for controlling the electrically actuated control valve (24) associated with said hydraulic circuit, depending on a plurality of engine operating parameters, including engine load and engine rotational speed, said method being characterized in that:

- said cam (14) associated with said at least one intake valve (V1 , V2) of each engine cylinder is so shaped as to generate, when the pressure chamber (C) is always kept under pressure, a lift profile of said at least one intake valve, as a function of the crank angle, having a boot conformation, with a first part defining a substantially constant first level of lift, followed by a second bell-shaped part, which substantially starts from the BDC and has a maximum point defining a second level of lift equal to at least 2.5 times said first level of lift, said process comprising governing, by means of said electronic controller, the electrically actuated control valve (24) associated with said hydraulic circuit of each cylinder, under given engine operating conditions, so that:

10. The method according to claim 9, characterized in that said electronic controller (25) is configured to govern the electrically actuated control valve (24) so that said at least one intake valve is actuated with a first opening period and a second opening period, respectively at said first part and said second bell-shaped part of said boot profile, with an intermediate phase in which said at least one intake valve is kept closed, while the respective piston is descending towards its BDC, so as to create said depression in the respective cylinder generating said jet of air entering the cylinder during the subsequent second opening period of said at least one intake valve, wherein said first opening period of said at least one intake valve of each cylinder begins substantially when the respective piston is at its TDC and ends when the piston is substantially midway between its TDC and its BDC, and wherein said second opening period of said at least one intake valve of each cylinder is of a first type (2A) or a second type (2B), wherein said second opening period of the first type (2A) begins when the piston has passed 4/5 of its path from the TDC to the BDC and has not yet reached its BDC, preferably at not more than 30° crank angle before the BDC, and at not less than 20° crank angle before the BDC, wherein said second opening period of the first type (2A) ends when the piston has passed the BDC and is in the process of rising to the TDC, preferably at least 20° crank angle after the BDC, and at not more than 30° crank angle after the BDC, wherein the second opening period of the second type (2B) begins when the piston has passed the BDC and is in the process of rising to the TDC, preferably at least 20° crank angle after the BDC, and at not more than 30° crank angle after the BDC, and wherein said second opening period of the second type (2B) ends at least 80° crank angle after the BDC, and at not more than 100° crank angle after the BDC.

11. The method according to claim 9, characterized in that, by means of said electronic controller (25), the pressure chamber (C) is always kept under pressure, so that said at least one intake valve (V1 , V2) remains constantly open during the intake stage in each operating cycle of each cylinder, and in that the boot profile of the cam has said first level that is low enough so that during the descent of the respective piston towards its BDC inside the cylinder the pressure drops by at least 0.4 bar (40,000 Pa), while, at the second bell-shaped part of the boot profile, starting when the piston reaches the BDC and begins to ascend towards the TDC, said at least one intake valve opens further, allowing said jet of air to enter the cylinder.