Internal combustion engine with air jet system for cylinder filling, and method for controlling the engine

Abstract

In an internal combustion engine, one or more intake valves (V1, V2) associated with each cylinder (1) of the engine are each controlled by a cam (70, 71) of a camshaft by means of a mechanical transmission. The actuating cam (70, 71) of each intake valve (V1, V2) comprises two or more axially adjacent cam portions (70, 71), having different profiles, and which can be selectively activated depending on the engine operating conditions. At least a first cam portion (70) has a profile having two lobes (70A, 70B) arranged and configured so as to cause, in each cylinder operating cycle, a first opening period of the corresponding intake valve (V1, V2) which begins when the respective piston is in the vicinity of TDC and ends when the respective piston is in an intermediate position between TDC and BDC, a second opening period of the intake valve (V1, V2), which begins when the respective piston is in the vicinity of BDC, or has exceeded BDC, and ends when the piston is in at intermediate point between BDC and TDC. The intake valve (V1, V2) actuated by said first cam portion is fully closed during an intermediate period in which the respective piston is still descending towards its BDC, so as to create a vacuum in the respective cylinder (1) that generates a jet of air entering the cylinder during the subsequent second opening period of each intake valve (V1, V2). A second cam portion (71) has a single-lobe profile. The first cam portion (70) having a profile with two lobes (70A, 70B) is activated when the crankshaft rotation speed is below a predetermined threshold value.

Description

[0001] Internal combustion engine with air jet system for cylinder filling, and method for controlling the engine

[0002] Field of the invention

[0003] The present invention relates to internal combustion engines of the type comprising:

[0004] - one or more cylinders and a piston movable in each cylinder between a Top Dead Center (hereinafter “TDC”) and a Bottom Dead Center (hereinafter “BDC”) and operatively associated with a crankshaft, wherein each cylinder of the engine has respective operating cycles including an intake stage, a compression stage, an expansion stage and an exhaust stage,

[0005] - one or more intake valves associated with each cylinder of the engine, to control a flow of intake air from one or more respective intake ducts in each cylinder operating cycle,

[0006] - wherein each intake valve is controlled, via a tapper, by a cam of a camshaft driven by the crankshaft,

[0007] - wherein each intake valve of each cylinder is controlled by the respective tappet, against the action of a return spring, by means of a mechanical transmission, so that each intake valve has a lift profile as a function of the crank angle that is invariable and related to the profile of the respective cam.

[0008] Prior art

[0009] In the past, several solutions have been proposed both in an attempt to reduce pumping losses in an internal combustion engine at low engine loads, and in order to reduce the effective compression ratio, maintaining a high expansion ratio, so as to avoid the risk of detonation (knocking) in the case of using high values of the geometric compression ratio. Typical solutions of this type are represented by engines operating according to the Miller cycle (early closing of the intake valves) or according to the Atkinson cycle (delayed closing of the intake valves).

[0010] Both of said known solutions have not, however, proved to be completely satisfactory, particularly from the point of view of combustion efficiency and reduction of harmful emissions. The Miller cycle is useful for reducing pumping losses, but presents problems at very low engine loads, as the pressure in the cylinder reaches extremely low values (less than 0.4 barA) with a consequent increase in the risk of blow-by through the oilscraper rings associated with the piston. Furthermore, the Miller solution involves a strong reduction of the Turbulent Kinetic Energy (“TKE”) which is extremely important in order to support the propagation of the flame during combustion.

[0011] The Atkinson cycle is also penalized by low values of “tumble” (the swirling movement of the air in the cylinder around an axis orthogonal to the axis of the cylinder) and TKE. The Atkinson cycle also has a problem in relation to the risk of a backflow of the air-fuel mixture in the intake ducts. The fuel is introduced into the combustion chamber by direct injection (which is necessary to counteract knocking) when the intake valves are open, so there is a significant risk of a backfire and poor control of the air / fuel ratio in the cylinder, with the further consequence of an increase in harmful emissions).

[0012] In Italian patent applications IT 10 2022 000025410, IT 10 2023 000003450 and IT 10 2023 000013266 (still secret at the priority date of the present invention), the Applicant presented progresses of an electronically controlled hydraulic system for the variable actuation of the intake valves of an internal combustion engine, aimed at obtaining a drastic increase in the TKE in the cylinder. In these known solutions, the intake valves are actuated by cams having a profile with two lobes arranged and configured such that, if the respective hydraulic circuits are kept under pressure, in each cycle of each cylinder the intake valves have a first opening period and a second opening period interspersed with an intermediate period in which the valves are closed. The first opening period begins when the respective piston is in the vicinity of its TDC and ends when the respective piston is still descending toward the BDC, approximately halfway through the stroke. The second opening period begins when the respective piston is in the vicinity of the BDC and ends when the piston is already rising toward the TDC. Thus, each intake valve is fully closed in an intermediate period when the respective piston is still descending toward its BDC, so as to create a vacuum in the respective cylinder that generates a jet of air entering the cylinder during the subsequent second opening period of the intake valve.

[0013] Said “air-jet” system, i.e. obtaining a jet of air entering the cylinder with the second opening period of the intake valves, increases combustion efficiency, especially at low engine speeds, since the jet of air gives rise to a high TKE, which in conventional engines is proportional to said rotational speed.

[0014] The availability of high TKE, not obtainable in the intake cycles of said known solutions, also allows to increase the quantity of recirculated exhaust gases (“EGR”, from Exhaust Gas Recirculation) inside the cylinder, even at the lowest loads. The introduction of internal EGR allows to increase the mass of the gases to be compressed, as well as the temperature in the cylinder when the piston is at TDC, with a reduction in pumping losses. The difficult combustion of the internal EGR is compensated by the higher swirling.

[0015] When you want to use the air jet to support combustion and efficiency at low loads, in practice the piston of each cylinder behaves like an air spring that first creates a vacuum in the cylinder and then, during the rise from BDC to TDC, pushes the piston towards TDC thanks to the vacuum previously generated, so the compression work in the cylinder begins only when the pressure returns to being greater than 1 barA: this means that the later said second opening period begins, the greater the pumping work that is recovered.

[0016] On the contrary, the air jet is able to increase the trapped air and therefore the engine load at maximum loads, thanks to the greater inertia of the column of incoming air, which allows a greater quantity of air to be trapped in the cylinder (in practice the air jet offers a sort of overpressure equal to 0.2 bar).

[0017] Further studies and experiments of the Applicant on the engine, including said electro-hydraulic valve actuation system, according to this new proposal have shown that there may however be problems in producing cams with two lobes capable of operating correctly in all operating conditions, in particular also at high engine rotations, and such as to generate intake valves lifts sufficiently high to ensure adequate filling of the cylinder. In particular, the main problems are due to pressure oscillations that are induced in the hydraulic circuit immediately after the first opening period of the intake valves: this occurs in particular at higher rotational speeds.

[0018] There is therefore a need for further improvements in this field.

[0019] Object of the invention

[0020] 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 a high combustion efficiency in all engine operating conditions and specifically also at low engine loads.

[0021] In particular, the invention aims to retain all the advantages of the previously proposed “air-jet” solution, overcoming at the same time the drawbacks due to the pressure oscillations that are induced in the hydraulic circuit that is provided in said previous solutions, immediately after the first opening period of the intake valves.

[0022] Summary of the invention

[0023] In order to achieve said purposes, the invention has as its object an internal combustion engine having the features of claim 1 , and a control method having the features of claim 6.

[0024] Preferred and advantageous features of the invention are indicated in the dependent claims.

[0025] Detailed description of the invention

[0026] Further features and advantages of the invention will be apparent from the following description with reference to the attached drawings, provided purely by way of non-limiting example, in which:

[0027] - figure 1 is a schematic perspective view of the upper part of a cylinder of an example of an internal combustion engine according to the invention, with the respective intake and exhaust ducts,

[0028] - figure 2 is a further schematic perspective view illustrating the intake and exhaust valves associated with each cylinder,

[0029] - figure 3 is a schematic sectional view of the actuating device associated with each intake valve of each cylinder of the engine according to the invention, - figure 4 is a perspective view of an example of a device of a known type in which the actuating cam of an intake valve has different profiles that can be selectively activated,

[0030] - figure 5 is a block diagram of the engine control system according to the invention,

[0031] - figure 6 is a diagram showing examples of ways of actuating the intake valves in the engine according to the invention, and

[0032] - figure 7 is a diagram showing a way of actuating the intake valves in a further embodiment of the invention.

[0033] With reference to figures 1 , 2, the number 1 indicates as a whole a cylinder of an internal combustion engine, for example a petrol engine, with which a combustion chamber 2 is associated in which two intake ducts 3A, 3B and two exhaust ducts 4A, 4B are lead, respectively controlled by two intake valves V1 , V2 and two exhaust valves V3, V4. Figure 1 also shows the supports 5, 6 acting as guides for the intake valves V1 , V2 and the exhaust valves V3, V4.

[0034] Figures 1 , 2 refer to an example in which a fuel injector I, arranged at the centre of the combustion chamber, coaxially with the cylinder 1 , and a spark plug SP arranged on one side of the combustion chamber face the combustion chamber 2 of cylinder 1. Figure 2 also shows the piston P movable in the cylinder between a TDC and a BDC and operatively connected, by means of a connecting rod assembly (only partially illustrated in figure 2) to the crankshaft (not illustrated).

[0035] Of course, the present invention is of general application and can be used for any type of engine, including a diesel engine or a gas engine, and whatever the number of cylinders and the number of intake valves associated with each cylinder, including the case of a single intake valve.

[0036] With reference to figure 3, the intake valves V1 , V2 are each controlled by a cam of a camshaft 7, actuated, in a way known per se, by the crankshaft by means of a transmission of any known type (for example a toothed belt or chain), according to the usual transmission ratio 1 :2 for which the camshaft 7 completes one rotation every two revolutions of the crankshaft.

[0037] With reference to figure 4, in the engine according to the invention the actuating cam of each intake valve is of a type, known per se, comprising at least a first and a second cam portion 70, 71 , adjacent to each other, having different profiles and selectively activated by means of an actuator device C, also of any known type.

[0038] The present disclosure and the accompanying drawings do not contain an illustration of the construction details of the actuator device C, since such construction details can be achieved in any known way and, separately taken, do not fall within the scope of the present invention. For example, cams with several cam portions having different profiles, which can be selectively activated, are described and illustrated in documents EP 1 724 447 A1 , EP 1 992 795 A2, EP 1 367 228 A1 .

[0039] Regardless of which of the two portions 70, 71 of each cam is the activated portion in a specific operating condition of the engine, the activated cam portion actuates the respective intake valve V1 or V2 via a tappet 8 (see figure 3) which in turn controls the respective intake valve via a mechanical transmission. This is to indicate that between the tappet 8 and the intake valve V1 or V2 there is no hydraulic pressurized chamber, as in the case of the previous proposals of the Applicant. The mechanical transmission between the tappet 8 and the intake valve V1 or V2 can be of any type, including the case illustrated in figure 3, in which the tappet 8 is in direct contact with one end of the stem of the respective intake valve.

[0040] The action of the cam on the tappet 8 causes the opening of each intake valve against the action of a respective return spring 9 (or a plurality of return springs 9).

[0041] Figure 3 shows a condition in which the activated portion of each cam is a first cam portion 70 having a profile with two lobes 70A, 70B, configured to generate the “air-jet” effect that has been discussed above. In particular, the two lobes 70A, 70B of the cam portion 70 associated with each intake valve are configured so as to cause, in each cylinder operating cycle:

[0042] - a first opening period of each intake valve V1 , V2 that begins when the respective piston is in the vicinity of TDC and ends when the respective piston is in an intermediate position between TDC and BDC, and

[0043] - a second opening period of each intake valve V1 , V2 that begins when the respective piston is in the vicinity of BDC or has exceeded BDC and ends when the piston is at an intermediate point between BDC and TDC, so that in each cylinder operating cycle each intake valve V1 and V2 is fully closed during an intermediate period between said first opening period and said second opening period, when the respective piston is still descending towards its BDC, so as to create a vacuum in the respective cylinder that generates a jet of air entering the cylinder during the subsequent second opening period of each intake valve.

[0044] Thanks to said arrangement, all the advantages of the “air-jet” system discussed above are achieved.

[0045] According to the invention, the second cam portion 71 of each cam has a single-lobe profile, of a conventional type, so that, when the second cam portion of each cam is activated, the intake valves V1 , V2 have a single opening period, of a conventional type, in each operating cycle of the respective cylinder.

[0046] According to the invention, an electronic controller E (figure 5) configured to control the actuator device C is provided, which allows the selection of the cam portion suitable for each operating condition of the engine. In particular, according to the invention, the electronic controller E can be configured to activate the first portion 70 of each cam, having the profile with two lobes 70A, 70B, only in certain engine operating conditions, in which the air-jet effect is desired. To this end, the electronic controller E is configured for receiving signals S indicative of one or more engine operating parameters, including at least the crankshaft rotational speed.

[0047] In the invention, the cam portion 70 having a profile with two lobes 70A, 70B is activated when the engine rotational speed is below a predetermined threshold value, regardless of the engine load value, while in other engine operating conditions, the cam portion 71 , which determines a conventional type of intake valve actuation cycle, is activated.

[0048] According to a further preferred feature, the engine according to the invention also comprises a timing variator device F (figure 5) of any known type, associated with the camshaft 7, which controls the timing of the rotation of the camshaft with respect to the rotation of the crankshaft. In this example, the timing variator device F is controlled by the electronic controller E such that, when said first cam portion 70, having a profile with two lobes 70A, 70B, is activated, the first opening period and the second opening period of each intake valve V1 and V2 can be shifted in time, delayed or advanced with respect to a basic operating mode.

[0049] Figure 6 shows as an example the lift profile of each intake valve of each cylinder of the engine according to the invention, as the crank angle varies.

[0050] In the diagram of figure 5, the value of 360° of the crank angle is considered to correspond to the TDC of the piston at the beginning of the conventional intake stage in each cylinder operating cycle. The crank angle of 540° corresponds to the BDC.

[0051] In the diagram of figure 6, the line REF indicates a conventional lift profile of each intake valve of the engine, obtainable by activating the cam portion 71 , without any intervention of the timing variator device F.

[0052] In particular, the reference profile can be used at higher rotational speeds when normally the TKE generated by the piston intake motion is sufficient to support a fast combustion, while the air jet mode profiles may not have sufficient angular duration to ensure maximum cylinder filling.

[0053] Assuming for example an engine whose geometric compression ratio is 12.5±1 , the line indicated by A indicates a lift profile obtainable with the first cam portion 70, having the two lobes 70A, 70B, in a basic operating mode, in which the first opening period A1 begins at a crank angle of about 360° ± 10°, and ends at a crank angle of about 480° ± 10° and in which the second opening period A2 begins at a crank angle of about 540° ± 10° and ends at a crank angle of 620° ± 10°. (these values are given only as an example and are not limiting, but please check them and possibly correct them for the better)

[0054] Line B shows a delayed operating mode, in which, by means of the timing variator device F, the entire valve lift profile is shifted horizontally by approximately 40° ± 10°, so that both the lift B1 in the first opening period and the lift B2 in the second opening period are delayed by this value.

[0055] Diagram C instead indicates an advanced mode, in which both the first lift C1 and the second lift C2 are advanced, for example by 15° ± 5° compared to the basic operating mode.

[0056] Again, it is possible to indicate preferred values for the valve lift:

[0057] • during the first opening A1 , duration about 100°-110°, the cam is preferably sized to open the valve by approximately 4-5 mm.

[0058] • during the second opening A2, duration about 60-80°, the cam is preferably sized to open the valve by 2-2.5 mm.

[0059] Thanks to the above features, the following advantages can be obtained:

[0060] 1 ) An over-expanded cycle is implemented (compression ratio lower than the expansion ratio) as an alternative to the Miller cycle (which provides for advanced closing) and Atkinson cycle (which provides for delayed closing), overcoming the drawbacks of these known solutions due to the low swirling of the Miller cycle and the risk of fuel backflow of the Atkinson cycle.

[0061] 2) The over-expanded cycle is obtained with a system having an extremely simple and economical structure.

[0062] 3) The “air-jet” effect is obtained with a solution that is constructively much simpler and has a much lower cost than the previously proposed solutions using an electronically controlled hydraulic system.

[0063] 4) The use of cams with cam portions having different profiles and selectively activatable allows in any case to use the cam portion with a two- lobe profile only when necessary, in particular when the crankshaft rotational speed drops below a predetermined threshold value, regardless of the engine load value, so as to obtain the desired increase in TKE in these operating conditions.

[0064] 5) When the cam portions with two lobes are active, a shift of the intake valve lift profile, by means of the timing variator device F, which causes a delay with respect to the basic operating mode, causes a delay in the closing of the intake valve at the end of the second opening period which results in a lower effective compression ratio, compared to the geometric ratio, which produces better resistance to knocking.

[0065] 6) This delayed operating mode can be used when there is a risk of knocking (at high engine loads and low engine speeds or at high engine loads and maximum engine speeds). Alternatively, this delayed operating mode can be used to adjust the amount of trapped air so as to allow a certain amount of air to flow back into the intake duct. Compared to the Atkinson cycle, this amount of air flowing back into the intake duct is lower. Furthermore, thanks to the higher TKE, it is possible to inject fuel after the intake valve has closed since the air-fuel homogenization is strongly supported.

[0066] 7) In other words, the invention proposes to increase the TKE at low crankshaft rotational speeds, independently of the engine load, generating the “air-jet” effect. However, this result is obtained with a constructionally simple system that allows the “air-jet” effect to be used only when necessary, through the adoption of cams including different cam portions having different profiles.

[0067] 8) Still referring to figure 6, the operating mode advanced with respect to the basic mode allows to maximize the cylinder filling even if during the first opening period it is possible that some exhaust gas enters the cylinder. This advanced operating mode can be used to achieve an “ultra-lean” combustion, which requires a high temperature, as well as a homogeneous mixture, to ensure a robust combustion; again, in ultra-lean mode it is necessary to maximize the cylinder filling and this is possible by minimizing the valve closing delay. Alternatively, the advanced operating mode can be used to achieve a dilute combustion with internal EGR, which is suitable for operating conditions with partial engine loads, where there is no risk of knocking, and a higher mixture temperature is actually beneficial to promote a correct combustion.

[0068] The cams associated with the two intake valves can have their first portions 70 equal or different from each other.

[0069] The use of different profiles for the cam portions 70 associated with the two intake valves of each cylinder can be useful to generate a “swirl” motion of the air charge in the cylinder. The swirl motion can be useful to achieve stratified combustion in certain engine operating conditions.

[0070] Figure 7 of the attached drawings shows the lift profiles of the intake valves V1 and V2 in an embodiment in which the first cam portion 70 of the valve V1 has the profile mentioned above, with two lobes, so as to generate the two lifts A1 and A2 (lower part of figure 7), while the first valve cam portion V2 has a single lobe, configured and sized to generate a single lift A corresponding to the lift A2 of the valve V1 .

[0071] In this embodiment, the single lift A of valve V2 causes a reduction in swirl during the opening period A, but the swirl is instead very intense before the opening A and allows the hot spots in the combustion chamber to be cooled.

[0072] Differently, the lift profile of valve V2 can be delayed with respect to the profile A2 of the first intake valve V1 (by about 15-20°), to have a further strengthening of the swirl which would then be maintained up to TDC and would allow stratified combustion. The choice between these two types of profile for the cam portion 70 associated with valve V2 depends on several factors. For example, an engine with a centrally positioned fuel injector configuration certainly lends itself to stratified combustion and therefore would deserve the most delayed profile for valve V2. The other option, i.e. the one exactly represented in figure 7, is suitable for a hydrogen engine, or for an engine that deserves a strong cooling through intense swirl, during the descent of the piston from TDC to BDC, but then requires a tumble motion in the combustion chamber to better support the combustion.

[0073] Again, it is worth remembering that in the case of engines with a particularly high geometric compression ratio (greater than 14) to maximize efficiency, for example in the case of an engine dedicated to a “series hybrid” or “range extender” application, the values of the actuation of the profile A2 must be shifted to the right by about 10-20°.

[0074] Or again, the only opening period A of the second intake valve V2 can be advanced with respect to the profile A2 of the first intake valve V1 in the case where swirl motion is desired only during the stroke of the piston from TDC to BDC and not at ignition.

[0075] In the engine according to the invention, the actuator device C that allows to select the cam portion that actuates each valve can also be used to generate swirl in the air charge introduced into the engine.

[0076] In a variant, it can be provided that each of the two intake valves is opened more than once before the piston reaches the BDC. Such an operating mode can be useful to create repeated swirl and tumble motions that, for example in the case of a hydrogen engine, could be advantageous to cool the hot spots of the cylinder.

[0077] Of course, notwithstanding the principle of the invention, the construction details and the embodiments may vary widely with respect to what is described and illustrated purely by way of example, without thereby departing from the scope of the present invention, as defined in the attached claims.

[0078] For example, the cam may comprise three different cam portions that can be activated selectively: a first cam portion 70 with two lobes 70A, 70B, a second cam portion with a single lobe and a third cam portion with two lobes, where the first cam portion is configured such that the duration of the second opening period is 80° ± 10°, while the third cam portion is configured such that the duration of the second opening period is 110° ± 15°. In this case, the profile with the longest duration could be used both for higher rotational speeds and also for cases of extremely low workloads. A solution with a three-profile selector device would eliminate the need for a timing variator.

[0079] The proposed invention remains valid even if a simple hydraulic tappet is used (i.e. not an electro-hydraulic device), capable of amplifying the valve movement with respect to the movement imparted by the cam.

Claims

CLAIMS1. An internal combustion engine, comprising:- one or more cylinders (1 ) and a piston (P) movable in each cylinder (1 ) between a TDC and a BDC and operatively associated with a crankshaft, wherein each cylinder (1 ) of the engine has respective operating cycles including an intake stage, a compression stage, an expansion stage and an exhaust stage,- one or more intake valves (V1 , V2) associated with each cylinder (1 ) of the engine, to control a flow of intake air from one or more respective intake ducts (3A, 3B) in each cylinder operating cycle,- wherein each intake valve (V1 , V2) is controlled, via a tappet (8), by a cam (70, 71 ) of a camshaft (7) driven by the crankshaft,- wherein each intake valve (V1 , V2) of each cylinder (1 ) is controlled by the respective tappet (8), against the action of a return spring (9), by means of a mechanical transmission, so that each intake valve (V1 , V2) has a lift profile as a function of the crank angle that is related to the profile of the respective cam (70, 71 ),- wherein the actuating cam (70, 71 ) of each intake valve (V1 , V2) comprises two or more axially adjacent cam portions (70, 71 ), having different profiles, and an actuator device (C) configured to selectively activate one of said cam portions (70, 71 ),- an electronic controller (E) configured for receiving data related to engine operating parameters, including at least the rotational speed of the crankshaft, and for selectively activating one of said cam portions (70, 71 ), via said actuator device (C), depending on said engine operating parameters, said engine being characterised in that:- at least a first cam portion (70), of said cam portions (70, 71 ) which can be selectively activated, which is associated with at least one intake valve of each cylinder, has a profile having two lobes (70A, 70B) arranged and configured so as to cause, in each cylinder operating cycle:- a first opening period of said at least one intake valve (V1 , V2) that begins when the respective piston is in the vicinity of TDC and ends when the respective piston is in an intermediate position between TDC and BDC,- a second opening period of said at least one intake valve (V1 , V2), that begins when the respective piston is in the vicinity of, or has exceeded BDC, and ends when the piston is at an intermediate point between BDC and TDC, so that in each operating cycle of the cylinder (1 ) said at least one intake valve (V1 , V2) is fully closed during an intermediate period when the respective piston is still descending towards its BDC, so as to create a vacuum in the respective cylinder (1 ) that generates a jet of air entering the cylinder during the subsequent second opening period of each intake valve (V1 , V2),- wherein a second cam portion (71 ), of said cam portions (70, 71 ) that can be selectively activated, has a profile having a single lobe, and- wherein said electronic controller (E) is configured for activating said first cam portion (70) with a profile having two-lobes (70A, 70B) only when the rotational speed of the crankshaft is below a predetermined threshold value.

2. The engine according to claim 1 , characterised in that it comprises a timing variator device (F) associated with said camshaft (7) and configured to vary the timing of the rotation of the camshaft (7) with respect to the rotation of the crankshaft, said timing variator device (F) being controlled by said electronic controller (E) such that, when said first cam portion (70) with a profile having two lobes (70A, 70B) is activated, the first opening period and the second opening period of said at least one intake valve (V1 , V2) can be shifted in time, delayed or advanced with respect to a basic operating mode.

3. The engine according to claim 2, characterized in that said electronic controller (E) is configured to control said timing variator device (F) so as to implement, when the first cam portion (70) with a profile having two lobes (70A, 70B) is activated, one of the following operating modes:- said basic operating mode, in which:- the first opening period begins at a crank angle of about 360° ± 10°, and ends at a crank angle of about 480° ± 10°, the values of 360° and 540° of the crank angle corresponding to the TDC and BDC of the piston at the beginning and at the end, respectively, of a conventional intake stage in the operating cycle of each cylinder,- the second opening period begins at a crank angle of about 540° ± 10° and ends at a crank angle of 620° ± 10°,- a delayed operating mode, where the first opening period and the second opening period are shifted in delay, with respect to the basic operating mode, by a crank angle of 40° ± 10°, and- an early operating mode, where the first opening period and the second opening period are shifted in advance, with respect to the basic operating mode, by a crank angle of 15° ± 5°.

4. The engine according to claim 3, characterized in that it has a geometric compression ratio of 12.5 ± 1.

5. The engine according to claim 1 , characterized in that it comprises a first intake valve (V1 ) and a second intake valve (V2) and that the first cam portions (70) associated with the two intake valves have identical profiles.

6. The engine according to claim 1 , characterized in that it comprises a first intake valve (V1 ) and a second intake valve (V2) and that the first cam portions (70) associated with the two intake valves (V1 , V2) have different profiles.

7. The engine according to claim 6, characterized in that the first cam portion (70) associated with the first intake valve (V1 ) has said profile having two lobes (70A, 70B) so as to generate said first opening period (A1 ) and said second opening period (A2), while the first cam portion (70) associated with the second intake valve (V2) has a profile having a single lobe configured to generate a single opening period (A) of the second intake valve (V2), corresponding to said second opening period of the first intake valve (V1 ).

8. The engine according to claim 7, characterized in that said single opening period (A) of the second intake valve (V2) can be configured so as to be simultaneous with said second opening period (A2) of the first intake valve (V1 ) or delayed or advanced with respect to said second opening period (A2) of the first intake valve (V1 ).

9. The engine according to claim 3, characterized in that the cam comprises three different cam portions which can be selectively activated: a first cam portion (70) with two lobes (70A, 70B), a second cam portion with a single lobe, and a third cam portion with two lobes, wherein the first cam portion is configured such that the duration of the second openingperiod is 80° ± 10°, while the third cam portion is configured such that the duration of the second opening period is 110° ± 15°.

10. A method for controlling an internal combustion engine, in which the engine comprises:- one or more cylinders (1 ) and a piston (P) movable in each cylinder (1 ) between a TDC and a BDC and operatively associated with a crankshaft, wherein each cylinder (1 ) of the engine has respective operating cycles including an intake stage, a compression stage, an expansion stage and an exhaust stage,- one or more intake valves (V1 , V2) associated with each cylinder (1 ) of the engine, to control a flow of intake air from one or more respective intake ducts (3A, 3B) in each cylinder operating cycle,- wherein each intake valve (V1 , V2) is controlled, via a tappet (8), by a cam (70, 71 ) of a camshaft (7) driven by the crankshaft,- wherein each intake valve (V1 , V2) of each cylinder (1 ) is controlled by the respective tappet (8), against the action of a spring of return (9), by means of a mechanical transmission, so that each intake valve (V1 , V2) has a lift profile as a function of the crank angle that is related to the profile of the respective cam (70, 71 ),- wherein the actuating cam (70, 71 ) of each intake valve (V1 , V2) comprises two or more axially adjacent cam portions (70, 71 ), having different profiles, and an actuator device (C) configured to selectively activate one of said cam portions (70, 71 ),- an electronic controller (E) configured for receiving data related to engine operating parameters, including at least the rotational speed of the crankshaft, and for selectively activating one of said cam portions (70, 71 ), via said actuator device (C), depending on said engine operating parameters,- said method being characterised in that:- at least a first cam portion (70) of said cam portions (70, 71 ) which can be selectively activated, which is associated with at least one intake valve of each cylinder, is provided with a profile having two lobes (70A, 70B) arranged and configured so as to cause, in each cylinder operating cycle:- a first opening period of said at least one intake valve (V1 , V2) that begins when the respective piston is in the vicinity of TDC and ends whenthe respective piston is in an intermediate position between TDC and BDC,- a second opening period of said at least one intake valve (V1 , V2), that begins when the respective piston is in the vicinity of, or has exceeded BDC, and ends when the piston is at an intermediate point between BDC and TDC, so that in each operating cycle of the cylinder (1 ) said at least one intake valve (V1 , V2) is fully closed during an intermediate period when the respective piston is still descending towards its BDC, so as to create a vacuum in the respective cylinder (1 ) that generates a jet of air entering the cylinder during the subsequent second opening period of each intake valve (V1 , V2),- wherein a second cam portion (71 ), of said cam portions (70, 71 ) that can be selectively activated, is arranged with a single-lobe profile, and- wherein the method includes activating said first cam portion (70) with a profile having two lobes (70A, 70B), by means of said electronic controller (E), only when the rotational speed of the crankshaft is below a predetermined threshold value.

11. The method according to claim 10, characterized in that a timing variator device (F) is associated with said camshaft (7) for varying the timing of the rotation of the camshaft (7) with respect to the rotation of the crankshaft, said timing variator device (F) being controlled by said electronic controller (E) such that, when said first cam portion (70) with a profile having two lobes (70A, 70B) is activated, the first opening period and the second opening period of said at least one intake valve (V1 , V2) can be shifted in time, delayed or advanced with respect to a basic operating mode.

12. The method according to claim 11 , characterized in that it comprises controlling said timing variator device (F) so as to implement, when the first cam portion (70) with a profile having two lobes (70A, 70B) is activated, one of the following operating modes:- said basic operating mode, in which:- the first opening period begins at a crank angle of about 360° ± 10°, and ends at a crank angle of about 480° ± 10°, the values of 360° and 540° of the crank angle corresponding to the TDC and BDC of the piston at the beginning and at the end, respectively, of a conventional intake stage in the operating cycle of each cylinder,- the second opening period begins at a crank angle of about 540° ± 10° and ends at a crank angle of 620° ± 10°,- a delayed operating mode, where the first opening period and the second opening period is shifted in delay, with respect to the basic operating mode, by a crank angle of 40° ± 10°, and- an early operating mode, where the first opening period and the second opening period are shifted in advance, with respect to the basic operating mode, by a crank angle of 15° ± 5°.

13. The method according to claim 12, characterized in that said delayed operating mode is implemented at engine loads above a threshold with rotational speeds below a threshold.

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