Internal combustion engine with variable actuation intake valves and with cylinders controlled to operate independently with ultra-lean combustion or stoichiometric combustion, and a control method therefor

Abstract

An internal combustion engine has at least one intake valve (V1, V2) for each cylinder, actuated by a cam (14) adapted for provide a lift profile having two maximum points spaced apart from each other, for example a profile with two lobes, or a boot profile. Each intake valve is actuated by a respective cam via a hydraulic circuit that can be pressurized or discharged by an electrically actuated control valve (24), controlled by an electronic controller. The controller can actuate each intake valve (V1, V2) with a first opening period and a second opening period interspersed with a phase in which the intake valve is completely closed or almost completely closed, while the respective piston is descending toward its BDC, so as to create a depression in the respective cylinder that generates an air jet entering into the cylinder during the subsequent second opening period. The second opening period begins when the piston is close to pass or has passed the BDC and is ascending toward the TDC. In this way, during a first part of the second opening period, while the piston is ascending toward the TDC, said air jet entering into the cylinder is created due to the depression created in the cylinder, while during a second part of the second opening period a portion of air is pushed back into the intake duct by the piston. However, the controller can anticipate the end of the second opening period, so that the amount of air returning to the intake duct is reduced or eliminated altogether. In this way, each engine cylinder can be controlled, independently of the other cylinders, to operate with stoichiometric combustion or with ultra-lean combustion.

Description

[0001] “Internal combustion engine with variable actuation intake valves and with cylinders controlled to operate independently with ultra-lean combustion or stoichiometric combustion, and a control method therefor”

[0002] ****

[0003] TEXT OF THE DESCRIPTION

[0004] Field of the invention

[0005] The present invention refers to internal combustion engines of the type indicated in the preamble of claim 1 .

[0006] 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.

[0007] Prior art

[0008] 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.

[0009] 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.

[0010] 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, 5 and two exhaust ducts 6 lead. The connection of the two intake ducts 4, 5 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 .

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] When the solenoid valve 24 is opened, the chamber C is connected with the channel 23, so that 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, so that the valve 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 exhaust channel 23, it is therefore possible to vary, as desired, the opening time and stroke of each intake valve 7.

[0017] 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 .

[0018] 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 said body 19 of the preassembled group 20, to the advantage of speed and ease of assembly of the engine.

[0019] 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.

[0020] 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 .

[0021] 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 exhaust 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.

[0022] 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.

[0023] 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.

[0024] In the known system described above, it is possible provide 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 of the engine camshaft.

[0025] In this case, if it is desired to actuate the two intake valves of the same cylinder in a differentiated way, 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 both to the pressure chamber and to 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 hereto, corresponding to figure 4 of the document cited above).

[0026] Alternatively, it is possible to provide the further solution known from document EP 3 832 078 A1 , also from the same Applicant, which provides for 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, one communicating with the hydraulic line between the two solenoid valves and the other with the pressure chamber (see figure 20 attached hereto, corresponding to figure 11 of EP 3 832 078 A1)

[0027] However, for the purposes of the present invention, it is also possible to 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 is possible to benefit from total flexibility in differentiating the openings of the two intake valves of each cylinder.

[0028] In the Italian patent applications IT 102022000025410, IT 102023000003450 and 102023000013266 the Applicant has presented progression of the known system described above.

[0029] Starting from these previous proposals, the Applicant has conducted further studies in order to further improve them.

[0030] To increase combustion efficiency, particularly at medium-low engine loads and at low engine rotational speeds, a technique already proposed is to operate with an ultra-lean combustion (“ultralean”), i.e. with a dosing for which the lambda parameter (ratio between the actual air / fuel dosing and the stoichiometric dosing) is higher than 1 .6 and preferably is higher than 1.7.

[0031] One of the main advantages of ultra-lean combustion is that the amount of heat that is transmitted to the cylinder walls (and that is consequently dispersed) is reduced, which benefits efficiency. Combustion is colder, so the amount of NOx in the exhaust is also reduced. At the same time, however, combustion is more difficult, so HC in the exhaust increases. This situation is reflected in the diagram in figure 12 of the attached drawings, which shows the variation of harmful emissions as the lambda value varies.

[0032] Another drawback of ultra-lean combustion lies in the risk of a failure to activate the catalyst associated with the engine, due to the fact that the catalyst is unable to reach the threshold temperature (in the order of 500°C) necessary for its operation.

[0033] A further disadvantage of ultra-lean combustion is that, if a turbo (turbine and compressor) is present to increase the specific power of the engine, the enthalpy upstream of the turbine, proportional to the temperature of the exhaust gases exiting the cylinders, may be limiting to adequately increase the intake pressure (Pboost), with consequent reduction of the load at which ultra-lean combustion can be implemented.

[0034] Due to the impossibility of working with a lambda parameter value between 1 and 1.6, a further problem of activating ultra-lean combustion is the management of the transition from the ultra-lean combustion operating mode to the stoichiometric combustion operating mode. The engine cylinders cannot work in a mutually unbalanced way as a load (i.e. with unbalanced values of the mean pressure (pmi) in the cylinder, so as not to generate vibrations and dangerous stresses on the crankshaft. On the other hand, it is not possible to instantly reduce, i.e. from one engine cycle to the next, the pressure in the intake manifold, common to all cylinders, from the (higher) value relating to the operation with ultra-lean combustion to the (lower) value relating instead to the operation in stoichiometric mode.

[0035] Therefore, there is a need for improvements in this field.

[0036] Object of the invention

[0037] The main object of the invention is to provide an internal combustion engine of the type indicated at the beginning of this disclosure that is characterized by high combustion efficiency in all engine operating conditions.

[0038] In particular, one object of the invention is to provide an internal combustion engine in which the intake valves of each cylinder can be controlled according to an innovative strategy, which opens up new possibilities for operating the engine with ultra-lean combustion, while overcoming the drawbacks of ultra-lean combustion.

[0039] Finally, an important aim of the invention is to achieve the above objects with a solution that is simple in construction and easy to apply even on an already existing engine.

[0040] Summary of the invention

[0041] 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 according to claim 6.

[0042] Preferred and advantageous features of the invention are indicated in the dependent claims.

[0043] Detailed description of the invention

[0044] 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:

[0045] - 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,

[0046] - 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 actuated by two separate cams, via respective tappets, respective pumping plungers and respective hydraulic circuits, and in which each cam is shaped with two lobes,

[0047] - figure 3 is a schematic view of the variable actuation system of the engine intake valves, in a second embodiment of the present invention, in which the two intake valves of each cylinder are controlled by two separate cams, via respective tappets, respective pumping plungers and respective hydraulic circuits, and in which each cam is shaped with a “boot” type profile,

[0048] - figure 4 is a schematic view of the variable actuation system of the intake valves of the engine, in a third embodiment of the present invention, in which the two intake valves of each cylinder are controlled by a single cam, via a single tappet, a single pumping plunger and a single pressure chamber communicating with the hydraulic actuators of the two intake valves, and

[0049] - figure 5 is a schematic view of an example of an engine cylinder, with two intake valves and two exhaust valves, a fuel injector and a spark plug,

[0050] - figure 6 is a plan diagram of a four-cylinder engine with fuel supply injectors in the cylinders,

[0051] - figures 7-11 are diagrams illustrating the operating principle of the engine according to the invention and the advantages associated with it,

[0052] - figure 12 illustrates the variation of harmful emissions of an engine as the lambda value of the ratio between the actual air / fuel dosing and the stoichiometric dosing varies, and

[0053] - figure 13 is a diagram illustrating an example of a gradual transition mode from ultralean combustion in all the engine cylinders to stoichiometric combustion in all the engine cylinders.

[0054] Starting from the known solution described above with reference to figure 1 , the invention allows efficient combustion to be achieved even at low engine loads and even in the case in which the engine operates with a fuel having a lower vapor pressure than that of gasoline, such as methanol; alternatively, the operating principle of the invention is also applicable to gaseous fuels

[0055] 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.

[0056] In this example, each engine cylinder has two intake valves V1 , V2. 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 intake valve, a respective two-position solenoid valve 24 capable of controlling the communication between the pressure chamber C and a pressure accumulator 270 which is also in communication with a low- pressure circuit of the engine lubrication oil.

[0057] In this embodiment, furthermore, each of the two cams 14 that control the two intake valves V1 , V2 of each engine cylinder is provided with two lobes 14A, 14B, configured to tend to cause two opening periods of the respective valve V1 or V2 for each rotation of the cam 14.

[0058] The two lobes 14A, 14B can be an integral part of a single cam body or be part of two separate bodies, coupled onto the camshaft in such a way as to be integral in rotation with each other and with the camshaft. Preferably, the two lobes 14A, 14B are offset from each other in the direction of the camshaft axis and the tappet plate 15 (figure 1 ) is sufficiently extended to be able to cooperate with both lobes

[0059] Thanks to the provision of a cam 14 with two lobes 14A, 14B for each intake valve of each engine cylinder, and thanks to the provision of a respective hydraulic circuit interposed between each cam and each intake valve, both intake valves of the engine can have a first opening period and a second opening period, spaced apart, corresponding to a conventional opening phase of an intake valve.

[0060] In the second embodiment illustrated in figure 3, each of the two cams 14 that control the two intake valves V1 , V2 of each engine cylinder is configurated so that, when the pressure chamber is constantly maintained under pressure, a lift profile of said at least one intake valve is generated, as a function of the engine crank angle, having a boot shape with a first part defining a first lift level, having a first maximum point, followed by a second part defining a second maximum point, higher than the first maximum point.

[0061] In all of the above embodiments, in given engine operating conditions, the electronic controller 25 governs the electrically actuated control valve 24 such that each intake valve opens at each engine cycle, in a first opening period and in a second opening period, with an intermediate period in which the intake valve is completely closed while the respective piston is descending toward its BDC, thereby creating a depression in the respective cylinder that generates an air jet entering into the cylinder during the subsequent second opening period of the intake valve.

[0062] In the case of the embodiment of figure 2, where the cams have double-lobe profiles, with zero lift between one lobe and the other, said result is achieved by always keeping the control valve 24 closed.

[0063] In the case of the solution of figure 3, where the cams have boot-type profiles, where the lift does not return to zero between the first maximum point and the second maximum point of the cam profile, the result is achieved by keeping said control valve 24 closed at the first opening period and the second opening period and open in an intermediate period between the first opening period and the second opening period, so as to keep the respective intake valve closed.

[0064] With reference to figure 7, the first opening period of each intake valve of each cylinder begins substantially when the respective piston is at its Top Dead Center (TDC) and ends when the piston is substantially midway between its TDC and its Bottom Dead Center (BDC). In this description, in the claims that follow, and in the attached drawings the convention has been adopted so that the engine crank angle is considered equal to 360° and 540° when the piston is at TDC and BDC, respectively, at the beginning and end of a conventional intake stage.

[0065] The second opening period of each intake valve begins when the piston is close to pass or has passed the BDC and is ascending toward the TDC.

[0066] As a result of the above features, during a first part of the second opening period, while the piston is ascending toward the TDC, an air jet is created entering into the cylinder due to the depression created in the cylinder between the first opening period and the second opening period, while during a second part of the second opening period the depression in the cylinder is progressively reduced until it is eliminated and a portion of air is pushed back into the intake duct by the piston continuing to ascend toward the TDC.

[0067] In the present invention, each cylinder is capable of operating, independently of the other cylinders, either in a first mode with ultra-lean combustion or in a second mode with stoichiometric combustion.

[0068] In the combustion mode with stoichiometric mixture of a given cylinder, the electronic controller 25 is configured for governing the electrically actuated control valve 24 associated with the cylinder such that the intake valves V1 , V2 of the cylinder open in said first opening period and in said second opening period in a manner corresponding to the profile of the cam associated with the cylinder (see the solid line in the diagram of figure 7).

[0069] In the combustion mode with ultra-lean mixture, the electronic controller 25 is configured to switch the electrically actuated control valve 24 associated with each cylinder to an open condition in advance of when the intake valves V1 , V2 of the cylinder would close due to the cam 14, so that said second opening period has a shorter duration (see dotted line in figure 7) and the portion of air that is pushed back into the intake duct by the piston in the terminal portion of the second opening period is reduced or eliminated altogether, so that the amount of air fed into the cylinder is greater.

[0070] The difference in the amount of air fed into the cylinder in the two said operating modes is made evident in the diagram of figure 8.

[0071] The diagrams of figures 7-11 show a comparison of three operating modes:

[0072] - the “Air Jet” (AJ) solution of the invention, i.e. with generation of the air jet during the second opening period of the intake valves, in a combustion mode with ultra-lean mixture;

[0073] - the “Air Jet” (AJ) solution of the invention, i.e. with generation of the air jet during the second opening period of the intake valves, in a combustion mode with stoichiometric mixture;

[0074] - the conventional solution (Otto cycle) with stoichiometric combustion.

[0075] The diagram in figure 7 shows the opening profile of both intake valves, i.e. the variation of the valve lift as a function of the engine crank angle.

[0076] The diagram in figure 8 shows the variation of the gas mass trapped in the cylinder; it can be seen how the two stoichiometric cases trap the same amount of air, less than the case with ultra-lean combustion.

[0077] The diagram in figure 9 shows the variation of the average turbulence in the combustion chamber: it is noteworthy how the two cases with Air Jet actuation are both characterized by a significantly higher TKE than the standard case (almost double at 700°). The higher TKE at the ignition angle is the determining factor that allows the stability of the combustion in ultralean mode.

[0078] The diagram in figure 10 shows the variation of the AHRR (Apparent Heat Release Rate) parameter, whose unit of measurement is Joule for each degree of rotation of the crankshaft, and therefore represents the power with which the fuel is transformed into heat. In other words, it is an index of how quickly and efficiently combustion is occurring: combustion that is too slow (duration greater than 40° of rotation of the crankshaft) is an indicator of instability and poor efficiency. It can be seen from the diagram in figure 10 that the stoichiometric AJ case has a very fast combustion, almost similar to an ideal Otto cycle, while the ultra-lean AJ case has a slower combustion, but comparable to that of the stoichiometric Otto operation. Therefore, the diagram confirms that the ultra-lean mode thus created has sufficient stability.

[0079] Finally, the diagram in figure 11 shows the variation of the temperature of the gases in the cylinder as a function of the position of the piston and therefore of the volume of the chamber delimited by the piston (T x V diagram): it is clear that ultra-lean combustion is a sort of “cold” combustion characterized by lower temperatures than the stoichiometric case. The same diagram shows how the temperatures of the exhaust gases in the stoichiometric case are almost 400°K hotter than in the case with ultra-lean combustion.

[0080] In the engine according to the invention, thanks to the possibility of operating each cylinder, independently of the others, with ultra-lean combustion or with stoichiometric combustion, the operating condition of the engine can be gradually varied between different operating conditions corresponding to a different distribution between the cylinders operating with ultra-lean combustion and the cylinders operating with stoichiometric combustion.

[0081] In a given operating condition, for example at a particularly low load, the engine can have for example the majority of the cylinders in combustion mode with ultra-lean mixture, so as to obtain all the advantages discussed above deriving from this mode, and for example only one cylinder in stoichiometric mode, to eliminate the drawbacks of ultra-lean combustion. The cylinder in stoichiometric mode will cause an increase in the temperature in the cylinder, which will result in a decrease in HC at the exhaust and in the elimination of the risk of a failure to activate the catalytic converter device associated with the engine. In this way, overall the operation of the engine will be very efficient and the emissions of unburned materials limited. In another operating condition, at medium-high load, two cylinders operate in ultra-lean mode and two others in stoichiometric mode, so as to be able to ensure the turbine of the turbo the adequate enthalpic contribution to support the compressor: in this way, while working at relatively high loads (for example 10-13 bar IMEP), the engine would still be characterized by a higher efficiency than the standard operation with 4 cylinders in stoichiometric mode.

[0082] In each operating condition, the electronic controller is configured to dynamically determine which and how many cylinders are in stoichiometric mode and how many and which cylinders are in ultra-lean mode, based on one or more engine operating parameters. In particular, the parameters on which this control is based include first of all the engine load, and preferably also the temperature of the engine exhaust gases and / or the temperature of the catalyst device.

[0083] The variation of the engine from one state to another can be very gradual. Figures 5, 6 and 13 refer to an example of a spark-ignition engine, with four in-line cylinders C1 , C2, C3 and C4. Each of the cylinders, in this example, has two intake valves V1 , V2 and two exhaust valves V3, V4. Each cylinder is associated with an electromagnetic injector I, in a central position in the cylinder and a spark plug S. The injectors I are controlled by an electronic fuel supply control unit. The intake valves V1 , V2 of each cylinder are controlled by the electronically controlled hydraulic system described above so as to have an opening cycle according to one or the other of the two operating modes (ultra-lean combustion and stoichiometric combustion) described above with reference to figure 7.

[0084] Figure 13 shows an example of a gradual transition from a condition in which all the cylinders are in a first mode to a condition in which all the cylinders are in a second mode. At each operating cycle of the engine one of the cylinders passes from one mode to the other, according to the scheme illustrated in figure 13.

[0085] It is clear, however, that figure 13 shows only an example of the operation of the engine according to the invention. In fact, completely different modes can be provided, based on the engine operating parameters. For example, it can be provided that in certain operating conditions, particularly at low loads, only one of the cylinders is in stoichiometric mode and all the other cylinders are in ultra-lean mode, making a different cylinder operate in stoichiometric mode at each engine cycle, or every n engine cycles.

[0086] In general, the electronic controller is configured to vary, in turns, the cylinders that operate in a certain mode (stoichiometric or ultra-lean) every n revolutions of the crankshaft

[0087] As can be seen, the invention opens the way to a large number of different operating modes that allow the electronic controller that governs the intake valves to be programmed to adapt the engine to each specific operating condition, so as to obtain high efficiency and at the same time reduced emissions harmful for the exhaust in each operating condition.

[0088] According to a further important feature of the invention, the electronically controlled fuel supply system can be configured to supply each cylinder, independently of the other cylinders, at each cylinder operating cycle, a relatively smaller amount of fuel when the cylinder is in said first combustion mode with an ultra-lean mixture and a relatively larger amount of fuel when the cylinder is in said second combustion mode with a stoichiometric mixture.

[0089] Another important advantage of the invention lies in the fact that the variation of the amount of air introduced into the cylinder, in order to obtain stoichiometric combustion or ultra-lean combustion, is achieved without varying the pressure of the intake air (for example by controlling a butterfly valve arranged in the intake manifold: this translates into a reduction in pumping losses compared to the standard Otto cycle case.

[0090] In the case where two intake valves are provided for each cylinder, and where the two valves are actuated by means of hydraulic cylinders separated by two separate chambers, it is still possible to actuate the two intake valves in an identical manner, corresponding to what is illustrated in figure 5, even if in the case of this embodiment the two intake valves of the cylinder can also be controlled differently, for example by opening both valves in the first opening period, and by opening only one valve in the second opening period.

[0091] 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 by the attached claims.

[0092] For example, the engine could have a number of cylinders other than four, without prejudice to the applicability of the invention. Furthermore, it could be possible to implement the intake valves in an asymmetric mode, with the aim of generating a predefined swirling flow field, but still maintaining the possibility of working in a stoichiometric or ultra-lean mode.

[0093] Finally, in the stoichiometric case, it could be considered to introduce a certain percentage of EGR, either recirculated from the inside (i.e. from the previous combustion cycle) or from the outside, i.e. recovered from the exhaust gases and reintroduced into the intake manifold.

Claims

CLAIMS1. An internal combustion engine, comprising:- a plurality of cylinders (C1 , C2, C3, C4), each with a piston movable in the 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 (14) to drive said at least one intake valve (V1 , V2) of each engine cylinder, via a tappet (15),- wherein said at least one intake valve (V1 , V2) 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 exhaust channel (23) communicating with a pressurized fluid accumulator (270), such that when said control valve (24) is opened, pressurized fluid drains from the pressure chamber (C) into said low-pressure channel and said at least one intake valve (V1 , V2) is closed by 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, based on a plurality of engine operating parameters, including engine load and engine rotational speed, and- an electronically controlled fuel supply system, comprising a fuel injector (I) associated with each cylinder (C1 , C2, C3, C4), said engine being characterized in that:- said cam (14) associated with said at least one intake valve (V) of each engine cylinder is configured so as to generate, when the pressure chamber (C) is maintained under pressure, a lift profile of said at least one intake valve, as a function of the engine crank angle, having two maximum points spaced apart from each other,- said electronic controller is configured for governing the electrically actuated control valve (24) associated with said hydraulic circuit of each cylinder, in given engine operating conditions, so as to cause, at each cylinder operating cycle, a first opening period and a second opening period of said at least one intake valve (V1 , V2), with an intermediate period in which said at least one intake valve is completely closed or almost completely closed while the respective piston is descending toward its BDC, so as to create a depression in the respective cylinder that generates an air jet entering into 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 begins when the piston is close to pass or has passed the BDC and is ascending toward the TDC,- so that during a first part of said second opening period, while the piston is ascending toward the TDC, said air jet entering into the cylinder is created, due to the depression created in the cylinder between the first opening period and the second opening period, while during a second part of the second opening period the depression in the cylinder is progressively reduced until it is eliminated and a portion of air is pushed back into the intake duct by the piston continuing to ascend toward TDC, wherein said electronic controller is configured such that each cylinder (C1 , C2, C3, C4) can operate either in a first combustion mode with stoichiometric mixture or in a second combustion mode with ultra-lean mixture, wherein:- in said first combustion mode with stoichiometric mixture, the electronic controller (25) is configured for governing the electrically actuatedcontrol valve (24) associated with each cylinder in such a way that said at least one intake valve (V1 , V2) of the cylinder is opened in said first opening period and in said second opening period in a manner corresponding to the profile of the cam associated with the cylinder (C1 , C2, C3, C4),- in said second combustion mode with ultra-lean mixture, the electronic controller (25) is configured to switch the electrically actuated control valve (24) associated with each cylinder to an open condition in advance of when said at least one intake valve (V1 , V2) would close due to the effect of the cam (14), so that said second opening period has a shorter duration, and the portion of air that is pushed back into the intake duct by the piston in the terminal portion of the second opening period is reduced or eliminated altogether, said electronic controller being also to cause each cylinder to operate either in the first combustion mode with stoichiometric mixture, or in the second combustion mode with ultra-lean mixture, depending on one or more engine operating parameters, and independently of the combustion mode with which the other cylinders operate.

2. The engine according to claim 1 , characterized in that the electronically controlled fuel supply system (E, I) is configured to supply each cylinder (C1 , C2, C3, C4), independently of the other cylinders, at each cylinder operating cycle, a relatively smaller amount of fuel when the cylinder is in said second combustion mode with ultra-lean mixture and a relatively larger amount of fuel when the cylinder is in said first combustion mode with stoichiometric mixture.

3. The engine according to claim 1 , characterized in that said electronic controller (25) is configured to dynamically select the operating mode of each cylinder, at each cylinder operating cycle, between said first combustion mode with stoichiometric mixture and said second combustion mode with ultra-lean mixture based on one or more engine operating parameters chosen among engine load, engine exhaust gas temperature and temperature of a catalytic converter device associated with the engine.

4. The engine according to claim 1 , characterized in that in given engine operating conditions, the electronic controller (25) is configured to cause one or more cylinders to operate in said first combustion mode with stoichiometric mixture and one or more other cylinders to operate in saidsecond combustion mode with ultra-lean mixture at the same time.

5. The engine according to claim 1 , characterized in that the electronic controller (25) is configured to vary, in turns, the cylinders operating in a given mode of said first and second modes, every n revolutions of the crankshaft.

6. A method for controlling an internal combustion engine, wherein the engine comprises:- a plurality of cylinders (C1 , C2, C3, C4), each with a piston movable in the 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 (14) to drive said at least one intake valve (V1 , V2) of each engine cylinder, via a tappet (15),- wherein said at least one intake valve (V1 , V2) 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 exhaust channel (23) communicating with a pressurized fluid accumulator (270), such that when said control valve (24) is opened, pressurized fluid drains from the pressure chamber (C) into said low-pressure channel and said at least one intake valve (V1 , V2) is closed by the respective return spring (9), regardless of the action of the respective cam,- an electronic controller (25) for controlling the electrically actuated control valve (24) associated with said hydraulic circuit, based on a plurality of engine operating parameters, including engine load and engine rotational speed, and- an electronically controlled fuel supply system, comprising a fuel injector (I) associated with each cylinder (C1 , C2, C3, C4), said method being characterized in that:- said cam (14) associated with said at least one intake valve (V) of each engine cylinder is configured so as to generate, when the pressure chamber (C) is maintained under pressure, a lift profile of said at least one intake valve, as a function of the engine crank angle, having two maximum points spaced apart from each other,- said electronic controller is configured for governing the electrically actuated control valve (24) associated with said hydraulic circuit of each cylinder, in given engine operating conditions, so as to cause, at each cylinder operating cycle, a first opening period and a second opening period of said at least one intake valve (V1 , V2), with an intermediate period in which said at least one intake valve is completely closed or almost completely closed, while the respective piston is descending toward its BDC, so as to create a depression in the respective cylinder that generates an air jet entering into 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 begins when the piston is close to pass or has passed the BDC and is ascending toward the TDC,- so that during a first part of said second opening period, while the piston is ascending toward the TDC, said air jet is entering into the cylinder is created due to the depression created in the cylinder between the first opening period and the second opening period, while during a second part of the second opening period the depression in the cylinder is progressively reduced until it is eliminated and a portion of air is pushed back into the intake duct by the piston continuing to ascend toward the TDC, wherein further:- each cylinder (C1 , C2, C3, C4) can operate either in a first combustion mode with stoichiometric mixture or in a second combustionmode with ultra-lean mixture,- a cylinder (C1 , C2, C3, C4) operates in said first combustion mode with stoichiometric mixture when the electronic controller governs the electrically actuated control valve (24) associated with the cylinder in such a way that said at least one intake valve (V1 , V2)) of the cylinder is opened in said first opening period and in said second opening period in a manner corresponding to the profile of the cam (14) associated with the cylinder,- a cylinder (C1 , C2, C3, C4) operates in said second combustion mode with ultra-lean mixture, when the electronic controller (25) switches the electrically actuated control valve (24) associated with the cylinder to an open condition in advance of when said at least one intake valve (V1 , V2) would close due to the effect of the cam (14), so that said second opening period has a shorter duration, and the portion of air that is pushed back into the intake duct by the piston in the terminal portion of the second opening period is reduced or eliminated altogether, wherein, further, each cylinder (C1 , C2, C3, C4) is caused to operate, by means of said electronic controller (25), either in the first combustion mode with stoichiometric mixture, or in the second combustion mode with ultra-lean mixture, depending on one or more engine operating parameters, and independently of the combustion mode with which the other cylinders operate.

7. The method according to claim 6, characterized in that the electronically controlled fuel supply system feeds into each cylinder, independently of the other cylinders, at each cylinder operating cycle, a relatively smaller amount of fuel when the cylinder is in said second combustion mode with ultra-lean mixture and a relatively larger amount of fuel when the cylinder is in said first combustion mode with stoichiometric mixture.

8. The method according to claim 6, characterized in that said electronic controller (25) is configured to dynamically select the operating mode of each cylinder, at each cylinder operating cycle, between said first combustion mode with stoichiometric mixture and said second combustion mode with ultra-lean mixture based on one or more engine operating parameters including engine load, engine exhaust gas temperature and the temperature of a catalytic converter device associated with the engine.

9. The method according to claim 6, characterized in that in given engine operating conditions, one or more cylinders operate in said first combustion mode with stoichiometric mixture and at the same time one or more cylinders operate in said second combustion mode with ultra-lean mixture.

10. The method according to claim 6, characterized in that the cylinders operating in a given mode of said first and second modes are varied, in turns every n revolutions of the crankshaft.

11. The method according to claim 6, characterized in that the switching of a cylinder between the first mode and the second mode is achieved without varying the pressure of the intake air.

Images