Internal combustion engine supplying gaseous fuel and control method thereof

Abstract

An internal combustion engine for supplying gaseous fuel, in particular hydrogen, comprises, for each cylinder (1), a first intake duct (3A) and a second intake duct (3B) controlled by a first intake valve 5 (VA) and a second intake valve (VB), respectively. In each working cycle of the cylinder, first only the opening and closing movement of the first intake valve (VA) is activated while the second intake valve (VB) remains closed, and then only the opening and closing movement of the second intake valve (VB) is activated while the first intake valve (VA) remains closed. Due to the cooling obtained in the opening phase of the first intake valve, the gaseous fuel 10 is supplied directly to the cylinder or into the second intake duct only when the second intake valve is opened, thereby obtaining regular combustion without pre-ignition or tempering phenomena.

Description

Technical Field

[0001] The present invention relates to an internal combustion engine supplied with gaseous fuel.

[0002] In particular, the invention relates to an internal combustion engine supplied with gaseous fuel, of the type comprising one or more cylinders and respective pistons sliding in the cylinders between top dead center (TDC) and bottom dead center (BDC) and operatively connected to a crankshaft, the engine being configured to actuate subsequent intake, compression, expansion and exhaust phases during each working cycle of each cylinder,

[0003] Wherein, for each cylinder, the engine comprises:

[0004] a first intake duct and a second intake duct which open into the cylinders and both communicate with the same intake manifold so as to receive air at the same pressure,

[0005] a first intake valve and a second intake valve associated with the cylinder to control the flow of intake air from the first intake duct and from the second intake duct into the cylinder, respectively, during each working cycle of the cylinder,

[0006] - actuating means of the first intake valve and of the second intake valve for controlling the opening movement and the subsequent closing movement of the first intake valve and of the second intake valve during each working cycle of the cylinder, and

[0007] A gaseous fuel supply system comprising at least one injector associated with each cylinder for injecting gaseous fuel directly into the respective cylinder or into an intake duct associated with the respective cylinder during each working cycle of the cylinder. Background Art

[0008] In engines of the above-mentioned type, there is the problem of avoiding backfire phenomena in the intake duct and knocking and / or pre-ignition phenomena in the combustion chamber.

[0009] One known method of solving this problem is to use non-combustible gas (fresh air) to cool the exhaust gases remaining on the hottest surfaces of the cylinder and combustion chamber (e.g., surfaces near the exhaust valves) at the beginning of the intake stroke. This prevents the combustible mixture from being exposed to high temperatures that could produce pre-ignition and / or prevents the formation of other ignition points corresponding to so-called "hot spots" of the combustion chamber during combustion triggered by the spark plug electrode, which could lead to irregular combustion.

[0010] Furthermore, if the fuel is hydrogen, which is characterized by its ability to burn in extremely lean mixtures and in very confined spaces (such as the volume defined between the cylinder, the piston rings and the piston), it may happen that the combustion is not completed and that combustion of the incoming mixture may be triggered during the intake stroke, with subsequent backfire towards the intake duct: also in this case, the high-speed movement of the fresh air (in particular the swirl movement) may extinguish these residual combustion before the fuel is introduced into the intake duct.

[0011] Another way to eliminate backfire is to inject gaseous fuel directly (or DI) into the combustion chamber when the intake valve is closed, rather than port fuel injection (or PFI): this strategy can avoid backfire toward the intake manifold, but it may not be sufficient to ensure extinguishing of residual combustion in the gap and cooling of the exhaust valve.

[0012] Even in the case of ultra-lean burn, which is useful for improving thermal efficiency, misfires can occur in the case of PFI as well as in the case of DI, leading to undesirable backfire or ignition during the subsequent air and fuel intake stroke.

[0013] If the gaseous fuel is hydrogen, it should also be taken into account that the minimum ignition energy is about ten times lower than for gasoline. This characteristic, together with the tendency of hydrogen to burn in a wide flammability range and the high diffusivity, determines a sharp increase in the risk of flashback in the intake duct. Therefore, countermeasures that are usually effective for gasoline or natural gas engines may not be effective in the case of engines supplied with hydrogen.

[0014] Furthermore, engines supplied with hydrogen need to be run with an ultra-lean dosage to avoid nitrogen oxide emissions and knock phenomena (due to the low ignition energy of hydrogen). Lean mixtures usually burn more slowly, which means that when the intake valve opens, there may still be some air and unburned fuel in the cylinder, which leads to backfire. This is more likely to happen if the engine is running at low load or with intake throttling. In this case, the pressure in the intake duct is lower than the pressure in the combustion chamber, and part of the exhaust gas and unburned hydrogen may flow back into the intake duct.

[0015] An attempt to solve this problem has been proposed in document US 7,980,220 B2. This known solution provides an engine of the above-mentioned type, with two intake ducts and two corresponding intake valves per cylinder. The injection of hydrogen is carried out in only one of the two intake ducts. At least at higher engine loads and speeds, during each working cycle of the cylinder, the intake valve of the intake duct from which the hydrogen is injected is controlled to open with a delay relative to the other intake valve. In this way, only fresh air enters the combustion chamber during the phases in which only the intake valve of the intake duct not associated with the injector is open. In theory, this solution should eliminate the risk of flashback, thanks to the introduction of fresh air for cooling during the first part of the intake stroke. However, this type of solution is not entirely satisfactory, since the above-mentioned cooling is obtained only during the short first part of the intake stroke: in fact, the air flow introduced by the second valve counteracts the swirl movement and the hot spot cooling as soon as the second valve opens.

[0016] Documents US 2012 / 023935 A1 and JP S60 212619 A disclose conventional internal combustion engines supplied with liquid fuel, wherein each cylinder has two intake valves having different opening times during each working cycle of the engine.

[0017] Purpose of the Invention

[0018] The object of the present invention is to provide an internal combustion engine supplied with gaseous fuel in which the risk of flashback is significantly reduced.

[0019] A further object is to provide an internal combustion engine supplied with a gaseous fuel, in particular hydrogen, in which the risk of knocking and pre-ignition (the so-called "big knock" phenomenon) is also significantly reduced.

[0020] In particular, the present invention is directed to an internal combustion engine supplied with gaseous fuel, which is capable of:

[0021] Effectively cool hot spots in the combustion chamber (to avoid detonation and macroknock);

[0022] Prevent fuel from flowing to the intake manifold (to avoid backfire).

[0023] Furthermore, another object of the invention is to provide an internal combustion engine supplied with gaseous fuel in which nitrogen oxide emissions are significantly reduced and which also makes it possible to operate with ultra-lean dosing using the increase in turbulent kinetic energy in the cylinder, thus causing better homogeneity of the fuel-air mixture. Summary of the invention

[0024] In order to achieve one or more of the above objects, the present invention relates to an internal combustion engine supplied with gaseous fuel, the type of which is indicated at the beginning of this specification, and which is also characterized in that:

[0025] - the actuating means of the first intake valve and of the second intake valve are configured to control, during each working cycle of the cylinder, at least under certain working conditions of the engine, only a first opening movement and a subsequent closing movement of the first intake valve, while the second intake valve remains closed, and then only an opening movement and a subsequent closing movement of the second intake valve, while the first intake valve remains closed, and

[0026] - The gas fuel supply system is configured such that:

[0027] - in each working cycle of each cylinder, when the first intake valve is open, no gaseous fuel is supplied,

[0028] - In each working cycle of the cylinder, gaseous fuel is supplied directly into the cylinder or into said second intake duct associated with the cylinder only when the second intake valve is open.

[0029] Due to the above characteristics, for the entire opening phase of the first intake valve, the cylinder receives only fresh air. The swirl motion of the air flow introduced into the cylinder during the opening of the first intake valve has a strong positive effect on cooling the hot spots in the combustion chamber, reducing the tendency to knock. The reduction in the risk of knock allows operation at higher compression ratios, which has a positive benefit for the indicated thermal efficiency. The same strong swirl motion also has the significance of extinguishing the combustion of any hydrogen remaining in the gap between piston and cylinder, thus avoiding backfire and / or pre-ignition phenomena.

[0030] In the case of port fuel injection (PFI), the high pressure gradient created by the opening of the second intake valve after the closing of the first intake valve avoids the backflow of gaseous fuel into the intake manifold and reduces the risk of flashback.

[0031] In contrast, in the case of direct injection of gaseous fuel into the combustion chamber, the opening of the first and second intake valves in a subsequent stage produces a combination of swirl and tumble motions of the air flow, which reduces the risk of backflow of gaseous fuel in the supply duct and thereby reduces the risk of flashback.

[0032] Furthermore, at the end of the intake stroke, due to the opening of the two intake valves in separate and subsequent stages, the swirl movement which can be adjusted according to the engine operating point has a significant effect on the homogeneity of the air / fuel mixture, resulting in a reduction in nitrogen oxide emissions, and has a significant effect on the reduction of heat losses during combustion (on the contrary, during combustion, too high a swirl would increase heat losses due to the contact of the flow with the walls of the combustion chamber).

[0033] The Applicant's studies and experience have shown that the opening of the two intake valves per cylinder of the engine at a later time generally allows to adjust the intensity of the macroscopic movements carried out in the combustion chamber, namely the so-called swirl (airflow rotating around the cylinder axis), tumble (airflow rotating around an axis perpendicular to the cylinder axis), cross-tumble (airflow rotating around an axis perpendicular to the cylinder axis and perpendicular to the tumble) movements, so as to contribute to the homogeneity of the charge in the case of direct injection of fuel into the combustion chamber and to benefit in terms of nitrogen oxide emissions. The above advantages derive from the fact that the two intake ducts flow into the cylinder in a tangential direction relative to the cylinder axis. In the initial phase of the intake stroke, starting when the piston is approximately at TDC, only the first intake valve is open. The air flow introduced into the cylinder through the first intake duct generates a fluid dynamic field comprising a high swirl component as well as tumble and cross-tumble components. After the closing of the first intake valve, which occurs before the piston has completed 2 / 3 of its intake stroke, the second intake valve opens in conjunction with the injection of fuel into the intake duct; this produces a dual effect:

[0034] 1) A flow field is created that opposes the flow field component of the vortex generated by the opening of the first intake valve, so that the pre-existing vortex weakens or even reverses its direction of rotation. This phenomenon is particularly desirable at higher engine loads, where in fact the temperatures of the walls of the combustion chamber and of the exhaust valve are higher; high swirl is also desired in the presence of a mixture whose low ignition energy is sufficient to initiate combustion, such as in the presence of hydrogen. Therefore, the high intensity swirl generated by opening the first valve is desirable, since it effectively cools these hot surfaces with the aim of avoiding knock and overknock. The purpose of the delayed opening of the second valve is to weaken the intensity of the previously generated vortex in order to avoid excessive heat exchanges during combustion. In fact, one problem associated with a flow field with a strong swirl component is that the swirl is not very sensitive to the position of the piston: when the piston moves towards TDC, the tumble motion is converted into turbulent kinetic energy, the swirl and the kinetic energy associated with it continue to exist even after TDC and are the cause of a higher heat dissipation through the walls of the combustion chamber during combustion. Unlike the turbulent kinetic energy that is desired to increase the combustion propagation rate, the kinetic energy associated with the swirl is therefore undesirable, and the attenuation of the swirl obtained by opening the second intake valve is desirable. In addition, when the piston approaches TDC and supports the combustion of an ultra-lean mixture, especially when the fuel is hydrogen, the transformation of the swirl into a macroscopic motion performed by a tumble component (or cross-tumble, but in any case about an axis orthogonal to the cylinder axis) allows the transfer of kinetic energy from the swirl to the tumble, and the possible conversion of turbulent kinetic energy, which is useful for increasing the combustion rate.

[0035] 2) Due to the low pressure generated in the cylinder during the angular interval in which both intake valves are closed, a high instantaneous flow is generated from the corresponding intake duct towards the cylinder when the second valve opens: during this interval, or even a few moments before the opening, the fuel is injected into the intake manifold, and all the fuel mixed with the air is sucked into the cylinder without any risk of backflow towards the intake manifold. In this way, due to the high relative ratio of the incoming air / fuel mixture with respect to the gases present in the cylinder, the risk of backflow of the fuel towards the intake manifold and subsequent backfire is avoided. Moreover, due to the high travel speed of the air and fuel mixture of the valves, an extremely homogeneous mixture between air and fuel is also obtained, with great advantages in terms of nitrogen oxide emissions.

[0036] In one embodiment, the engine includes a variable actuation device for a first intake valve and a second intake valve, and the variable actuation device is configured to realize a first operating mode, a second operating mode, or a third operating mode under different operating conditions of the engine. In the first operating mode, at a subsequent time, first only the first intake valve is opened, and then only the second intake valve is opened. In the second operating mode, the first intake valve and the second intake valve are opened at the same or substantially the same crank angle as each other, and the first intake valve and the second intake valve are closed at the same or substantially the same crank angle as each other. In the third operating mode, only the second intake valve is opened and closed.

[0037] The variable actuation device of the intake valve may be an electrically controlled hydraulic actuation device or a device having an electromagnetic or electro-pneumatic actuator or a device comprising an actuation cam having different profiles which can be selectively activated.

[0038] The invention also relates to a method for controlling an engine as described above.

[0039] Further advantageous features of the invention are pointed out in the dependent claims. DETAILED DESCRIPTION

[0040] Further features and advantages of the invention will emerge from the following description, which is given by way of non-limiting example only, with reference to the accompanying drawings, in which:

[0041] Figure 1 , Figure 2 are perspective and plan views of a combustion chamber of a cylinder of an internal combustion engine according to the present invention,

[0042] Figure 3 is a perspective view of a conventional actuation system for an intake valve associated with a cylinder of a conventional internal combustion engine,

[0043] Figure 3A Shows Figure 3 corresponding to a variant of an embodiment of the motor according to the invention,

[0044] Figure 4-Figure 6 is a schematic diagram showing various examples of opening and closing cycles of two intake valves associated with a cylinder of an engine according to the present invention, compared with a conventional cycle,

[0045] Figure 7-Figure 11 is a schematic diagram illustrating the advantages of the present invention, and

[0046] Fig.12 is a schematic diagram of a variable actuation system for an intake valve of a known type developed by the applicant and sold under the MultiAir trademark, which may be used in embodiments of the present invention.

[0047] Reference Figure 1 , Figure 2 , reference numeral 2 denotes the entire combustion chamber associated with a cylinder 1 of an internal combustion engine supplied with hydrogen according to the invention.

[0048] Figure 1 , Figure 2 Two intake ducts 3A, 3B are shown, which open into the combustion chamber 2 and are shaped and oriented according to any known technique so as to direct respective air flows into the cylinder in directions spaced apart from the cylinder axis C1. Both ducts 3A, 3B communicate with the same intake manifold 30 (only partially shown) and therefore receive air at the same pressure from the air supply line to the engine. Figure 1 , Figure 2 Also shown are two exhaust ducts 4A, 4B which are associated with the cylinder 1 and which converge in an exhaust manifold 5 of the internal combustion engine (at Figure 2 The middle part is visible).

[0049] The engine according to the invention is provided with a hydrogen supply system of any known type. The hydrogen supply system comprises a hydrogen injector H2 for each cylinder, such as Figure 2 As shown, it is arranged and configured to supply hydrogen into the intake duct 3B. The injector H2 is only Figure 2 Schematically shown in the drawings, because it can be manufactured according to any known technology. In the accompanying drawings, only the parts relevant to the purpose of the present invention are shown, and it should be understood that the structure and overall construction of the engine can also be manufactured in any known manner.

[0050] Associated with the two intake ducts 3A, 3B are two conventional mushroom-type intake valves having valve stems and rounded heads: a first intake valve VA and a second intake valve VB.

[0051] As will be further explained below, for the purposes of the present invention, the actuation system of the intake valves VA, VB may be of any known type. Figure 3An actuating device for an intake valve VA, VB of a conventional type is shown, comprising a camshaft 6 which is rotatably supported in the cylinder head structure of the engine and whose rotation is controlled in a conventional manner by a drive device (such as a toothed belt drive device) of a crankshaft (not shown) of the internal combustion engine. Figure 3 Only a part of the camshaft 6 is shown in FIG. 1 , and the camshaft includes two cams 6A and 6B, which are used to operate the first intake valve VA and the second intake valve VB, respectively. Figure 3 In the conventional example shown, two cams 6A, 6B actuate two valves VA, VB by means of respective rocker arms 7A, 7B, each rocker arm being mounted in a swinging manner at one end on a support 8 carried by the cylinder head structure and the opposite end acting on a respective intake valve.

[0052] In the case of the first embodiment of the present invention, the cams 6A, 6B are Figure 3A The method shown is improved to obtain Figure 4-Figure 6 A lift curve of the type shown in any of the accompanying drawings.

[0053] Figure 4 The lift curves of two intake valves VA, VB according to the first embodiment of the invention are shown. Figure 4 The diagram in shows the displacement of each intake valve according to the engine rotation angle. In the convention used here, an engine rotation angle of 360° corresponds to the state when the piston in the cylinder is at TDC. The piston position when it is at BDC corresponds to a crank angle of 540°.

[0054] exist Figure 4 In FIG. 1 , the line LC shows the lift diagram of the intake valves in the case of a conventional engine equipped with a conventional cam. In the case of the conventional solution, the two intake valves VA, VB are controlled simultaneously and synchronously according to the curve LC. It can be seen that in the conventional solution, the two valves start to open before TDC, reach a maximum open state near a crank angle of approximately 470°, and close again at a crank angle of approximately 600°. In order to obtain this result, the two cams 6A, 6B have the same profile, thereby generating a lift curve LC, and have the same angular position on the camshaft 6, as shown in FIG. Figure 3 shown.

[0055] exist Figure 4 In the embodiment of the present invention shown, the cams 6A and 6B have different structures (eg Figure 3A ) and are oriented at different angles on the camshaft 6. The two cams 6A, 6B are constructed and oriented so that the inlet valves VA, VB are respectively generated by Figure 4 The lift curves represented by LA and LB in (it should be noted that Figure 4The lift curves LA and LB shown in the following figures have only qualitative values).

[0056] The first important feature to be observed is that during each working cycle of the cylinder, first only the opening and closing movement of the first intake valve VA is activated, while the second intake valve VB remains closed, and then only the opening and closing movement of the second intake valve VB is activated, while the first intake valve VA remains closed.

[0057] The second important feature to be noted is that the hydrogen supply system is controlled so that in each working cycle of each cylinder, when the first intake valve VA is open, no hydrogen is supplied, so that the air flow introduced into the cylinder with the opening of the first intake valve VA is used to cool the cylinder by utilizing the swirl motion of the air flow, thereby avoiding subsequent pre-ignition or flashback phenomena, and in each working cycle of the cylinder, hydrogen is supplied by the injector H2 into the intake duct 3B associated with the cylinder only when the second intake valve VB is open. Figure 4 In FIG. 1 , the activation phase of the hydrogen injector H2 is indicated by the dashed line H2.

[0058] In this way, due to the cooling obtained during the opening phase of the first intake valve VA, the fresh air flow entering the cylinder alone during the opening of the first intake valve VA produces effective cooling of the combustion chamber, which allows normal combustion to be obtained without pre-ignition or backfire phenomena.

[0059] Figure 4 A particularly preferred embodiment is involved, in which the first intake valve VA starts to open when the piston in the cylinder approaches TDC (or in the vicinity thereof, just before or after TDC), and then closes before 540° crank angle (i.e., when the piston in the cylinder is still moving in the direction of BDC and has not yet reached BDC). On the other hand, the second intake valve VB opens when the piston in the cylinder approaches BDC (or in the vicinity thereof, just before or after BDC), and closes after a further rotation of the crankshaft (e.g., equal to about 90°).

[0060] Refer again Figure 4 According to a preferred solution, closing of valve VA occurs when the piston is in the middle of its stroke, while opening of valve VB occurs at BDC. Figure 5 , Figure 6 is similar to Figure 4 A schematic diagram showing a further embodiment of the present invention is shown, which differs from the shape of the lift curve in that Figure 4 .

[0061] All of the above embodiments have in common that during the intake stroke in the cylinder, at a first time, substantially only the first intake valve VA opens while the second intake valve VB remains closed, and at a second time, only the second intake valve VB opens while the first intake valve VA remains closed.

[0062] When only the air from the intake duct 3A is introduced into the cylinder, the introduced air flow generates a fluid dynamic field having a swirl component (flow rotating around the cylinder axis C1).

[0063] As mentioned previously, a flow field characterized primarily by swirl motion allows the hot spots of the combustion chamber to be significantly cooled, thus having anti-knock implications.

[0064] The opening of the second intake valve VB determines a powerful admission of air from the second intake duct 3B into the cylinder due to the low pressure generated in the cylinder by the early closing of the valve VA: during the opening of the valve VB, the gaseous fuel injected into the corresponding intake duct 3B is immediately transported to the inside of the cylinder by the air flow. By means of the high instantaneous flow into the cylinder, the presence of gas in the same intake duct 3B after the closing of the valve VB is avoided, and therefore the risk of backfire is avoided.

[0065] As described above, the present invention can be implemented both with an internal combustion engine having an actuation device for the intake valves of a conventional type, in which the lift curves of the two intake valves are fixed and predetermined, and with an internal combustion engine equipped with a variable actuation system for the intake valves.

[0066] So, for example, reference Figure 6 The cam 6A determining the opening and closing of the first intake valve VA can be controlled by means (for example electrohydraulic means) so as to have a profile determining the lift curve denoted by LA1. In fact, due to the separation of the cam movement from the intake valve movement by hydraulic means, the law of movement of the intake valve may not be monotonic. However, the same variable actuation system can be used to vary the lift curve of the valve VA, for example according to the curve LA2. Similarly, the actuation means combined with the cam 6B can be controlled according to Figure 6 The lift curve LB1 shown produces a law of motion. However, the engine may be equipped with a variable actuation system which allows obtaining the effective lift of the valve VB according to the curve LB2.

[0067] In one example, the invention is applied to an internal combustion engine equipped with a variable actuation system for the engine intake valves of the type developed by the same applicant and sold under the MultiAir trademark.

[0068] Fig.12 An example of a variable actuation system MultiAir is schematically shown. In this case, each intake valve VA, VB ( Fig.12The device associated with the valve VA is shown to be actuated by the corresponding cam 6A or 6B by means of an electrically controlled hydraulic device 8. The cam 6 actuates a tappet 9 which is kept in contact with the cam 6 by a return spring 10. The tappet 9 is associated with a pumping piston 11 of a master cylinder which delivers pressurized fluid from a chamber 12 to a chamber of a slave cylinder 13, the piston 14 of which serves as actuator for the intake valve VA. The intake valve VA is pulled by a spring 15 towards a closed position of the intake duct 3A. All the above-mentioned components are carried by the engine cylinder head structure 16. The electrically controlled valve 17 is controlled by an electronic control unit E. When the electrically controlled valve is in the closed state, it interrupts the communication between the pressurized fluid chamber 12 and the low-pressure environment 18, communicating with a fluid accumulator 19 and an inlet 20 intended to communicate with the engine lubrication circuit. If the electrically controlled valve 17 is in the closed state, the pressurized fluid chamber 12 is isolated, so the movement of the tappet 9 imparted by the cam 6 can be transmitted to the intake valve VA via the fluid in the chamber 12 and the slave cylinder 13. In the case where the cam 6 keeps the intake valve VA open, the opening of the electrically controlled valve 17 actuated by the electronic control unit E causes the discharge of the pressurized fluid chamber 12, and the intake valve VA is subsequently closed due to the action of the return spring 15. In this case, the intake valve VA is insensitive to the movement of the tappet 9 imparted by the cam 6.

[0069] The description provided here is merely to illustrate the basic working principle of the MultiAir system. The applicant has developed various embodiments of the system, which have been the subject of various patent publications, including those already mentioned above.

[0070] It will be appreciated that the invention may also be used in conjunction with any known type of variable actuation system for an intake valve, such as an electromagnetic actuation system or a variable actuation system of the multi-profile cam type.

[0071] In the case of a variable actuation system, it is possible that the above-described operating mode, i.e., at subsequent times, firstly only actuation of the first intake valve and subsequently only actuation of the second intake valve, is actuated only in response to certain conditions of the engine operating conditions, while under other engine operating conditions the two intake valves per cylinder are controlled in a conventional manner so that they open and close simultaneously.

[0072] Figure 7-Figure 11 It shows the main benefits obtainable with the present invention compared to conventional standard actuation where both intake valves open at TDC and close at BDC.

[0073] Figure 7 : is a graph showing the change in the average value of turbulent kinetic energy (TKE) in the combustion chamber in the case of the standard example and in the case of the present invention. Figure 7The diagram in shows how the delayed opening of the second intake valve creates a new increase in turbulent kinetic energy near BDC, so that even if it is dissipated at the next TDC, the TKE value is much higher than in the standard case.

[0074] Figure 8 a. Figure 8 b and Figure 8 c schematically shows the different macroscopic movements occurring in the combustion chamber. These figures show a reference system in which the axis X lies in the plane of symmetry of the intake valve guide and coincides with the air introduced into the combustion chamber. Therefore, the movement in the plane orthogonal to the unit vector Y is defined as tumble movement; the movement in the plane orthogonal to the unit vector X is defined as cross tumble movement. The movement in the plane orthogonal to the unit vector Z is defined as swirl. The so-called tumble, cross tumble and swirl index are defined as follows:

[0075] Tumble index = ω 滚流 / ω 发动机

[0076] Cross tumble index = ω 交叉滚流 / ω 发动机

[0077] Eddy current index = ω 涡流 / ω 发动机

[0078] Among them, ω 发动机 is the engine speed, ω 滚流 ,ω 交叉滚流 and ω 涡流 is the average angular velocity of the corresponding motion [rad / sec].

[0079] Fig. 9 The effect of the opening of the second intake valve on the swirl generated by the previous opening cycle of the first intake valve is shown: the dashed line shows the trend that the swirl index would have had if no opening of the second intake valve had occurred after the opening movement of the first intake valve. It can be seen that the swirl intensity during combustion is also considerable, as well as the increase in heat exchange and the deterioration of the engine efficiency. The solid line shows the advantage of the invention with regard to the swirl: as soon as the second intake valve starts to open, the swirl decreases in proportion to the air introduced during the opening of the second intake valve. It can be inferred that having an actuating device for the intake valve capable of varying the curve of the opening movement of the second intake valve makes it possible to adjust the intensity of the swirl present in the combustion chamber during combustion. The dash-dotted line refers to a standard embodiment which obviously does not provide for swirl formation.

[0080] Fig.10The effect of the opening of the second intake valve on the tumble flow generated by the previous opening cycle of the first intake valve is shown: It should be noted that at the beginning of the opening of the second intake valve (which occurs at 500° crank angle in this example), the tumble index increases significantly, which is consistent with the sudden decrease in the swirl index (at Fig. 9 In a manner similar to the standard case, starting from a crank angle of 660°, the tumble index decreases to zero due to the movement of the piston compressing the tumble flow, with the associated conversion of kinetic energy into turbulent kinetic energy: see also Figure 7 : From crank angles of 660° to 700°, TKE is no longer dissipated further because the dissipated TKE is replaced by the dissipation resulting from the elimination of the tumble flow.

[0081] Fig.11 shows the trend of the cross-tumble index: similar to the vortex ( Fig. 9 ), which does not exist in the standard implementation. Fig.10 ), due to the transition of the swirl motion, the opening of the second intake valve after the opening motion of the first intake valve correspondingly activates the cross tumble motion. Similar to the case of the tumble motion, the cross tumble motion also helps to support the TKE in the range between 660° and 700° of the crank angle.

[0082] In one embodiment, the first intake duct and the second intake duct are sized such that under high engine load, closing of the first intake valve generates a pressure wave that travels up the first intake duct and through the common intake manifold in the second intake duct, thereby maximizing cylinder charging.

[0083] In another example, the first intake duct and the second intake duct have different diameters and different lengths, which are selected in such a way that, in the case of maximum engine filling and the second intake valve fully opened, the air flow entering the combustion chamber with the opening of the second intake valve does not offset the swirl motion of the air flow previously introduced into the combustion chamber with the opening of the first intake valve.

[0084] In another example, the actuation means of the first intake valve and of the second intake valve are configured to control the lift of the first intake valve significantly lower than the lift of the second intake valve, so that the filling of the cylinder is mainly obtained due to the opening of the second intake valve: in this movement, the intensity of the initial swirl is reduced, but the mass of air and fuel introduced by decompression during the second opening is increased. This situation may be desirable for engines with a high compression ratio, since a high compression ratio has an extremely negative effect on the flow field (and therefore on the combustion rate), and at low loads, when the heat transferred to the walls is irrelevant, the residual swirl is necessary to increase the flow field and TKE during combustion.

[0085] In the engine according to the invention, the above advantages are added to those related to reduced risk of flashback, reduced nitrogen oxide emissions and reduced risk of pre-ignition and knocking which would otherwise occur as a result of the supply of hydrogen.

[0086] It should be noted that, as described above, the closing of the first intake valve occurs when the piston has not yet reached BDC, and in particular, the preferred embodiment provides that the first intake valve is closed when the piston is approximately in the middle of its intake stroke, whereby the subsequent movement of the piston generates a low pressure in the cylinder, which low pressure favors the entry of the gas flow from the second intake duct when the second intake valve opens. The greater the delay in the opening of the second intake valve VB relative to the closing of the first intake valve VA, and the greater the low pressure generated in the cylinder with the movement of the piston after the closing of the first intake valve VA, and the greater the flow generated by the opening of the second intake valve VB. A greater flow corresponds to a higher momentum, a higher turbulent kinetic energy and a greater inlet flow velocity: this higher velocity counteracts the motion already present in the combustion chamber, which motion already present may favor the backflow of gaseous fuel into the intake duct.

[0087] In the case of a variable actuation device for the intake valve, the degree of opening of the second intake valve VB can be adjusted in order to adjust the intensity of the swirl motion in the combustion chamber after the second intake valve is opened. The adjustment of the swirl motion has a significant effect on the other components of the flow motion in the combustion chamber (tumble and cross tumble) and on the instantaneous generation of turbulent kinetic energy, which in turn increases the combustion rate.

[0088] Therefore, the optimal swirl intensity value that can be adjusted by adjusting the lift curve of the second intake valve depends on various factors, in particular: the gaseous fuel injection system (PFI or DI) and, in the case of direct injection (DI), the injector position in the combustion chamber (lateral or central), the crank angle at the start and end of the gaseous fuel injection, the engine load and the desired combustion type, stratified or homogeneous charge.

[0089] The strong swirl generated in the cylinder as the first intake valve opens has a strong cooling effect on the hot spots in the combustion chamber.

[0090] In the case of port fuel injection (PFI), the advantage is obtained that, due to the low pressure in the cylinder, the outgoing flow from the second intake duct is high, which avoids the risk of backflow towards the intake duct and the risk of flashback.

[0091] In case variable actuation of the intake valves is used, the advantage is obtained that the lift curve of each valve can be "substantially rectangular" with particularly high valve closing speeds, which is particularly important for achieving the advantages of the invention.

[0092] Both solutions described above, port fuel injection (PFI) or direct injection (DI), are advantageous for several reasons: in the case of PFI, the injection system is simple and low cost, while in the case of direct injection (DI), the gaseous fuel injection system is relatively more complex, but certainly preferable in cases where the specific power of the engine is higher.

[0093] Naturally, without prejudice to the principle of the invention, the embodiments and construction details may vary widely with respect to what has been described and shown purely by way of example, without thereby departing from the scope of the invention as defined in the appended claims.

[0094] Specifically, the present invention is effective even if the intake duct is common to both intake valves; furthermore, it is possible to start injecting fuel into the intake duct before the second intake valve opens (even during the last part of the opening of valve 1).

[0095] Likewise, the fuel injector may be located at a portion of the intake manifold that is common to both intake valves.

Claims

1. An internal combustion engine supplied with a gaseous fuel, in particular hydrogen, comprising one or more cylinders (1) and respective pistons sliding in said cylinders between TDC and BDC and operatively connected to a crankshaft, said engine being configured to actuate subsequent intake, compression, expansion and exhaust phases during each working cycle of each cylinder, in, For each cylinder, the engine comprises: a first intake conduit (3A) and a second intake conduit (3B), the first intake conduit and the second intake conduit leading to the cylinder and both communicating with the same intake manifold (30) to receive air at the same pressure, a first intake valve (VA) and a second intake valve (VB) associated with the cylinder (1) to control the flow of intake air from the first intake duct (3A) and from the second intake duct (3B) into the cylinder during each working cycle of the cylinder, actuating means (6) of the first intake valve (VA) and of the second intake valve (VB) for controlling the opening movement and the subsequent closing movement of the first intake valve (VA) and of the second intake valve (VB) during each working cycle of the cylinder, and a gaseous fuel supply system comprising at least one injector associated with each cylinder for injecting gaseous fuel directly into the respective cylinder or into an intake duct associated with the respective cylinder during each working cycle of the cylinder, The engine is characterized by: The actuating means of the first intake valve (VA) and the second intake valve (VB) are configured to, in each working cycle of the cylinder, at least under given working conditions of the engine, first control only the opening movement and the subsequent closing movement of the first intake valve (VA) while the second intake valve (VB) remains closed, and then control only the opening movement and the subsequent closing movement of the second intake valve (VB) while the first intake valve (VA) remains closed, and The gas fuel supply system is configured such that: In each working cycle of each cylinder (1), when the first intake valve is open, no gaseous fuel is supplied. In each working cycle of the cylinder (1), gaseous fuel is supplied directly into the cylinder or into the second intake duct associated with the cylinder only when the second intake valve (VB) is open.

2. The internal combustion engine according to claim 1, characterized in that The gas fuel injector (H2) is disposed in the second intake duct (3B).

3. The internal combustion engine according to claim 1, characterized in that The gaseous fuel injector (H2) is arranged to inject the gaseous fuel directly into the combustion chamber (2) of each cylinder (1).

4. The internal combustion engine according to claim 1, characterized in that It includes a variable actuating device (8) of a first intake valve (VA) and a second intake valve (VB), and the variable actuating device is configured to provide a first operating mode or a second operating mode or a third operating mode under different operating conditions of the engine. In the first operating mode, at a subsequent time, first only the first intake valve (VA) is opened, and then only the second intake valve (VB) is opened. In the second operating mode, the first intake valve (VA) and the second intake valve (VB) are opened at the same or substantially the same crank angle as each other, and the first intake valve (VA) and the second intake valve (VB) are closed at the same or substantially the same crank angle as each other. In the third operating mode, only the second intake valve is opened and closed.

5. The internal combustion engine according to claim 4, characterized in that The first operating mode is activated when the engine load is above a predetermined threshold and the ratio between the effective air / fuel dosage and the stoichiometric dosage is above 0.

5.

6. The internal combustion engine according to claim 1, characterized in that The midpoint of the opening phase of the first intake valve (VA) is located in the first half of the intake stroke of the piston from TDC to BDC, while the midpoint of the opening phase of the second intake valve (VB) is located at the bottom dead center BDC, coincides with the bottom dead center or substantially coincides with the bottom dead center.

7. The internal combustion engine according to claim 4, characterized in that The variable actuation device of the first intake valve (VA) and the second intake valve (VB) is an electro-hydraulic actuation device (8) or a device having an electromagnetic or electro-pneumatic actuator, or a device comprising actuation cams with different profiles, which can be selectively activated.

8. The internal combustion engine according to claim 1, characterized in that The gaseous fuel supply system is configured to inject fuel into the intake duct (3B) at any time before the second intake valve (VB) opens but after the first intake valve (VA) closes, and preferably a single common fuel injector is provided.

9. A method for controlling the operation of an internal combustion engine comprising one or more cylinders (1) and respective pistons sliding in said cylinders between TDC and BDC and operatively connected to a crankshaft, wherein: During each working cycle in each cylinder, the subsequent intake, compression, expansion and exhaust phases are actuated, Wherein, for each cylinder, the engine comprises: a first intake conduit (3A) and a second intake conduit (3B), the first intake conduit and the second intake conduit leading to the cylinder and both communicating with the same intake manifold (30) to receive air at the same pressure, a first intake valve (VA) and a second intake valve (VB) associated with the cylinder (1) to control the flow of gas from the first intake duct (3A) and from the second intake duct (3B) into the cylinder, respectively, during each working cycle of the cylinder, actuating means (6) of the first intake valve (VA) and of the second intake valve (VB) for controlling the opening movement and the subsequent closing movement of the first intake valve (VA) and of the second intake valve (VB) during each working cycle of the cylinder, and a gaseous fuel supply system comprising at least one injector (H2) associated with each cylinder for injecting gaseous fuel directly into the respective cylinder or into an intake duct associated with the respective cylinder during each working cycle of the cylinder, The method is characterized in that it comprises: By means of the actuating means of the first intake valve (VA) and of the second intake valve (VB), during each working cycle of the cylinder, at least under given working conditions of the engine, firstly only the opening movement and the subsequent closing movement of the first intake valve (VA) are controlled, while the second intake valve (VB) remains closed, and then only the opening movement and the subsequent closing movement of the second intake valve (VB) are controlled, while the first valve inlet (VA) remains closed, and The gas fuel supply system is controlled so that: In each working cycle of each cylinder, when the first intake valve is open, no gaseous fuel is supplied. Gaseous fuel is supplied directly into the cylinder (1) or into the second intake duct (3B) associated with the cylinder only when the second intake valve (VB) is open in each working cycle of the cylinder.

10. The method according to claim 9, characterized in that It includes a variable actuating device for a first intake valve (VA) and a second intake valve (VB), and under operating conditions of the engine involving the risk of backfire or irregular combustion, with the help of the variable actuating device, the variable actuating device activates a first operating mode or a second operating mode or a third operating mode. In the first operating mode, at a subsequent time, first only the first intake valve (VA) is opened and closed, and then only the second intake valve (VB) is opened and closed. In the second operating mode, the first intake valve (VA) and the second intake valve (VB) are opened at the same or substantially the same crank angle as each other, and the first intake valve (VA) and the second intake valve (VB) are closed at the same or substantially the same crank angle as each other. In the third operating mode, only the second intake valve (VB) is opened and closed.

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