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TEXT OF THE DESCRIPTION

Field of the Invention

The present invention relates to internal combustion engines . More speci fically, the invention has been developed with reference to hydrogen spark ignition internal combustion engines , having an architecture derived from a compression-ignition internal combustion engine .

State of the Art

The current trend towards a massive reduction of carbon dioxide emissions into the atmosphere influences , on various levels , the production chain in the automotive industry, and requires each stage to implement solutions - which were not previously envisaged - in order to comply with the increasingly strict regulations both on a national and on a European level .

The desired reduction of CO2 emi ssions , down to a complete elimination, requires i . a . the implementation of automotive systems which do not involve combustion ( e . g . electric vehicles ) , or which employ carbon- free fuels such as hydrogen .

Among the main features of hydrogen combustion it may be mentioned a very high resistance to knocking and a speed of the flame front which is much higher than in the case of carbon-based fuels .

Such features lead to a high pressure peak during combustion, and therefore require an engine having a structure adapted to withstand the consequent mechanical stresses .

In this regard, with reference to Figure 1 , the implementation of hydrogen- fuelled engines having architectures derived from diesel engines , or generally speaking from compression-ignition engines , is particularly advantageous , because such engines are generally dimensioned in such a way as to withstand higher mechanical stresses than the equivalent spark ignition engines , therefore enabling maximi zing the thermo-dynamic ef ficiency during hydrogen combustion . Reference E in Figure 1 generally designates a compression-ignition internal combustion engine o f the prior art ; the Figure shows a plan view of the internal volumes of a cylinder 2 and of intake conduits 4 ( tangential intake , so-called " swirling port" ) , 6 ( so- called " filling port" , inletting air in a helical flow) .

However, such a choice is not devoid of technical problems . By means of example , a primary technical problem concerns the flow field of the charge within the combustion chamber . It is well-known that the intake conduits of a diesel compression-ignition engine are configured to generate a rotational movement of air around the cylinder axis ( so-called " swirl motion" ) which, albeit guaranteeing very good results as regards mixing and combustion when the fuel employed is diesel oil , in the case of hydrogen combustion is very inef fective , even counterproductive , and may even lead to mis fire events . The problem is closely connected to the properties of hydrogen : hydrogen is an extremely light gas , speci fically lighter than nitrogen and oxygen which are present in the combustion air . The swirl motion induced in the combustion chamber, which is necessarily also imparted to the hydrogen inj ected into the combustion chamber, leads to the rotation of a charge comprising hydrogen, oxygen and nitrogen . As a result , the latter two gases are proj ected ( and concentrated) towards the periphery of the combustion chamber, by means of a centri fugal ef fect , due to the mass thereof being heavier than hydrogen, while hydrogen is not subj ected to the same proj ection and remains substantially around the inj ection point . This constitutes an obstacle to ef fectively mixing combustion air and hydrogen, due to centri fugal separation, and hinders the mixture ignition ( or at least prevents the complete combustion thereof ) .

A further problem concerns the general architecture of compression-ignition engines ; the head surface facing the cylinder is generally planar or nearly planar, and generally such that the axes of the valves are approximately perpendicular to the edge of the head ( or, similarly, parallel to the longitudinal axis of the cylinder ) ; therefore , the angle comprised between the valves approaches zero . This is not generally a problem for the ( central/coaxial ) installation of a diesel inj ector I , but it becomes an issue for the installation of a spark plug together with a hydrogen inj ector : i f the spark plug were located centrally, it would be possible to cover a volume region o f the combustion chamber where the hydrogen would be nearly certainly located, because the centri fugal forces are not particularly ef fective on hydrogen .

Due to such installation di f ficulties , solutions are known of hydrogen- fuelled engines which are based on compression-ignition engines , envisaging installing the spark plug in an eccentric position with respect to the cylinder axis , a position which typically coincides with the position originally envisaged for the installation of a glow plug GP of the compression- ignition engine , the architecture of which has been borrowed . This inevitably leads to a compromise solution : the spark plug is located in an area where , irrespective of the possible hydrogen scarcity, the charge speed due to the swirl motion is so high as to hinder in any case an optimal ignition of the mixture .

Obj ect of the Invention

The present invention aims at solving the technical problems outlined in the foregoing . Speci fically, the obj ect of the invention is to provide a hydrogen spark ignition internal combustion engine which borrowing the architecture thereof from a compression-ignition internal combustion engine , but which does not exhibit the previously described technical problems .

Summary of the Invention

The obj ect of the present invention is achieved by means of an internal combustion engine having the features set forth in the claims that follow, which form an integral part of the technical disclosure provided herein in relation to the invention .

Brief Description o f the Figures

Figure 1 shows the profile o f the head of an internal combustion engine of the prior art , and the flow field of the charge therewithin,

Figure 2 shows the intake and exhaust volumes , as well as the volumes of the cylinder, within an internal combustion engine according to the invention,

Figure 3 is a representation similar to Figure 1 , but referring to an engine according to the invention, and

Figure 4 is a cross-sectional view along the cylinder axis of an engine according to the invention .

Detailed Description

Reference 1 in Figure 2 generally denotes a hydrogen spark ignition internal combustion engine according to the invention . The reference numbers adopted in Figure 2 are consistent with the reference numbers adopted in the description of the known solution of Figure 1 , essentially in order to remark that the internal combustion engine according to the invention borrows its architecture from an engine of the prior art , while introducing the modi fications which will be described in the following .

The engine 1 comprises at least one cylinder 2 , having a longitudinal axis Z2 along which a piston P is reciprocally movable ( Figure 4 ) . Each cylinder 2 is closed at the top by a head surface 3 which is part of a cylinder head (which is not shown, as it is known in itsel f ) and which faces the cylinder 2 in order to define a combustion chamber together with the piston P, speci fically with a crown thereof . The engine 1 comprises , in association with each cylinder, a first and a second intake conduits 4 , 6 and at least one exhaust conduit 8 , which in the present case is bi furcated and includes a first section 8A and a second section 8B, which are located substantially facing and opposing the conduits 4 and 6 , speci fically at the positions of the connections thereof to the head surface 3 . Referring to Figures 2 and 3 , the first intake conduit 4 is arranged side by side with the second intake conduit 6 and, referring to the geometry of the cylinder head shown in Figures 2 and 3 , i f the head surface is ideally divided into pseudo-quadrants ("pseudo" - because general ly the diameter of the intake valves and the diameter of the exhaust valves are not the same ) , conduit 4 and conduit 6 open into adj oining and consecutive quadrants .

Similarly to the engine E , the conduit 4 is a so- called " swirling port" , configured to input air at a position peripheral with respect to axis Z2 and in a tangential direction, so as to cause and maintain the swirl motion of the air ( and globally, of the charge ) within cylinder 2 . The conduit 6 has a helical geometry ( so-called " filling port" ) and it is conf igured to input air with a helical movement at a position peripheral with respect to axis Z2 ( and having the axis of the helical movement parallel or substantially parallel to the same axis Z2 ) so as to introduce the inlet air (which has a speed lower than the inlet speed of the air moving through conduit 4 ) into the main swirl motion induced by conduit 4 .

In combination with the aforementioned features , the first intake conduit 4 is configured to inlet air into the cylinder 2 at a first radial distance D4 from the longitudinal axis Z2 of cylinder 2 , and the second intake conduit 6 is configured to inlet air into cylinder 2 at a position having a second radial distance D6 from the axis Z2 of cylinder 2 , wherein the second radial distance D6 is smaller than the first radial distance D4 .

References 10 and 12 designate a pair of bores having a position corresponding to the position of the bores accommodating the inj ector I and the glow plug GP . However, in the engine 1 , the hole 10 - which has a central position ( coaxial with the axis Z2 or having a minimum eccentricity with respect thereto , mainly depending on the diameter of the valves ) - provides a seat for an ignition spark plug, while the hole 12 provides a seat for an inj ector 16 configured to inj ect hydrogen into the cylinder 2 . In this way, the inj ector 16 is arranged between the intake conduits 4 and 6, therefore in an eccentric position with respect to the axis Z2, while the spark plug 14 is arranged centrally, i.e., coaxially with axis Z2 or with a minimum eccentricity with respect thereto, mainly depending on the diameter of the valves. If an eccentricity is present, the latter is generally equal to or smaller than 10 mm.

According to the invention, the injector 16 comprises a respective longitudinal axis A16 which is inclined, at installation, at an angle a equal to or greater than 40° with respect to a reference plane P3 orthogonal to the axis Z2. Plane P3 is preferably chosen - but it will be appreciated that any plane parallel thereto yields the same result - as a plane tangential to the head surface, i.e., to the coupling surface between the head and the cylinder block accommodating the cylinder (s) 2.

In embodiments, the centers of the bores 10 and 12 are aligned along a radial direction, so that the axis A16 is incident to the axis Z2, i.e., it is not skew thereof .

In such embodiments, the projection of the longitudinal axis A16, which defines the angle a together with plane P3, intersects the axis Z2 of the cylinder 2 and of the spark plug 14 at a point P16, i.e., in other words, the axis A16 and the axis Z2 may be incident and not skew. In such instances, the longitudinal axis A16 of the injector 16 intersects the longitudinal axis Z2 at a point P16 below a bottom of the bowl-shaped recess 20, when the piston P is at a top dead centre (see Figure 4) .

However, embodiments are generally preferred wherein the axis A16 and the axis Z2 are skew, i.e., wherein the bore 12 and the bore 10 are not aligned along a radial direction ( the hole 10 is arranged centrally or with minimum eccentricity, as described in the foregoing) . The skew arrangement of axis A16 with respect to axis Z2 leads to an introduction of hydrogen along a direction which is globally oriented along a chord of the circumference of the bowl-shaped recess 20 , and which generally is not incident to or passing through a centre or anyway central areas .

I f the axes are skew, the condition of Figure 4 is always valid, therefore point P16 is located below a bottom of the bowl-shaped recess 20 when the piston P is at a top dead centre , but point P16 is to be understood as the intersection of a proj ection of axis A16 onto axis Z2 ( or vice versa ) .

With reference to Figure 4 , the inj ector 16 comprises a tip end 16T , configured to deliver hydrogen into cylinder 2 and there fore facing into cylinder 2 itsel f ( thereby implementing a direct fuel inj ection) , and the spark plug 14 comprises a tip end 14T including a discharge section, configured to ignite a mixture of air and hydrogen into cylinder 2 ( therefore , also the end 14T faces into cylinder 2 ) . Solutions which do not envisage the direct inj ection of hydrogen into cylinder 2 are not generally adopted, because the low density of hydrogen makes an inj ection upstream of the intake valve extremely prone to a backf low of hydrogen towards the mani fold, with a consequent backfiring .

With reference to Figure 4 , reference 18 generally designates a combustion chamber of engine 1 at a position where the piston P is at the top dead centre . The combustion chamber 18 is defined by the head surface 3 ( including the surfaces of the intake and exhaust valves facing into the cylinder 2 ) , in a lesser or negligible way by the walls of the cylinder 2 ( in the part which is exposed at the moment of ignition, which however varies along the range of crank degrees along which combustion takes place ) and to a greater extent by a bowl-shaped recess ( so-called "bowl" ) 20 provided at the top of piston P . The bowl-shaped recess 20 preferably has an omega-shaped profile .

As is already known, a piston P compris ing the bowl-shaped reces s 20 consequently also comprises , at the crown, an annulus 22 defining a squish area, which is equal to the area of the annulus itsel f and which, when piston P approaches the top dead centre , imparts to the charge in cylinder 2 a toroidal squish motion schematically represented by vortices S22 in Figure 4 .

The squish motion, as is generally known, originates from the variation of the local compression ratio while approaching the top dead centre , and speci fically from the local variation of the compression ratio present between the squish area 22 and the bowl 20 . The gas being compressed by piston P during the compression stroke vents radially from the squish area 22 (where the local compression ratio is greater ) towards the bowl-shaped recess 20 (where the local compression ratio is smal ler ) while approaching the top dead centre , therefore generating a toroidal whirling motion represented by the vortices S22 , the formation whereof is supported and determined by the profile of the bowl-shaped recess 20 .

As can be observed, the axis A16 of inj ector 16 - which defines an initial inlet direction of hydrogen - intersects the volume region whereat the vortices S22 and the overall squish motion of the charge are established, thereby favouring the mixing of hydrogen thanks to the strong mixing induced by the vortices S22 .

The operation of engine 1 according to the invention is as follows . In the engine 1 , at the opening of the intake valves , the air enters cylinder 2 with a swirl motion imparted by conduits 4 and 6 . In this regard, the conduits 4 and 6 are preferably shaped and si zed in such a way as to induce a swirl motion which is less marked than would normally be obtained in the internal combustion engine wherefrom the present architecture is borrowed; speci fically, the conduit 4 , which i s the main conduit , generates a swirl motion with tangential inlet and a speed V4 which i s adapted to generate a vortex having a clockwise rotation direction around the axis Z2 , whereas the conduit 6 is configured to generate an air inlet into cylinder 2 having a tangential direction and a speed V6 which is adapted to generate a vortex with an opposite ( anti-clockwise ) rotation direction around the axis Z2 . Given that the radial distances D4 and D6 from the axis Z2 of the resultant speeds V4 and V6 are di f ferent from each other, because D6 < D4 ( due to the arrangement of the conduits 4 and 6 astride of the axis Z2 and due to the asymmetry of the conduits 4 and 6 themselves ) , the vortex generated by the conduit 6 attenuates the vortex generated by the conduit 4 , therefore obtaining a swirl motion which is less marked than in the case of the engines of the prior art , such as engine E , and which is generally more favorable to hydrogen combustion .

It will be observed, with reference to Figure 3 , that to this end the conduit 6 may preferably be shaped in such a way as to have a helical geometry less marked than the conduit 6 in the engine E , or else it may be shaped in such a way as to eliminate the helical geometry in favor of so-called "plunger" geometries , thereby enhancing the mitigating ef fect on the swirl motion, which is functional to the hydrogen combustion .

Such swirl motion is generally characteri zed by a global swirl ratio equal to or lower than 1.5, which results from a compound of the swirl motion induced by conduit 4 (having an average swirl ratio of 2) and the swirl motion induced by conduit 6, having an average swirl ratio equal to or lower than a unit) .

The hydrogen injection through injector 16 starts after (and generally near) the closing of the intake valves, essentially for two purposes: avoiding hydrogen leaks towards the intake manifold, with the consequent risk of so-called backfiring events, which would destroy the engine 1, concentrating the injection in the ranges of crank angle degrees as close as possible to those exhibiting squish and vortices S22, because the latter favour the mixing of hydrogen with combustion air.

In any case, it shall be borne in mind that in given situations, which essentially depend on the operating point of the engine, it is possible to envisage a start of the hydrogen injection through injector 16 slightly in advance of the closing of the intake valves. Especially when air enters cylinder 2 at high speeds, the dynamic effect of the inlet air dragging the hydrogen flow introduced into cylinder 2 prevails on the possible reflux events during the intake .

By way of example only, the start timing of hydrogen injection through injector 16 covers a range of crank angle degrees between 110 crank angle degrees (start of injection after the closing of the intake valves) and 300 crank angle degrees (start of injection before the closing the intake valves) of the combustion top dead centre.

In the internal combustion engine 1 according to the invention, the motion within the cylinder 2, which favours the mixing of hydrogen with combustion air, only develops during the last tens of crank angle degrees before the top dead centre , and therefore the inj ection at such a stroke section of piston P - around the closing of intake valves , s lightly in advance or slightly later, according to the load - guarantees the best combustion performances . The squish motion which appears close to the top dead centre , moreover, concentrates the mixture of air and hydrogen at the tip end 14T of spark plug 14 , where the ignition electrodes are arranged, therefore creating the optimum conditions not only for mixing but also for igniting the mixture of air and hydrogen .

In this regard, the skew arrangement of axes Z 16 and Z2 is particularly ef fective for mixing, because in the range of crank degrees where the toroidal whirling motion of vortices S22 takes place there is also the ef fect of the swirl motion, which does not ebb as compression proceeds (unlike what would happen, for example , with a tumble motion) .

Schematically, this means that the flow lines of the flow field of the fluid experiencing the toroidal whirling motion do not develop along radial planes , but rather along planes which are "deflected" with respect to the radial direction of the tangential component brought about by the swirl motion . Globally, the vortices S22 form their convolutions along chordal planes , and hence the skew arrangement of axis A16 in a chordal , and not diametral , plane is very ef fective in introducing hydrogen and in mixing it with combustion air, because by so doing the hydrogen is blended in along the natural flow paths of the combustion air, thus avoiding local overleaning or undermixing due to undesirable interactions on incident paths of hydrogen and combustion air .

Globally, the invention provides a method for the combustion of hydrogen and air in the internal combustion engine 1 comprising the following steps :

- controlling an opening of a f irst intake valve and of a second intake valve , which are respectively associated with the first intake conduit 4 and with the second intake conduit 6 ,

- intaking combustion air into cylinder 2 through the first and the second intake conduits 4 , 6 by means of a movement of piston P towards a bottom dead centre , controlling a closing of the first and the second intake valves ,

- compressing the combustion air in cylinder 2 by means of a movement of piston P towards a top dead centre , wherein compressing the combustion air moreover comprises forcing the combustion air to a radial movement ( squish motion) towards the bowl-shaped recess 20 , in order to generate a toroidal whirling motion of the combustion air with axis oriented according to the longitudinal axis Z2 of cylinder 2 ( and therefore being parallel or substantially coincident ) , and vortices S22 having an axis substantially orthogonal to the longitudinal axis Z2 of cylinder 2 , the vortices S22 developing in the bowl-shaped recess 20 ,

- introducing hydrogen into cylinder 2 by means of inj ector 16 in correspondence of the bowl-shaped recess 20 and of the toroidal whirling motion of the combustion air, in order to mix hydrogen with combustion air, controlling an ignition of a mixture of combustion air and hydrogen by means of spark plug 14 .

Considering the timing information provided in the foregoing, the step of introducing hydrogen into cylinder 2 does not necessarily define the starting moment of inj ection, but it simply indicates that the inj ection is taking place . In other words , at least a portion of the hydrogen flow is introduced into cylinder 2 at the position of the bowl-shaped recess 20 and of the toroidal whirling motion of the combustion air, in order to mix hydrogen therewith, while other fractions of the hydrogen flow delivered by inj ector 16 may be ( and generally are ) introduced into cylinder 2 in conditions wherein the flow field of the fluid is di f ferent .

More speci fically, i f the timing of the start of inj ection is very early, and i f it takes place before closing the intake valves , a part of the hydrogen flow enters cylinder 2 at an instant when the toroidal whirling motion has not developed yet ( such a motion derives from the squish motion of the mixture in the cylinder, which appears towards the end of compression) . I f the timing of the inj ection start takes place after closing the intake valves , a more signi ficant part of the hydrogen flow is introduced into cylinder 2 when the toroidal whirling motion has started or has already developed .

The arrangement of point P16 below the bottom of the bowl-shaped recess 20 when piston P is at the top dead centre enables - together with the choice of angle a - an ef fective introduction of the hydrogen flow into the toroidal whirling motion which develops in the bowl-shaped recess ( 20 ) . A solution which is preferred for its ef fectiveness is obtained by adding a skew arrangement of the axes Z2 and A16 , for the reasons described in the foregoing .

This constitutes a solution to the problem o f mixing and igniting the mixture of air and hydrogen which af fects the engines of the prior art having an architecture similar to engine E, while requiring very small modi fications of the head of the internal combustion engine E to accommodate inj ector 16 and spark plug 14 . Such modi fications essentially comprise a slight reshaping (widening) of the seat of glow plug GP, in order to enable the introduction of inj ector 16 , and an equally slight widening of the seat which normally accommodates inj ector I , by employing, e . g . , spark plugs 14 with long protrusion and small diameter, like those which are already used in small-bore spark ignition internal combustion engines .

Of course , the implementation details and the embodiments herein may widely vary with respect to what has been described and illustrated herein, without departing from the extent of the present invention as set forth in the annexed claims .