More specifically, the invention relates to a reciprocating internal combustion engine, particularly of the direct- injection type, in which a plurality of intake ducts extending through the cylinder head of the engine are associated with each cylinder of the engine, each intake duct having an inlet portion which is arranged substantially parallel to the plane of the top of the cylinder block and which has a first end for connection to an intake manifold of the engine and a second end connected to an outlet portion of the duct opening into the cylinder in a respective intake- valve seat, the intake ducts of each cylinder comprising a main intake duct oriented in a manner such that the fluid emerging therefrom is subject to a swirling motion coaxial with the axis of the cylinder, and at least one filling intake duct which opens into the cylinder in a valve seat disposed upstream of the valve seat associated with the main intake duct, with reference to the direction of rotation of the swirling motion, the outlet portion of the at least one . filling duct being scroll-shaped and the inlet portion having a mouth portion which is substantially straight and a guide portion which is interposed between the mouth portion and the scroll-shaped portion of the duct, and which has an arcuate shape with opposite curvature to that of the scroll-shaped portion.

In known engines of the type defined above, various methods are used for arranging the intake ducts so as to generate a high degree of turbulence in the fluid admitted to the cylinder in order to achieve rapid and efficient combustion and to try to keep the structure of the ducts, and hence of the engine as a whole, fairly simple.

In particular, appended Figures la, lb and lc show three specific solutions adopted for the ducts associated with the valves of the cylinders of engines with four valves per cylinder.

With reference to Figure la, the seats 5 and 7 of the intake valves may be arranged with their axes in a plane perpendicular to the longitudinal plane P of the cylinder block (or at a small angle to the perpendicular). In this case, an intake duct 8a associated with the seat 5 comprises an inlet portion 9 substantially parallel to the plane of the top of the cylinder block, for connection to the intake manifold of the engine (not shown), and an outlet portion 11 by means of which the fluid is supplied to the cylinder 1.

The duct 8a may be of the plunging type and may generate a swirling motion of the fluid admitted to the cylinder due to its position relative to the axis of the cylinder, or may have a scroll-shaped outlet portion with a helix winding in the direction of the swirling motion to be induced in the fluid.

Owing to the shape and arrangement of the duct 8a, the fluid supplied to the cylinder 1 is in any case subject to a . swirling motion coaxial with the axis A of the cylinder 1 and having an anticlockwise sense of rotation with reference to the drawings.

Another intake-valve seat 7 is disposed immediately upstream of the seat 5 with reference to the direction of the swirling motion of the fluid supplied through the duct 8a. A second duct 8b is associated therewith and, when its outlet portion 15 is scroll-shaped, it winds in the same direction as the swirling motion generated by the duct 8a. A moderately arcuate inlet portion 13 extends from the outlet portion 15 towards the intake manifold of the engine. This known arrangement has, however, been found inefficient because the two fluid flows supplied to the cylinder 1 through the two ducts 8a and 8b interact with one another negatively, causing a considerable reduction in the effective turbulence index due to the two ducts in combination, in comparison with the theoretical turbulence indices of the two ducts considered separately. Moreover, the shape of the intake ducts of this arrangement is relatively complex to produce, since the core used to produce the cavity of the interspace for the passage of the cooling water of the engine head has to be formed in several separate parts and the ducts also have individual cores, substantially increasing production costs; moreover, the exhaust duct 4, which is a single duct for the two exhaust-valve seats 3, is inefficient since it is affected by a reduced permeability during the operation of the engine.

The known arrangement of Figure lb, in which the same reference numerals as in Figure la are used to indicate identical or similar parts, on the other hand, provides for the axes of the two seats 5 and 7 of the intake valves to be arranged in a plane parallel to the longitudinal plane P of the cylinder block, which constitutes an advantage from the point of view of the construction of the engine head. In this case, a main intake duct 8a of the plunging type (but which may alternatively have a scroll-shaped end portion) is associated with the seat 5 and a filling intake duct 8b with a scroll-type outlet portion 15 is associated with the seat 7. An inlet portion 13 substantially parallel to the plane of the top of the cylinder block is connected to the outlet portion 15 and has a first end 14 for connection to the intake manifold of the engine and a second end 16 for connection to the portion 15.

In particular, the portion 13 comprises a mouth portion 13a which is substantially straight or has a very small curvature, and a guide portion 13b which terminates at the end 16 of the inlet portion 13 and is thus interposed between the mouth portion 13a and the scroll-shaped portion 15. The guide portion 13b has a slight curvature in the opposite direction to that of the portion 15 with a radius of curvature about ten times greater than that of the seat 7, so that the entire inlet portion 13 is formed approximately as a tangent to the scroll-shaped portion 15, although it is slightly arcuate.

This arrangement enables the intake ducts, and also the exhaust ducts 4a and 4b to be kept well separated, at least in their first portions, the exhaust ducts 4a and 4b then converging in a single duct 4. This arrangement has the advantages of rendering the structure of the engine head fairly simple so that it can be produced with a one-piece core, and of ensuring good efficiency of the exhaust ducts.

However, it still does not represent an optimal solution. In fact, because of the geometry of the duct 8b and of its position relative to the cylinder 1, which is practically essential owing to the presence of the other ducts and of the members for connecting the head to the cylinder block, such as holes for the passage of its clamping screws (not shown in the drawings), the duct 8b provides poor fluid-dynamic performance characterized by an unfavourable turbulence index so that, in practice, it does not actually contribute to the generation of a swirling motion in the fluid supplied to the cylinder 1 through the intake ducts 8a and 8b. The turbulence index is a parameter which varies in dependence on the angle ç of the positioning of the scroll-shaped portion 15 relative to the axis A of the cylinder 1. In practice, the angle ç is formed between the plane extending through the axis A of the cylinder 1 and the axis of the valve of the seat 7 and a plane parallel to the end portion 16 of the duct portion 13, that is, to the initial section of the scroll- shaped portion 15, which passes through the axis of the valve of the seat 7.

Figure 2 is a graph which shows qualitatively the values adopted by the turbulence index T (the curve shown by a solid line) and by the mean discharge coefficient E (the curve shown by a dotted line) for a scroll-shaped intake duct, with variations of the angle. As can be seen, whereas the coefficient E varies little with variations in the angle (p, the index T is very sensitive to variations thereof. In particular, the index T adopts a minimum value for values of the angle 9 of about 280°-300°.

In practice, a scroll-shaped intake duct of standard geometry is used to evaluate the variation of T as a function of cp, the angular position of the intake duct being modified relative to a valve seat positioned at a fixed point relative to the head of the respective cylinder. In the specific case, the value of the angle ç is actually about 290° which corresponds, as shown in Figure 2, to the region close to the minimum of the turbulence index, so that the performance of the duct 8b is disadvantageous with regard to the swirling motion to be induced in the fluid supplied to the cylinder 1.

This is due basically to the fact that the resultant of the velocity vector of the fluid which passes through the end section 16 of the mouth portion of the duct 8b, that is, the inlet section of the scroll-shaped portion 15, is directed substantially towards the axis A of the cylinder 1, or is even deflected to the opposite side of the axis A to the . valve seat 5 and hence in the opposite direction to the swirling motion generated by the fluid admitted to the cylinder 1 through the duct 8a. In this situation, the duct 8b does not effectively contribute to the generation of the swirling motion of the fluid inside the cylinder 1, adversely affecting the performance of the engine in comparison with the theoretical advantage which should result from the presence of the filling intake duct. The terms"main"and "filling"are derived from the fact that, in this configuration, the first duct is that which substantially generates the turbulent motion, whereas the second is limited to providing a conribution in terms of air-flow; its shape therefore has the main function of not generating turbulence in the opposite direction to that of the turbulence induced by the first duct.

In an attempt to overcome the limitations of the typical arrangement shown in Figure lb, a filling intake duct 8b having the shape shown in Figure lc, in which the same reference numerals as those of Figures la and la are used, has been proposed. This duct 8b comprises an inlet portion 13 of which the mouth portion 13a is parallel to the plane of the top of the cylinder block and is disposed in a raised position relative to the main intake duct 9, and of which the guide portion 13b, connected to the scroll-shaped portion 15, extends like an elbow from the portion 13a and constitutes an almost vertical ramp.

This solution considerably reduces the value of the angle(p in comparison with that of the arrangement of Figure lb, but makes the geometry of the duct 8b excessively complex so that the ducts for the cooling water in the engine head are more tortuous and the head is very bulky vertically.

In order to overcome the problems of the prior art, the subject of the invention is an engine of the above-mentioned type, characterized in that the inlet portion of said at least one filling duct is shaped in a manner such that the resultant of the velocity vector of the fluid which passes through its second end is directed in a direction such as to be deflected, relative to the axis of the cylinder, in accordance with the swirling motion generated by the fluid admitted to the cylinder through the main intake duct.

By virtue of this concept, it is possible to use a filling intake duct with a geometry close to that of more conventional filling ducts, that is, with the entire inlet portion of the duct extending substantially parallel to the plane of the-top of the cylinder block, permitting minimal modification of the structure of the engine head and of the ducts for the cooling water, good permeability of the exhaust ducts, and a positioning angle o of the scroll-like portion relative to the axis of the cylinder which differs substantially from the range between 280° and 300°, so as to achieve a relatively high turbulence index for the intake ducts and hence optimal engine performance.

A further subject of the invention is a filling intake duct for a cylinder of a reciprocating internal combustion engine, particularly of the direct-injection type, comprising an inlet portion to be arranged substantially parallel to the plane of the top of the cylinder block, and having a first end for connection to an intake manifold of the engine and a second end connected to a scroll-shaped outlet portion of the duct opening into the cylinder in a respective intake-valve seat, in which the inlet portion has a mouth portion which is substantially straight and a guide portion having an arcuate shape with opposite curvature to the scroll-shaped portion and interposed between the mouth portion and the scroll- shaped portion, characterized in that its guide portion has a mean radius of curvature of the same order of magnitude as the radius of the respective valve seat.

Further characteristics and advantages of the invention will . become clearer from the following detailed description given with reference to the appended drawings, provided purely by way of non-limiting example in which: Figures la, 1b and lc, which have been described above, are similar schematic plan views showing respective known solutions of the architecture of the intake and exhaust ducts of the cylinders of reciprocating internal combustion engines with four valves per cylinder, Figure 2 is a graph showing qualitatively the variation of the turbulence index and of the mean discharge coefficient for a scroll-shaped intake duct, as a function of the angle of positioning of the scroll-shaped portion relative to the axis of the respective cylinder, and Figure 3 is a schematic elevational view of a filling intake duct according to the invention.

Figure 3 shows a filling intake duct 8b according to the invention, associated with a valve seat 7 of a cylinder 1 of a reciprocating internal combustion engine with direct fuel- injection, for example of the Diesel type.

The general arrangement of the other ducts associated with the cylinder 1 is not shown in Figure 3 in order not to detract from the clarity of the drawing, since this general arrangement does not fall within the scope of the invention and since it is wholly analogous to that shown in Figure lb.

For an engine with four valves per cylinder, the arrangement provides, in short, for a main intake duct of the plunging type, associated with the valve seat 5, and for a pair of exhaust ducts associated with a pair of exhaust-valve seats 3 which then converge into a single duct 4. In particular, the two exhaust-valve seats 3 and the two intake-valve seats 5 and 7 are arranged, respectively, in planes substantially parallel to the longitudinal plane P of the cylinder block.

Naturally, the arrangement of the intake and exhaust valves may be varied with respect to that shown, for example, for an engine in which the cylinders have more than four valves per cylinder, by small modifications within the capabilities of a skilled person.

As in the case of Figure lb, the duct 8b has a scroll-shaped outlet portion 15 winding in the same direction as the rotation induced in the fluid in the cylinder 1 by the main intake duct 8a, which is anticlockwise with reference to Figure 3, and an inlet portion 13 substantially parallel to the plane of the top of the cylinder block. This portion 13 is shaped in a manner such that the resultant R of the velocity vector of the fluid emerging from the end section 16 is directed in a direction such as to be deflected substantially, relative to the axis A of the cylinder 1, to the side on which the valve seat 5 is disposed, that is, in a manner such as to contribute actively to the swirling motion induced in the fluid supplied to the cylinder 1 through the valve seat 5 by means of the main supply duct.

The inlet portion 13 has, in particular, a mouth portion 13a which is substantially straight or has a small curvature and which extends from its first end 14 as far as a curved guide portion 13b which terminates in the region of the end section 16 of the portion 13 and has a pronounced curvature in the opposite direction to that of the scroll-shaped portion 15.

The guide portion 13b has, in particular, a relatively small mean radius of curvature, that is, of the same order of magnitude as the radius of the respective valve seat 7, for example, about twice the radius of the valve seat 7, in contrast with the radius of curvature of the guide portion 13 of the known solution shown in Figure lb, which was almost ten times greater than the radius of the seat 7.

By virtue of the configuration of the duct 8b according to the invention, the positioning angle ç adopts a value differing substantially from the range of between 280° and 300°, for example, a value of about 260°, as shown in Figure 2, so that the turbulence index T for the duct 8b is satisfactory and such as not to penalize the overall turbulence index of the intake ducts 8a and 8b of the cylinder 1, so as to achieve optimal engine performance by virtue of the fact that the duct 8b contributes effectively to the generation of a swirling motion in the fluid supplied to the cylinder 1.