CH711318A2 - reciprocating engine Atkinson cycle. - Google Patents

Abstract

Alternative internal combustion engine comprising one or more groups of V or star cylinders and comprising an epicyclic crank mechanism comprising a fixed crown (4) coaxial with the crankshaft and with an internal toothing of radius Rc, a pinion (5) with a toothing of radius Rp which engages said crown, an eccentric (6) and at least one crank (7) of radius Rm, said eccentric (6) being rigidly connected to said pinion (5) and pivoted to said crank (7), in which the pistons are connected to the eccentric by connecting rods (11, 12).

Description

Application fields

[0001] The invention concerns the technical field of reciprocating internal combustion engines; in particular the invention relates to an engine having an expansion ratio greater than the compression ratio.

Known art

[0002] The term alternative engine means a cylinder and piston engine, i.e. comprising one or more pistons (pistons) sliding with reciprocating motion within respective cylinders.

[0003] The vast majority of alternative engines use an ordinary crank mechanism centered to connect the piston to the crankshaft. Said crank mechanism is formed by the known connecting rod-crank system which constrains the piston to a predetermined and constant stroke between a lower dead point (pmi) and a top dead center (pms). Said stroke determines the compression ratio of the engine and obviously the displacement.

[0004] The resulting thermodynamic cycle (for example Otto or Diesel) has a compression ratio equal to the expansion ratio, a consequence of the fact that the stroke of the piston between the pmi and the pms is always the same. It is known that a limit of these thermodynamic cycles lies in the fact that the gases expelled from the cylinder still have a significant fraction of the energy supplied by the fuel, ie in other words the expansion phase cannot fully exploit the energy released by the combustion .

[0005] To overcome this limitation, a thermodynamic cycle has been proposed with an expansion ratio greater than the compression ratio, known as the Atkinson cycle. The Atkinson cycle is able to extract more energy from the flue gases before expelling them, compared to a conventional Otto cycle, and consequently has a higher yield. It is therefore very attractive from a thermodynamic point of view; however it requires a crank mechanism able to give the piston a differentiated stroke depending on the phase of the cycle, in particular longer during the expansion phase.

[0006] The realization of such a crank mechanism introduces some complications and so far has slowed down the spread of engines operating with this principle. For example, a known type of crank mechanism capable of carrying out the Atkinson cycle provides a toggle system which however is almost inapplicable in today's internal combustion engines due to weight and bulk. Another possible solution is represented by a crank mechanism comprising an epicyclic gear. The planetary systems are more compact and efficient than a toggle system, but they are still expensive and are plagued by internal leaks (friction and viscous resistance) greater than the original crank mechanism.

[0007] These difficulties have so far discouraged the use of Atkinson cycle engines. The ordinary crank mechanism is now very consolidated and is preferred despite its limitations.

[0008] In the automotive field, especially in hybrid vehicles, there are engines that, although using ordinary crank mechanisms, make a variant of the Atkinson cycle increasing the opening time of the intake valves. This arrangement reduces the compression ratio even if the piston stroke between the pms and the pmi is always the same; therefore it is not able to fully exploit the energy of the flue gases leaving a margin of improvement. It also tends to reduce power and the increase in valve opening time is feasible only within certain limits. Other ways of exploiting the residual energy of exhaust gases include for example supercharging or, in cogeneration systems, heat recovery, but require additional components outside the engine, and are therefore expensive and not always applicable.

Summary of the invention

[0009] The invention proposes to solve the problem set out above, of how to realize a reciprocating motor with differentiated stroke, more specifically with an expansion stroke greater than the suction / compression stroke, capable of realizing the thermodynamic cycle known as the cycle of Atkinson, efficiently and with costs, dimensions and low weight.

[0010] The object is achieved with a reciprocating internal combustion engine comprising: a drive shaft; at least two cylinders with 120-degree angled axes; a respective piston slidable in each of said cylinders; an epicyclic crank mechanism that connects the pistons to the crankshaft and that transforms the reciprocating motion of the pistons into a rotary motion, characterized by the fact that: said epicyclic crank mechanism comprises a fixed crown coaxial with the driving shaft and having an internal toothing, a pinion which engages said crown, at least one eccentric and at least one crank with a respective crank pin; said eccentric is rigidly connected to said pinion and has an eccentricity distance with respect to the axis of the pinion; the eccentric is pivotally pivoted to said crank pin so as to rotate around the pinion axis, and the pistons are connected to the eccentric via connecting rod connections.

[0011] Preferably the pinion has a radius Rp which is equal to twice the crank radius Rm, while the crown has a radius Rc equal to three times the crank radius Rm. Said radii Rp and Rc are to be understood as referring to the primitive circumference of the external toothing of the pinion and, respectively, of the inner toothed crown toothing. The crank radius Rm is the distance between the axis of the crankshaft and the axis of the crank pin.

[0012] The crank mechanism also has, advantageously, a suitable angular offset between crank and eccentric. This phase shift can be defined as the angle formed between the crank and a line joining the center of the pinion and the center of the eccentric. Preferably, said phase shift is 90 ° or about 90 °.

[0013] In a 4-stroke engine, the kinematic system of the invention obtains an alternating motion of the piston with an expansion stroke greater than the compression stroke. Consequently the expansion ratio is greater than the compression ratio. Said motion of the piston results from the combination of the rotation of the crank with the rotation of the eccentric around the crank pin, and from the fact that the pistons are bound to the eccentric itself. More in detail, the difference between compression and expansion runs is related to eccentricity.

[0014] The main advantage of the invention is that the kinematic mechanism is simple, compact and light; it also allows to connect to the same eccentric 2 120 ° V pistons or 3 star pistons.

[0015] 1 mentioned ratios between crank radius Rm, radius of the pinion Rp (equal to 2 Rm) and radius of the crown Rc (equal to 3 Rm) bring the following advantages: the pinion has a relatively large diameter, which reduces the stresses on the teeth during the combustion phase; the viscous friction losses are limited and comparable with those of an ordinary crank mechanism due to the fact that the relative speed between connecting rod and eccentric is relatively low, being half the speed of the crankshaft, while the pin between crank and eccentric, which rotates to higher speed, has a small diameter that limits losses.

[0016] It should also be noted that the crank mechanism of the invention comprises only two additional components with respect to the ordinary crank mechanism, namely the fixed crown and the pinion with an integral eccentric. Said two components can be made with known and not particularly expensive processes, and therefore the cost of the crank mechanism of the invention is similar to the cost of an ordinary crank mechanism.

[0017] Still more advantageously, the motor comprises two groups of cylinders arranged in a V or star shape on opposite sides with respect to said fixed crown of the crank mechanism. These realizations can be defined as double V or double star respectively.

[0018] In a double-V or double-star engine, the crank mechanism preferably comprises two eccentrics fixed to the same pinion, i.e. an eccentric for each of the two groups of cylinders. The invention therefore allows the realization of multi-cylinder engines (up to 6 cylinders) by exploiting a single epicycloidal crank mechanism and a single crank, with very limited axial dimensions and much less than the bulk of conventional V or in-line engines. The shorter crankshaft is less subject to torsional stresses and, more generally, the reduced axial dimensions make the motor housing easier.

[0019] It should be noted that the engine of the invention is modular and can be made with an even greater number of cylinders. An engine up to 6 cylinders can be made with a single crank, so for example with two cranks 12 cylinders can be operated obtaining an extremely compact and light construction in relation to the fractionation. The multi-cylinder realizations are particularly attractive because the crank mechanism of the invention can have a slightly higher cost than a conventional crank mechanism; in a multi-cylinder engine, however, this additional cost is offset by the great advantage of compactness, as well as by the improved performance of the Atkinson cycle. A multi-cylinder engine according to the invention has a high efficiency and a favorable weight / power ratio which makes it very competitive with the traditional technique.

[0020] The arrangement of the star cylinders is particularly preferred, allowing to have 3 cylinders (single star) or 6 cylinders (double star) for a single crank; however the engine of the invention may comprise one or more groups of 2 cylinders at 120 degrees (V-shaped arrangement).

[0021] The connecting rod connections between the pistons and the eccentric can comprise a single connecting rod for each piston or a connecting rod system and connecting rods. By this term we mean a system comprising a suitably shaped mother connecting rod, which has a foot connected to one of the pistons and a head pivoted directly to the eccentric, and respective rods for the other pistons, called rods being pivoted to the main rod, instead of directly to the eccentric. This variant also applies to engines with a plurality of V or stars.

[0022] The advantages of a connection to a mother rod and connecting rods are the reduction of the axial dimensions and the constructive simplification, since only the parent rod is connected to the eccentric. The connection to the eccentric must be accurate and typically requires a hydrodynamic bearing, while the connection between rods and the connecting rod is generally achievable with less expensive means. A hydrodynamic bearing introduces losses due to viscous friction; the invention also allows these losses to be reduced since there is only a hydrodynamic bearing for 3 cylinders.

[0023] The invention allows to produce internal combustion engines superior to the currently common engines, characterized by a high efficiency, thanks to the implementation of the Atkinson cycle, and by a high power in relation to dimensions and weight, thanks to the compact arrangement of the cylinders, all with costs similar to those of conventional engines.

[0024] Another considerable advantage is given by the fact that good performances are obtained even with relatively low compression ratios: this is an advantage in the case of use of low quality fuels having a low octane number. Moreover with low compression ratios the maximum temperatures are reduced, to the advantage of the durability of the "hot" components of the engine which are subjected to less thermal stress.

[0025] The invention is applicable to internal combustion engines for any use and of small or large size, including for example stationary uses, operation of operating machines, cogeneration units, and vehicle traction. In the field of vehicles, the invention can be successfully applied in the automotive field but also in the railway and naval field; in these last two sectors the reduction in the quantity of fuel required (thanks to better performance) has a significant impact. Furthermore, an engine according to the invention is fully compatible with the various technical measures already known in the field of internal combustion engines (eg supercharging, etc.) to increase power or efficiency or to reduce consumption, which may be applied as required.

[0026] The advantages will be even clearer with the following description which illustrates some preferred embodiments, with the help of the figures.

Description of the figures

[0027] <tb> Fig. 1 <SEP> is a front view of the main components of a reciprocating piston engine according to a way of carrying out the invention. <tb> Fig. 2 <SEP> shows the components of fig. 1 exploded. <tb> Fig. 3 <SEP> is a diagram that illustrates some operating parameters of the crank mechanism according to the invention. <tb> Fig. 4 <SEP> is a diagram that illustrates another parameter of the crank mechanism, in particular the phase shift of the eccentric. <tb> Fig. 5 <SEP> is an enlarged detail of fig. 3 <tb> Fig. 6 <SEP> is a diagram illustrating the movement of the piston in an engine according to the invention. <tb> Fig. 7 <SEP> shows the components of an engine according to another embodiment, comprising two groups of cylinders on a single crank. <tb> Fig. 8 <SEP> is an example section of a motor of the type of fig. 7

Detailed description

[0028] Fig. 1 shows the components of a three-cylinder 120 ° star engine, and indicates the cylinder axles 1.1, 1.2 and 1.3. In the cylinders respective pistons 2.1, 2.2 and 2.3 run. The crankshaft is perpendicular to the plane of fig. 1 and has an axis of rotation passing through the point 3 of intersection of the cylindrical axes.

[0029] The motor head (valves, ducts, etc.) is not essential for the purposes of the invention and can be made according to the technique known per se, therefore it will not be described in detail.

[0030] For an understanding of the invention, on the other hand, it should be noted that the motor comprises an epicyclical crank mechanism which converts the reciprocating motion of the pistons 2.1, 2.2 and 2.3 into a rotary motion of the driving shaft around the axis 3. Said crank mechanism essentially includes a crown 4, a pinion 5, an eccentric 6 and a crank 7.

[0031] The crown 4 has an internal toothing 8 of primitive radius Rc; the pinion 5 meshes said crown 4 with an external toothing 9 of primitive radius Rp, which is coupled to the toothing 8. The crown 4 is fixed, that is for example is integral with the frame or the base of the engine.

[0032] The eccentric 6 is rigidly connected to said pinion 5 and has an eccentricity distance ε with respect to the axis of the pinion itself.

[0033] The crank 7 carries a pin 10 and has a crank radius Rm, which corresponds to the distance between the axis 3 and the axis of said pin 10. The eccentric 6 is mounted rotatably on said pin 10. In particular the pin 10 is received in the seat 22 which, as can be seen in fig. 2 is offset from the substantially cylindrical body of the eccentric 6 and coincides with the axis of the pinion 5.

[0034] The pistons are connected to the eccentric 6 by connecting rod connections which in the example of fig. 1 are made with a connecting rod (or main connecting rod) 11 for the piston 2.1 and two connecting rods (or secondary connecting rods) 12 for the remaining pistons 2.1 and 2.3. The connecting rod 11 is pivoted directly to the eccentric 6 while the connecting rods 12 are connected to the mother link 11 with pins 13. The connection of the connecting rod to the pistons can be carried out in a conventional manner, for example with pins 18.

[0035] The components of the crank mechanism are even better visible in the exploded view of fig. 2, which shows the bench pins 14, corresponding to the motor axis 3, and the pin 10 on which the eccentric element 6 is rotatable. The particular shape of the head 15 of the connecting rod 11 is also noted, which for example has pairs of fins 16 which carry the pins 13 of the connecting rods 12, and the bearing 17 which realizes the rotary coupling between the head of the connecting rod connecting rod 11 and the eccentric 6.

[0036] The main quantities that define the geometry of the crank mechanism are the aforementioned Rc, Rp and Rm rays and the eccentricity ε. Said sizes are shown in the diagram of fig. 3 and are even better visible in fig. 5

[0037] The following relations exist between the Rp, Rm and Rc rays: Rp = 2 Rm; Rc = 3 Rm.

[0038] Fig. 4 shows another geometric parameter of the crank mechanism, ie the δ phase shift between eccentric and crank. Preferably, said phase shift is equal to 90 ° as in the figure.

[0039] In fig. 3 and 4 for simplicity, only one piston is shown and the connecting rods 12 are not shown.

[0040] Observing in particular the figs. 3 - 5, the operation of the crank mechanism of the invention is understood.

[0041] Suppose for example that the crank has a rotation speed (angular velocity) ω around the motor axis 3. The pivot 10 rotates with said velocity ω running along a circular trajectory 19 (fig. 5) of radius Rm, set from the crank link. The eccentric 6, being mounted on the pin 10, runs along the same trajectory and furthermore, due to the effect of the meshing with the fixed crown 4, it rotates around the pin 10 in the opposite direction to said speed ω. Due to the dimensional relationships between the spokes, the rotation speed of the eccentric 6 with respect to the fixed reference is half the rotation speed of the crank 7. Therefore: angular velocity of the crank = ω angular velocity of the eccentric = –ω / 2.

[0042] Since the connecting rod 11 is pivoted on the eccentric 6, the connecting rod head follows the movement of the eccentric which is the composition of the circular motion of the pin 10 and of the rotation of the eccentric 6 around the same pin with half speed and discordant .

[0043] As a result of the existing constraints, as well as of the eccentricity ε, the resulting motion of the piston 2.1 approximately derives from the composition of: a harmonic motion of amplitude Rm and pulsation ω, generated by the motion of the crank pin 10 with respect to the fixed frame; a harmonic motion of amplitude ε and pulsation ω / 2 generated by the rotation of the eccentric on the pin 10.

[0044] This relation is approximate since it neglects the inclination of the connecting rod 11 with respect to the cylinder axis but still allows to understand the operating principle. With a suitable phase between these two motions, determined by the geometry of the system and in particular the phase shift between the crank and the eccentric, the resulting motion of the piston reaches alternatively, every two revolutions, a first bottom dead center and a second bottom dead center, at a different distance from the upper dead point.

[0045] Fig. 6 represents a graph with the crankshaft rotation angle from 0 to 720 ° (two turns) on the horizontal axis and the linear displacement of the piston on the vertical axis. The dashed lines I and II represent the two harmonic motions mentioned above, and the continuous line III represents the motion resulting from their composition (motion of the piston).

[0046] Introducing the time t and the rotation angle α defined as α = ω t as well as the angular phase shift δ between the crank and the eccentric, the position of the piston along axis 1 can be described by the equation: ε sin (α / 2) + Rm sin (α + δ) which corresponds to line III. The difference between the compression and expansion strokes is 2 ε.

[0047] The motor operates with the known suction, compression, expansion and discharge phases. The first bottom dead center is reached at the end of the suction phase, while the second bottom dead center is reached at the end of the expansion phase. As a result, the expansion stroke is greater than the compression stroke and the engine achieves the desired Atkinson cycle.

[0048] In the figures, and in particular fig. 2, the compactness of the engine according to the invention is appreciated.

[0049] Fig. 7 shows an embodiment with two groups of pistons 20, 21 on opposite sides of a single fixed crown 4. In the example the two groups 20, 21 have three pistons each, respectively the first group 20 comprises the pistons 2.1–2.3 and the second group includes the pistons 2.4–2.6. Pistons 2.1–2.3 and 2.4–2.6 respectively are arranged in a 120-degree star; consequently there is a double-star 6-cylinder engine.

[0050] The group 20 is similar to that of Figs. 1 and 2 and is mounted on eccentric 6; the assembly 21 is constructively identical to the group 20 and is mounted on a second eccentric 6a (Fig. 8), also integral with the pinion 5. This embodiment is extremely compact and gives the motor considerable power in relation to weight and dimensions, allowing the six pistons 2.1–2.6 to be operated with a single crank. In practice the embodiment of fig. 7 allows to double the number of cylinders by adding only one eccentric.

[0051] It should be noted that to obtain the correct kinematics the angular phase shift between the two eccentrics 6 and 6a must have a value defined on the basis of the angular offset of the two groups of cylinders. For example, given a generic angular displacement β between said two eccentrics, the pistons of the second group connected to the second eccentric 6a (and respective cylinders) have offset axes of a suitable angle φ with respect to the pistons and cylinders of the first group connected to the eccentric 6, and said angle (in degrees) can be calculated with the formula: φ = 120 ° - 2/3 β.

[0052] From the point of view of balancing, it can be observed that each group of 3 star cylinders has balanced 1st order inertia forces (by counterweights on the driving shaft); those of the 2nd order can be balanced for example with a counter-rotating shaft.

[0053] In other versions (not shown) the engine of the invention can comprise one or more groups of 2 cylinders at 120 degrees (V-shaped arrangement). Therefore it is possible to realize two or more «stars» or «V» of cylinders.

[0054] The invention achieves the aims set out above, providing an attractive solution for implementing the Atkinson thermodynamic cycle.

Claims (9)

1. Alternative internal combustion engine comprising: a crankshaft; at least two cylinders, in which the axes of the cylinders are angled at 120 degrees, a respective piston slidable in each of said cylinders with reciprocating motion along the cylinder axis, an epicycloidal crank mechanism that connects the pistons to the crankshaft, designed to transform the reciprocating motion of the pistons into a rotary motion, characterized by the fact that: said epicyclic crank mechanism comprises a fixed crown (4) coaxial with the driving shaft and having an internal toothing, a pinion (5) which engages said crown, at least an eccentric (6) and at least one crank (7) with a respective crank pin. (10), said eccentric (6) is rigidly connected to said pinion (5) and has an eccentricity distance (ε) with respect to the axis of the pinion itself, the eccentric (6) is pivotally pivoted to said crank pin (10) and is arranged so as to rotate around the axis of said pinion (5), and the pistons are connected to the eccentric by means of connecting rod connections ( 11, 12). 2. Engine according to claim 1, characterized in that: the internal toothing of the crown (4) has a radius Rc; said pinion (5) has an external toothing of radius Rp; said crank (7) has a crank radius Rm, and in that: said radius Rp of the pinion is equal to twice the crank radius Rm, and said radius Rc of the crown is equal to three times the crank radius Rm. 3. Engine according to claim 1 or 2, comprising at least three cylinders arranged in a star shape and angled at 120 degrees. 4. Engine according to claim 1 or 2, having a phase shift between crank (7) and eccentric (6) of about 90 degrees. 5. Motor according to any one of the preceding claims, comprising two groups of cylinders (20, 21) on opposite sides with respect to said crown (4). 6. Motor according to claim 5, said two groups of cylinders being both connected to said pinion (5) by means of two respective eccentrics (6, 6a) fixed to said pinion (5), and the two eccentrics being rotatably mounted on a same pin ( 10) of crank. 7. Engine according to claim 5 or 6, wherein each group of cylinders comprises two 120-degree V-shaped cylinders or three star-shaped cylinders spaced 120 degrees apart. 8. Engine according to claim 6, wherein said two groups of cylinders have offset axes of an angle equal to φ = 120 ° - 2/3 β, wherein p is the angular phase shift in degrees between said two eccentrics. 9. Engine according to any one of the preceding claims, in which the connecting rod connection between a group of pistons and the eccentric comprises a main connecting rod (11) with a foot connected to one of the pistons (2.1) of the unit and head (15) pivoted directly to the eccentric, and one or more secondary connecting rods (12) that connect the other pistons (2.2, 2.3) of the group to the parent rod (11).