Reciprocating piston mechanism and a method of increasing internal EGR in an internal combustion engine

A reciprocating piston mechanism comprises a crankcase and a crankshaft having at least a crankpin The crankshaft is rotatable about a crankshaft axis. The mechanism comprises a crank member which is rotatably mounted on the crankpin, and comprises at least a bearing portion which is eccentrically disposed with respect to the crankpin. The bearing portion has an outer circumferential wall which bears the big end of a connecting rod such that the connecting rod is rotatably mounted on the bearing portion of the crank member via the big end. The crank member is provided with a crank member gear which meshes with a first auxiliary gear being an external gear. The first auxiliary gear is fixed to a second auxiliary gear via a common auxiliary shaft The auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about an auxiliary shaft axis which extends parallel to the crankshaft axis. The second auxiliary gear meshes with a central gear having a center line which coincides with the crankshaft axis. The crank member gear is an internal gear.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a national stage filing of International patent application Serial No. PCT/EP2009/051702, filed Feb. 13, 2009, and published as WO 2009/101173A1 in English.

BACKGROUND

Aspects of the present invention relate to a reciprocating piston mechanism comprising a crankcase, a crankshaft having at least a crankpin, said crankshaft being supported by the crankcase and rotatable with respect thereto about a crankshaft axis, at least a connecting rod including a big end and a small end, a piston being rotatably connected to the small end, a crank member being rotatably mounted on the crankpin, and comprising at least a bearing portion which is eccentrically disposed with respect to the crankpin, and having an outer circumferential wall which bears the big end of the connecting rod such that the connecting rod is rotatably mounted on the bearing portion of the crank member via the big end; wherein the crank member is provided with a crank member gear which meshes with a first auxiliary gear being an external gear, wherein the first auxiliary gear is fixed to a second auxiliary gear via a common auxiliary shaft, which auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about an auxiliary shaft axis extending parallel to the crankshaft axis, which second auxiliary gear meshes with a central gear having a center line which coincides with the crankshaft axis.

Such a reciprocating piston mechanism is known from EP 0 184 042. The known mechanism comprises a crank member which is driven by the crankshaft through first and secondary auxiliary gears and additional gears.

SUMMARY

In an aspect of the invention, the crank member gear is an internal gear. The reciprocating piston mechanism according to an aspect of the invention provides the opportunity to build a compact mechanism, which does neither require driving means for rotating the central gear at a certain rotation frequency nor relative large gear dimensions in case of a central gear which is not rotated at a certain rotation frequency.

The auxiliary shaft axis is spaced from the crankshaft axis which means that upon rotating the crankshaft the first auxiliary shaft is rotated about the crankshaft axis at similar speed as the crankshaft. Depending on the configuration and dimensions of the central gear, the second auxiliary gear, the first auxiliary gear and the crank member gear, the crank member is driven at a certain speed and direction by the mentioned cooperating gears. In one embodiment, the crank member rotates at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft.

In a practical embodiment the second auxiliary gear is an external gear and the central gear is an internal gear. In order to rotate the crank member at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft and in the same direction thereof, the central gear may stand still to obtain a compact mechanism. This means that no additional driving means or additional gears are necessary to drive the second auxiliary gear. Instead, the second auxiliary gear is driven by means of rolling off along the teeth of the central gear upon rotating the crankshaft. The gear ratio between the central gear and the second auxiliary gear may be half as high as the gear ratio between the crank member gear and the first auxiliary gear. This condition may be achieved by a certain choice of diameters and gear teeth modulus.

In one embodiment the auxiliary shaft extends through a crank arm of the crankshaft, and the first and second auxiliary gears are disposed at opposite sides of the crank arm. In practice the crank arm is a joint element which is disposed between the crankpin and a central cylindrical part of the crankshaft which is often a bearing of the crankshaft. The crank arm serves to keep the crankpin at an eccentrical position with respect to the cylindrical part. The advantage of this embodiment is that the distance between two crank arms can be short.

The second auxiliary gear may be disposed adjacent to at least a crankshaft bearing portion as seen in axial direction of the crankshaft axis. This further improves compactness of the mechanism in axial direction of the crankshaft axis.

In one embodiment, the second auxiliary gear is disposed between two crankshaft bearing portions as seen in axial direction of the crankshaft axis, because this provides a more stable bearing condition of the crankshaft.

The central gear may be rotatably mounted in the crankcase. This provides the opportunity to turn the central gear in order to adjust the position of the crank member with respect to the crankshaft at each position of the crankshaft. This means that the crank member may be rotated at substantially half of the speed of the crankshaft, but that the position of the crank member with respect to the position of the crank shaft at an arbitrarily selected crank angle position may vary within a certain range. For example, the position of the crank member can be adjusted such, that in top dead center the position of the piston with respect to the crankcase may be varied. Applying this embodiment in an internal combustion engine provides the opportunity to vary compression ratio, and to use a longer expansion stroke in combination with a shorter exhaust stroke for increasing internal EGR (Exhaust Gas Recirculation).

The central gear may be drivable by a driving means, such as an electric motor using a transmission including gears, for example.

The crankshaft may be provided with a crankshaft bearing, wherein the auxiliary shaft extends within the outer circumference of the crankshaft bearing. In practice, the auxiliary shaft may extend within the inner side of the crankshaft. The advantage of this configuration is that a very compact structure is obtained.

The crankshaft may comprises at least a second crankpin which is angularly spaced with respect to the crankpin about the crankshaft axis, and at least a second crank member rotatably mounted on the second crank pin, wherein the second crank member is provided with a second crank member gear being an internal gear which meshes with a third auxiliary gear being an external gear, wherein the third auxiliary gear is fixed to a fourth auxiliary gear via a common second auxiliary shaft, which second auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about a second auxiliary shaft axis extending parallel to the crankshaft axis, which fourth auxiliary gear meshes with the central gear. In terms of an internal combustion engine this mechanism may be applied in a four-cylinder engine having two cylinders per crankpin. The advantage of this embodiment is that the driving means for driving the crank member and the second crank member are centrally disposed between the first and second crankpin as seen in axial direction of the crankshaft axis. This means that the engine can be built compactly by applying a relatively simple mechanism, whereas no parts for driving the crank member and second crank member are necessary on the engine at opposite end portions of the crankshaft.

The mechanism may be adapted such that the fourth auxiliary gear is integrated in the second auxiliary gear, and the second auxiliary shaft is integrated in the auxiliary shaft such that the auxiliary shaft axis and the second auxiliary shaft axis coincide. In this case, only a single auxiliary shaft is necessary, whereas the fourth auxiliary gear is in fact eliminated. This further simplifies the mechanism.

In this embodiment the auxiliary shaft and/or the second auxiliary shaft may extend within the outer circumference of the crankshaft bearing.

The invention also relates to a method of increasing internal EGR in a four-stroke internal combustion engine, which engine comprises a crankcase, a crankshaft having at least a crankpin, said crankshaft being supported by the crankcase and rotatable with respect thereto about a crankshaft axis, at least a connecting rod including a big end and a small end, a piston being rotatably connected to the small end, a crank member being rotatably mounted on the crankpin, and comprising at least a bearing portion which is eccentrically disposed with respect to the crankpin, and having an outer circumferential wall which bears the big end of the connecting rod such that the connecting rod is rotatably mounted on the bearing portion of the crank member via the big end, wherein the crank member is substantially rotated at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft, and wherein the crank member is positioned with respect to the crankpin such that in top dead center of the piston the maximum eccentricity is angled with respect to a first plane extending through the crankshaft axis and a center line of the crankpin, and with respect to a second plane extending perpendicular to the first plane and parallel to the crankshaft axis. Due to these features the engine can have a long expansion stroke in combination with a short exhaust stroke since top dead center of the piston at the end of the exhaust stroke is lower than at the end of the compression stroke. As a consequence a relatively high internal EGR rate can be achieved.

An aspect of the invention also relates to a reciprocating piston mechanism comprising a crankcase, a crankshaft having at least a crankpin, said crankshaft being supported by the crankcase and rotatable with respect thereto about a crankshaft axis, at least a connecting rod including a big end and a small end, a piston being rotatably connected to the small end, a crank member being rotatably mounted on the crankpin, and comprising at least a bearing portion which is eccentrically disposed with respect to the crankpin, and having an outer circumferential wall which bears the big end of the connecting rod such that the connecting rod is rotatably mounted on the bearing portion of the crank member via the big end, wherein the crank member is provided with a crank member gear, being an external gear, which meshes with a first auxiliary gear, being an external gear, wherein the first auxiliary gear is fixed to a second auxiliary gear, being an external gear, via a common auxiliary shaft, which auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about an auxiliary shaft axis extending parallel to the crankshaft axis, which second auxiliary gear meshes with an intermediate auxiliary gear, being an external gear, which intermediate auxiliary gear also meshes with a central gear, being an internal gear, having a center line which coincides with the crankshaft axis. The intermediate auxiliary gear serves to rotate the auxiliary shaft in opposite direction.

An aspect of the invention also relates to a reciprocating piston mechanism comprising a crankcase, a crankshaft having at least a crankpin, said crankshaft being supported by the crankcase and rotatable with respect thereto about a crankshaft axis; at least a connecting rod including a big end and a small end; a piston being rotatably connected to the small end; a crank member being rotatably mounted on the crankpin, and comprising at least a bearing portion which is eccentrically disposed with respect to the crankpin, and having an outer circumferential wall which bears the big end of the connecting rod such that the connecting rod is rotatably mounted on the bearing portion of the crank member via the big end; wherein the crank member is drivably connected to an auxiliary shaft via a first transmission wherein the auxiliary shaft is mounted to the crankshaft and rotatable with respect thereto about an auxiliary shaft axis extending parallel to the crankshaft axis, wherein the auxiliary shaft is drivably connected to a central ring via a second transmission, wherein the central ring has a center line which coincides with the crankshaft axis; wherein the first and second transmission are adapted such that the crank member rotates at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft when the central ring has a fixed position with respect to the crankcase. The advantage of this mechanism is that it is relatively simple because it is not necessary to drive the central ring.

In a practical embodiment the first transmission comprises a crank member gear being an internal gear and mounted to the crank member, and a first auxiliary gear being an external gear and fixed to the auxiliary shaft, wherein the crank member gear and the first auxiliary gear mesh with each other.

The second transmission may comprise a second auxiliary gear being an external gear and mounted to the auxiliary shaft, which second auxiliary gear meshes with an internal gear of the central ring.

In an alternative embodiment the first transmission may comprise a crank member gear being an external gear and mounted to the crank member, and a first auxiliary gear being an external gear and fixed to the auxiliary shaft, wherein the crank member gear and the first auxiliary gear mesh with each other.

The second transmission may comprise a second auxiliary gear being an external gear and mounted to the auxiliary shaft, which second auxiliary gear meshes with an intermediate auxiliary gear, being an external gear, which intermediate auxiliary gear also meshes with an internal gear of the central ring. The intermediate auxiliary gear serves to direct the rotational direction of the crank member in the same rotational direction as the crankshaft.

In a further alternative embodiment the first transmission comprises a wheel being mounted to the crank member and an auxiliary wheel being fixed to the auxiliary shaft, wherein the wheel and the auxiliary wheel are drivably connected to each other through an endless driving element. In practice the wheel and the auxiliary wheel are sprocket wheels and the endless driving element is a chain. Furthermore, the second transmission may comprise a second auxiliary gear being an external gear and mounted to the auxiliary shaft, which second auxiliary gear meshes with an internal gear of the central ring including. The advantage of the latter embodiment is that due to the configuration of the first transmission, the second transmission does not require an intermediate auxiliary gear in order to reverse the direction of rotation.

DETAILED DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENTS

FIG. 1-4show an embodiment of a reciprocating piston mechanism1according to an aspect of the invention, which is suitable for an internal combustion engine. The reciprocating piston mechanism1comprises a crankcase (not shown), which supports a crankshaft2by crankshaft bearings3. The crankshaft2in the embodiment includes a crankpin4and is rotatable with respect to the crankcase about a crankshaft axis5.

Furthermore, the mechanism1comprises a crank member6which is rotatably mounted on the crankpin4. The crank member6is provided with two bearing portions7which are disposed eccentrically with respect to the crankpin4. Each of the bearing portions7has an outer circumferential wall which bears a big end8of a connecting rod9. Thus, the connecting rod9is rotatably mounted on the crank member6via its big end8. The connecting rod9also includes a small end10to which a piston11is rotatably connected.

The crank member6is provided with a crank member gear12which meshes with a first auxiliary gear13. The first auxiliary gear13is fixed to a second auxiliary gear14via a common auxiliary shaft15. The auxiliary shaft15is mounted to the crankshaft2and is rotatable with respect to the crankshaft2about an auxiliary shaft axis which extends parallel to the crankshaft axis5. This means that the auxiliary shaft axis is spaced from the crankshaft axis5. In this embodiment the auxiliary shaft15extends through a crank arm16such that the first auxiliary gear13and the second auxiliary gear14are disposed at opposite sides of the crank arm16. In this case the crank arm16and a crankshaft bearing3are integrated such that the auxiliary shaft15extends through both. Thus, the auxiliary shaft14extends within an outer circumference of the crankshaft bearing3.FIG. 3shows that the first gear13is disposed at the side of the crankpin4of the crank arm16.

The second auxiliary gear14meshes with a central gear17having a center line which coincides with the crankshaft axis5. In this embodiment the central gear17is an internal gear and the second auxiliary gear14is an external gear.

According to an aspect of the invention the crank member gear12is an internal gear and the first auxiliary gear13is an external gear. Due to this configuration the reciprocating piston mechanism1can be built in a compact way and is simpler than those known in the art.

As can be seen inFIG. 3the second auxiliary gear14is disposed between two crankshaft portions18. In this case a sealing ring (not shown) is supported by one of the portions18and a flywheel is mounted to a frontal end of the crankshaft2near the crankshaft portions18. It is also conceivable that the crankshaft portions18form crankshaft bearing portions, between which the second auxiliary gear14is disposed as seen in axial direction of the crankshaft axis5. In that case one crankshaft bearing3is formed by two separate crankshaft bearing portions18.

The advantage of the configuration as shown inFIG. 1-4is that the mechanism is compact in axial direction of the crankshaft axis5, and the crankshaft2is symmetric and relatively small between the crankshaft bearings3.FIG. 3-4show how the mechanism1can be assembled by fixing two parts of the crankshaft to each other. Alternatively, the crankshaft can be made of a single piece, such as disclosed in patent application EP 07102584.5.

FIG. 5-7show an alternative embodiment of the reciprocating piston mechanism1according to an aspect of the invention. In this embodiment the crankshaft2comprises a second crankpin19, which is angularly spaced with respect to the crankpin4about the crankshaft axis5. The mechanism1further comprises a second crank member20comparable to the crank member6as described hereinbefore. The second crank member20is provided with a second crank member gear21which meshes with a third auxiliary gear22.

In this embodiment the third auxiliary gear22is fixed to a second auxiliary shaft24(not visible inFIG. 5), to which auxiliary shaft24a fourth auxiliary gear25is fixed, as well. The second auxiliary gear14and the fourth auxiliary gear25both mesh with the central gear17and engage therewith at locations spaced from each other in circumferential direction of the central gear17. The second auxiliary shaft24is mounted to the crankshaft2and rotatable with respect to the crankshaft2about a second auxiliary shaft axis. Both the auxiliary shaft axis and the second auxiliary shaft axis extend parallel to the crankshaft axis5and within an outer circumference of the crankshaft bearing3, which in this case comprises two crankshaft portions as seen along the crankshaft axis5. The auxiliary shaft15and the second auxiliary shaft24fit in respective holes26in the crank arm16at the crankshaft bearing3, seeFIG. 7.

FIG. 5-7show that the mentioned gears form driving means for driving both the crank member6and the second crank member20, wherein the driving means are located at the center of the reciprocating piston mechanism1as seen along the crankshaft axis5. In case of a four-cylinder internal combustion engine comprising such a mechanism1, this means that no parts of the driving means need to be located at the axial ends of the crankshaft2. Furthermore, no separate driving means is necessary for driving the central gear17at a certain rotation frequency. It is also noted that counterweights are omitted near the center of the crankshaft2as seen along the crankshaft axis5.

In the embodiment as shown inFIG. 5-7the central gear17is rotatably mounted in the crankcase and drivable by a driving mechanism for turning the central gear17within a predetermined angle, for example by an electric motor23via a transmission. This feature provides the opportunity of creating a relatively high rate of internal EGR in a four-stroke internal combustion engine if the mechanism1is applied therein. The central gear17can be angularly positioned with respect to the crankcase such that in top dead center of the piston11the maximum eccentricity is angled with respect to a first plane extending through the crankshaft axis5and a center line of the crankpin4, and with respect to a second plane extending perpendicular to the first plane and parallel to the crankshaft axis5. In other words, when the piston11is in top dead center, the maximum eccentricity of the crank member6is angled between 0 and 90 degrees with respect to the upward position of the associated crankpin4, and is thus neither exactly aligned with the crank arm nor at an angle of 90 degrees thereto.FIG. 15shows a pressure (P)-volume (V) diagram of a cycle of such an internal combustion engine, wherein the central gear17is turned to a position as described. At the end of the compression stroke the top dead center of the piston11is higher than at the end of the exhaust stroke, whereas its bottom dead center at the end of the expansion stroke is lower than that at the end of the inlet stroke. This means that the engine in this case combines a long expansion stroke with a short exhaust stroke. In particular, the short exhaust stroke leads to a relative high rate of internal EGR which is advantageous under certain engine conditions.

FIG. 8shows a part of another alternative embodiment of the reciprocating piston mechanism1according to an aspect of the invention. In this embodiment the third auxiliary gear22is fixed to the auxiliary shaft15, to which auxiliary shaft15the first and second auxiliary gears13,14are fixed, as well.FIG. 8also shows that the crank arm16has a single hole26for receiving the auxiliary shaft15. It can be seen that the auxiliary shaft15extends within the outer circumference of the crankshaft bearing3, resulting in a compact structure. Thus in assembled condition three pairs of gear combinations are present behind each other as seen along the crankshaft axis5: the crank member gear12meshing with the auxiliary gear13, the central gear17(not shown inFIG. 8) meshing with the second auxiliary gear14, and the third auxiliary gear22meshing with the second crank member gear21. Thus, when the embodiment as shown inFIG. 5-7is modified such that the auxiliary shaft axis and the second auxiliary shaft axis coincide, and the fourth auxiliary gear25is integrated in the second auxiliary gear14, the embodiment as shown inFIG. 8is obtained.

The way in which the different gears mesh with each other is illustrated inFIG. 9related to the embodiment as shown inFIG. 8, and inFIG. 10related to the embodiment as shown inFIG. 5-7.FIG. 10shows that both the second auxiliary gear14and the fourth auxiliary gear25mesh with the central gear17, whereasFIG. 9shows that only the second auxiliary gear14meshes with the central gear17. In the case of a single auxiliary shaft15as shown inFIG. 9, the auxiliary shaft15must be positioned such that the first auxiliary gear13and the third auxiliary gear22mesh with the crank member gear12and the second crank member gear21, respectively. At the same time, the second auxiliary gear14has to mesh with the central gear17, which means that the positions as well as the dimensions of the different gears must be selected accurately in order to obtain a desired rotation frequency of the crank member6and the second crank member20with respect to the crankshaft2.

FIG. 11shows an alternative embodiment of the mechanism1according to an aspect of the invention. In this embodiment the crank member gear12is an external gear meshing with the first auxiliary gear13(not visible inFIG. 11). The first auxiliary gear13is fixed to the second auxiliary gear14via the auxiliary shaft15. In this case the second auxiliary gear14does not mesh with the central gear17, but it meshes with an intermediate auxiliary gear27. The intermediate auxiliary gear27serves to direct the rotational direction of the crank member6and the second crank member20in the same rotational direction as the crankshaft2. Basically, its dimension is not relevant, but it preferably fits within the central gear17together with the other gears14,25. The intermediate auxiliary gear27meshes with the central gear17. In the same way, the second crank member gear21is an external gear which meshes with the third auxiliary gear22. The third auxiliary gear22and the fourth auxiliary gear25are fixed to the second auxiliary shaft24. Again the auxiliary shaft axis and the second auxiliary shaft axis extend parallel to the crankshaft axis5and preferably extend within an outer circumference of a crankshaft bearing (not shown). The gear dimensions can be selected such that the crank member6and the second crank member20rotate in the same direction as the crankshaft2and at half speed thereof. The gear ratio between the central gear17and the second auxiliary gear14may be half as high as the gear ratio between the crank member gear12and the first auxiliary gear13. This condition may be achieved by a certain choice of diameters and gear teeth modulus.

FIG. 12shows another alternative embodiment, wherein the second and fourth auxiliary gears14,25of the embodiment as shown inFIG. 11are integrated in the second auxiliary gear14. The first auxiliary gear13, second auxiliary gear14and the third auxiliary gear22are fixed on the auxiliary shaft15. The second auxiliary gear14meshes with the intermediate auxiliary gear27, which on its turn meshes with the central gear17.

It is noted that the mechanism according toFIGS. 11 and 12can be applied for a two-cylinder reciprocating piston mechanism1as shown inFIG. 1-4, as well. Due to the external gears of the crank member12and the second crank member21, helical gears may be preferred for minimizing noise.

FIGS. 13 and 14illustrate the way in which the different gears mesh with each other in the embodiment as shown inFIGS. 11 and 12, respectively.FIG. 13shows that only the intermediate auxiliary gear27meshes with the central gear17, whereas the first auxiliary gear13meshes with the crank member gear12, and the second auxiliary gear14fixed to the auxiliary shaft15meshes with the intermediate auxiliary gear27. Similarly, the third auxiliary gear22meshes with the second gear member21, and the fourth auxiliary gear25meshes with the intermediate auxiliary gear27.

FIG. 14illustrates the embodiment in which the intermediate auxiliary gear27meshes with the central gear17on the one hand and with the second auxiliary gear14on the other hand. The second auxiliary gear14, the first auxiliary gear13, and the third auxiliary gear22are fixed to the single auxiliary shaft15, whereas the first auxiliary gear13and the third auxiliary gear22mesh with the crank member gear12and the second crank member gear21, respectively.

FIGS. 16-18show an alternative embodiment of the reciprocating piston mechanism1according to an aspect of the invention. Similar to the embodiments as described hereinbefore, the crank member6is drivably connected to the auxiliary shaft15via a first transmission. In this case the first transmission comprises a crank member sprocket wheel28which is mounted to the crank member6, an auxiliary shaft sprocket wheel29which is mounted to the auxiliary shaft15, and a chain30which drivably connects the first and second sprocket wheels28,29to each other. The auxiliary shaft15is mounted to the crankshaft2and rotatable with respect thereto about an auxiliary shaft axis extending parallel to the crankshaft axis5. Similar to the embodiments as described hereinbefore the auxiliary shaft15is also drivably connected to the central ring or central gear17via a second transmission. In this case the second transmission comprises the second auxiliary gear14which is an external gear and mounted to the auxiliary shaft15. The second auxiliary gear14meshes with the internal gear of the central gear17. The central gear17has a center line which coincides with the crankshaft axis5. The central gear17also has an external gear for adjusting its position with respect to the crankcase, but similar to the other embodiments as described hereinbefore, the first and second transmissions are adapted such that the crank member6rotates at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft2when the central gear17has a fixed position with respect to the crankcase. The crank member6and the crankshaft2have the same direction of rotation.

The mechanism in the embodiments as illustrated inFIGS. 16-18differ from the embodiments as illustrated inFIGS. 1-10in that the first transmission comprises sprocket wheels28,29which are drivably connected to each other through a chain30instead of an internal gear12and an external gear13meshing with each other. In terms of costs, manufacturing a sprocket wheel28is less expensive than manufacturing an internal crank member gear12.

The mechanism in the embodiments as illustrated inFIGS. 16-18differ from the embodiment as illustrated inFIGS. 11-14in that the first transmission comprises the sprocket wheels28,29and the chain30instead of an external gear12of the crank member6and an external first auxiliary gear13meshing with each other, and in that the second transmission comprises the external second auxiliary gear14and the internal gear of the central gear17instead of the external second auxiliary gear14and the intermediate auxiliary gear27which meshes both with the second auxiliary gear14and the internal gear of the central gear17. This means that with the mechanism as illustrated inFIGS. 16-18the same ratio of rotation speed between the crank member6and the crankshaft2can be achieved as with the embodiment as illustrated inFIGS. 11-14without the necessity of the intermediate auxiliary gear27.

It is noted that combinations of different features of the embodiments as described hereinbefore may be combined.

Furthermore, it is noted that the embodiments of the mechanisms as described hereinbefore can be applied such that the bearing portion of the crank member is not eccentrically disposed with respect to the crankpin, but wherein the bearing portion has a circular cross-section. This configuration is similar to that of a conventional reciprocating piston mechanism, but the crank member, which is disposed between the crank pin and the big end, is rotated with respect to the crankshaft. It has surprisingly been found that internal friction of the total mechanism is reduced due to the fact that friction reduction at the big end/crank member/crankpin outweighs friction increase due to the first and second transmissions. In this case it is not necessary that the first and second transmissions are adapted such that the crank member rotates at a rotation frequency with respect to the crankcase which is substantially half of that of the crankshaft when the central ring has a fixed position with respect to the crankcase. The ratio may be different from a half.

From the foregoing, it will be clear that aspects of the invention provides a relatively simple reciprocating piston mechanism which offers the possibility of designing a compact embodiment of the mechanism.

The invention is not limited to the embodiments shown in the drawings and described hereinbefore, which may be varied in different manners within the scope of the claims and their technical equivalents. For example, the central gear and the second and fourth gear may have different gear configurations than shown in the above embodiments. Furthermore, the central gear may be driven at a certain rotation frequency. The mechanism may be applied for crankshafts having more than two crankpins.