GB2245646A

GB2245646A - (64) Variable compression ratio i.c. engine

Info

Publication number: GB2245646A
Application number: GB9014160A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1990-06-26
Filing date: 1990-06-26
Publication date: 1992-01-08
Anticipated expiration: 2010-06-26
Also published as: GB2245646B; GB9014160D0

Abstract

An eccentric sleeve 12 is positioned between a crank pin 14 and a connecting rod 10 and a ball detent (20, Fig. 2) retains the eccentric sleeve 12 in either of two stable angular positions relative to the connecting rod, each position corresponding to a predetermined compression ratio. An arm 18 projecting laterally from the sleeve 12 can collide with movable abutments 50 arranged at two positions adjacent the crankshaft for selecting the compression ratio. When the arm 18 collides with an abutment the eccentric is dislodged from the detent and is rotated from one stable position to the other. <IMAGE>

Description

Title VARIABLE COMPRESSION INTERNAL COMBUSTION ENGINE Field of the invention The present invention relates to internal combustion engines and is concerned in particular with the cranking mechanism of a reciprocating piston engine which permits variation of the compression ratio of the engine.

DescriPtion of the prior art It is desirable for various reasons which need not be discussed within the context of the present application but which are well documented in the prior art to be able to vary the compression ratio of an engine depending upon the operating conditions.

The swept volume of the combustion chamber cannot be changed to have any significant effect upon the compression ratio and to achieve variable compression it is necessary to vary the volume of the combustion chamber at top dead centre.

There have been proposals in the prior art to achieve variable compression ratio by various alterations to the combustion chamber but it has also been suggested to vary the compression ratio by alteration of the cranking mechanism. The latter approach relies on varying the distance from the crank pin to the piston crown and the most appropriate manner of achieving this objective without adding undue complexity and increasing the mass of the moving parts is to include an eccentric between the crank pin and the connecting rod.

It has been suggested that the eccentric be mounted on the crankshaft so that it does not normally rotate relative to the crank pin but a hydraulic mechanism is provided to alter the phase of the eccentric relative to the crank pin to vary the compression ratio. This design weakens the crankshaft and the hydraulic control of the position of the eccentrics of several cylinders is both complex and insufficiently precise.

Another approach described in DE 3 127 760 is to attach a linkage to the eccentric so that it rotates on the crank pin as the crankshaft turns but also oscillates relative to the connecting rod. In this German patent, the linkage consists of a rod rigidly connected at one end to the eccentric and having at its other end a head which is guided in a slide. The slide is linear but the head and rod can oscillate from side to side relative to the axis of the slide, this being essential in view of the geometry. To vary the compression ratio in this construction, it is necessary to alter the position of the slide by moving the slide in a circular path generally centred on the axis of the crankshaft. This proposal is difficult to construct having regard to the space requirement.It also presents difficulty when controlling several cylinders simultaneously and suffers from frictional wear due to the movement of the head of the rod in the slide.

A still further known system is described in G.B. Patent Specification No. 2 190 959, which is concerned with a reciprocating piston internal combustion engine in which an eccentric sleeve is positioned between a crank pin and a connecting rod of the engine which eccentric sleeve is arranged to oscillate about the crank pin during rotation of the crankshaft, wherein a lever arm is secured non-rotatably to the eccentric sleeve and is movably linked to a control lever pivoted at its other end about a stationary anchorage pin.

The mechanism in the latter patent results in the eccentric sleeve oscillating relative to the connecting rod and thereby varying the piston crown to crank pin distance cyclically during rotation of the crankshaft.

In all the prior art, the eccentric requires continuous application of an external force, be it mechanical or hydraulic, to rotate it in relation to the crank pin.

Summary of the invention According to the present invention, there is provided a reciprocating piston internal combustion engine comprising an eccentric sleeve positioned between a crank pin and a connecting rod of the engine, detents for retaining the eccentric sleeve in each of two stable angular positions relative to the connecting rod, each position corresponding to a predetermined compression ratio, the maximum thickness of the eccentric in each stable position being retained in line with the centres of the big end and small end bearings of the connecting rod, an arm projecting laterally from the eccentric sleeve and movable abutments are arranged at two positions adjacent the crankshaft for selecting the compression ratio, each abutment when positioned to collide with the laterally projecting arm during rotation of the crankshaft, serving to dislodge the eccentric from the respective detent and to rotate the eccentric sleeve from one stable position to the other.

The invention makes use of the fact that the connecting rod is in compression during part of a four stroke cycle and in tension during other parts of the cycle to rotate the eccentric sleeve. The variation in load along the connecting rod are caused by the inertial forces of the reciprocating masses connected to the crankshaft. When the eccentric sleeve is in a dead centre position, this load does not apply a turning moment to rotate the eccentric sleeve but in any other position, the turning couple of the load tends to rotate the eccentric sleeve into the other dead centre position. Thus an abutment is only required to move the eccentric out of a dead centre position and thereafter the sleeve is self driven until is reaches the other stable position.

On reaching the other stable position, the turning couple is reduced to zero and the detent acts on the sleeve to make sure that it does not overshoot and that it remains in line with the big end and small end bearings as the connecting rod oscillates from side to side.

Preferred features of the invention As the load along the connecting rod changes in direction, there will be times during change over when the net load on the eccentric sleeve will be small or negligible. It is advantageous to position the abutments such that the collisions between the laterally projecting arm with the abutments take place during these favourable crank angle positions as this reduces wear and noise as little effort is required to reposition eccentric sleeve at these crank angles.

Preferably, each abutment comprises a pivoted mass urged by means of a spring into a position in which it will collide with laterally projecting arm when the sleeve is in one of its stable positions. If the repositioning of the sleeve is successful, the laterally projecting arm will not collide with the sprung mass in subsequent cycles since the mass will lie outside the new locus of the arm. If repositioning of the sleeve does not take place, for example if the attempt to reposition the sleeve takes place during a power stroke when the load on the connecting rod is excessive on account of the combustion pressure, then collision will recur in subsequent cycles until such time as the desired new variable compression ratio is achieved.

Because of this mode of operation, the change over from one compression ratio to the other does not require careful timing and it is only necessary to move the abutments simultaneously. Once moved to their new positions, the abutments need not be moved again until the compression ratio is again to be changed.

Preferably, the abutments are ganged for movement at the same time. For example, the abutments may be mounted on a common laterally movable carriage or they may be arranged on separate shafts disposed one on each side of the crankshaft, the shafts being geared or linked for rotation at the same time as one another and in the appropriate directions.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of the crank mechanism of an engine constructed in accordance with the present invention, Figure 2 shows a section through a connecting rod assembly, and Figures 3a and 3b show an end view and a side view of the halves of an eccentric bearing fitted between the crank pin and the connecting rod.

In Figure 1, the connecting rod is represented schematically by the line 10. At its lower end, the connecting rod 10 is journalled by an eccentric bearing 12 about a crank pin 14. The large circle 16 represents the locus of the centre of the crank pin 14 as the crankshaft rotates.

In the invention, a change in compression ratio is achieved by altering the effective length of the connecting rod 10. The eccentric bearing 12 can have its lobe in line with the axis of the connecting rod in one of two ways and a detent is used to retain it in these positions. In the position shown in Figure 1, the effective length of the connecting rod, that is to say the distance between the centres of the big end and little end bearings, is long and this provides a higher compression ratio. Conversely, when the eccentric 12 is rotated by 1800, the effective length of the connecting rod is reduced resulting in a lower compression ratio.

The construction of the eccentric bearing is shown in Figure 2, 3a and 3b. The bearing 12 is made up of two half shells 12a and 12b which mate on a diameter. Each shell has a laterally projecting arm 18a or 18b by means of which the eccentric bearing can be turned. In Figure 3b, the shells are shown separated and lying side by side whereas in Figure 3a they are shown in their mating condition which they adopt when fitted between the crank pin and the connecting rod 10. As can be seen from the section of Figure 2, a ball catch 20 acts as a detent and engages in either of two notches in the bearing 12 to maintain the bearing lobe in a dead centre position in relation to the connecting rod 10. This means that in operation the eccentric bearing 12 rocks on the crank pin 14 but remains stationary relative to the connecting rod 10.

Even in the absence of the ball catch 20, the bearing has only two stable positions in relation to the connecting rod because the force along the connecting rod and the reaction of the crank pin in any other position of the bearing will form a couple acting on the eccentric bearing tending to bring it into a position aligned with the axis of the connecting rod 10. If an impact force of sufficient magnitude is applied to the eccentric through the laterally projecting arm 18, it will dislodge the bearing 12 from the ball catch 20 and drive it towards its other stable position whereupon it will be captured by the centring effect of the reaction couple until the ball catch engages in the opposite notch. Thus a step change in compression ratio can be achieved by simply applying an impact force to the projecting arm 18 as the crankshaft is rotating.

It will be noted that arm 18 will project either to the left or to the right of the circle 16 as the crankshaft rotates depending on the prevailing setting of the compression ratio. If an abutment is placed in the path of the arm 18 to the right of the circle 16 in Figure 1 then it will only contact the arm 18 if the engine is running with high compression ratio and conversely an abutment arranged to the left of the circle 16 in Figure 1 will only contact the arm 18 if the engine is running with low compression ratio. To change from one ratio to the other, one merely needs to place an obstruction in the way of the arm 18 as it rotates on the appropriate side of the crankshaft 10. The impact with such an abutment or obstruction will be sufficient to dislodge the eccentric from the ball catch and propel the bearing towards the other compression ratio setting.

In the illustrated embodiment, the abutment is not an immovable bar but a sprung weight 50. The weight is mounted on a pivoted plate 52 which is urged by a spring 54 towards a stop 56. The entire arrangement is mounted on an arm 58 which can be moved from left to right as represented by an arrow between a position (as shown) where the mass 50 will obstruct the path of the arm 18 and a position in which it will allow the arm 18 to pass unimpeded.

A weight operating in an analogous manner is also provided on the other side of the crankshaft and the two weights can be ganged so that as one is moved out of the way of the arm 18 on one side of the crankshaft, the other weight is brought into play on the other side of the crankshaft.

The eccentric bearing 12 has two surfaces which require lubrication and as with a conventional big end bearing the inner surface can be supplied with oil under pressure through a bore in the crank pin. The other surface can be lubricated by providing a bore through the shells to allow oil to flow from one surface to the other. Conveniently, an oil groove 60, 62 is formed in each of the two surfaces and small holes may connect the grooves on the opposite sides of the shells.

The inertia of the connecting rod and piston will result in the connecting rod being stressed and the stress will change between tension and compression as the crankshaft rotates. At the change-over between tension and compression, the load on the eccentric bearing is at a minimum and it is advantageous to position of the sprung weights 50 so as to collide with the projecting arm 18 at a crank angle of minimum loading, which in practice usually lies between 600 and 900 on either side of the top dead centre position of the crankshaft.

Claims

1. A reciprocating piston internal combustion engine comprising an eccentric sleeve positioned between a crank pin and a connecting rod of the engine, detents for retaining the eccentric sleeve in each of two stable angular positions relative to the connecting rod, each position corresponding to a predetermined compression ratio, the maximum thickness of the eccentric in each stable position being retained in line with the centres of the big end and small end bearings of the connecting rod, an arm projecting laterally from the eccentric sleeve and movable abutments are arranged at two positions adjacent the crankshaft for selecting the compression ratio, each abutment when positioned to collide with the laterally projecting arm during rotation of the crankshaft, serving to dislodge the eccentric from the respective detent and to rotate the eccentric sleeve from one stable position to the other.

2. An engine as claimed in claim 1, wherein the abutments are positioned in such a manner that collision between the laterally projecting arm and abutments takes place at values of crank angle corresponding to minimum loading of the connecting rod as the inertial forces acting along the connecting rod change in direction.

3. An engine as claimed in claim 1 or 2, wherein each abutment comprises a pivoted mass urged by means of a spring into a position in which it will collide with laterally projecting arm when the sleeve is in one of its stable positions.

4. An engine as claimed in claim 3, wherein the pivoted sprung masses on each side of the crankshaft are ganged for synchronised movement with one another.

5. A variable compression ratio reciprocating piston internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.