GB2258271A

GB2258271A - Variable compression ratio i.c.engine.

Info

Publication number: GB2258271A
Application number: GB9116580A
Authority: GB
Inventors: D Frost; Kenneth Elgar Southall
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-08-01
Filing date: 1991-08-01
Publication date: 1993-02-03
Also published as: GB9116580D0

Abstract

A plurality of pistons are each connected to a respective crankpin 16 on a common crankshaft 10 by way of a respective connecting rod 12. Eccentric collars 28 are arranged between the crankpins 16 and the connecting rods 12 and a pin or pins 22 rotatable within axial bores 20 in the crankshaft may rotate the collars 28 to vary the compression ratio by drive between pin and collar teeth 24, 26. The pin or connected pairs of pins with teeth 24 at their free ends may be rotated by a spring-biased drive wheel which is moved to one angular end position by the spring and is held in the other angular end position by a detent. <IMAGE>

Description

Variable Compression Ratio Internal Combustion Engine Field of the invention The present invention relates to an internal combustion engine in which the effective length of the connecting rod may be varied during use to alter the compression ratio.

Background of the invention The optimum compression ratio (or to be more precise the expansion ratio) of an engine varies with the operating conditions. Various proposals have been put forward in the past to enable this ratio (between the volumes of the combustion chamber at the piston top dead centre and bottom dead centre positions) to be altered during use. For example, one may alter the volume at top dead centre by selectively connecting an auxiliary chamber to the main combustion chamber under certain conditions. Another proposal has been to move the entire crankshaft towards and away from the cylinder.

A further proposal, upon which the present invention is based, has been to modify the effective length of the connecting rod by placing an eccentric collar at the big end of the connecting rod which may be rotated about the crankpin to vary the position of the point of maximum eccentricity of the collar relative to the axis of the connecting rod. Rotation of the eccentric collar will thus change the distance between the centre of the crankpin and the piston crown to vary both the minimum and maximum volumes of the working chamber by the same amount thereby varying the compression ratio.

The above proposal can readily be applied to a single cylinder engine but there is no teaching in the prior art as to how all the eccentric collars of a multi-cylinder engine may be rotated at the same time about their respective crankpins.

Obiect of the invention The present invention seeks to enable eccentric collars mounted in the big ends of the connecting rods of a multicylinder internal combustion engine to be rotated at the same time to permit the effective lengths of all the connecting rods to be varied.

Summarv of the invention According to the present invention, there is provided a multi-cylinder reciprocating piston internal combustion engine comprising a plurality of pistons each connected to a respective crankpin on a common crankshaft by way of a respective connecting rod, eccentric collars arranged between the crankpins and the connecting rods and means for rotating the eccentric collars in unison to vary the effective lengths of the connecting rods and thereby change the engine expansion ratio, wherein the means for rotating the eccentric collars in unison comprise through bores in the main bearings of the crankshaft concentric with the axis of the crankshaft, pins rotatably received within the bores in the crankshaft main bearings, pinions arranged at the ends of the pins, and toothed sectors fast in rotation with the eccentric collars and meshing with the pinions.

To enable the collars to be assembled about the crank pins, they may be formed of two semi-cylindrical halves. Only one of the halves need be formed with teeth and this is desirable from the point of view of easing assembly.

The two halves need not be clamped to one another and may be held in position only by virtue of being trapped between the crank pin and the big end bearing shells fitted to the connecting rod. Locating formations are however preferably provided on the ends of the half collars for aligning the halves with one another.

The pinion on the ends of the pins located on the axis of the crankshaft are preferably of smaller outer diameter than the pins so that the pins may be formed in one piece with the pinions. If the pinions cannot pass through the bores receiving the pins, then the pins may be formed from separable halves, joined to one another within the axial bores.

It is advantageous to provide a locating screw or spring biassed ball to prevent axial displacement of the pins within their bores to prevent the pins from sliding and obstructing the path of the connecting rod.

In order to ensure adequate lubrication of the big end bearings, holes may the formed in the eccentric collar. It will be appreciated that the collar at any given compression ratio does not rotate relative to the crank pin and the only bearing surfaces are between the big end bearing shells and the outer surface of the eccentric collar. During changes of compression ratio, the eccentric collars are rotated by 180 about the crank pins.

To rotate the pins, it is possible to provide a spring biassed drive wheel connected to the end pin and means for braking the wheel to cause it to turn relative to the crankshaft. A detent may lock the drive wheel in its end positions. When the detent is released, the spring acting on the drive wheel will return it to its other end position.

Being able to alter the expansion ratio is of value in improving the efficiency of a spark ignited engines but it also finds application in diesel engine, where it can be used to improve starting.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing which is an exploded perspective view of part of the crankshaft of an engine of the invention.

Detailed description of the Preferred embodiment The drawing shows the crank of only one of the pistons of a multi-cylinder internal combustion engine. The crankshaft 10 is formed with coaxial main bearing sections 14 and a plurality of eccentric cranks 16 each extending between a respective pair of cheeks 18. Each crank 16 is connected by a connecting rod 12a, 12b to a respective one of the pistons (not shown), the bearing surface of the connecting rod being lined by two half shells 12c of the big end bearing which are stationarily mounted within the connecting rod. As so far described, the crankshaft and the connecting rods are conventional.

To enable the expansion ratio of the engine to be varied, an eccentric collar made up of two halves 28a, 28b is fitted between the bearing shells 12c and the crank pin 16. The eccentric collar has an uneven thickness because its cylindrical inner and outer surfaces are not coaxial.

Each collar 28a, 28b also has toothed sectors 26 which are formed on only one (28b) of the halves of the collar and which are centred on the axis of the crank pin 16. These toothed sectors mesh with pinions 24 on the ends of pins 22 rotatably mounted with bores 20 formed on the axis of the crankshaft 10.

The pin 22 at one end of the crankshaft 10 is externally rotated by a suitable actuation mechanism when the expansion ratio is to be changed. Rotation of this pin turns the collar 28 with which it meshes and drive is transmitted through this collar to the next pin and the next collar, this process being repeated to cause all the collars to be turned in unison.

As illustrated, the teeth of the pinions are too large to pass through the bores 20 and the pins must therefore be assembled from two part which are suitably joined together within the bores 20. It is alternatively possible however for the teeth to lie on a smaller diameter so as to be capable of passing through the bores 20. In this case, all the pins 22 may be assembled by being pushed in from one end of the crankshaft and they may be held against axial displacement by a grub screw or other retaining means engaging in an annular groove in the pins 22.

In order to assemble the crankshaft and connecting rods, the pins 22 are first fitted in one of the ways described above.

The collar halves 28b formed with the toothed sector 26 are then placed over the crank pins 16 and they are all turned until their teeth just begin to mesh with their respective pinions. When all the collars are in mesh with their pinions, the latter can be rotated and all the collars will then lie with the eccentric at the same angular position in relation to the crankshaft. For each connecting rod, the second half 28a of the collar is next then placed over the crank pin 16 and is aligned relative to the other half by mating formations on the ends of the two halves. The bearing shells 12c are next positioned in the connecting rod and the cap of the connecting rod 12b is bolted to the remainder of the rod 12a in the usual manner.

In operation, the collars 28a, 28b are stationary relative to the crank pins except during adjustment of the expansion ratio. Likewise the bearing shells 12c are stationary relative to the connecting rod 12a. The shells 12e rotate about the collars and bearing surface is lubricated by oil pumped through passages in the crankshaft, in the conventional manner.

To change the expansion ratio, the end pin 22 is rotated to cause the remaining pins 22 and all the collars to rotate in manner earlier described. This alters the position of the eccentric and thereby changes the effective length of the connecting rod, that is to say the distance from the piston crown to the centre of the crank pin.

The most convenient manner to rotate the end pin 22 is to use a brake and a strong return spring acting on a drive wheel. In this way, the engine itself provides the force required to vary the expansion ratio. If the brake is not applied, then the pins 22 rotate with the crankshaft and the expansion ratio remains constant. When the brake is applied, the pins 22 are decelerated relative to the crankshaft and cause rotation of the collars 28a, 28b against the action of the return spring. After rotating through 180o, a spring loaded detent acts to lock the drive wheel in position. The spring loaded detent may be released mechanically of electrically and, upon it release by the detent, the drive wheel will turn under the action of the spring to return the collars to their other stable end positions.

As an alternative, a phase change mechanism can be connected between the pins and the crankshaft and driven electrically, hydraulically, mechanically or pneumatically.

The control of the positions of the eccentric collars 28 may be continuous but this makes for greater complexity as the pins 22 will need to be held firmly in most positions.

However, in the two positions of the collars 28 in which the maximum eccentricity lies on the axis of the connecting rod, the collars are not subjected to a turning moment and it is preferred to control the collars 28 so that they move from one of their stable positions to the other.

Claims

1. A multi-cylinder reciprocating piston internal combustion engine comprising a plurality of pistons each connected to a respective crankpin on a common crankshaft by way of a respective connecting rod, eccentric collars arranged between the crankpins and the connecting rods and means for rotating the eccentric collars in unison to vary the effective lengths of the connecting rods and thereby change the engine expansion ratio, wherein the means for rotating the eccentric collars in unison comprise through bores in the main bearings of the crankshaft concentric with the axis of the crankshaft, pins rotatably received within the bores in the crankshaft main bearings, pinions arranged at the ends of the pins, and toothed sectors fast in rotation with the eccentric collars and meshing with the pinions.

2. An engine as claimed in claim 1, in which the collars are formed of two semi-cylindrical halves.

3. An engine as claimed in claim 2, in which only one of the halves of the collars is formed with a toothed sector.

4. An engine as claimed in claim 2 or 3, in which the two halves are not be clamped to one another and are held in position only by virtue of being trapped between the crank pin and big end bearing shells fitted to the connecting rod.

5. An engine as claimed in claim 5, in which locating formations are provided on the ends of the half collars for aligning the halves with one another.

6. An engine as claimed in any preceding claim, wherein the pinion on the ends of the pins located on the axis of the crankshaft are of smaller outer diameter than the pins.

7. An engine as claimed in claim 6, wherein a locating screw or spring biassed ball is provided to prevent axial displacement of the pins within their bores to prevent the pins from sliding and obstructing the path of the connecting rod.

8. An engine as claimed in any one of claims 1 to 5,wherein the pinions lie on too large a circle to pass through the bores receiving the pins, the pins being formed from separable halves and being joined to one another within the axial bores.

9. An engine as claimed in any preceding claim, comprising a spring biassed drive wheel connected to the end pin, means for braking the drive wheel to cause it to turn relative to the crankshaft, and a detent for locking the drive wheel in an end positions, whereby, when the detent is released, the spring acting on the drive wheel returns the end pin to its other end position.

10. A multi-cylinder 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.