GB2251457A

GB2251457A - I.c.engine with variable compression ratio

Info

Publication number: GB2251457A
Application number: GB9100202A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-01-05
Filing date: 1991-01-05
Publication date: 1992-07-08
Also published as: GB9100202D0; WO1992012337A1

Abstract

A reciprocating piston internal combustion engine is disclosed having an eccentric (16) arranged between each piston (10) and its connecting rod (12) and a spring (22) for applying a biassing torque to urge the point of maximum eccentricity on the eccentric (16) to a rest position lying on a line transverse to the longitudinal axis of the connecting rod. The eccentric is free to oscillate about the rest position as a result of the forces acting between the piston (10) and the connecting rod (12) during engine operation and this varies dynamically the instantaneous effective length of the connecting rod.

Description

Title ENGINE WITH VARIABLE COMPRESSION RATIO Field of the invention The present invention relates to a reciprocating piston internal combustion engine in which the distance between the piston crown and the centre of the crank pin, that is to say the effective length of the connecting rod, can be varied during engine operation.

Background of the invention Various attempts have already been made to vary the compression ratio of an engine by modifying the distance from the crown of the piston to the centre of the crank pin.

In a paper published by the Society of Automotive Engineers (SAE 900229) there is disclosed a two part piston in which the crown and the body of the piston are movable relative to one another and a hydraulic self-jacking mechanism is disposed within the piston to raise the crown when the compression ratio is to be increased. The mechanism is self-jacking in that the inertial forces acting on the crown are relied upon to draw fluid into the working chamber of the jacking mechanism and a pressure relief valve is provided to release fluid from the working chamber when an excessive pressure is reached, as occurs when knocking commences. In this way, the compression ratio is maintained at its maximum level consistent with there being no knocking under the prevailing operating conditions.

Several problems arise with such a hydraulic mechanism amongst them the increase in the reciprocating mass, cooling of the piston crown, noise and wear as the piston crown collides at high speed with its end stops.

In GB-2 190 959 there is descried a-4reciprocating piston internal combustion engine in which an eccentric sleeve is positioned between the crank pin and the connecting rod of the engine. The eccentric sleeve is arranged to oscillate about the crank pin axis during rotation of the crankshaft by the action of a lever arm which 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 oscillation of the eccentric sleeve modifies the cranking motion and varies the compression ratio and the expansion ratio dynamically but the amount of oscillation and its phasing is limited by the geometry of the control linkage. It is necessary in this case to employ a sophisticated external control system to match the compression ratio to the engine speed and load in order to ensure maximum efficiency and that the engine is not damaged by being operated with excessive compression ratio under high load.

Object of the invention The present invention seeks to provide an internal combustion engine in which the compression ratio can be varied by modifying the effective length of the connecting rod but which does not suffer from the disadvantages of the known prior art, as discussed above.

Summary of the invention According to the present invention, there is provided a reciprocating piston internal combustion engine having an eccentric arranged between each piston and its connecting rod and means for applying a biassing torque to urge the point of maximum eccentricity on the eccentric to a rest position lying on a line transverse to the longitudinal axis of the connecting rod, the eccentric being free to oscillate out the rest position a a result of the forces acting between the piston and the connecting rod during engine operation so as to vary dynamically the instantaneous effective length of the connecting rod.

Preferably, the means for applying a biassing torque comprises a spring acting between the inner surface of the piston and the outer surface of the connecting rod.

In one embodiment of the invention, the gudgeon pin is provided with bearing surfaces for the piston and the connecting rod which are eccentric with respect to one another, the gudgeon pin being fitted to the piston by means of separate bearing sleeves or end caps as used to retain the cross of a universal joint in its forks.

In an alternative embodiment, the gudgeon pin has a concentric bearing for the connecting rod but is mounted in the piston by means of eccentric bearing rings which are fast in rotation with the gudgeon pin but can rotate relative to the piston.

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 section through a piston and the top end of a connecting rod of an engine of the invention, Figure 2 is a section along the line II-II in Figure 1, Figure 3 is a section similar to that of Figure 1 through an alternative embodiment of the invention, and Figure 4 is a section along the line IV-IV in Figure 3.

Description of the ;eserred embodiments In Figure 1, a piston 10 is mounted on a connecting rod 12 by means of a gudgeon pin 14 which has a raised eccentric bearing 16 rotatably received within the connecting rod 12.

Because the bearing 16 prevents the gudgeon pin 14 from passing through a bearing hole in the piston 10 having the same diameter as the end bearings 18 of the gudgeon pin 14, a larger hole is formed in the piston 10 at least at one end of the gudgeon pin 14 and the latter is retained in the piston 10 by means of a support sleeve or end cap which is push fitted in the hole in the piston 10 but in which the gudgeon pin 14 can rotate freely. A spring 22 is held in place by dimples 24 and 26 on the inner surface of the piston 10 and the outer surface of the connecting rod 12, respectively.

The arrangement described permits the gudgeon pin 14 to rotate freely relative to the connecting rod 12 and the piston 10. The movement has two limit positions at which the maximum eccentricity lies on the longitudinal axis of the connecting rod (the line connecting the centres of the small end and beg end bearings), in one case increasing the effective length of the rod and in the other case decreasing it.

In operation, the position adopted by the eccentric on the gudgeon pin 14 is determined by the forces acting between the piston and the connecting rod and the force of the spring 22. If the engine is stationary and when the net force on the piston is negligible, the spring 22 tends to force the point of maximum eccentricity as far away from it as possible, that is to say to the 3 o'clock position as viewed in Figure 1. When the engine is operating, however, a dynamic force acts between the piston 10 and the connecting rod 12 which tends to realign the eccentric and dynamically vary the effective length of the connecting rod 12.

T e~-e are three main components to the dynamic force acting between the piston and the connecting rod. First, the piston has inertia and there is a reaction force when trying to make it reciprocate in the cylinder bore which varies generally sinusoidally with crank angle being maximum upwards at top dead centre, maximum downwards at bottom dead centre of the crankshaft and negligible half way up the bore. Of course the movement of the piston is not exactly sinusoidal because of the finite length of the connecting rod but this approximation is adequate for an understanding of the invention.

The second force acting upon the piston is the reaction force from the compression of the gases in the combustion chamber. This force varies with the engine stroke and piston position, being maximum at top dead centre of the compression stroke and minimum at the bottom dead centre of the induction stroke.

Lastly, there is the force acting downwards on the piston to drive the crankshaft as a result of combustion of the gases in the combustion chamber.

The maximum eccentricity of the gudgeon pin will always tend to align away from the net force acting on it, this force being the resultant of the three dynamic forces and the static force of the spring 22. The torque which causes this rotational motion is caused by the fact that the net force acts through the centre of the eccentric bearing 16 which is offset from the centre of rotation of the gudgeon pin 14 as determined by the end bearings 18. The turning moment which is the product of the net force and the offset is enough to overcome any frictional torque on the bearing surfaces of the gudgeon pin 14. The gudgeon pin will therefore naturally oscillate about its spring biased rest position as the engine operates in its various strokes.

The inertial force aloXie tends to increase the compression ratio towards the end of the compression stroke and exhaust stroke and to decrease compression (or increase expansion ratio) at the end of the power stroke and induction stroke.

At low load, this force is dominant and this results in increased thermal efficiency and volumetric efficiency.

As engine load is increased, the pressure becomes increasingly important and movement towards the maximum compression ratio position are opposed and the compression ratio is progressively decreased as engine load is increased.

This automatically prevents excessive combustion pressure.

The engine therefore always runs at the highest safe compression ratio towards the end of the compression stroke.

The combustion pressure can also bring about a rapid movement of the piston relative to the connecting rod if the combustion pressure is excessive and this limits the peak pressure and reduces the tendency to knock and the risk of resultant damage to the engine.

The arrangement can therefore be seen to achieve dynamic variation of the instantaneous effective length of the connecting rod which is beneficial not only during the compression and exhaust strokes but also during the intake and expansion strokes. Furthermore, the arrangement requires no external control being actuated by the forces which occur naturally during operation of the engine.

The oscillation of the gudgeon pin bearing 16 can be regarded as superposing on the reciprocating motion of the piston caused by the crank shaft a smaller amplitude oscillation of varying magnitude but which is always correctly synchronised with the crankshaft motion. This can be contrasted in two ways with the prior art proposal contained in SAE 900229. First, the length of the connecting rod varies dynamically during an engine cycle whereas prior 1; prior art the length is substantially constant during a cycle but may be varied from one cycle to the next. Second, the changes in effective length are not carried out abruptly by moving the piston against end stops but by a gradual self-limiting rotation of the eccentric.

The noise and wear problems of the prior art can thus be circumvented.

The dynamic variation of the effective length of the connecting rod brings about a change in the working cycle of the engine which has four beneficial aspects. First, a high compression temperature is maintained by the increase in compression ratio. Second, excessive peak pressure is clipped to avoid knocking. Third, the volume of the combustion chamber is minimised at the end of the exhaust stroke thereby improving scavenging. Last, the maximising of the expansion ratio during induction increases the effective swept volume and the mass of the trapped intake charge.

It will also be noted that the invention adds little to the reciprocating masses of the engine and does not require special lubrication or cooling. The invention can furthermore be implemented inexpensively, in that it requires only minor modification to the piston/connecting rod assembly and does not add significantly to the component count nor to the complexity of assembly.

The embodiment of Figures 3 and 4 operates in the same manner as that of Figures 1 and 2 and differs only in the manner in which the mutually eccentric bearings are formed and assembled.

The piston 10 and connecting rod 12 can, as in the case of the first embodiment, be conventional. In this embodiment, the gudgeon pin 14' can itself also be conventional but instead of being a push fit into the connecting rod 12 it is required to acts as a bearing for the connecting rod. The ends of the gudgeon pin 14' X eccentric sleeves 20' which are a push fit over the ends of the gudgeon pin 14'.

The sleeves 20' are free to rotate relative to the piston 10 and are assembled in situ onto the gudgeon pin 14' in a jig which ensures that their points of maximum eccentricity are aligned with one another. In this embodiment, the only additional components required over and above a conventional engine are the two eccentric sleeves 20' and the spring 22. The holes in the piston and the connecting rod are of course also dimensioned differently but that does not affect the manufacturing cost.

Claims

1. A reciprocating piston internal combustion engine having an eccentric arranged between each piston and its connecting rod and means for applying a biassing torque to urge the point of maximum eccentricity on the eccentric to a rest position lying on a line transverse to the longitudinal axis of the connecting rod, the eccentric being free to oscillate about the rest position as a result of the forces acting between the piston and the connecting rod during engine operation so as to vary dynamically the instantaneous effective length of the connecting rod.

2. An internal combustion engine as claimed in claim 1, wherein the means for applying a biassing torque comprises a spring acting between the inner surface of the piston and the outer surface of the connecting rod.

3. An internal combustion engine as claimed in claim 1 or claim 2; wherein a gudgeon pin is provided having bearing surfaces for the piston and the connecting rod which are eccentric with respect to one another, the gudgeon pin being fitted to the piston by means of separate bearing sleeves or end caps.

4. An internal combustion engine as claimed in claim 1 or claim 2, wherein a gudgeon pin is provided having a bearing for the connecting rod coaxial with the gudgeon pin, the gudgeon pin being mounted in the piston by means of eccentric bearing rings which are fast in rotation with the gudgeon pin but can rotate relative to the piston.

5. A reciprocating piston internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to, and as illustrated in, Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.