GB2251456A

GB2251456A - I.c. engine with variable compression ratio

Info

Publication number: GB2251456A
Application number: GB9100201A
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: GB9100201D0

Abstract

An eccentric ring 16 is arranged between the connecting rod and its crank pin. An hydraulic mechanism is provided for setting the angular position of the eccentric ring 16 to vary the effective length of the connecting rod and thereby determine the compression ratio. Oil is drawn into portions of a groove 26 in the connecting rod on either side of a piston 38 on the ring 16 through non-return valves 28, 58 so that the pressure tends to build-up in the right-hand groove portion until a relief valve 48 is operative when the pressure exceeds a predetermined limit. Springs 20, 22 bias the ring 16 to an unpressurised rest position and the ring is rotated in operation until the maximum combustion chamber pressure corresponds to the limit set by the valve 48. <IMAGE>

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 selfjacking 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 described a reciprocating 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 nonrotatably 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.

Obiect 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 ring arranged between each connecting rod and its crank pin and a hydraulic self-jacking mechanism for locating the angular position of the eccentric ring relative to the connecting rod to vary the effective length of the connecting rod and thereby determine the compression ratio, wherein hydraulic fluid is drawn into the jacking mechanism in a direction to increase the compression ratio by the action of the inertial forces acting on the connecting rod and piston assembly and a relief valve is operative to drain fluid from the hydraulic mechanism when the pressure exceeds a predetermined limit whereby the compression ratio is reduced in response to an excessive gas pressure in the combustion chamber.

Preferably, the hydraulic mechanism comprises an arcuate groove in the connecting rod defining a working chamber for a double acting piston formed as a radial projection on the eccentric ring, the parts of the arcuate working chamber on each side of the piston being connected to one another by means of a non-return valve whereby the eccentric is only permitted to move in response to inertial forces acting in a direction tending to increase the compression ratio, the pressure relief valve being being provided in the higher pressure working chamber to permit the eccentric ring to move in a direction to reduce the compression ratio in response to excessive gas pressure, as occur during detonation.

Advantageously, springs are provided in the two parts of the working chamber for urging the point of maximum eccentricity on the eccentric ring to a position lying on a line transverse to the longitudinal axis of the connecting rod and for preventing the position of the point of maximum eccentricity from becoming aligned with the longitudinal axis of the connecting rod.

In place of springs, end stops may be used to prevent the eccentric from being aligned with the connecting rod but springs are preferred in order to avoid the noise and wear associated with abrupt stopping of the eccentric ring.

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 an end elevation of a connecting rod fitted with a split eccentric ring, Figure 2 is a partially section view of the connecting rod and eccentric ring shown in Figure 1, Figure 3 is a plan view of the split eccentric ring when removed from the connecting rod, and Figure 4 is a plan view from above of the end cap of the connecting rod.

Descrintion of the preferred embodiment A generally conventional connecting rod 10 has a separable end cap 12 to enable it to be fitted about a crank pin secured to the connecting rod 10 by means of bolts 14. In a conventional engine, big end bearing shells would be secured directly to the connecting rod so that axis of the crank pin would be centred within the circular aperture in the lower end of the connecting rod.

A split eccentric ring 16 is mounted within the connecting rod 10. The crank pin can rotate within the eccentric ring 16 and the ring can itself oscillate relative to the connecting rod 10. The ring 16 is shown in Figure 1 in its rest position when no dynamic or hydraulic forces act upon it. The point of maximum eccentricity of the ring lies at the 3 o'clock position as viewed. This point lies a line transverse to the longitudinal axis of the connecting rod, which is taken to be the line connecting the centres of the small end and big end bearings.

It can be seen from Figure 1 that if the ring 16 is rotated counter-clockwise as viewed to move the point of maximum eccentricity towards the 12 o'clock position, then the effective length of the connecting rod is increased and, conversely, if it is rotated clockwise towards the 6 o'clock position then the effective length of the connecting rod is reduced.

Movement of the eccentric ring 16 in response to the dynamic conditions is controlled by a hydraulic self jacking mechanism acting between the eccentric ring and the connecting rod. An arcuate working chamber 26 is formed in the connecting rod 10 by a groove in which springs 20, 22 are received. The springs 20, 22 urge the eccentric ring 16 towards the rest position and, when fully compressed limit the movement of the eccentric ring to each side of its rest position. A projection 30 from the eccentric ring 16 which sealingly engages the surface of the arcuate groove 26 serves as a double acting piston which divides the arcuate working chamber 26 into two parts. The two parts of the working chamber 26 on each side of the projection 30 are connected to one another by a non-return valve 28 formed in the connecting rod 10. A non-return valve 58 in the low pressure half of the working chamber supplies oil to keep the chamber full from the supply of lubricating oil available at the big end bearing shells and a relief valve 48 allows oil to escape from the high pressure half of the chamber 26.

The eccentric ring 16 is split on its centre line to enable it to be assembled about the crank pin. The two halves of the ring 16 are interlocked at one end and held together at the other end by means of a clip 38 which fits over the two halves of the projection 30 and acts as the piston. As shown in Figure 3, the eccentric ring 16 has two outer bearing surfaces 34 and 36 and a central land 32 which is received in the groove 26 to seal off the groove and guide the eccentric ring 16.

In operation, the reciprocation of the engine piston and connecting rod assembly will apply a force to the eccentric ring 16 tending to align it with the direction of the resultant force. Depending upon the part of the engine cycle this force will vary between an upwards and a downwards force and in the absence of hydraulic control, the eccentric ring would oscillate.

The action of the hydraulic mechanism is to allow rotation in only one direction, as determined by the sense of the one way valve 28. In the embodiment illustrated, the valve allows oil to flow from the part of the working chamber shown in the left in the drawing to the other half but not vice versa. When the dynamic forces acting on the eccentric ring 16 try to turn it counter-clockwise, it moves in a gradual manner being damped by the action of the oil forced through the non-return valve. Thus the eccentric will be allowed to adopt a position to increase the effective length of the connecting rod 10 and the engine compression ratio. Rotation of the eccentric ring 16 in the opposite direction will not normally occur and the engine will always try to operate at the maximum possible compression ratio.

If the engine is operated with high compression ratio and high load, however, excessive gas pressures will build up and cause an excessive fluid pressure in the part of the working chamber 26 shown to the right in the drawing.

Should this occur, the relief valve 48 will open automatically to allow the eccentric ring 16 to rotate clockwise under the action of the gas pressure transmitted through the engine piston. In other words, without external control, the hydraulic self-jacking mechanism always urges the eccentric into the maximum safe compression ratio, regardless of engine load and speed.

The principle of operation of the engine is essentially the same as that of the engine described in the SAE paper 900229 mentioned above and all the benefits of that proposal are retained. However, there is no significant increase in the reciprocating mass because a convention engine piston connecting rod assembly can be used. The noise and wear problems are furthermore mitigated by the fact that the movements of the eccentric ring in the present invention are gradual and self-limiting and one does not rely on unbuffered stops to bring the movement to an abrupt end in either direction.

It will also be noted that the invention not add to the problem of achieving adequate lubrication and cooling. The invention can also be implemented inexpensively, in that it requires only minor modification to the connecting rod and big end bearing assembly and does not add significantly to the component count nor to the complexity of assembly.

Claims

1. A reciprocating piston internal combustion engine having an eccentric ring arranged between each connecting rod and its crank pin and a hydraulic self-jacking mechanism for locating the angular position of the eccentric ring relative to the connecting rod to vary the effective length of the connecting rod and thereby determine the compression ratio, wherein hydraulic fluid is drawn into the jacking mechanism in a direction to increase the compression ratio by the action of the inertial forces acting on the connecting rod and piston assembly and a relief valve is operative to drain fluid from the hydraulic mechanism when the pressure exceeds a predetermined limit whereby the compression ratio is reduced in response to an excessive gas pressure in the combustion chamber.

2. A reciprocating piston internal combustion engine as claimed in claim 1, wherein the hydraulic mechanism comprises an arcuate groove in the connecting rod defining a working chamber for a double acting piston formed as a radial projection on the eccentric ring, the parts of the arcuate working chamber on each side of the piston being connected to one another by means of a non-return valve whereby the eccentric is only permitted to move in response to inertial forces acting in a direction tending to increase the compression ratio, the pressure relief valve being being provided in the higher pressure working chamber to permit the eccentric ring to move in a direction to reduce the compression ratio in response to excessive gas pressure, as occur during detonation.

3. A reciprocating piston internal combustion engine as claimed in claim 2, wherein springs are provided in the two parts of the working chamber for urging the point of maximum eccentricity on the eccentric ring to a position lying on a line transverse to the longitudinal axis of the connecting rod and for preventing the position of the point of maximum eccentricity from becoming aligned with the longitudinal axis of the connecting rod.

4. A 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.