GB2251455A - I.c.engine with variable compression ratio - Google Patents

Abstract

A reciprocating piston internal combustion engine is disclosed having an eccentric ring (16) arranged between each connecting rod (10) and its crank pin. Springs (20, 22) apply a biassing torque to urge the point of maximum eccentricity on the eccentric ring (16) to a rest position lying on a line transverse to the longitudinal axis of the connecting rod (10). The eccentric ring (16) is free to oscillate about the rest position as a result of the forces acting between the connecting rod and the crank pin during engine operation and dynamically varies 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 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 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.

Summarv 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 means for applying a biassing torque to urge the point of maximum eccentricity on the eccentric ring to a rest position lying on a line transverse to the longitudinal axis of the connecting rod, the eccentric ring being free to oscillate about the rest position as a result of the forces acting between the connecting rod and the crank pin 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 radial projections on the outer surface of the eccentric ring and the inner surface of the connecting rod.

Advantageously, two compression springs applying biassing torques in opposite senses act between the connecting rod and the eccentric ring.

Conveniently, means are provided for oil damping the movement of the eccentric ring relative to the connecting rod.

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.

Description 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 forces act upon. 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 (the distance between the small end and big end bearings) 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.

Springs 20, 22 are arranged between the eccentric ring 16 and the connecting rod 10 to urge it into the rest position, this being the position in which the eccentric ring lies when no dynamic forces act between the connecting rod and the crank pin. The connecting rod is formed with a annular groove 26 in which the springs 20, 22 are received. A radial projection 28 into the annular groove 26 provides abutment surfaces for the ends of the two springs 20, 22. The other ends of the springs 20, 22 act on a radial projection 30 of the eccentric ring 16 also received with the groove 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. As shown in Figure 3, the eccentric ring has two outer bearing surfaces 34 and 36 and a central protruberant land 32 which is received in the groove 26. The land 32 acts to guide the eccentric ring 16 and also seals off the groove to form a closed oil chamber. Oil is supplied to this chamber through passages (not shown) leading from the crank pin, through the big end bearing shells (which are fitted between the ring 16 and the crank pin) and passages in the ring 16 itself.Oil present in this chamber must be pumped from one side of the connecting rod to the other across the projections 28 and 38 and this will damp the oscillation of the eccentric ring within the connecting rod under the action of the dynamic forces which will be described below.

In operation, the position adopted by the eccentric ring 16 is determined by the forces acting between the connecting rod and the crank pin and the force of the springs 20, 22. If the engine is stationary and when the net force on the connecting rod, the springs 20, 22 tend to force the point of maximum eccentricity to the 3 o'clock position as viewed in Figure 1. When the engine is operating, however, a dynamic force acts between the connecting rod 10 and the crank pin which tends to realign the eccentric ring 16 and dynamically vary the effective length of the connecting rod 10.

There are three main components to the dynamic force acting between the connecting rod 10 and the crank pin.

First, the piston and connecting rod assembly has inertia and there is a reaction force when trying to make it reciprocate in the cylinder bore. This force itself has components arising from the rotating mass and the reciprocating mass of the assembly. The sum of these forces varies cyclically with crank angle being maximum upwards at top dead centre, maximum downwards at bottom dead centre of the crankshaft and zero at some points part way up and down the bore.

The second force acting upon the piston and connecting rod assembly is the reaction force from the compression of the gases in the combustion chamber. This force varies in magnitude 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 eccentric ring 16 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 springs 20, 22. The torque which causes this rotational motion is caused by the fact that the net force acts through the centre of the eccentric ring 16 which is offset from the centre of the crank pin. 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 between the eccentric ring 16 and the connecting rod 10. The eccentric ring 16 will therefore naturally oscillate about its spring biased rest position as the engine operates in its various strokes.

The inertial force alone 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 connecting rod relative to the crank pin 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 eccentric ring 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 effective length of the connecting rod varies dynamically during an engine cycle whereas in the 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 oil damping mentioned above further assists in minimising erratic movement and wear.

A further effect of the oil damping is to counteract the inertial forces if they should become excessive at high engine speeds, given that they vary as the square of the engine speed.

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 connecting rod and big end bearing assembly and does not add significantly to the component count nor to the complexity of assembly.

Claims (6)

1. A reciprocating piston internal combustion engine having an eccentric ring arranged between each connecting rod and its crank pin and means for applying a biassing torque to urge the point of maximum eccentricity on the eccentric ring to a rest position lying on a line transverse to the longitudinal axis of the connecting rod, the eccentric ring being free to oscillate about the rest position as a result of the forces acting between the connecting rod and the crank pin 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 radial projections on the outer surface of the eccentric ring and the inner surface of the connecting rod.

3. An internal combustion engine as claimed in claim 2, wherein two compression springs are provided applying biassing torques in opposite senses act between the connecting rod and the eccentric ring.

4. An internal combustion engine as claimed in claim 3, wherein the springs are received in an annular groove defined between the connecting rod and the eccentric ring.

5. An internal combustion engine as claimed in claim 4, wherein the groove is sealed and filled with oil and acts as part of damping means for oil damping the movement of the eccentric ring relative to the connecting rod.

6. An internal combustion engine constructed, arranged and adapted to operate substantially as herein described with reference to, and as illustrated in, the accompanying drawings.