GB2223292A

GB2223292A - Pistons

Info

Publication number: GB2223292A
Application number: GB8822893A
Authority: GB
Inventors: Roger Harvey Slee
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1988-09-29
Filing date: 1988-09-29
Publication date: 1990-04-04
Anticipated expiration: 2008-09-29
Also published as: GB8822893D0; GB2223292B

Abstract

A variable compression ratio piston of multipart construction in which a centre crown portion of the piston is movable relive to the remainder of the piston to facilitate the compression ratio variations.

Description

Improvements in and relating to pistons This invention relates to pistons, and in particular to a piston incorporating a device to facilitate a variable compression ratio.

The value of variable compression ratio (VCR) devices in alleviating the damaging effects of very high cylinder pressure on the bearings and cranked drive scantlings of highly rated four stroke diesel engines has long been known.

Various features capable of providing a VCR effect have previously been described. Some features involving variable length connecting rods or variable stroke cranks (via eccentric bearings) do not readily provide a viable VCR solution for highly rated multicylinder diesel engines, whilst designs involving variable height cylinder heads or variable-volume cylinder head sub chambers, adversely influence gas flows, turbulence and flame propagation.

A piston incororating VCR features is described in German Patents Nos. 1,062,981 and 1,256,941 using a system of controlled oil flow to raise or lower the outer jacket of an essentially two-piece piston, relative to an inner member fixedly attached to the connecting rod by a gudgeon pin in the usual manner.

The oil flow used to actuate the VCR feature is obtained from the engine lubrication system via the connecting rod, and subsequently transferred via an oil collector to check valves in an upper or lower control chamber. These check valves may be of the spring assisted non-return type, allowing only an oil flow into the chamber.

If excessive combustion gas chamber pressure is encountered, to allow the piston outer jacket to be lowered relative to the piston inner member oil is discharged from the upper control chamber via a discharge valve to the crankcase. This discharge valve, therefore acts as a pressure relief valve, and may be preset to 'crack' or open at a pressure corresponding to the maximum gas pressure permissible.

Obviously during steady state running the piston outer jacket progressively hunts relative to the piston inner member over the course of a few cycles. Thus, for a load increase the piston height with respect to the gudgeon pin is effectively lowered, and conversely raised for a load decrease.

These features have, however, a serious disadvantage, in that diesel combustion is heavily reliant upon intense incylinder turbulence.

Simply using the variable height piston described above causes a severe loss in turbulent mixing of the fuel and air under high load conditions, since the piston crown does not rise completely to the top of the cylinder under these conditions; hence, the annulus of air surrounding the combustion chamber cavity remains relatively quiescent, slowing combustion and leading to the fomulation of NO emissions which are difficult to treat x by subsequent exhaust reaction.

We have now found a means of achieving the advantages of a variable height piston without the disadvantages of the previous known constructions.

Thus according to the present invention a piston for an internal combustion engine is of a multi-part construction comprising a piston shell pivotally mounted on a gudgeon pin, and a centre crown portion slidably mounted within the piston shell and movable between a raised position. in which the compression ratio produced in use is at a maximum, and a lowered position in which said compression ratio is at a minimum, the crown of the piston being made up of a central area on said centre crown portion and an annular area on said piston shell.

The centre crown portion is preferably raised and lowered relative to the piston shell by means of an oil chamber beneath it, and may be made of the same material as the remainder of the piston, (eg metal alloy) or may be a quite different material such as a ceramic material.

The piston of this invention has a conventional crown profile when the centre crown portion is in its fully raised position. As the centre crown portion of the piston is lowered the combustion volume within the piston is increased to lower the compression ratio, whilst the piston shell continues to rise completely to the top of the cylinder forcing the annulus of air around the combustion chamber cavity to flow into the combustion chamber cavity, thereby improving turbulence in the combustion chamber.

The invention will now be described, by way of example only, with reference to the accompanying drawings in which Figure 1 is a cross-section of a piston in accordance with the invention.

Figure 2 is a diagram showing the piston profile in its maximum compression configuration and Figure 3 is a diagram showing the piston profile in its minimum compression configuration.

As shown in Figure 1 the piston consists of an outer shell 1, carrying three piston rings 2, an inner member 4 fitting closely into the outer shell 1, and a centre crown member 13 slidably mounted within the outer shell 1.

The outer shell 1 is pivotally mounted on a gudgeon pin 3, carried by a connecting rod 7, and contains a stepped centre bore within which the inner member 4 and the centre crown member 13 are located. A piston ring carrier 16 is incorporated into the outer shell 1 for the topmost piston ring.

The inner member 4 abuts against a shoulder 17 in the stepped bore of the outer shell, and carries fluid sealing rings 11 to provide a tight seal within the bore. A one-way fluid valve 5 and spring-loaded oil connecting shoe 6 are filled within another stepped bore, this one at the centre of the inner member 4, and an adjustable fluid pressure relief valve 12 is also carried by the inner member, in an offset position relative to the other valve.

The oil collecting shoe 6 is urged by spring 8 against a mating machined surface on the end of the connecting rod 7, and the rod is provided with an internal bore 18 and small end bearing bush 9 which allows oil to flow up inside the connecting rod, around the bush 9 and out into the collecting shoe 6.

The centre crown member 13 carries a sealing ring 14 and a gas seal/heat transfer ring 15 which makes a seal against the inner surface of the stepped bore in the outer shell 1. A shoulder 19 in the stepped bore, and corresponding shoulder on the centre crown member 13 serve to act as a stop to mark the maximum extent to which the centre crown member 13 may slide away from the inner member 4. The resulting volume 10 between the two members 4 and 13 is filled with oil entering through one way valve 5 which provides the means by which the centre-crown member is raised and lowered when in use.

In operation, oil is caused to flow up the connecting rod 7 around bearing bush 9 and into variable volume 10 via shoe 6 and valve 5.

As the variable volume 10 fills with oil, the intermediate member 13 is 'jacked up', reaching a maximum compression ratio position when volume 10 is full.

Back-leakage through valve 5 is prevented by a light spring.

If the combustion forces on the firing stroke of the engine produce pressures in excess of those desired, the oil within the variable volume 10 will be subject to correspondingly high pressures, allowing the pressure relief valve 12 to open and bleed some of the pressurised oil in the variable volume 10 to the engine crankcase.

The pressure at which the relief valve opens is preset to an appropriate value.

This bleeding" of the oil out from variable volume 10 allows the centre crown member 13 to descend somewhat relative to the main outer piston shell 1, thus increasing the effective volume available to the combustion charge, and automatically lowering the cylinder compression ratio.

In steady state running, the intermediate member 13 hunts around some notional intermediate position.

The effect on the piston profile of the movement of the centre crown portion is illustrated diagrammatically in Figures 2 and 3. Figure 2 illustrates the cross-sectional profile of the piston 20 with the centre crown portion in its fully raised position, the volume of the combustion chamber 21 with the piston at TDC being at its minimum with the piston in this configuration.

The opposite situation is illustrated in Figure 3, in which the cross-sectional profile of the piston 20 is that resulting from the centre crown portion being in its fully lowered position, and the volume of the combustion chamber 22 can be seen to be considerably greater than that in Figure 2. It can also be seen, however, that because only the centre of the piston crown has moved. the volume of the annulus 23 of gas around the combustion chamber is unaffected, so that squish and turbulence requirements in the combustion chamber can still be fully satisfied.

As has been mentioned above, it is to be expected that during steady state running conditions the centre crown portion of the piston will settle into a location intermediate the extremes illustrated in Figures 2 and 3.

Claims

1. A piston for an internal combustion engine, said piston being a multi-part construction comprising a piston shell pivotally mounted on a gudgeon pin, and a centre crown portion slidably mounted within the piston shell and movable between a raised position in which the compression ratio produced in use is at a maximum, and a lowered position in which said compression ratio is at a minimum, the crown of the piston being made up of a central area on said centre crown portion and an annular area on said piston shell.

2. A piston according to claim 1 in which said centre crown portion has beneath it an oil chamber by means of which the centre crown portion may be raised and lowered relative to the piston shell.

3. A piston according to claim 3 or 2 in which the oil chamber has a one-way valve set to permit oil to enter the chamber and a pressure-relief valve to allow oil to be expelled from the chamber under a predetermined pressure.

4. A piston substantially as described herein with reference to, and as illustrated in, the accompanying drawings.