GB2189005A - Engines - Google Patents

Abstract

An engine includes a ceramic piston 10 reciprocating within a ceramic cylinder 11. Since ceramic materials have much lower coefficients of thermal expansion than the metals of which pistons and cylinders are customarily made, the clearance between the ceramic piston and the ceramic cylinder is small, possibly 25 microns. Upper and lower piston rings 18, 20 are provided which, together with the piston skirt 15, and auxiliary seals define enclosed regions or cavities 24 on both the thrust and non-thrust faces of the piston. A passage 23 is provided for the supply of a pressurised gas, such as air, to the cavity to provide supporting gas film for the motion, as the air leaks past the upper and lower piston rings. The gas can be supplied from the combustion chamber or from a separate source. <IMAGE>

Description

SPECIFICATION Engines The invention relates to engines and in particular to engines in which a ceramic piston reciprocates with a ceramic liner.

The terms "ceramic piston" and "ceramic cylinder", as used throughout this specification, refer to these components when made wholly of ceramic materials and also when made with only their working surfaces of ceramic materials; for example when the ceramic material is coated on metal surfaces.

In recent years, much attention has been directed to the use of engines including ceramic piston and cylinder arrangements. The benefit of such arrangements is that they reduce the loss of heat to the coolant, so allowing combustion and exhaust temperatures to be increased, so improving the efficiency of the engine and reducing fuel consumption. This also allows the cooling system to be of reduced capacity or to be eliminated. The efficiency of such engines is also improved by the lower coefficients of thermal expansion of the ceramic materials, as compared with metals, which allow ceramic pistons to be run in ceramic cylinders with clearances which are much less than the clearances of metal pistons in metal cylinders, so reducing the incidence of piston slap.

As a result of the elevated operating temperatures generated with a ceramic piston and a ceramic cylinder, conventional oii lubrication systems do not provide satisfactory iubrication of such ceramic pistons and cylinders. There is a need, therefore,. for a system especially adapted to suit engines having ceramic pistons and cylinders.

According to a first aspect of the invention, there is provided an internal combustion engine comprising a piston of a ceramics material reciprocable within a cylinder of a ceramics material, the piston having a crown and a generally annular skirt and carrying at least two piston rings which engage the associated cylinder, at least one ring being towards the crown end of the piston and at least one ring being towards the lower edge of the skirt, a supply line passing through the engine and terminating at one end in the cavity between the piston and the cylinder bounded by the piston rings and terminating at an opposite end at a source of pressurised gas for passage to said space.

The source of pressurised gas may be a combustion chamber formed by the cylinder above the crown, the passage leading from the combustion chamber through the piston to said cavity, for the supply to said space of combustion gas under pressure, said passage including a non-return valve preventing return flow from said cavity to the combustion chamber.

The crown may be formed with a shaped combustion bowl, said passage leading from said combustion bowl to said cavity.

The piston may have a gudgeon pin bore, in which case, two passages may be provided which lie in a common plane including the piston axis and normal to the gudgeon pin bore axis.

In an alternative embodiment, a pump or compressor may form the source of gas, the pump or compressor being connected by said passage to said cavity.

In this case, and where the piston is carried at one end of a connecting rod, the other end of which is connected to a crankshaft, the pump or compressor may be external to the piston and cylinder with the passage passing through the crankshaft and the connecting rod to the cavity.

A little end of the connecting rod may be connected to the piston by a gudgeon pin, the portion of the passage in the connecting rod emerging at the little end and the passage continuing as a radial portion extending from an outer surface of the gudgeon pin, which engages the little end, to an axis of the gudgeon pin, the passage then extending along the gudgeon pin axis to emerge in said cavity.

In an alternative embodiment including a pump or compressor, the passage may pass from the cavity through the piston and include linked tubes connected between said portion of the passage within the piston and said pump or compressor, the linkage accommodating reciprocation of the piston.

Where the source of pressurised gas is the combustion chamber, and where the cylinder is formed in an engine block, the passage may extend from the combustion chamber through the engine block to emerge in said cavity.

In any of the foregoing embodiments, two opposed seals may be provided on the piston, each seal extending axially along the skirt between the piston rings and contacting the associated cylinder, to divide said cavity into two semi-cylindrical zones, two passages being provided, each leading to an associated one of the two zones.

According to a second aspect of the invention, there is provided a method of providing a supporting film for the reciprocation of a ceramic piston in a ceramic cylinder, the piston having a crown and a skirt with two piston rings arranged respectively towards the crown and towards a lower edge of the skirt, the method comprising supplying to a cavity between the piston and the skirt and bounded by the piston rings, a fluid under pressure.

Preferably, the supply of fluid is continuous during reciprocation of the piston.

Alternatively, however, the supply of fluid to the cavity may be intermittent.

The following is a more detailed description of some embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a schematic cross-sectibn through a first form of ceramic piston and cylinder arrangement in an internal combustion engine and including a passage for gas, Figure 2 shows, to the left-hand side, a part-section through a second form of ceramic piston and cylinder arrangement in an internal combustion engine including a passsge for gas extending through a connecting rod of the engine, and to the right-hand side, a part-section of said piston and cylinder arrangement in a plane at right angles to the sectional plane of the left-hand side.

Figure 3 shows, to the right-hand side, a third form of ceramic piston and cylinder arrangement in an internal combustion engine and including linked tubes for gas, and, to the left-hand side, a fourth form of ceramic piston and cylinder arrangement in an engine embodying a passage for gas leading through an engine block, and Figure 4 is a cross-section through the piston and cylinder of the embodiments of any one of Figures 1 to 3, showing the provision of a cavity divided by seals.

The engines now to be described with reference to the drawings contain pistons and cylinders which are made of ceramic materials.

As mentioned above, the terms "ceramic pistons" and "ceramic cylinders" cover the case where both the piston and the cylinder are made wholly of ceramics materials or where they have only their working surfaces made of ceramic materials with the ceramic materials being carried on a substrate of, for example, a metal. In either case, the ceramic materials used for the piston and the cylinder may be the same or different.

Any suitable ceramic material can be used for the pistons and cylinders such as reaction bonded silicon nitride, hot pressed silicon nitride, sintered silicon carbide, hot press silicon carbide and partially stabilised zirconia.

The engine can be of any particular type, for example a four-stroke engine, or a twostroke engine. The engine may be in-line or horizontally opposed or of any other configuration and may be a spark ignition or compression ignition engine. The piston needs to be long in relation to the stroke.

Referring first to Figure 1, the engine of this embodiment hasa ceramic piston 10 mounted for reciprocation in a ceramic cylinder 11. The piston has a crown 12 formed with a combustion bowl 13. An upper ring band 14 extends around the crown with a skirt 15 depending from the upper ring band 14 and terminating at a lower ring band 16 which extends around the lower edge of the piston 10.

The upper ring band 14 is provided with two piston ring grooves 17 each containing an associated piston ring 18 which is in sliding contact with the associated cylinder 11.

The lower ring band 16 is also provided with two piston ring grooves 19 each receiving an associated piston ring 20 which is in sliding contact with the associated cylinder 11.

The diameter of the skirt is less than the diameter of the ring bands 14, 16, and the upper and lower piston rings 18, 20, the cylinder 11 and the skirt 15 together define an annular cavity 24 extending around the piston 10.

Since the coefficient of thermal expansion of ceramic materials is substantially less than that of metals, the gap between the piston 10 and the cylinder 11 is substantially less than the gap in conventional metal pistons and cylinders. For example, the gap may be 25 microns or less. Thus, the volume of the cavity 24 is small.

The piston 10 is provided with a pair of gudgeon pin bosses 21 which define a gudgeon pin bore 22 for receiving a gudgeon pin (not shown).

A pair of passages 23 extend through the piston 10 between the combustion bowl and the cavity 24. The passages 23 lie in a common plane including the piston axis 25 and normal to the gudgeon pin bore axis 26 and are angled downwardly and outwardly from the combustion bowl 13 to emerge at the surface of the skirt 15 within the cavity 24.

Each passage 23 contains a non-return ball valve 27.

In use, the ceramic piston 10 reciprocates with in the ceramic cylinder 11 with fuel combustion taking place in the combustion bowl 13 to drive the piston 10. The ball valves 27 are such that the passages 23 open only when combustion has taken place and the pressure in the combustion bowl 13 is at a predetermined high level. When the passages 23 open, combustion gases pass down the passages 23 to the cavity 24 to fill the cavity 24 with combustion gases under pressure.

These gases provide a pressurised supporting gas film between the piston 10 and the cylinder 11 during the reciprocation of the piston 10 and disperse by slow leakage past the piston rings 18, 20.

In view of the very narrow clearance between the piston 10 and the associated cylinder 11, the volume of the cavity is small, as explained above, and the pressure of the combustion gases and their rate of leakage past the piston rings 18 is such that sufficient pressurised gas is always in the cavity 24, even though it is replenished only at one point during each cycle.

Referring next to Figure 2, parts common to Figures 1 and 2 will be given the same reference numerals and will not be described in detail In this engine embodiment, the piston 10 is connected to a crankshaft (not shown) by a connecting rod 30. A little end 31 of the connecting rod engages a gugdeon pin 32 carried by the gudgeon pin bosses 21 (see right-hand side of Figure).

An auxiliary pump or compressor (not shown) is provided whose output is connected to a passage whose initial section passes through the crankshaft to the big end (not shown) of the connecting rod. The passage then includes a section 33a which extends along the connecting rod and emerges at the little end 31. At the point of emergence on the bearing, the passage is then continued by a circumferential section 33b of the little end bearing which is of increased diameter. The passage then continues with the section 33c provided in the gudgeon pin 32 and leading from the curved surface of the gudgeon pin 32 at the increased diameter portion 33b, to the axis of the gudgeon pin 32, where it continues with an axial section 33d which emerges at the end of the gudgeon pin at the cavity 24.

Seals 34 are provided at the axially spaced ends of the little end bearing.

In use, the piston 10 reciprocates in the cylinder 11 in a combustion cycle. The auxiliary pump is driven and supplies air under pressure through the passage 33a from whence it passes around the increased diameter section 33b, through the radial section 33c, along the axial section 33d to enter the cavity 24. The seals 34 prevent leakage of air past the ends of the little end bearing. In this way, the cavity 24 has a constant supply of air under pressure which proyides a supporting gas film between the piston 10 and the cylinder 11 and which leaks out of the cavity 24 past the piston rings 18, 20.

In the embodiments of Figures 1 and 2, the cavity 24 is described as extending around the whole circumference of the piston. It will be appreciated, however, that the two separate generally rectangular pockets could be provided on the thrust and non-thrust surfaces of the skirt respectively. These pockets could be provided with respective perimeter seals, in order to retain the gas within the pockets.

The supply of gas is directed accordingly.

Referring next to the two embodiments shown in Figure 3 of the drawings, parts common to these embodiments and to the embodiments of Figures 1 and 2 will not be described in detail and are given the same reference numerals. In the embodiment of the right-hand side of Figure 3, a separate auxiliary pump or compressor (not shown) is provided, as in the embodiment of Figure 2. In this case, however, the air under pressure is led from the pump to the cavity 24 by a passage formed with a flexible linkage. Thepassage extends through linked tubes 35a, 35b connected by a knuckle joint 36, to accommodate reciprocation of the piston 10.

The second tube 35b is connected by a swivel joint 37 to a continuation 35c of the passage through the body of the piston 10 to emerge in the cavity 24.

An alternative method is to supply air via a passage 35d extending from the auxiliary compressor or pump through the material of the cylinder 11 to emerge at the cavity 24. A similar passage (not shown) is, in this method, provided at the other side of the cylinder.

The cavity is formed by two pockets 24a, 24b, provided on the thrust and non-thrust surfaces of the piston. Seals (not shown) may be provided around the perimeters of the pockets.

in use, as the piston 10 reciprocates in a combustion cycle, air under pressure is supplied continuously through the passage 35a, 35b, 35c, 35d to produce a supply of air under pressure in the pockets 24a, 24b. As with the embodiment of Figure 2, this air provides a pressurised supporting air film between the piston 10 and the cylinder 11 and leaks gradually past the piston rings 18, 20.

The knuckle joint 36 and the swivel joint 37 accommodate the relative movement between the piston 10 and the cylinder 11.

Referring next to the left-hand side of Figure 3, in this embodiment, a passage 38 is provided in a cylinder block 39, in which the cylinder 11 is formed, and leads from a combustion chamber 40 at the top of the cylinder through the cylinder block 39 to the pockets 24a, 24b. A one-way needle valve 41 is provided for preventing reverse flow and for controlling the pressure of the gas provided to the cavity 24. This may be adjusted by means of a spring-loaded adjusting screw 42 which moves the needle valve through a heat insulating push rod 43. This needle valve 41 may be replaced by a ball valve (not shown).

In use, the valve 41 opens during the compression cycle of the engine to admit combustion gases into the pockets 24a, 24b. In view of the small volume of the pockets 24a, 24b, the amount of combustion gas necessary to maintain a required pressure within the pockets 24a, 24b, is small and does not affect significantly the operation of the engine.

As in the embodiments of Figures 1, 2 and the left-hand side of Figure 3, this gas under pressure provides a pressurised gas film between the piston 10 and the cylinder 11 and leaks past the piston rings 18, 20.

Since the angle of the connecting rod relative to the piston axis 25 varies throughout the engine cycle, the connecting rod 30 exerts a lateral force on the piston which tends, during upward movement of the piston, to urge the piston 10 towards one side of the cylinder 11, and which, as the piston moves downwardly, urges the piston 10 towards an opposite side of the cylinder 11. The piston may assume various degrees of tilt and attitude with respect to the cylinder at various points in the cycle, depending on inertia, friction, and the sinematics of the system. This force acts in a direction normal to the plane including the piston axis 25 and the gudgeon pin bore axis 26. The effect of this is to reduce the clearance on the side of the piston 10 urged towards the associated cylinder 11 and to increase the clearance on the opposite side.

This has the disadvantage that air under pressure within the pockets 24a, 24b would reduce in pressure as a result of the increased clearance, since there is greater leakage of the air past the piston rings 18, 20 on the side of the piston 10 with the greater clearance.

In order to overcome this, the piston may, as shown in Figure 4, be provided with two seals 44 extending axially along the skirt 15 between the upper and lower ring bands 14, 16. The seals 44 are on diametrically opposite sides of the piston and are disposed about a common plsne including the piston axis 25 and the gudgeon pin bore axis 26.

The effect of these seals 44 is to divide the cavity 24 into two zones on opposite sides of the plane including the piston axis 25 and the gudgeon pin bore axis 26. Two passages 45 are provided for supplying fluid. The result of this is that any leakage of air under pressure from one side of the piston 10, during reciprocation, affects the pressure in one zone only and the pressure in the other zone remains unaffected.

It will be appreciated that there are other ways in which a lubricating fluid under pressure could be supplied to the cavity 24. For example, the passage could terminate in an open end which, as the piston 10 approaches bottom dead centre, enters an outlet of a supply of air under pressure, and opens a valve which allows the air under pressure to enter the passage and pass to the cavity. As with the embodiment of Figure 1, the intermittent replenishment of the cavity 24 is not a problem because the leakage of air past the piston rings 18, 20 is not substantial.

It will also be appreciated that the auxiliary pump or compressor of the embodiments of Figures 2 and 3 (righthand side) may be driven in any required way. For example, it can be driven off the engine transmission or it could be driven by reciprocation of the piston 10.

Claims (15)

1. An internal combustion engine comprising a piston of a ceramics material reciprocable within a cylinder of a ceramics material, the piston having a crown and a generally annular skirt and carrying at least two piston rings which engage the associated cylinder, at least one ring being towards the crown end of the piston and at least one ring being towards the lower edge of the skirt, a supply line passing through the engine and terminating at one end in a cavity between the piston and the cylin- , der bounded by the piston rings, and terminating at an opposite end at a source of pressurised gas for passage to said space.

2. An engine according to claim 1 in which the source of pressurised gas is a combustion chamber formed by the cylinder above the crown, the passage leading from the combustion chamber through the piston to said cavity, for the supply to said space of combustion gas under pressure, said passage including a non-return valve preventing return flow from said cavity to the combustion chamber.

3. An engine according to claim 2 wherein the crown is formed with a shaped combustion bowl, said passage leading from said combustion bowl to said cavity.

4. An engine according to claim 2 or claim 3 and in which the piston has a gudgeon pin bore, two passages being provided which lie in a common plane including the piston axis and normal to the gudgeon pin bore axis.

5. An engine according to claim 1 in which a pump or compressor is the source of gas, the pump or compressor being connected by said passage to said cavity.

6. An engine according to claim 5 and in which the piston is carried at one end of a connecting rod, the other end of which is connected to a crankshaft, the pump or compressor being external to the piston and cylinder with the passage passing through the crankshaft and the connecting rod to the cavity.

7. An engine according to claim 6 wherein a little end of the connecting rod is connected to the piston by a gudgeon pin, there being a portion of the passage in the connecting rod which emerges at the little end, the passage continuing as a radial portion extending from an outer surface of the gudgeon pin, which engages the little end, to an axis of the gudgeon pin, the passage then extending along the gudgeon pin axis to emerge in said cavity.

8. An engine according to claim 5 wherein the passage passes from the cavity through the piston and includes linked tubes connected between said portion of the passage within the piston and said pump or compressor, the linkage accommodating reciprocation of the piston.

9. An engine according to claim 1 wherein the source of pressurised gas is the combustion chamber, the cylinder being formed in an engine block with the passage extending from the combustion chamber through the engine block to emerge in said cavity.

10. An engine according to any one of claims 1 to 9 wherein two opposed seals are provided on the piston, each seal extending axially along the skirt between said piston rings bounding the cavity and contacting the associated cylinder, to divide said cavity into two semi-cylindrical zones, two passages being provided, each leading to an associated one of the two zones.

11. An internal combustion engine substantially as hereinbefore described with reference to any one of Figures 1 to 3 or to any one of Figures 1 to 3 as modified by Figure 4 of the accompanying drawings.

12. A method of providing a supporting film for the reciprocation of a ceramic piston in a ceramic cylinder, the piston having a crown and a skirt with two piston rings arranged respectively towards the crown and towards a lower edge of the skirt, the method comprising supplying to a cavity between the piston and the skirt and bounded by the piston rings, a fluid under pressure.

13. A method according to claim 12 wherein the supply of fluid is continuous during reciprocation of the piston.

14. A method according to claim 12 wherein the supply of fluid to the cavity is intermittent.

15. A method of providing a supporting film for the reciprocation of a ceramic piston in a ceramic cylinder substantially as hereinbefore described with reference to any one of Figures 1 to 3 or to any one of Figures 1 to 3 as modified by Figure 4 of the accompanying drawings.