CYLINDER LINERS

Abstract: Cylinder liners (10) for internal combustion engines, particularly diesel engines, are described the liners comprising a metal outer sleeve (11) having therein an inner sleeve (13) comprising a ceramic material which is held by compressive hoop stresses under all normal engine operating conditions, the ineer sleeve (13) extending over at least a part of the axial length of the liner. In a preferred embodiment the inner sleeve (13) comprises a composite material of two different ceramic materials. For example an inner sleeve (13) is described having a silicon nitride working surface with a layer (14) of partially stabilized zirconia thereon.

Description: The present invention relates to cylinder liners for internal combustion engines and particularly to composite cylinder liners comprising metals and ceramic material for insulation purposes. It is well known that the thermal efficiency of an internal combustion engine may be improved by reducing the quantity of heat rejected from the engine during running. One method of achieving such a reduction is to improve the heat insulating ability of the cylinder. The use of ceramic cylinder liners has been proposed but ceramic materials suffer from being unreliable when subjected to tensile stresses. GB-A-643435 describes the use of a ceramic sleeve on to which a metal outer sleeve is shrink-fitted. However, the use of a single ceramic material means that it is difficult to achieve both good insulating properties and an acceptable running surface for the cylinder liner. It is not feasible to fit monolithic ceramic cylinder liners into existing engines having cast iron liners due to the strength limitations of most ceramics. EP-A-0110488 describes the use of two concentric sleeves of ceramic material which may be fitted within a metal sleeve. The ceramic sleeves are slidable relative to one another to relieve stresses. The ceramic sleeves are preferably of the same material but may be of different materials. Such an arrangement requires very accurate machining to allow the sleeves to be fitted one within another. It is an object of the present invention to provide a cylinder liner having the running surface with the necessary properties of good thermal shock-resistance and corrosion-resistance while providing an enhanced coefficient of thermal conductivity and which can be easily manufactured. According to the present invention a cylinder liner for an internal combustion engine comprises a metal outer sleeve having therein a composite ceramic sleeve extending over at least a part of the length of the liner and which composite sleeve is held by compressive hoop stresses under all normal engine operating conditions characterised in that the composite ceramic sleeve comprises an inner sleeve of a first ceramic material having deposited on the outer surface thereof by plasma spraying a layer of a second ceramic material having a lower coefficient of thermal conductivity than the first ceramic material. The composite sleeve may comprise, for example, a sleeve of silicon nitride having on the outer diameter thereof a layer of zirconia. In such a construction the silicon nitride sleeve provides the running surface of the cylinder and has the necessary properties of good thermal shock-resistance and corrosion-resistance whilst the zirconia layer provides much enhanced thermal insulation due to the coefficient of thermal conductivity of the plasma sprayed material being approximately 10% that of silicon nitride. Preferably the metal outer sleeve is made of steel. The composite ceramic sleeve may be shrink-fitted into the metal sleeve by heating of the latter sleeve. The mechanical interference between the outer sleeve and the ceramic sleeve should be such that the ceramic sleeve is held under compressive stress for all engine operating conditions. In order that the present invention may be more fully understood examples will now be described with reference to the accompanying drawings of which: Figure 1 shows a section, including the axis, through a cylinder liner according to the present invention; and Figure 2 shows part of a second embodiment of a cylinder liner according to the present invention. Reference is now made to Figure 1 in which the cylinder liner is shown generally at 10. The cylinder liner comprises a steel outer sleeve 11 having a composite ceramic sleeve 12 therein extending over the working length of the cylinder liner and which composite sleeve itself comprises two materials. The sleeve 12 comprises an inner cylinder sleeve 13 of reaction-bonded silicon nitride (RBSN) having a layer 14 of partially stabilised zirconia (PSZ) on the outer surface thereof. The layer 14 is formed by plasma spraying. The composite sleeve 12 is shrink-fitted to the steel sleeve 11. The sleeve 11 is first heated to 260 degrees C whereupon the inner sleeve 12 is inserted so that the lower edge 16 of the sleeve 12 abuts the step of the inwardly turned flange 17. The degree of hoop stress imposed by the steel sleeve 11 on the ceramic sleeve 12 is very high in the cold condition and may be close to the yield point of the steel and the compressive strength of the ceramic sleeve. This ensures that the ceramic is held in compression even when the steel has heated up during running of the engine. A cylinder liner as described above having a bore of about 80mm, an 817M40 to BS970 steel sleeve 11 of wall thickness about 6mm, an RBSN wall thickness of 4mm and a PSZ wall thickness of 2mm was constructed for a single cylinder diesel test engine. The diesel engine used for the test normally has a cast iron cylinder liner. The engine was further equipped with a monolithic ceramic piston and a partially insulated cylinder. Running temperatures of the cylinder liner in the upper cylinder region were increased by 130 degrees C, an increase of 60% over the conventional cylinder liner when using the same piston and cylinder head. During one test of the engine the piston failed leaving the connecting rod to thrash about in the cylinder. No damage other than scoring of the cylinder liner was caused. The cylinder liner on the running surface may be treated with, for example, a chromium oxide containing material as a porosity sealant to improve fuel consumption and to reduce friction between the rings and liner. Although the invention has been described with reference to RBSN and PSZ other ceramics may of course be used and may, for example, include sintered silicon nitride, sialon or silicon carbide. As is shown in Figure 2 the composite cylinder sleeve 12 may extend only partially along the total cylinder length. The liner may be in the form of an internal cuff ending above the extent of travel of the topmost piston ring (not shown). The steel used for the outer sleeve 11 will be dependent upon the relative thicknesses of the metal outer sleeve and the composite ceramic sleeve and the stress which it is desired to impart to the ceramic. Where the outer sleeve is relatively thick a mild steel may suffice.

Claims: A cylinder liner (10) for an internal combustion engine, the liner comprising a metal outer sleeve (11) having therein a composite ceramic sleeve (12) extending over at least part of the length of the liner and which composite sleeve is held by compressive hoop stresses under all normal engine operating conditions characterised in that the composite ceramic sleeve comprises an inner sleeve (13) of a first ceramic material having deposited on the outer surface thereof by plasma spraying a layer (14) of a second ceramic material having a lower coefficient of thermal conductivity than the first ceramic material. A cylinder liner according to Claim 1, characterised in that the composite ceramic sleeve extends over the working length of the liner. A cylinder liner according to either Claim 1 or Claim 2, characterised in that the outer sleeve is made of steel. A cylinder liner according to any one preceding claim, characterised in that the inner sleeve of the composite ceramic sleeve comprises silicon nitride. A cylinder liner according to any one preceding claim, characterised in that the layer of second ceramic material comprises zirconia. A cylinder liner according to any one preceding claim, characterised in that the composite ceramic sleeve is shrink-fitted inside the metal outer sleeve.

Inventors: EVANS, DAVID CHARLES, THORPE, WILLIAM ANTHONY
Applicants: AE PLC
Application year: 1988
Main classification: F02F1
IPC codes: F02F7, F02B3, F16J10