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.
It is not feasible to fit monolithic ceramic cylinder liners into
existing engines having cast iron liners due to the strength limitations of most
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
wherein the composite ceramic sleeve comprises an inner sleeve of a first ceramic
material having deposited on the outer diameter thereof a layer of a second ceramic
material having a lower coefficient of thermal conductivity than the first ceramic.
The composite sleeve may comprise, for example, a sleeve of silicon
nitride having on the outer diameter thereof a layer of zirconia.
The zirconia may, for example, be deposited by plasma spraying.
In such a construction the silicon nitride sleeve provides the running
surface of the cylinder and has the necesssary 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.
Referring now to Figure 1 where 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) thereon.
The layer 14 may be formed by plasma spraying, for example. The composite
sleeve 12 is shrink-fitted to the steel sleeve 11. The sleeve 11 is first heated
to 260°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°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
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.