The present invention relates to cylinders for internal combustion
engines and particularly to such cylinders manufactured from ceramic materials.
It is desirable to be able to manufacture some components of internal
combustion engines, especially diesels, from ceramic material. Such components
include the pistons and cylinders of cylinder liners which may be subject to the
action of hot combustion gases and abrasive fuels. It is generally considered
that the ability to use ceramic materials for some components will allow higher
temperatures to be employed in the combustion chamber region, such higher temperatures
being advantageous in lowering noxious emissions from the engine.
Another advantage of some ceramic materials is the ability to operate
without the need for external lubrication at high temperatures.
The cylinder of an internal combustion engine has to withstand high
mechanical stresses due to the gas pressures generated by the burning fuel charge
at the moment of ignition and also very high thermal stresses due to temperature
fluctuations over short time periods.
Not only are such stresses continuously varying with time in the
region of the cylinder near to the combustion chamber but they are also varying
depending on position in all other parts of the cylinder. There is, for example,
a stress gradient due to gas pressure variations throughout the working cycle
and also a thermal stress gradient due to temperature variation from top to bottom
of the cylinder.
Such stress variations especially when combined with physical discontinuities
in ceramic cylinder components can lead to catastrophic failures. For example,
in the case of a diesel engine having horizontally opposed pistons where a fuel
injector port is sited in the centre of the axial length of the cylinder, it has
been found that failure of the cylinder occurs by cracks emanating from the fuel
injector port which acts as a stress concentrator.
Ceramic materials are generally better at withstanding compressive
stresses than tensile stresses. In order to minimise the tensile stresses imposed
upon the ceramic material of the cylinder it has been proposed to fit the complete
ceramic cylinder inside a metal supporting sleeve by interference or shrink fitting.
A problem with this is that due to the thermal and mechanical stresses involved
and to differences in the coefficient of thermal expansion between the metal sleeve
and the ceramic piece the ceramic liner invariably works loose.
According to the present invention a ceramic cylinder for an internal
combustion engine comprises at least two portions including at least one main cylinder
portion and at least one combustion region portion the at least two portions being
contained within a metal housing and wherein the at least one combustion region
portion has a generally axially directed slit therein, the slit passing through
the whole thickness of the ceramic material.
The portion of liner in the combustion region has to withstand the
highest thermal and mechanical stresses. The slit allows this portion to deform
elastically in compression and to accommodate any expansion effects in the supporting
metal housing without loosening.
The metal housing may comprise a metal sleeve.
Where necessitated by the specific engine design, features such as
injection ports etc. may be sited on the line of the slit thereby minimising any
stress raising tendencies such features may otherwise promote.
Stresses due to gas pressure within the cylinder are borne in compression
since the combustion chamber portion of the cylinder liner is able to be fully
supported by the metal housing without the generation of significant tensile stresses.
In order that the invention may be more fully understood examples
will now be described by way of illustration only with reference to the accompanying
drawings, of which:
- Figure 1 shows a schematic perspective view of a combustion chamber region
portion of a ceramic cylinder liner according to the present invention for an
opposed-piston 2-stroke diesel engine;
- Figure 2 shows a section in elevation of a ceramic cylinder liner assembly
according to the present invention;
- Figure 3 shows a section through the line AA¹ of the cylinder liner of
Figure 2 looking in the direction of the arrows;
- Figure 4 shows a section in elevation of a schematic representation of another
type of reciprocating piston diesel engine having a cylinder according to the present
- Figure 5 which shows a section through the line BB¹ of Figure 4 looking
in the direction of the arrows.
Referring now to Figures 1 to 3 and where the same features are denoted
by common reference numerals.
A complete cylinder liner assembly is denoted at 10. The assembly
comprises a steel outer sleeve 11, two outer main cylinder portions 12, 13 and
a combustion region portion 14. One outer portion 12 includes exhaust ports 15
which pass through co-operating ports 16 in the sleeve 11 whilst the other outer
portion 13 includes inlet ports 17 which similarly co-operate with ports 18 in
the sleeve 11. The combustion region portion 14 includes a fuel injection port
19 which is situated in an axial slit 20 which passes through the wall thickness
of the portion 14.
The portions 12, 13 and 14 are made from silicon nitride material
produced by the reaction bonding process. Other materials may of course by used
including, for example, sintered silicon nitride, Syalon (trade mark), silicon
The portion 14 is compressed such that the opposite faces of the
slit 20 approach each other and is permanently elastically deformed whilst in the
sleeve 11 even allowing for temperature effects. Siting of the injection port 19
in the slit 20 prevents the port from becoming a significant stress-raising feature.
Because the portion 14 is permanently in compression harmful tensile forces are
never able to develop whatever the temperature and gas loading conditions.
The ceramic liner portions are assembled into the sleeve 11 and any
steps present at the interfaces between the portions are removed by a grinding
operation which results in the cylinder achieving the desired finished bore dimension.
Any slight step which may be generated during running may be accommodated by appropriate
tapers or chamfers on the associated pistons (not shown) and piston rings (not
shown) if used. To allow for inevitable variations in differential expansion between
the steel sleeve and ceramic cylinder portions, heat degradable spacers may be
positioned between the cylinder portions. Plastics materials such as acrylic plastic,
for example, may be used. Such shims or spacers burn away in operation to leave
the desired axial spacing of the ceramic cylinder portions. The ash produced is
ejected via the exhaust ports.
The portion 14 may be of relatively short axial length since the
rate of gas pressure and temperature reduction along the cylinder axial length
is very high. The portions 12 and 13 are fully able to accommodate the gas pressure
and temperature variations without the need for an axial slit since the average
levels of pressure and temperature are much lower. Furthermore, there is no extra
gas leakage due to the slit since the effect of the slit is only to add a slight
additional volume to the combustion chamber, the slit being effectively sealed
at the interfaces between the three portions.
Figures 4 and 5 show a cylinder in an engine the cylinder comprising
a cast iron support 40 formed by the engine block, a main cylinder portion 41 and
a combustion region portion 42. The combustion region portion 41 comprises a slit
43 in the axial direction. Fuel injection means (not shown) are included in the
cylinder head 44. A piston 45 traverses the joint interface 46 between the two
portions 41 and 42.