GB2148144A

GB2148144A - Thermally active coating

Info

Publication number: GB2148144A
Application number: GB08327882A
Authority: GB
Inventors: Mervyn Charles Lovell; Dr Peter John Moss
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1983-10-18
Filing date: 1983-10-18
Publication date: 1985-05-30
Also published as: GB8327882D0; GB2148144B

Abstract

The surfaces of a heat transfer surface are coated with a material which undergoes a transition with increasing temperature such that its thermal conductivity increases. The thermal activity of the coating will assist in the cold start performance of i.c. engines, giving reduced warm-up times and exhaust emissions.

Description

SPECIFICATION Thermally active coating The present invention relates to thermally active coatings particularly where such thermally active coatings are used to control temperatures within an internal combustion engine.

The combustion zone of an internal combustion engine is necessarily surrounded by a structure designed to resist the thermal and mechanical stresses associated with the production of power by the engine. This structure may be either directly cooled by air or indirectly by air with water as a transfer medium. Whichever cooling method is used, the cooling system is used not only to prevent parts of the structure becoming overheated, but also to ensure that all parts of the structure are maintained at a suitable operating temperature.

When an internal combustion engine is started from cold, however, the structure and the cooling system act as a thermal sink preventing the rapid attainment of the operating temperature of the cylinder wall, head and piston crown. Operation of the engine with these components below their optimum operating temperatures will result in increased wear of those components and increased fuel consumption, which leads to increased levels of pollutants in the engine exhaust.

An object of the present invention therefore is to provide a thermally active coating on the cylinder wall, head and piston crown to assist in the rapid attainment of the optimum operating temperature by the surface of these components, with no reduction in the required rate of cooling when the optimum operating temperature has been achieved.

According to the present invention there is provided a thermally active coating on a heat transfer surface for use in controlling the heat flow from a variable temperature heat source, the thermally active coating having a thermal conductivity below a transition temperature contributed to substantially by lattice vibrations and a thermal conductivity above a transition temperature contributed to by lattice vibrations and conduction electrons.

The ratio of the thermal conductivities above and below the transition temperature may be varied by varying the grain size and the grain boundary composition of the thermally active medium. These variations may be produced during growth of the medium, or during subsequent treatment.

Athermally active coating for the internal walls of a combustion chamber, the coating having a lower thermal conductivity than the combustion chamber walls below a transition temperature and an increased thermal conductivity above a transition temperature, the transition temperature being lower than the steady state operating temperature of the combustion chamber walls.

The thermally active coating may comprise a transition-metal compound, a mixture of transitionmetal compounds or silver compounds.

The thermally active coating may be one or more of the oxides of vanadium.

The thermally active coating may comprise a thermally inactive binder incorporating a thermally active material or mixture of materials.

An embodiment of the invention will now be described by way of example only with reference to the following drawings, in which: Figure 1 shows in cross section a schematic representation of the combustion zone of a spark ignition engine with the piston at bottom dead centre.

In Figure 1,the piston 1, connected buy a gudgeon pin 2 to a connecting rod 3 for transmission of motion to and from the crankshaft (not shown), is at the bottom dead centre (B D C) position in the cylinder 4. At the B D C position on the induction stroke the inlet valve 5 would usually be at least partly open while at B DC on the power stroke the exhaust valve 6 would usually be at least partly open. Both inlet and exhaust valve are shown closed for clarity. The valves are contained within the head 7 which also contains inlet 8 and exhaust 9 tracts a spark plug 10 and cooling water channels 11. The head is sealed to the block 12 surrounding the cylinder by an interposed gasket 13.The block contains water cooling channels 14, linked to the channels in the head 11 by further water cooling channels (not shown).

The combustion zone comprises the volume bounded by the interior of the head and valves, cylinder and piston crown 15. These surfaces are provided with a coating 16ofV02, indicated buy a dashed line. The VO2 coating is thermally active and exhibits a low thermal conductivity below 70"C and a high thermal conductivity above 70"C. When the engine is started or run with the VO2 coating at a temperature of 70"C and below, the VO2 coating provides a resistance to the flow of heat from the gas in the combustion zone to the thermal sink of the surrounding structure and ensures that the surfaces bounding the combustion zone rise rapidly to a temperature of 70"C reducing wear on the moving parts in the combustion zone and promoting reduced fuel consumption while the engine warms. When the VO2 coating has reached 70"C the transition to a high thermal conductivity state takes place allowing heat transfer from the combustion zone to the cooling water, while the surface of the combustion zone remains at a temperature of at least 70"C.

It will thus be seen that the presence of the thermally active coating provides a means for reducing the deleterious effects of operating an engine with the combustion zone walls in contact with a large low temperature thermal sink.

Persons skilled in the art will be aware that the action of the thermally active coating is analogous to the action of a thermostat in an engine water cooling system, which promotes rapid warming of the coolant adjacent to the combustion zone before allowing excess heat to be carried by the circulation of coolant to the radiator. It will also be clear that a thermally active coating of the type described above will be useful as a coating on the exterior of the heat transfer fins of an air cooled engine or the surface of a radiator for a water-air heat exchanger, or any heat exchanger where heat has to be extracted from a source at a controlled temperature.

Claims

1. A thermally active coating on a heat transfer surface for use in controlling the heat flow from a variable temperature heat source the thermally active coating having a thermal conductivity below a transition contributed to substantially by lattice vibrations and a thermal conductivity above a transition temperature contributed to by lattice vibrations and conduction electrons.

2. A thermally active coating as claimed in claim 1, the ratio of the thermal conductivity above the transition temperature to that below the transition temperature being variable with the grain size and the grain boundary composition of the thermally active coating.

3. Athermally active coating for the internal walls of a combustion chamber, the coating having a lower thermal conductivity than the combustion chamber walls below a transition temperature and an increased thermal conductivity above a transition temperature, the transition temperature being lower than the steady state operating temperature of the combustion chamber walls.

4. A thermally active coating as claimed in claim 2 or claim 3 comprising a transition metal compound, our a mixture of transition metal compounds or silver compounds.

5. A thermally active coating as claimed in claim 2 or claim 3 in which the coating is one or a mixture of the oxides of vanadium.

6. A thermally active coating as claimed in claim 2 or claim 3 in which the coating comprises a thermally inactive binder incorporating a thermally active material or mixture of thermally active materials.

7. A combustion chamber of an internal combustion engine having a thermally active coating substantially as hereinbefore described and as illustrated with reference to Figure 1.