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Heat transfer

valvesWith the dominance of four-stroke engines in racing, and the near-monopoly that poppet valves have in such engines, it is little surprise that these parts have become well developed. While most racing is still based on naturally aspirated engines, it is likely that a much greater proportion of race engines in future will be turbocharged, following the continuing trend in series production engines toward smaller boosted power units. There are compelling reasons for doing this; not only is engine efficiency improved, but a small, light engine

requires a smaller supporting structure, and the resulting car is much lighter and more fuel efficient, and requires less outright engine output to perform well.

By turbocharging any engine, we increase cylinder pressures and temperatures, and in doing so we make increased demands on a number of components, from spark plugs to pistons to bearings and valves. The increased operating temperatures for valves in turbocharged engines means that material selection and management of heat become more critical.

Many turbocharged engines will use valves made of high-temperature 'superalloy' materials. Inconel and Nimonic are commonly used, and these superalloy materials retain a large proportion of their room-temperature mechanical properties at high temperature. Beyond certain temperature limits, the more usual valve materials, such as titanium and the martensitic and austenitic steels lose a large proportion of their ambient temperature mechanical properties, limiting engine performance. There are a lot of high-temperature materials to choose from, and motorsport benefits from the development of materials for aero gas turbine engines by seeing new materials coming through which are suitable for our use.

The management of heat in the valve, in any type of race engine, is critical to the durability of the component. Hollow valves with an internal coolant are very successfully used in many race series to remove heat from the valve head, transferring it to the valve guide and thence to the engine coolant via the cylinder head. The internal coolant is often pure metallic sodium, although others have been used with success.

Some valve guides are directly liquid-cooled, intersecting liquid-cooling channels in an attempt to make the transfer of heat more efficient.

In valves with no internal coolant, a large proportion of heat energy is transferred via the valve seat interface during the period when the valve is closed. A wider seat improves heat transfer, but this can be at the expense of flow coefficients. The choice of seat insert material is important, and materials combining good strength with high thermal conductivity are ideal candidates.

As with much in engine design though, improvements made to the cooling of the valve do not come without penalty. Improved cooling via the valve seat causes the tensile stresses in the periphery of the valve head to increase, and this can have an effect on the durability of the valve, and in some cases can lead to the appearance of fatigue cracks.

A number of people are reported to have used, or to be using, thermal barrier coatings on the face of the valve to minimise the heat transfer through the valve and to reduce valve operating temperatures.

Fig. 1 - This valve has a hollow stem; partly filling this with a coolant (typically sodium) improves the transfer of heat from the valve head (Courtesy of Zanzi SpA)

Written by Wayne Ward

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