Powder-metal valve seats
Modern race engines place very harsh demands on valve seats for a number of reasons, all of which are concerned with extracting the absolute maximum from the engine. In comparison to a passenger car engine, as an example, racing seats are expected to deal with a greater amount of heat transfer per unit volume, higher temperatures, greater stresses on valve seating and higher seating velocities. We expect to run thin seat areas in order to improve gas flow characteristics at a given valve lift, and we tend to like shutting valves quickly after keeping them open as long as possible at any given crankshaft angle.
What we need is a material with temperature resistance, high thermal conductivity, thermal expansion which is close to that of the cylinder head and good mechanical properties, especially fatigue resistance. Often, as engineers, we find ourselves ‘painted into a corner’ in one of these areas. We may need higher thermal conductivity but can only find it in a material with lower strength, or we may find another characteristic that we like but find that the thermal expansion coefficient puts the cylinder head at risk due to high stresses. In order to solve another problem, we may need to lower stresses by widening the valve-to-seat contact, which can harm performance.
The traditional materials have been copper alloys, namely different types of bronzes. In recent years, beryllium-copper alloys have become a firm favourite; often we find a mix of two alloys in the same engine with one alloy – usually a high-strength type – specified for the inlet seats, and a different, lower strength but high thermal conductivity alloy used for the exhaust seats.
Sometimes, however, we find that we can’t get the seat material to live for long enough. The long-life engines, which once comprised only endurance racers but nowadays include series such as Formula One and MotoGP, may need a different class of material altogether. Powder-metal valve seats are based on a ‘pre-form’, which consists of a sintered metal (often steel) that is deliberately made porous. This is then infiltrated with an alloy that lends the structure stiffness and thermal conductivity. The infiltration alloy is generally a copper or bronze.
The thermal conductivity of the powder-metal seat material, compared to one of the normal bronze candidates, is compromised owing to the amount of material now displaced by the pre-form (which is of lower conductivity). However, because a significant proportion of its volume consists of a high-strength material, with increased heat resistance, the combined properties of the material mean its strength is markedly increased, as is its resistance to deform gradually at the higher end of the operating temperature range.
Such materials are essentially a type of metal-matrix composite (MMC) but in powder-metal valve seat materials, the reinforcement is in much greater proportion and of much greater size than in a traditional MMC. It is an unusual material in that the strengthening reinforcement and the matrix (if we wish to consider the softer, weaker material as the matrix) are both continuous structures, whereas traditional powder reinforcements in MMCs are designed to be discontinuous and evenly distributed within the matrix.
Written by Wayne Ward