Seats and Guides - Materials Requirements
The poppet valve in an internal combustion engine is subjected to some very harsh operating conditions, with rapidly changing stress and thermal cycles. There are generally four other components that are in contact with the valve - the lash cap, stem seal, valve guide and valve seat. Some engines run without stem seals though, and lash caps were dealt with in another RET-Monitor article recently.
Although they have a very similar cycle of operation, the inlet and exhaust valves are subjected to very different thermal cycles, with the exhaust valve running much hotter than the inlet. This is because the inlet valve will have relatively cool air or air-fuel mixture intermittently flowing past it, whereas the exhaust valve is heated by very hot burnt exhaust gases flowing past. Consequently, the exhaust valve has much higher demands in terms of material temperature resistance and has greater cooling requirements.
The valve seats are the primary means of removing heat from a valve, and therefore one requirement for the seat material is high thermal conductivity. The combination of valve-to-seat contact area and thermal conductivity dictates the potential rate of heat removal for a seat.
Valve seats are often produced from copper alloys, with beryllium copper being popular owing to its combination of strength and thermal conductivity. It is common practice to produce the inlet seat from a stronger material that is less thermally conductive. Copper alloys are also excellent bearing materials: they are very resistant to seizure and do not react chemically with any of the fluids passing through the ports and combustion chamber.
The thermal limit for copper exhaust seats is sometimes approached for exhaust valve seats in boosted engines with high exhaust gas temperatures, where the seat can sometimes begin to melt. In production engines, valve seat inserts produced by sintering powdered ferrous materials are often used, and some companies have developed sintered materials that are infiltrated with copper to improve thermal conductivity.
The valve guides are also commonly made from copper alloys. As less heat is transferred through the guide than the seat, their requirements in terms of thermal conductivity are lower, and the materials used are generally not the very expensive alloys containing beryllium. Having said this, engines using sodium-cooled valves transfer a much greater proportion of heat through the guide; the liquid sodium is very effective at taking heat from the valve head and transferring it to the stem. The strength requirement is also lower; in an ideal situation there should be no lateral load on the valve, although in the real world there is often a degree of side load from valve actuation and thermal distortion of the head.
While the guide and seat are machined concentrically with great accuracy, we would be naïve to think that they remain perfectly concentric in use. The lateral loads are low enough to allow the use of other materials. Aluminium is one possible candidate, as it has low density and high thermal conductivity. Cast-iron guides are widely used in heavy-duty production engines, and powder-metal ferrous metal guide materials have been developed in which lubricating fluids are stored in the pores of the sintered structure. There are also sintered guide materials that are infiltrated with solid lubricants.
Written by Wayne Ward