Plasma/thermal sprayed valve seatsTags : coatings
The subject of valve seat materials has been dealt with in RET-Monitor before; an article published in 2012 briefly discussed the requirements for seat materials. There are several important requirements for a valve seat material. It is generally responsible for the vast majority of heat transfer from the poppet valve, it needs to be strong enough not to be deformed by the action of the valve closing onto it, and it needs to be resistant to corrosion, seizure and wear in service.
The heat transfer from the valve to the water jacket (or cooling air) is improved greatly by bringing the valve and the water jacket (or cooling air flow) closer together. If the head itself were able to fulfil the requirements of deformation and wear resistance, race engine design and development engineers might be pleased to dispense with the valve seat inserts. Not only do the components offer a thermal resistance, so does any contact between components, and in order to get really good contact between components that are forced together, we need to approach the yield stress of the weaker material. As the head is usually the weaker out of the head and valve seat combination, we would have the head stressed in tension close to or beyond its yield point. So, in order to guarantee reliability, the head-to-seat contact offers more thermal resistance than it ideally would.
It is in this context, and for more reasons besides, that a sprayed valve seat looks very attractive. The sprayed seat can be very thin, and it requires no machined recesses with sharp internal corners into which the seats fit. The internal machined corners act as a significant stress concentration from which fatigue cracks can initiate. The distance between the valve-to-seat contact area and the water jacket can be minimised, and the high thermal resistance of the contact between head and valve is also minimised as there is a metallic bond between the seat material and the valve. Furthermore, as an interference fit is eliminated along with the consequential tensile stresses in the cylinder head, the fatigue life of the head is improved.
One of the limiting factors in the proximity of valves to each other in four-valve chambers is the stress in the area between adjacent valves. With a large part of the tensile stress eliminated, the designer might choose to use the lower stress to reposition and maybe resize the valves. Whatever he or she chooses, sprayed valve seats offer great latitude in design and development, or greater reliability.
The disadvantage of the sprayed valve seat though is manufacturing complexity and the cost for small production quantities. Their use has been a common practice for some time in production engines, and heads manufactured with thermal-sprayed seats are economical when produced in large quantities. However, for a race engine the cost can only be justified if the engineering reasons are good and the rewards for success are great. We should not be surprised to find that it is not widely used in racing at present, and those that have used it have generally been engine suppliers run by large car manufacturers – that is, those involved in Formula One. Even with their large budgets, this technique is not used universally.
Written by Wayne Ward