Heat transfer through valve seats
In designing a new race engine, or developing an existing one, the management of heat in poppet valves - and the transfer of heat from them - is only one small aspect, but it has an effect on performance and reliability.
The subject of hollow poppet valves that are cooled internally through the use of sodium or a similar material has been discussed previously, both in Race Engine Technology magazine and in my RET-Monitor articles. The aim here is to transfer heat from the valve head to the cooling system via the valve guide.
However, where internal cooling isn't feasible or allowed under the rules, we have to look to other ways to keep the heads of the valves cool. Exhaust valve cooling is important for reliability reasons. Exhaust valve temperature is an important limiting factor for exhaust valve materials, and is the driving force behind the use of superalloy materials and new titanium materials for exhaust valve applications. It is also important to keep the head of the inlet valve cool, because the transfer of heat from a hot valve head to the incoming charge affects volumetric efficiency. What we want to do is transfer heat from the valve head to the cooling system as efficiently as possible. So, what can we do to achieve this?
Increasing the width of the valve seat is an effective method of increasing heat transfer, but this can have a significant negative effect on flow coefficients, and seat contact widths are generally kept to a minimum for this reason.
The use of high thermal conductivity valve seat materials is another effective way of increasing heat transfer. Beryllium-copper alloys are commonly used for this purpose, but beryllium-free alternatives are available that offer similar thermal properties. A range of more exotic materials offer even higher thermal conductivity, and computing and power electronics applications are, in many cases, driving the development of high thermal conductivity materials.
The use of thin-section valve seats can be used to reduce the distance and the thermal resistance between the hot valve and the cooling water, although this has some practical limits when using machined valve seat inserts. The practice of plasma-spraying valve seat material directly onto the cylinder head before machining is widely used in production car engines, but has only found limited application in bespoke race engines. Although the technique is very effective, it is also expensive when setting up to process a very small number of cylinder heads.
Water cooling of valve seats is another possible method, and this is commonly used in very large engines and piston aero engines, where the technique has been used for many years. Simply connecting a machined recess to the water jacket by using one or more drillings will have only limited effect unless there is a pressure differential between the inlet and exit drillings to the valve seat to encourage water to flow around the seat.
We also need to be aware of the possible negative implications of seating a hot valve onto a well-cooled seat.
The contracting of the rapidly cooled seat surface on the valve which results when the valve contacts the cool seat can lead to some valve distortion and tensile stresses in the rim of the valve. These cyclic tensile stresses can lead to fatigue failure in the form of valve 'chipping' and cracking.
Fig.1 - This is a typical water-cooled valve seat insert from a piston aero engine. While this is not suited to compact race engines, the task of removing heat from the valve is an important consideration
Written by Wayne Ward