Sodium filled exhaust valves - not just a Norton issue

Recently I wanted to throw in a ‘curve ball’ type question to an interview for a young engineer to test their problem solving. The curve ball question I came up with related to the exhaust valve illustrated below.

Works Norton sodium filled valve

So this is no ordinary exhaust valve. 
It is a Works Norton Valve from one of their famous 500cc single cylinder machines. What is most notable is this is not a valve fitted to one of Norton’s manx production racers. The stem is the giveaway; and what a giveaway that is given that it is ½” in diameter!

Looking back now it is fair to say this is ghastly design. But this is where we need to put our problem solving hat on and consider why it has such a structure. The larger diameter is of course filled with sodium. Sodium is a group one metal which becomes liquid at relatively low temperatures. The liquid sodium can convect heat away from the valve head.

The exhaust valve is also noteworthy for its convex head. Once again this shape is chosen not to increase compression ratio (there are far easier ways to do this, e.g. material on the piston etc), but is instead incorporated to take more heat out of the combustion chamber.

So the convex head and large stem filled with sodium all points to increasing heat transfer away from the combustion chamber and most importantly away from the valve head itself. 
Why was this required?

Many far higher performance engines can operate perfectly satisfactorily with conventional solid exhaust valves.

Once again we need to look back at the period when the engine was racing; post WW2 when petrol supply was in short demand and resulted in low octane ‘pool’ type fuels. The lower octane fuels had a far greater tendency to result in engine ‘knock’. As we know there are a few mechanisms that cause uncontrolled combustion in a spark ignition engine (knock, pre-ignition), however one item that can be big influence is hot spots within the combustion chamber. It is no surprise that an exhaust valve is one of the hottest parts of any engine. 

So there we have it, on the very low quality petrol available post WW2 Norton tried all they could to prevent hot spots in the engine (hence heavily cooled exhaust valve) in order to prevent engine knock and allow them to use higher compression ratios and higher levels of ignition advance.

But the engineers of the time knew exactly this and were well aware that if different fuels were used, a sodium filled exhaust valve may not be required. In the words of Steve Lancefield when talking about Manx engines for 500cc racing cars which could run higher octane alcohol fuels having greater charge cooling characteristics:

“Yes, most of the Norton engines I prepare are fitted with ‘Sodium’ exhaust valves – it is open to question whether these are really necessary on such well-cooled engines using alcohol, particularly as very good results have been obtained with engines using non-sodium valves. However, for what it is worth, the sodium valve is a shade lighter in weight than its solid counterpart and the use of this type of valve is on this score alone worthy of consideration. When using hydrocarbon fuels, unhesitatingly – sodium-filled exhaust valves please!”

Sodium filled valves are however not just a remnant of engineering history.  Modern turbo gasoline direct injection (TGDI) engines with their very high specific outputs are also now requiring such technology once again to reduce valve head temperatures to prevent knock and allow higher compression ratios, higher boost pressures and more ignition advance. The images below details types of exhaust valves that can be used on engines and their influence on the valve surface temperature. The sodium filled valve (A) also utilise a hollow head filled with sodium to improve heat transfer – striking similarities to the issue and solution Norton came up with over 60 years previously! The sodium filled valve results in stronger valves (further from the fatigue line) with lower surface temperatures (better for knock) and also having a reduced weight compared to the solid equivalent.

Exhaust valve cross sections

Exhaust valve surface temperature and strength at peak power conditions in a modern TGDI engine