Sodium-Filled Exhaust Valves

Temperature is an important consideration to be made for the functioning of engines. A wrong estimate here or there could lead to problems such as knocking and pre-ignition and consequentially catastrophic failures. The usual working temperature for an engine is between 85-87 deg C. Below that threshold, the coolant absorbs too much energy from the cylinder and above it, the pressure conditions become unfavourable and all sorts of problems start to occur.

To maintain ideal working temperatures, the engine has built-in passages for the coolant to travel through and absorb heat from components. The hot coolant is then passed through the radiator and circulated back into the engine. The coolant passages weave through almost every core engine component except two – the valves. This may seem odd given the fact that both valves have to dip into the hottest part of the engine once per cycle, but due to the very fast periodic motion of the valves, it’s extremely complex and difficult to develop a solution that can keep up at higher RPMs. The valve seats are included in the coolant network but they can’t cool the valve heads which are the regions that are being shoved inside the combustion chamber.

However, there is one important thing to be noted here. The intake valve doesn’t require a coolant path since it’s being cooled from top to bottom by the fresh charge inducted into the engine every cycle. The main point of contention is the exhaust valve which is repeatedly smothered by extremely hot exhaust gases (around 850 deg C) as it vents them away at the end of each cycle. Exhaust valves often start glowing as the heads reach temperatures in excess of 750 deg C. And oftentimes, an overheating exhaust valve head is the primary reason for a knocking/self-igniting engine. Granted, the cylinder’s temperatures are closely monitored by the ECU and it can adjust the air-fuel lambda ratio to develop less heat, but that would also cause a drop in power from the engine. Valves made from materials with much higher thermal conductivity such as Inconel and titanium exist, but they cost exorbitant prices and there’s no hope of seeing such high-quality components in regular vehicles. Fortunately, there is another solution that’s been picking up steam recently which does not require any ECU intervention.

Back in 2018, Kia utilised a different type of poppet valve made by TRW Engine Components in the Stinger GT. This valve looked identical to a normal valve but was completely hollow from the inside and filled halfway with solid sodium.

Why sodium? It’s because sodium is an excellent conductor of heat and has a very low melting point of about 98 deg C and a very high boiling point of 883 deg C. As the valve head starts to heat up, the sodium inside turns to liquid and sloshes up and down with the periodic movement of the valve. Due to its high thermal conductivity, the sodium picks up the heat from the valve head and transfers it along the stem all the way to the valve seat which as stated before is cooled by the coolant. Thus the valve head temperatures are substantially lowered and the chances of knocking or pre-ignition are virtually eliminated. In the example of the Kia Stinger GT, the temperatures of the valve head were observed to have plummeted by around 150 deg C which is a huge difference. This handy illustration by TRW Engine Components is brilliantly reflective of the highly efficient nature of the sodium valve compared to an ordinary solid valve.

Now it must be said that this type of valve isn’t brand-new technology. In fact, the earliest appearances of the sodium-filled exhaust valve date back to Pegasus aircraft engines in 1936. They were also present in a handful of 1948 boats and even some Alfa Romeos and Ferraris in the 1960s. Mercedes even experimented with mercury-filled exhaust valves in their 1930s racing cars albeit leading to disappointing results. The big question here is why are sodium valves still the exception instead of the norm? The benefits are present in spades with virtually no downsides. The fabrication process is the same with the exception of the valve being hollow, and even then the manufacturer will save money in the long run by using marginally less metal for every valve. And it’s not as if sodium is a rare and expensive material. It makes up about 2.6% of the Earth’s crust which makes it the 4th most abundant material on the planet. The going rate for a kilogram of sodium today is $3 USD, maybe even less depending on the region. So why don't more car manufacturers include this supremely elegant technology in their engines? It’s an absolute mystery to me.