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Solid and Hollow Valves
Performance demands on today’s diesel engines to meet greenhouse gas (GHG) targets are increasing. To achieve greater efficiency, smaller displacement engines with higher power output use increased compression ratios that produce higher combustion pressures and temperatures. Valves have traditionally been made from heavier, solid materials, which, due to inertia, is usually not a problem in relatively slow-speed diesel operation. To enable valves to better dissipate heat from higher exhaust temperatures, engine makers are increasingly using hollow, sodium-filled valves Figure 28. Sodium is a solid metal used to fill a hollow valve stem or a combined hollow stem and valve head. Sodium is unique because it melts at close to 207.5° F (97.5° C). As a liquid, sodium can better conduct heat away from the valve head to the valve guide than a solid valve. While sodium-filled, hollow valves are costlier, they are still less expensive than moving to more exotic alternative materials to make more heat-resistant solid valves.
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SAFETY TIP |
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Sodium metal is highly reactive with water. If a valve stem or head containing sodium is opened, large pieces can melt and may even explode. The reaction produces caustic soda (sodium hydroxide) and flammable hydrogen gas. Be very cautious to never damage or break a hollow, sodium-filled valve. |
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FIGURE 09-28: Sodium is used to fill hollow exhaust valves. Sodium turns into liquid at high temperatures, enabling it to better conduct heat from the head to the valve guide.
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