1. Classification of Cylinder Heads and Valve Trains

1.2. Aluminum Versus Cast Iron

LO 9-02: Describe the functions, construction, composition, types, and applications of diesel engine cylinder head assemblies.

 

Aluminum Versus Cast Iron

Cylinder heads are commonly made of cast iron alloy because of its superior strength, ease of manufacture, machinability, and capability to withstand high temperatures. Aluminum is another material now gaining widespread use as a material of choice. At one time, aluminum was not considered for several reasons:

  • Tendency to warp and crack, especially when overheated
  • Difficulty in tolerating dirty oil, which can quickly wear away softer aluminum friction surfaces
  • Extra manufacturing steps required to install cast iron or steel alloy valve guides, valve seats, and bushings for overhead camshaft (if used)
  • Greater electrochemical corrosion in engines containing high aluminum content, making aluminum unable to tolerate old, contaminated coolant in the same way as cast iron
  • More costly to cast than iron

The higher expansion rate of aluminum at operating temperature compared to iron is another problem. The expansion coefficient of aluminum, which is four to seven times greater than that of cast iron, demands improvements in the selection and replacement of head gasket materials and greater precision in cylinder head fastener tightening procedures. Valve lash can be similarly affected by expansion. Cylinder head bolts must stretch too and require nonreusable torque-to-yield bolts.

The primary advantage for aluminum, however, is that it is lightweight. Unnecessary engine weight is the enemy of good braking, handling, and quick acceleration, and it displaces the vehicle’s capacity to carry load Figure 5. Because aluminum is 50% lighter than cast iron, aluminum cylinder heads can easily save more than 60–100 lb (27–45 kilogram [kg]) in some larger diesel engines. Aluminum also lends itself well to the manufacture of head casting with larger intake and exhaust ports to create more powerful engines.

Another favourable attribute of aluminum is its higher heat conductivity, which improves heat transfer from the combustion chamber to the coolant. This is especially important for emissions. Valve temperatures can be as much as 200° F (93° C) cooler in aluminum cylinder heads. This not only lowers the variations in valve lash clearances but also enhances durability of the valve seats and increases reliability of the engine as a whole. Aluminum carries away excess heat more rapidly, and localized hot spots are eliminated with aluminum cylinder heads, thus higher combustion pressures and temperatures can be sustained. Similarly, because high combustion chamber temperatures cause increases to NOx production, this emission can be reduced with the use of aluminum heads.

   Fig09_5

FIGURE 09-5: The cylinder heads on this 6.6 L General Motors Duramax engine are made from hardened aluminum.

 

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