1. Classification of Cylinder Heads and Valve Trains

1.8. Classification by Number of Cylinder Heads

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

 

Classification by Number of Cylinder Heads

The number of cylinder heads used by a diesel engine is influenced by a variety of factors. Using a single-piece casting enhances strength and rigidity of the engine, but this type of cylinder head does not have the flexibility to adapt to multiple engine platforms. Modular engine construction where fewer components can be assembled into multiple engine configurations of 2, 3, 4, or 6 cylinders, V engines, or in-line engines favour the use of individual cylinder head castings capable of being interchangeable for each cylinder. Engine application, gas flow, serviceability, and the location of the camshaft are other considerations that need to be factored in when selecting the type of cylinder head casting.

Single-Piece Cylinder Heads
Single-piece cylinder head castings cover the engine block completely using only one head. One-piece castings are relatively easy to manufacture, and their rigidity adds structural strength to a block. Single-piece castings do, however, have the greatest amount of creep when thermally cycled, which means they expand and contract at a significant rate across the block deck by sliding over the head gasket. This can translate into more head gasket failures at the rear of the engine where coolant temperatures are hottest. Servicing a single-piece casting can be labor intensive, particularly if it is unnecessary to service all engine cylinders but the entire cylinder head requires removal. Care in maintaining a liner projection is critical on sleeved engines using single-piece castings. Warping and cracking of heads and liner flanges will result because clamping forces are concentrated on the cylinder liner with the highest cylinder liner projection. On some high horsepower (high-kilowatt) Caterpillar engines, a patented spacer plate between the cylinder head and the block deck minimizes the loads exerted by the cylinder head against the liner flange for improved reliability Figure 15.

 

  Technicians Tip  
  Cylinder head gasket leaks caused by differences in relative movement between the cylinder block and the head usually occur at the hottest coolant temperature location. On most in-line six-cylinder engines, this means the rear of the engine is where most leaks are observed because it is usually farthest away from the water pump and radiator.  
  Fig09_15

FIGURE 09-15: A steel spacer plate between the block deck and the cylinder head on this Caterpillar C15 engine minimizes cylinder head clamping forces applied to the liner flanges.

 

Multiple-Piece Cylinder Heads
Multiple-piece cylinder heads use a separate cylinder head for one, or a group of two and three, cylinders in a multiple-cylinder engine Figure 16. These cylinder heads are ideal for modular engine designs where part castings shared between different engine models can be used to construct various engine configurations. This allows the same head to be used on a variety of cylinder arrangements. For example, a single cylinder head can be used to build single-, 2-, 4-, 6-, or 8-cylinder engines Figure 17. Servicing is simplified if only one or a few cylinders of an engine are being repaired. However, multiple-cylinder heads tend to move around more, wearing the top of liners or block decks. The result is more frequent head gasket leaks and damage to the block deck that may require engine removal to resurface the block at a machining facility. They cannot provide much structural rigidity to the engine either.

  Fig09_16

FIGURE 09-16: One cylinder head per cylinder is used on this V-12 locomotive diesel using a modular engine design. Multiple-piece cylinder heads may cover one, two, or even three separate cylinders.

   Fig09_17

FIGURE 09-17: This in-line six-cylinder engine uses a single head casting for two cylinders. Note the dowel pins to position the cylinder head on the block deck.

  

 

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