LO 8-3 Identify and describe the construction techniques and composition of cylinder blocks and outline the advantages and disadvantages of each design

1.3. Open Versus Closed Block Decks

Open Versus Closed Block Decks

Aluminum engine blocks are commonly made using a die cast process. This involves squeezing molten aluminum between two steel halves of a stamping die to produce a finished part. The advantages include improved block dimensional accuracy, smoother sealing surfaces, the ability to use thinner block walls, and eliminating secondary machining in comparison to cast iron or aluminum blocks. Die casting is a faster production method and less expensive than casting blocks using sand or fiberglass molds. Sleeves and steel inserts can also be cast in place. Aluminum used for the casting has a “mushy” consistency when it is injected into dies and pressurized before the dies are separated. Using this method, diesel engine cylinders can be separated with aluminum that is as thin as 0.315" (8 mm) to form a very compact engine that is 30% lighter than cast iron. These blocks are specially heat-treated and particle hardened, followed by a plasma-sprayed coating of hard iron alloy applied to the cylinder walls. Cast iron main bearing caps can be used in these engines. In contrast to cast iron blocks, the use of stamping dies requires die cast blocks to use what is known as an open-deck design Figure 21. Open decks are easily identified because the block deck (the top surface of the block in contact with the cylinder head) has no connecting metal between the block walls and the upper portion of the cylinder wall. Although coolant can better circulate in open-deck blocks, the design has the disadvantage of an absence of reinforcement in the upper cylinder walls. Without a metal bridge between the cylinder walls and the block deck, the weakly supported cylinder walls are capable of moving during service. Closed decks connect the block deck with the cylinder walls to form a stronger, more rigid cylinder structure Figure 22.

FIGURE 08-21: This die cast block for an off-road diesel application uses an open-block deck. There is no support at the top of the cylinder that bridges the cylinder walls with the rest of the block.

FIGURE 08-22: Closed blocks connect the top of the cylinder wall with the block deck by using a metal bridge. This bridge closes the gap between the cylinder and the rest of the block.