LO 8-4 Explain the purpose and methods used to increase the rigidity and strength of a diesel engine cylinder block

1.1. Deep-Skirt Blocks

Deep-Skirt Blocks

A deep-skirt block design refers to a block configuration in which the bottom edge of the block extends to well below the crankshaft’s centerline Figure 25. The sides of the crankcase are separated from the main bearing caps to minimize the transmission of combustion noise that has been converted to mechanical vibrations from the main bearing caps. Bearing caps are held in place with two or four vertically placed bolts. Lengthening the block walls past the main bearing caps enhances structural rigidity, operating smoothness, and even allows for a larger mating surface with the transmission. A reinforcing bedplate may also be attached to the oil pan rails and main bearing caps to enhance the rigidity and strength of the block’s crankcase Figure 26. High horsepower, deep-skirted engines will often tie the oil pan rails and bearing caps together into a single rigid structure to prevent vibration and distortion of the crankshaft bearing bores. Main bearing caps are held in place not only by bolts but also by using an interference machine fit. This means the cap is slightly wider than the machined ledge it sits in and some force is required to both remove and position the cap correctly in the bearing bore.

Deep-skirted blocks will also use what is called a serpentine block wall in the crankcase area. A serpentine or wavy shape with curves minimizes flexing compared to a flat surface and adds rigidity, much like a curve imparts this property to a body panel or shovel blade. The block wall can be made thinner without sacrificing strength, and the serpentine shape reduces noise transmission from the crankcase Figure 27. Adding ribs and webbing to the crankcase area provides strength to areas that cannot be easily curved.

FIGURE 08-25: Deep-skirt blocks extend the block walls below the main bearing caps.

FIGURE 08-26: A bedplate attached to the oil pan rails and the main bearing caps of this high-torque output, deep-skirted, International DT-466 engine block adds extra rigidity and strength to the block.

FIGURE 08-27: A serpentine block wall used by this DD15 engine adds rigidity to the crankcase and reduces noise transmission while allowing for use of a thinner block wall without sacrificing strength.