LO 8-2 Identify major cylinder block components and basic cylinder arrangements

1. Fundamentals of Cylinder Blocks

Fundamentals of Cylinder Blocks

The cylinder block can be described as the largest single part and backbone of the diesel engine. Its primary function is to enclose cylinder components and liners (if equipped) as well as major engine components such as the crankshaft and camshaft Figure 5. Main bearing caps, which are part of the block, hold the crankshaft in position along the crankshaft centerline Figure 6.

FIGURE 08-5: This engine uses replaceable cylinder liners, enabling it to be quickly rebuilt and restored to original specifications.

FIGURE 08-6: Main bearing caps hold the crankshaft in position along the cylinder block’s main bearing bore.

The transmission, cylinder head(s), flywheel housing, and all other major engine parts are bolted or connected to the cylinder block Figure 7. The cylinders in diesel engine blocks are arranged in-line, V-banked, or horizontally opposed Figure 8. The number of cylinders within a block are commonly configured in even numbers of 4-, 5-, 6-, 8-, and even 10- and 12-cylinder bores; however, many single, 2-, 3-, and even 5-cylinder blocks are produced as well. High cylinder pressures combined with a rapid combustion phase create powerful mechanical forces that vibrate and torsionally twist the block and crankshaft. Furthermore, high cylinder pressures and both tension and compression loading of connecting rods distort the crankshaft main bearing and the connecting rod bores. These operating characteristics of diesels require specialized features to reduce noise, vibration, and unpleasant operating harshness. Strong, rigid, vibration-free, lightweight, and durable designs are necessary for engines to perform well under all operating conditions. Because the reciprocating action of pistons within the cylinder bores can eventually wear out the engine, blocks are often constructed with replaceable cylinder liners to restore engines to their original specifications.

FIGURE 08-7: The flywheel housing on this engine also encloses the rear-mounted geartrain.

FIGURE 08-8: Engine block cylinder banks are configured in one of at least three basic ways: A. in-line, B. V-bank, or C. horizontally opposed.

1.1. Cylinder Block Structure

Cylinder Block Structure

The diesel engine cylinder block consists of these four distinct sections:

1. Main block structure: encloses the cylinder components
2. Cylinder component assemblies: includes the pistons, rings, wrist pins, and connecting rods
3. Crankcase: includes the area below the cylinders enclosing the crankshaft, bearing caps, and its bearings
4. Flywheel housing: may also enclose the rear-mounted geartrain

The geartrain may be located on either the front or the rear of the engine Figure 9. Geartrain rattle is an issue in diesel engines, affecting not only noise but also engine durability. Using scissor gears is one method to minimize gear rattle, a common problem in the long geartrains of overhead camshaft engines Figure 10.

FIGURE 08-9: Location of a scissor gear in a Cummins ISX-15 engine with dual overhead camshafts.

FIGURE 08-10: A rear-mounted geartrain reduces noise from geartrain rattle and provides greater durability of drive gears and more precise valve timing.

A scissor gear is actually two separate gears incorporated into a single unit. Zero lash between the two sets of gear teeth is maintained by a tensioned set of springs. High torsional vibration forces contribute to gear rattle and geartrain wear Figure 11. Firing impulses applied to the crankshaft cause sharp variations to its rotational velocity. The rhythmic change in crankshaft speed is referred to as torsional vibration Figure 12. With each alternating power and compression stroke, the crankshaft will slow and then accelerate several times during each revolution. The results are camshaft and crankshaft speed variation of between 3–6% during each crank rotation, which affects geartrain durability and noise emissions. At the rear of the engine, the heavy mass of an engine flywheel can smooth out torsional vibrations. Moving the geartrain to the rear of the engine, where torsional vibrations are reduced, can dramatically reduce geartrain noise and wear. Gear life and valve timing accuracy are improved, which explains why most manufacturers have adopted this arrangement in newer engine designs.

FIGURE 08-11: A scissor gear is actually two separate gears incorporated into a single unit. Zero lash between the two sets of gear teeth is maintained by a tensioned set of springs.

FIGURE 08-12: Torsional vibration is caused by the twisting action produced by power impulses in the engine and resistance to crankshaft movement by the driveline and compression strokes.