Chapter 07: Final Review & Key Terms

1. Ready for Review

• Cylinder components consist of the pistons, piston rings, piston pins, and connecting rods. Because these components convert combustion energy into mechanical power, they are the most highly stressed parts of the diesel engine.
• Cylinder components need to have a useful life that meets or exceeds EURO 6 and EPA emissions standards for durability. The EPA requires 435,000 miles (700,065 km) or 10 years for engines to remain emissions compliant. The latest EURO 6 HD diesel standards are 434,960 miles (700,000 km) or 7 years of useful life.
• Basic piston designs used in diesels include slipper skirt, trunk, articulating (MD and HD only), and steel alloy (MD and HD only).
• Slipper skirt pistons are used in lighter, more compact diesels while steel alloy pistons are used in the latest HD engines. Articulating pistons have a two-piece design and use an aluminum skirt attached to a steel crown with a piston pin. Trunk pistons are longer than they are wide.
• LCV pistons reduce emissions by minimizing the space between the piston crown and the cylinder wall above the compression ring. Articulating and steel alloy pistons are classified as LCV pistons because the top ring is closer to the crown and the clearance between the cylinder wall and the crown is reduced.
• While aluminum pistons are traditionally lighter than steel pistons, which minimizes inertia forces at the end of each stroke, aluminum pistons have several disadvantages. Aluminum pistons’ need for high crevice volume and larger operating clearances have enabled pistons with steel alloy crowns, which can withstand higher cylinder temperatures and pressures, to displace aluminum pistons in many applications.
• Pistons are cooled by several mechanisms, including transferring heat through the piston rings, during valve overlap when the intake air mass crosses over the piston, and by oil cooler nozzles, which spray engine oil aimed at the underside of the piston.
• Piston rings help form a gas-tight seal in a cylinder, distribute oil over the cylinder wall, transfer heat from the piston, and provide a low-friction, replaceable wear surface.
• Three types of piston rings are used in an engine: a compression ring, an intermediate ring, and an oil control ring. Each ring has its own unique shape and materials based on its primary function.
• Compression rings help form a gas-tight seal in the cylinder. Intermediate rings form a compression seal and control oil distribution. Oil control rings distribute a thin coat of oil on the cylinder walls.
• The introduction of low-tension, low-friction piston rings is the most recent change in ring technology to reduce engine wear and friction.
• Steel piston rings are more commonly used. Longer break-in periods are required due to harder ring materials.
• Piston pins are offset from the piston’s centerline to reduce noise when the piston crosses over TDC.
• Piston rings, pistons, and connecting rods all have directional markings that indicate which way they should be installed in an engine.
• Rolled threads, which are stronger than machine-cut threads, are used on connecting rod cap bolts.
• Prolonged engine idle produces engine slobber, which is a combination of lubricating oil, unburned fuel, and soot. Slobber can cause piston rings to stick and leaves damaging deposits on pistons.
• Prolonged engine idle time can also increase cylinder glazing and soot loading of engine oil. Glazing takes place when the fine lines on the cylinder wall, called crosshatch , are filled with oil that is then baked and smoothed over by the piston rings. When glazed, cylinder walls cannot properly retain oil, which leads to increased cylinder blowby and power loss.
• Crankcase pressure testing is the most effective way to non-intrusively evaluate how much an engine’s cylinders are worn.
• Most wear takes place in the cylinders in the ring turnaround area, which is the place in the cylinder above the highest point of travel of the oil control rings and below the compression ring. Cylinder wear is tapered, with the greatest wear at the top of the cylinders.
• No service life remains in a cylinder when the percentage of crosshatch that is worn away exceeds 20% of the stroke length. Maximum taper wear is 0.001" (0.025 mm) per 1" (25 mm) of cylinder bore diameter to a maximum of 0.005" (0.127 mm).
• After rebuilding an engine, the engine must be run-in under a heavy load to seat piston rings and minimize the likelihood of cylinder glazing.