Chapter 07: Final Review & Key Terms

1. Ready for Review

WRAP UP: 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.

 

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1.1. Key Terms

WRAP UP: Key Terms

 

anodizing A process used to harden aluminum by electrochemically reacting oxygen with aluminum.

articulating piston A two-piece piston design that uses a separate aluminum skirt connected to an alloy steel crown through a piston pin.

break-in period The operation of an engine after it is initially assembled or rebuilt when the piston ring, cylinder wall, bushing, and bearing surfaces have high initial wear as the moving surfaces conform to each other.

cam ground piston An elliptically shaped piston that expands to a round, symmetrical shape after it is warmed up.

compatibility A property that allows two metals to slide against each other with minimal friction or wear.

compression height The distance from the piston pin to the top of the piston crown.

compression testing A measure of the maximum pressure of engine cylinders when cranking.

crankcase pressure test A measurement of the amount of cylinder blowby; this indicates the sealing ability of the piston rings and cylinder walls.

crevice volume The area between the piston crown and the cylinder wall above the top compression ring.

crosshatch A cylinder wall finish of fine intersecting lines that are used to retain oil.

cylinder glazing A condition that occurs when lubricating oil is first baked and then smoothed into the crosshatch of the cylinder wall. Cylinder glazing prevents the cylinder wall from retaining oil and reduces the ability of rings to seal.

cylinder leakage The percentage of gas leakage past the rings. High leakage rates are usually due to worn cylinders and rings.

dial bore gauge A precision measuring tool used to measure taper wear in a cylinder.

durability requirements Legislated standards for engine durability that require noxious engine emissions to remain below set thresholds through the expected useful life of an engine.

dusting out A condition where dirt is drawn into an engine; this causes premature abrasive wear of the cylinder walls and rings.

forged pistons Pistons made from aluminum alloy billets that are stamped into shape by forging dies.

hydrostatic lock A condition that occurs when fluids in the cylinder of an engine prevent the engine from rotating.

hypereutectic piston A piston that has high silicon content (16–20% silicon).

Magnehelic gauge A gauge that measures both pressure and vacuum using inches or a water column as a measuring unit.

major side thrust Piston side thrust caused by cylinder pressure and the angle of the connecting rod during power.

micro-finish etching A piston skirt finish where many fine lines are machined into the skirt to retain oil.

minor side thrust Piston side thrust caused by compression pressure and the angle of the connecting rod.

mono steel pistons A one-piece piston constructed from steel.

MONOTHERM piston A one-piece piston design made entirely of alloyed steel, which has a compact height and a large reduction of material between the skirt and the crown.

Ni-Resist insert A stainless steel–nickel alloy insert placed in aluminum pistons in the compression ring groove. The insert minimizes groove wear caused by ring movement.

nitriding The process of hardening a metal’s surface by heating the metal and quenching it with cyanide salts.

piston slap A noise in the engine produced by large operating clearances between a piston and a cylinder wall. Piston slap is most often heard when an engine is cold.

shot-peened A technique that uses small steel balls to blast metal surfaces in order to close any small cracks or pores, which have the potential to become larger.

slipper skirt piston A piston design that has a portion of the skirt removed on both non-thrust sides of the piston to provide clearance for the crankshaft counterweights.

soot loading A condition that occurs when engine oil dissolves combustion soot.

split fracture connecting rod A technique used to form a mating surface between a connecting rod and a cap. The mating surfaces are not machined but are produced by fracturing the big end of the rod along a line scribed into the rod.

trapezoidal pin bushing A pin bushing design in which the bearing surfaces absorbing combustion forces are wider on the connecting rod and piston to improve piston and connecting rod load-bearing capabilities.

trunk piston A piston design that is dimensionally longer than it is wide and uses a full piston skirt.

 

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