LO 7-8 Explain the scope of cylinder component service and operating factors that accelerate cylinder wear

1.1. Engine Damage from Excessive Idle Time

Engine Damage from Excessive Idle Time

It is important to know that idling a diesel engine for prolonged periods of time shortens engine life and damages cylinder components.

Idling damages cylinder components in several ways:

1. Slobber production. Combustion and exhaust temperatures at idle in diesels are relatively “cold.” This happens because excess air combines with small quantities of fuel injected at idle to produce low cylinder temperatures and pressures. Pistons do not expand properly when cold, resulting in poor cylinder sealing. Unburned oil, fuel, and soot are produced from low-temperature, low-pressure combustion, which, when mixed, forms engine slobber. This slobber will accumulate in the exhaust system and stick to the exhaust ports, the exhaust valves, the piston rings, and in the ring belt area Figure 54. If this slobber flows around the piston rings and hardens in the ring grooves, the rings will stick in their grooves and become almost ineffective. Slobber is also one of the most common causes of premature EGR valve failure because it will contaminate, or gum up, EGR valves, causing them to stick. Direct injection (DI) engines are especially prone to slobbering because of their high thermal efficiency and low heat transfer rate to engine coolant. Slobber is a substance that must be purged from the cylinder during combustion or prevented from forming; otherwise, slobber residue can accumulate and cause engine damage. During loaded engine operation, heat hardens the deposits on pistons into very abrasive carbonized crystals that eventually score the cylinder walls Figure 55.

FIGURE 07-54: Engine slobber produced during engine idle dripped from the exhaust pipe joints of this truck.

FIGURE 07-55: This piston has operated for prolonged periods at idle, leading to excessive carbonizing in the ring belt area.

2. Excessive soot loading in engine oil due to idling and prolonged low engine load operation. Soot loading of engine oil is the result of poor combustion quality when the engine is at idle. Soot clings to the cylinder wall and is scraped down into the crankcase where it contaminates lubrication oil. A large amount of soot loading in the oil shortens the maintenance intervals between oil changes and requires that oil filters need to be changed more frequently. Soot is also an abrasive compound that will cause premature engine wear if allowed to reach more than 5% contaminant loading.

3. Cylinder glazing. Oil fills the crosshatch finish of the cylinder walls during idle and is baked onto the cylinder wall. Piston and ring travel smoothen the baked oil into a mirrorlike finish on the cylinder wall, which is then unable to retain oil. This process, called glazing , prevents the piston rings from effectively sealing, leading to a loss of compression through blowby. After an engine is rebuilt, it is especially critical to prevent the cylinder walls from becoming glazed by placing the engine under load soon after the engine starts. The deep cylinder wall crosshatch and poorly seated new rings will allow an engine to quickly glaze to the point where it is very hard to start and has low power. Shortly after starting, applying a heavy load and running the engine for at least 45 minutes to an hour helps seat the rings into the cylinder wall. This process, called break-in , uses high cylinder pressure to push the rings into the cylinder wall, where friction will smooth both the ring face and the cylinder wall surfaces to ensure that the rings better conform to the cylinder wall. Rapid break-in after rebuilding will ensure reliable and powerful new engine performance Figure 56 and Figure 57.

FIGURE 07-56: Crosshatch is observed by the fine intersecting lines on the cylinder walls. Crosshatching provides a cylinder wall finish, which retains engine oil needed for piston lubrication and to promote a gas-tight piston ring seal.

FIGURE 07-57: A ball-type hone can be used to apply new crosshatch finish to a cylinder.

Engines that operate under severe conditions, such as prolonged idle conditions in cold weather, require shorter service intervals between oil changes. Under these conditions, engine lubricating oil will contaminate faster, with soot produced by poor-quality combustion. Because soot contaminates the lubricating oil of diesel engines, that oil will blacken faster than gasoline engine lubricating oil.