03-08-2012, 01:00 PM
CAVITAION CORROSION IN DIESEL ENGINE CYLINDER LINERS
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ABSTRACT
Cavitation corrosion (liner pitting) became a major problem for wet sleeve diesel engines in the 1940s. For most engines, generous use of supplemental coolant additives (SCAs) adequately controlled pitting. In the mid 1980s, problems of SCA overtreatment such as silicate gelation, water pump seal seepage, and solder bloom raised awareness that too generous usage of SCAs was undesirable. This new awareness caused a general shift toward lower average SCA concentration.
In addition, engine power densities increased and load factors increased as speed limits increased. These factors increased the work load of the engine, resulting in a greater potential for liner pitting. All these factors have resulted in an increased incidence of liner pitting in recent years.
This report is an overview to help place all the various factors related to liner pitting in perspective. The effects of engine design and materials, cooling system hardware, maintenance practices, and coolant composition are discussed.
INTRODUCTION
Cavitation erosion is the progressive loss of material from a solid surface due to the impact jet formed by the collapsing of vapour bubbles. Vapour bubbles are formed when the pressure of the liquid goes below the vapour pressure of it at that particular temperature. If pressure of the liquid goes much higher than the vapour pressure of the liquid near a solid surface, then the bubbles collapse causing impulsive jets of fluid to strike the surface and erode away the material.
Cavitation corrosion (liner pitting) became a major problem for wet sleeve diesel engines in the 1940s. For most engines, generous use of supplemental coolant additives (SCAs) adequately controlled pitting. In the mid 1980s, problems of SCA overtreatment such as silicate gelation, water pump seal seepage, and solder bloom raised awareness that too generous usage of SCAs was undesirable. This new awareness caused a general shift toward lower average SCA concentration.
In addition, engine power densities increased and load factors increased as speed limits increased. These factors increased the work load of the engine, resulting in a greater potential for liner pitting. All these factors have resulted in an increased incidence of liner pitting in recent years.
“Department of ME, GEC, Thrissur”
CAVITATION CORROSION OF DIESEL CYLINDER LINERS
a section of an engine block with a replaceable wet cylinder liner. As the piston moves vertically, it also moves a small amount horizontally due to inertia and combustion loading and impacts the cylinder liner causing it to vibrate. When the vibration is severe, the coolant cannot "follow" the rapid motion of the liner wall. Vapor bubbles form due to creation of an instantaneous low pressure in the coolant as the liner moves away from the coolant. Immediately after the vapor bubbles form, the liner surface movement reverses direction creating a localized high pressure in the coolant causing the vapor bubbles to collapse.
Fig.1: Schematic of cylinder liner cavitation corrosion. The resulting forces from the collapsing bubbles act on the liner surface, along with corrosion, to remove metal in a pitting fashion (Fig. 2). Just as it is important to know what cavitation corrosion is, it is equally important to know what it is not. Figure 3 shows a liner with some cavitation corrosion, but it failed at 59715 miles due to another reason. The cavitation corrosion is in the circled center of the liner, but the failure occurred lower at the edge of the crevice seal groove. Figure 4 shows a typical deep pit with rough surfaces from the area of cavitation corrosion.
Cavitation corrosion in thrust side. Fig.3: Failure at 59715 miles due
to erosion behind the crevice seal.
This is in contrast to the smooth surface of the failure hole shown in Fig. 5. The seal was held away from its groove by steel shot (Fig. 6) found near the failure. The failure resulted from flow erosion due to coolant pumping action behind the rubber crevice seal. The smooth surface of the flow-eroded failure hole is in stark contrast to the rough surface of the cavitation corrosion pit, which is typical of cavitation corrosion.
Fig.6: Steel shot that held crevice seal away from its seat.
Casting defects and stray electrical currents are two other examples of what cavitation corrosion is not.
Casting porosity does not routinely occur only on the thrust side of the liner and less frequently elsewhere, as does cavitation corrosion. This rules out casting porosity.
As for stray electrical currents, it is difficult to imagine an external electrical current choosing to flow through the massive cast iron block to only small discrete areas of the liner and create pitting.
3. VARIOUS VIEWS ON CAVITATION CORROSION
Although work in the area of cavitation damage in general and cylinder liner cavitation damage in particular has supported the basic view that material damage is
caused by a combination of mechanical damage due to the forces resulting from vapor bubble collapse and chemical damage due to corrosion, not all agree.
Some of these other views are as follows: