Chemical or Electrical Cylinder Corrosion?
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The Merits of Two Theories of Cylinder Corrosion Investigated The Electrical-friction Hypothesis Adopted
AFTER many years' work by research organizations, both at home and overseas, the existence in automobile engines of some form of cylinder corrosion has now been generally accepted. Until quite recently, however, only the possibility of chemical corrosion has seriously occupied the minds of our chemists and engineers. For various reasons, their task has not been a light one and even to-day the useful evidence available remains sketchy and incomplete.
Faults in Chemical Theory.
Those investigators who claim that chemical corrosion is the primary cause of the cylinder wear, base their opinions on the assumption that the constituents of the hot gases and vaporous fuel promote oxidation of the cylinder walls. If this be the case, the corrosion should be more or less general throughout the whole combustion space.
With most won cylinders, however, the wear is in fact, confined to the area around the first compression ring, taken at top dead centre. Above this area, from the topmost point of the piston ring to the top of the cylinder, appears an unworn ridge of metal totally unaffected by corrosion.
Apparently, then, oxidation is restricted to the wearing surfaces of the cylinder walls, which, when properly lubricated, seldom come into contact with either the fuel mixture or fire exhaust gases. The incongruity of the chemical theory does not stop there. In the course of much research work, it has been found that not only is cylinder wear strictly localized, but the location varies in a peculiar manner with different engines.
For example, in one make of engine the wear is all on the thrust side of the cylinder block, on a second at the back of cylinders two and three, and on a third at the front of cylinders one and four. The explanation of this phenomenon remains as yet undiscovered. Meanwhile, the uncertainty materially weakens the arguments of those in favour of chemical corrosion.
Arising from this inability to explain the peculiar characteristics of , cylinder wear, a new and much more interesting hypothesis has been adopted by several investigators in
this country and in America. They believe that abnormal wear can be traced to electrical friction.
The infinitesimal currents which cause disintegration of the cylinder walls are said to be of the static variety, generated by the violent movement of the piston. Thus, whereas in a correctly lubricated cylinder the piston should be insulated from it by a fine film of oil, in actual practice certain. areas on the cylinder wall are denuded of lubricant by extreme heat or petrol dilution. These exposed areas allow electrical attrition or friction to be set up with the consequent excessive wear.
A Plausible Explanation.
The plausibility of this theory is borne out by the improvement in cylinder wear when graphite, or, as in the latest Castro' product, tin and chromium are added to the lubricating oil. In accordance with this theory, both these additions should help to reduce the static charge in the fast-moving piston by de-insulating it from the whole surface of the cylinder walls.
Finally, the abnormal wear of cylinders fitted with aluminium pistons can be accounted for, in this way, by the more powerful thermocouple provided by the two different metals.
So far, little direct experiment has been devoted to a study of the possibility of electrical corrosion. What has been done is promising, but whether the arguments against chemical cylinder corrosion can be successfully surmounted without introducing additional problems is still a matter of conjecture.