R. W. Wall, in one of the papers in the symposium previously mentioned, refers to laboratory experiments, the results of which indicated that corrosion at metal surfaces is accelerated by vibration and, in the case of iron in water, corrosion occurs in the form of isolated pits rather than in a uniform manner, suggesting that pitting could occur as a result of cylinder vibration only and may not necessarily be a consequence of cavitation in the water. Once pits develop, one would expect them to develop at a fairly rapid rate, due to the combination of a small anode and a large cathode. It is also thought that vibration increases the access of oxygen to the corroded surface, this being utilized in the cathode reaction to reduce the polarising action of hydrogen.
Damage to other portions of liners, that which is seen in the piston ring grooves, under flanges, or on the portion adjacent to the lower scaling ring (as seen on the liner shown in Figure 1) is considered to result, not from cavitation, but from differential aeration-a more common cause of corrosion, particularly where stagnant conditions may obtain.
Measures to prevent, or reduce, cavitation damage. should be considered firstly from the aspect of design, attention being given to methods of reducing the amplitude of the liner vibration. Piston slap can be reduced by minimizing the clearance, but attempts to reduce this too far could result in scuffing and increased wear. Other workers have suggested the use of a cam-ground piston with this end in view. Much greater success in reducing the severity of the piston impact has attended work done by the Admiralty Research Laboratories, using a specially designed piston incorporating an annular belt of oil between upper and lower rings, the oil acting as a viscous damper to cushion the motion of the piston and eliminating the tilting at the ends of the stroke.
In certain circumstances it may be possible to alter the design of the liner. In severe cases, longitudinal ribs can be employed or an additional support provided at about the mid-length, in an attempt to avoid resonant conditions. Measures of a palliative nature can be introduced by surface treatment of the liners, and in this respect the use of hard chromium plating or chromium plating on nickel has been found to be effective in certain conditions. Sprayed metals or the use of a ceramic coating based on alumina, applied by flame-spraying, have also been recommended. Other workers favour a resilient coating, such as nylon, but with materials of this type it may be difficult to effect a satisfactory bond and the attendant effect on heat conductivity must also be considered.
Attempts have been made to reduce the severity of attack by attention to the environment. Inhibitors, such as chromates, benzoate/nitrite mixtures, and emulsified oils, have been tried with varying success. Unfortunately, some inhibitors may not be suitable for use at temperatures approaching 100°C. and others may break down under conditions of cavitation; organic compounds, for instance, may be degraded and lead to precipitation.
Cavitation damage tends to fall off as the temperature is raised and, in one case, damage which was shown to be at a maximum at 50°C diminished significantly at 80°C. Raising the temperature of the cooling water is stated to be effective where anti-freeze additions based on ethylene glycol are employed.
Attempts have been made to reduce or prevent cavitation damage by the application of cathodic protection, and this has been found to be effective in certain instances of trouble on propellers. Its application to cylinder liners, however, is particularly difficult, and it is understood that no large measure of success has been obtained. Its effectiveness in any situation must be related to the particular parts played by the electrochemical and the mechanical factors in the case. The fact that cathodic protection has been found to be effective in counter-acting trouble with ships propellers, supports the view that cavitation damage results primarily from an electro-chemical, rather than a simple mechanical cause.
Little success has attended efforts to obtain relief by changes in the type of material. Iron with a higher elastic modulus should assist in reducing damage by modifying the vibration characteristics of the liner. More corrosion-resistant irons alloyed with chromium and nickel would possibly show an advantage, but unfortunately they possess inherently poor wear resistance.
Among other measures that have been advocated is pressurization of the cooling system, and this has effected a cure in certain cases experienced by the French Railways, possibly by preventing the formation of bubbles. It has also been suggested that it would be beneficial to remove the electrochemical factor entirely and to employ fuel oil as a coolant, but obviously a modification of this nature could only be adopted in certain instances.
In one case within our experience, corrosion in a six-cylinder diesel engine driving a small locomotive showed itself on cylinders Nos. 2, 5 and 6 after three years' service. After a further year, the No. 5 liner had to be replaced. The replacement gave only one further year's service, and the new one which was installed showed slight pitting after three months or so. The affected portion of the liners was in all cases adjacent to an internal rib on the engine frame as shown in Figure 4 and it is presumed that the trouble was associated with local turbulence, resulting from restriction in the water spaces. A slight attack was also shown on the frame itself in this vicinity. In this case, a 1% emulsified oil was added to the cooling water, which was also made alkaline, and no recurrence of the cavitation was seen during the subsequent six years.
In diesel engines trouble from cavitation damage is also found in coolant circulating pumps, main and big end bearings, and in the fuel injection equipment. With bearings, damage is thought to result from flow-induced cavitation, while in pumps and injection equipment it is associated with violent fluctuations that may occur when the flow is suddenly interrupted, It is found mainly in unloaded areas of bearings where the oil film pressure is low and, therefore, results from a cause different from that responsible for the more usual type of failure shown by these components. Its occurrence can be minimised by design changes, so that the oil pressure is maintained, or possibly by a change in material, a tin-base babbitt being more resistant than a lead-base alloy of the same hardness.
In conclusion, it would appear that the precise cause of the type of damage described is not yet known with certainty. Although-it is often ascribed to cavitation within the cooling water the evidence is largely of a circumstantial character based on the similarity between it and the damage shown by propellers and pump impellers. It is fairly certain that it stems basically from vibration of the liner, but whether or not mechanical forces are the primary or only factor is still a matter of doubt. With regard to alleviation of the trouble, it would appear that if the severity of either the mechanical or electrochemical factors is diminished, then the cavitation damage as such may be reduced to more tolerable proportions.
References
Corrosion/Erosion—Maritimer; Anti-corrosion, Volume 14, No. 2, Feb. 1967.
Cavitation Corrosion and its Prevention in Diesel Engines, Symposium Alexandra Palace, London, 10th Nov. 1965, British Railways Board.
Best Regards,
ROTASYON CYLINDER LINER & PISTON
Dry cylinder liners are among the basic piston protectors. They must withstand extremely high temperatures and guard against impurities, so they are constructed of high-grade materials, such as cast iron and ceramic-nickle plating. Dry liners are much thinner than their counterpart, wet liners. They do not interact with the engine coolant but instead provide a very close fit with the jacket in the cylinder block to protect the piston from heat and impurities.
Finned cylinder liners are constructed of the same type of heat and impurity-resistant metal. This type of liner is designed for the air-cooled engine, and in operation works much like the dry cylinder liner in that the cooling medium for the motor is air. However, these liners are fitted with tiny fins which allow the inflowing air to draw with great force around the cylinder to provide cooling.
