Fractured Heads.—Fracture of a cylinder head is a malady which appears to afflict all makes of Diesels, no matter what the design may be. There is no doubt that the more complicated heads fracture often, but even the simplest of head castings do give way, usually on heavier loads. Bad water, without ques tion, occasions the greater number of the fractured cylinder heads. This especially applies to the horizontal engine. Here, on shutting down, the water gives up its salts, which deposit on iron surfaces. The greater part settles on the lower surfaces, marked A in Fig. 69. However, a part of the soft sludge adheres to the vertical surfaces B. This coating accu mulates until it is as much as an inch thick. Since scale is an excellent non-conductor, no cooling effect is experienced on the hot cast-iron head wall which is in contact with the intense flame of the burning fuel. On cooling, the contraction of the iron gradually weakens the bond of the scale. This scale ulti mately drops off while the engine is under load, exposing a red hot iron surface to the cooling water. The sudden localized contraction of the iron on being chilled results in a fractured head. Evidently the horizontal head is more likely to shed the scale than is the vertical head. It becomes necessary for an engineer to inspect the cylinder head at stated intervals; if scale is present, it can be removed by scraping. If a solution of muriatic acid and water in the proportion of one to ten is allowed to remain in the jacket a few hours, all the scale can be washed out with a hose.
Heat stresses due to faulty design of the head also contribute to these fractures. This, however, is beyond the sphere of the operating force, although every engineer should endeavor to persuade the management, when new units are to be installed, to purchase only those engines whose design offers comparative freedom from operating difficulties.
Cylinder heads frequently crack through the bridge between the exhaust and fuel valve cavities. These fractures are due to improper cooling rather than to faulty design, although there are doubtless instances where casting fractures develop at these places as soon as the head is machined. To safeguard the engine head from such fractures that may develop in service, the flow of cooling water to the head must be positive and the temperature kept at a constant reading, the value of which can only be determined by experiment on the particular engine.
On marine Diesels the fracturing of cylinder heads is directly traceable to excessive overloads. In a heavy sea the propellor is at times exposed and immediately thereafter completely buried. This leads to engine hunting, and neither governor nor
manual control can cope with the situation. The fuel pumps deliver excessive charges to the cylinder, creating beyond the capacity of the heads to withstand.
Furthermore the marine engine, to reduce the weight per horsepower, is speeded far higher than the stationary Diesel.
This applies especially to the submarine and light cruising engines. The heat absorbed per sq. inch of surface by the heads of a 500 h.p. engine at 400 r.p.m. is double the amount absorbed by the heads of a 500 h.p. at 200 r.p.m., since the cylinder bore is practically one-half that of the latter engine and the total amount of heat absorbed is approximately the same. The cooling system, then, must be absolutely correct in design if fractures are to be avoided. The tendency of the salt water to scale is, of course, more prdnounced where the temperatures are as high as exist in the marine Diesel heads.
Repair of Fractured Cylinder Heads.—Oxyacetylene welding has not been a marked success in the repairing of cracked cylinder heads. The cylinder head contains a great weight of iron, and, in welding, the flame is localized. The consequence is that the metal immediately about the fracture is highly heated and ex pands. After the molten metal is added, closing the fracture, the head is allowed to cool. The mass of the head has not been heated, and so shows no contraction. The obvious result is the shrinkage of the metal at the edges of the fracture, reestablishing the fault.
To successfully weld a cylinder head, a furnace similar to Fig. 76 can be constructed of fire brick. The floor should also be made of fire brick supported by old grate bars or iron rods. After a coke fire has been burning about the head for twenty four hours, the entire casting becomes red-hot. The oxy acetylene flame is then applied; the fracture is enlarged to a trough shape, thus allowing the added metal to reach the bottom of the fracture. The new metal is deposited in small quantities and thoroughly welded to the cast iron before more is added. The head should then be left in the furnace to cool for forty eight hours. Since the entire furnace has been at a high tem perature, the cooling will be very gradual, thereby avoiding all shrinkage strains. If the fracture has been across a valve seat, the part must be machined and the valve refitted. The proc ess of welding here outlined has been followed with complete success, saving hundreds of dollars in a plant where five Diesels were installed.