Thickness of Cylinder design of cylinders, as well as of all other parts, is strictly a matter that pertains to engine manufacturing, and not to engine operation. Even so, an operator should see that certain precautions have been observed in order that he may be assured a satisfactory machine. In all these designs it is well for the operator or purchaser to note if the cylinder walls are thick enough to allow for at least one re boring. The walls should be calculated from the formula for a cylinder with thin walls.
pD t =k 28 where t = thickness of cylinder wall.
p = maximum explosion pressure.
D = diameter of cylinder. S = allowable fibre stress.
k = constant to allow for reboring, its value depending on cylinder diameter.
Diameter of cylinder 6 to 11 in. 12 to 20 in.
Value of k It is common practice with low-pressure engines to cast the cylinder liners with the jacket. It is undoubtedly the most sat isfactory method in engines of moderate size. It is very easy to provide material for reboring and at the same time keep the thickness down to a value where the cooling effect of the water is sufficient. It is seldom that the cylinder liner develops a frac ture, and so the replacement cost may be forgotten. As regards the reboring feature a few builders are following the practice of selling a new piston, oversize, and a rebored cylinder, taking as part payment the worn cylinder; this cylinder is then rebored at the factory and sold to the next customer.
Reboring a Cylinder.—Frequently the cost of a new cylinder is so excessive that the cheapest procedure is to rebore the worn cylinder. If the engine cylinder be under 10 inches in diameter, the job of reboring can be done by almost any machine shop. It is not necessary that the shop have a horizontal boring mill. The cylinder can be clamped onto the carriage of a large lathe, and a boring bar made of a piece of shafting with a boring tool clamped in a slot cut in the shafting. This can be placed between centers on the lathe and be driven by a lathe dog. No matter whether the job be done on a boring mill or lathe, a roughing cut should be first taken, followed up by a light finishing cut; the feed should be fine enough to allow the surface to be free from any undesirable tool marks. To complete the reboring job a block of brass or cast iron should have one side turned to the cylinder radius; wrapping fine emery cloth about this block, a vigorous rubbing will make the cylinder smooth as glass and pre serve the curvature. In reboring a worn cylinder, extreme care should be used in getting the center line. The average cylinder has the flange, where it is bolted to the frame, turned square with the original cylinder center line. This flange should be
used as 'a guidance in lining up. On large engines the cylinder cannot be handled on a lathe. The best procedure is to have the cylinder rebored by some firm that makes a specialty of reboring cylinders. This can be done by means of a portable boring mill without removing the cylinder from the engine.
Causes of Cylinder Wear.—The question of cylinder wear on the low-pressure oil engine is very vital. Probably this is raised more than any other where the purchase of such an engine is being considered. Looking back into the history of the develop ment of this type, one must concede that cut and scored cylinders figure quite prominently. A number of things were contributory causes. Water injection was blamed by many; lack of lubrica tion, poor grade of cast iron used in making the cylinder, and lack of cooling effect by others. Water injection may have some effect where kerosene is used or where the fuel oil is high in sul phur, but for the fuel or distillate oils ordinarily used it is open to dispute as to whether the water does cause cutting. A great number of engines have run from five to seven years without any decided cylinder cutting. In discussing water injection in semi-Diesel engines the statement was made that the nascent oxygen combined with the iron, forming a ferric oxide, and that this caused cylinder cutting. It must be borne in mind that the temperature attained in the semi-Diesel is much higher than in the low-pressure engine using fuel oil. On the other hand, where kerosene is used, the combustion is in the form of a rapid explosion, and the temperature does run much higher than when using fuel oil. So it is probable that water injection with kerosene does cause ferric oxide deposits. However, the chief cause of this cutting is the lack of proper lubrication of the piston and cylinder. It is a common occurrence to discover an operator using a light gas-engine or automobile oil, and these oils do not have sufficient body to lubricate an oil engine properly. Regardless of statements that the temperature of the cylinder does not exceed 250° Fahrenheit, an engineer knows that the temperature of the flame. in the cylinder runs up into the three thousand degrees Fahrenheit, and that this heat will burn a light body, low-fire test oil. A successful cylinder lubricant must have enough body to stay on the walls, a fairly high fire test, and must burn without leaving any deposit of ash or carbon. Many engineers, especially in small plants, use oils that have not been filtered properly; these oils will always leave a deposit of carbon.