For durability, and the ability to withstand high temperatures without warping, nothing has been found superior to cast iron, though the lightness of steel and aluminum alloys has commended them for aviation use, and in some cases for racing automobiles.
Early and still common practice in the building of multicylinder gasoline motors was the casting of cylinders separately, it being by this policy easier to secure sound castings, simpler to machine and finish them, and less troublesome to disassemble parts of the motor without disturbing the rest.
Fig. 5. Studebakcr Six-Cylinder Motor Showing the Block Castings of the Six Cylinders.
In a number of cases, where extremely light weight was desired, this method was followed but the cylinders were machined all over and a sheet-copper water-jacket was applied in assembling. This has been most successful in aeroplane work, and also for motorcars, but when the Cadillac changed to the form shown in Fig. 3, this construction lost its principal American adherent. In addition to this construction, there have been a number of motors built with an applied water-jacket of sheet metal, this being of the built-on form. These have shown splendid cooling abilities, but, under the twisting and racking of automobile frames, particularly in later years with the more flexible frames, have shown too much tendency toward leakage to become popular.
The great advantage of having the several cylinders of one motor cast together - en bloc, as the French term it - is that the alignment and spacing of the different cylinders is thus rendered absolute and permanent, regardless of any differences in adjustment that may otherwise occur in assembling.
This construction has been applied to a large proportion of the small and medium-sized fours, a fair proportion of the larger fours, and to a considerable number of sixes. One of the latter is shown in Fig. 5, this being the Studebaker six, which has a bore of 3½ inches and a stroke of 5 inches, rating at 29.6 horsepower but actually developing about 60. Some idea of the extent of this practice may be gained from the statistics for 1914, these showing that in a total of different motors 30 were cast in threes and 93 in block. As the three-cylinder construction is really a block modification, this gives a total of 123 as compared with the individual casting, 15, and the twin, 98.
Fig. 6. Ford Engine with Cylinders. Crankcase, and Gearbox in Two Parts Courtesy of Ford Motor Car Company, Detroit, Michigan.
Another advantage is that the water connections, exhaust and intake manifolds, etc., are rendered simpler both in their form and the number of their points of attachment.
In some advanced motor designs the passages for the incoming mixture and the exhaust gases, and in one case even the carbureter itself, are all incorporated in the main casting.
Another example of simple construction is that illustrated in Fig. 6, which depicts one of the latest Ford motors, in which cylinders, upper half of the crankcase, and the gearbox are all cast in one piece. The lower half of the crankcase and gearbox are similarly constituted of another simple pressed steel unit, while a second casting is used for the heads of the cylinders and the water connection.
The pistons of automobile motors have long been made of cast iron, with the piston pin held in bosses on the piston walls. For all ordinary service this construction, well carried out, serves every purpose, but with the development of very high-speed motors, with piston speeds twice and three times as high as past practice has sanctioned, there is a growing tendency to substitute steel for cast iron in this important reciprocating element.
Particularly in aviation motors has this been the case, the pistons of one well-known revolving motor, for example, being machined to the thinnest possible sections out of a high-grade alloy steel. In this motor the connecting rods are hinged to the head of the piston instead of to the walls, which thus can be made much thinner than . otherwise would be necessary. This practice has been followed to a slight extent by some automobile manufacturers. There are now a few stock cars of established quality provided with pressed-steel pistons.
In cars, too, the movement toward smaller bores and higher efficiency has brought about the use of much lighter pistons, this being done by making them thinner and shorter. The latest development has been the use not only of aluminum pistons and die-forged aluminum alloy connecting rods, but also of aluminum cylinders having cast-iron sleeves driven in to form the actual cylinder surfaces.
Cast iron for piston rings, long used to the exclusion of everything else, is in slight degree yielding its pre-eminence for this purpose also. This is because it has been found, in aviation motors with steel cylinders, that bronze affords greater durability and smoother running against the steel cylinder wall, for which reason bronze rings - with steel or cast-iron springs, or "bull rings", behind them - have been found most advantageous. Multiple rings, three or more in a groove, are finding favor. Their thinness necessitates the use of steel.