Fig. 14.—Sectional drawing of a Built-up Cylinder suitable for an Aeroplane Engine.
In Figs. [8] and [9] the valves are placed one on each side of the cylinder, this form of cylinder being known as a T-headed cylinder, but it is rather more usual here in England to place both valves on the same side of the cylinder and next to each other as indicated in Fig. [13], this form of cylinder being known as an L-headed cylinder. The chief object is of course to avoid the use of two valve shafts and also to produce a neater looking engine, but the T-headed design is better cleaned or scavenged by the passage of the inlet and exhaust gases. When a motor-car engine has two cylinders we frequently find them both in a single casting, having a common water-jacket, and then we say they are cast in pairs. A four-cylinder engine may thus have: (1) Cylinders cast separately; (2) Cylinders cast in pairs; (3) Cylinders cast en bloc; or all four in a single large casting. The third method is cheapest in first cost, but in the event of breakage will become the most expensive. The second method is a sound compromise.
An example of a built-up cylinder and water-jacket is shown in Fig. [14], the cylinder barrel being of steel tube with steel flanges, and the water-jacket being formed by copper tube slipped over the outside of the steel cylinder. Its great advantage lies in the reduction of weight, and it is thus largely used for aeroplane work. The valves would then be fitted in the top cover of the cylinder and driven by overhead gearing.
[CHAPTER III]
ENGINE DETAILS
The Piston is perhaps the most important detail to consider, for it is on the piston that the force of the explosion acts when the heat energy is converted into mechanical energy. It must be made sufficiently strong to withstand the bursting effect of successive explosions, and yet if we make the metal too thick it will retain too much of the waste heat and the piston may seize in the cylinder due to expansion. To understand why the piston is likely to seize in the cylinder we have only to remember that when a metal body is heated it gets larger in every direction, but if cooled it returns to its original size. Now if we make the metal of the piston too thick so that the waste heat cannot pass quickly through it and dissipate itself at cooler parts of the engine, then the bulk of this heat will be concentrated in the piston head, causing it to expand and become a tight fit in the cylinder, as the cylinder walls are fairly thin and in contact with the jacket water which keeps them fairly cool and prevents them expanding much above their normal size. The actual amount of expansion is very small of course, but there is very little clearance between the piston and the cylinder walls, even when the engine is all cold—perhaps five-thousandths of an inch. The piston therefore must be strong, yet as light as we can make it, having regard to the necessity for its being amply stiff and rigid and able to stand up to its work.
Fig. 15.—Two views of a Cast Iron Piston with Gudgeon Pin and Packing Rings.