Valves operated by push rods and rocker arms are also shown in [Figures 17], [18] and [19]; Figure 18 is a single-cylinder horizontal engine, while [Figure 19] is a horizontal double opposed engine, in which one cam operates a valve in each cylinder. [Figure 20] shows the valve mechanism of a vertical engine in which all parts, including the rocker arm, are enclosed to protect them from dust, and so they can run in oil.
A small space is always left somewhere between the cam and the valve stem, to give the valve stem room to lengthen, which it will do when it gets hot. If this space were not left, the valve stem, in lengthening as it became hot, would strike the part next to it, and the valve would be lifted from its seat. This would cause the engine to lose power. This space must be kept properly adjusted, and instructions for this will be found in [Chapter XII].
Fig. 17.—“Hart-Parr” Engine
A valve is held against its seat by a spring, which must be compressed when the valve is opened. If this spring is too weak, it will not hold the valve tightly on its seat, while if it is too stiff, the cam shaft and other parts will be needlessly strained in compressing it.
Friction between the cam and the end of the valve stem or push rod would cause rapid wear if these parts were not of hardened steel, and kept well oiled. Still further to reduce wear, there is usually a roller on the end of the push rod, as shown in [Figure 16] and some of the other illustrations. [Figure 15] shows a construction in which the end of the push rod is a flat disk, which rotates as the cam comes into contact with it.
Fig. 18.—“Oil-Pull” Engine
When the mixture burns, the top of the piston, the cylinder head, and the walls of the combustion chamber become heated, and if it is not prevented they will get so hot that they will expand sufficiently to cause the piston to stick, or seize. The upper part of the cylinder is, therefore, provided with a cooling system that keeps these parts from getting overheated. Channels are provided through which water is circulated; the water takes the heat from the metal parts, becoming heated itself, and then passes to a cooler, or radiator, where it gives up the heat to currents of air.
In addition to the channels, or water jackets, around the cylinder, a cooling system includes the radiator, the connections, and usually a pump that keeps the water in motion.