The valves may open into the cylinder in a variety of ways; both may be in one pocket, or one may be in a pocket and the other in the head, or each in a separate pocket, or both in the head. The first two illustrations show automatic inlet valves, and the third and fourth mechanically operated valves; when two mechanically operated valves are in the head, it is necessary to open them by means of rocker arms, for because of their position it would be impossible for the valve-lifter rod to act directly on their stems.
If after the explosion the burned gases were permitted to escape directly into the open air from the cylinder, the effect would be the same as the firing of a gun, and for the same reasons. The pressure in the cylinder being higher than that of the atmosphere, the sudden expansion of the gases would produce a report, and as this would be most undesirable for an automobile, provision is made by which the gases are cooled and permitted to expand gradually, so that when they reach the open air they are at its pressure, or nearly so. This is done in the muffler, or silencer, to which the exhaust pipe conducts the products of combustion. The muffler consists of a series of chambers of different sizes, one inside of the other; the gases pass from the smaller to the larger, expanding as they go, until from the largest they should escape without noise, having lost their heat and pressure.
While the pressure exerted during the power stroke depends on the heat of the gases, and it is necessary to have the engine hot in order that there may be as little loss of heat as possible, the temperature must not be permitted to rise to the point at which the lubricating oil would burn. Lubricating oil for gasoline engines is made to stand high heat, but if heated beyond its limit it will burn, and then, besides the loss of its property of lubrication, a deposit of carbon, hard or gummy, will form, fouling the combustion space or piston rings, and interfering with the operation of the engine. Overheating is prevented either by circulating water through channels surrounding the combustion space, or by directing a blast of air against it.
Fig. 13.—A, Force circulation water-cooling system; B, thermosiphon circulation water-cooling system. (Flow of water indicated by arrows.)
The channels, called water jackets, provided for the circulation of the water, are usually cast with the cylinder, or formed of sheet metal. Cool water enters at the bottom and escapes at the top, absorbing heat during its passage. Of the two systems of keeping the water in circulation, the most usual consists of a rotary pump, which forces the water through the jackets and then to a cooler, or radiator, which is so placed that it is exposed to the air currents set up as the car moves. In order to cool the water, the radiator must have a large surface exposed to the air, and the water must pass through it in small streams. The early types consisted of coils of small copper tubing, on which were strung disks of copper, the water flowing through the tubing, and the disks absorbing its heat and giving it up to the air, but these are being abandoned in favor of cellular or honeycomb radiators. These types, which are usually placed at the extreme front of the car, are made up of a great number of short lengths of small tubing, in any one of several shapes, placed side by side, and held together either by plates or by soldering their ends.
Fig. 14.—A, Centrifugal pump; B, vane pump; C, gear pump and cover.
Fig. 15.—Radiator Constructions. A, Spiral flange (water passes through the tube); B, cellular, and C, honeycomb (air passes through the tubes; water passes between the tubes).