Continuing from the previous chapter we can next investigate the cooling system, considering first the office of the cooling system. This system must cool the cylinder walls to such an extent as to permit of the proper lubrication and to prevent the carbonization of the lubricating oil. Preignition will also occur if the metal is permitted to heat to a red heat, causing the gas to ignite on the compression stroke. It is the general impression that the office of the cooling system is to abstract the heat from the gases within the cylinders, this heat having been generated by the explosion. This is not the case, for, as a matter of fact, the duty of the cooling medium is to keep the cylinder walls cool, the heat of the gases being converted into useful energy.
There are two general types of cooling systems, the direct or air system and the indirect or a system using a cooling medium such as water or oil. The term direct is applied to the air system, as there is no intermediate transfer of heat from the cylinder walls to the radiating surfaces by means of a cooling liquid. Air cooling is generally effected by cooling ribs or fins as they are sometimes called, cast integral with the cylinder walls and head. Some mechanical method, such as mounting a fan at front of the motor, or combining it with the fly wheel, is generally resorted too for inducing air circulation. Fig. 29 illustrates a typical air cooling system. This type of cooling system is most popular on the low-priced vehicles using two, three and four-cylinder motors. It may be used on either two or four-cycle motors.
The indirect cooling system as mentioned above involves the circulation of a liquid such as water or oil, to absorb the heat and deliver same to a current of air which is passed over the surface of the radiator within which the liquid in its heated state is circulated. At present water is used as the medium in all indirect systems. Oil was used in some cases, but it presented serious disadvantages. Water may be circulated in two ways, first by the natural system which is termed the thermo-syphon system, and second under pressure through the use of a suitable pump driven by the engine.
The indirect system may be described as follows: Water is circulated from the lower water tank of the radiator through a distributing manifold to the lowest point of the cylinder water jackets, and. as it becomes heated, its specific gravity decreases, rises and flows out through the outlet manifold located on the top of the cylinders, to the top of the radiator, passing into the upper tank. From here it is distributed to various water passages through which it passes to lower tank and is recirculated. These water passages are separated by air spaces for heat radiation.
In the forced circulation, a pump draws water from the lower tank and forces it through cylinders, as well as creating a pressure to force it through the water passages of the radiator. In the thermo-syphon system, the pump is eliminated and circulation is induced by the heat of the motor. The water under the influence of heat sets up a circulation. It can readily be understood that the heat replaces the pump and acts as the moving force on the water.
Fig. 29. Sectional View of an Air-Cooled Motor.
Many different types of radiators have been worked out since the early days of the industry. The types in use at present are termed the honeycomb, cellular, vertical tube and vertical tube built-up types. The radiator is necessarily comprised of an upper and lower water tank and the core in which the water is divided into small streams, which are separated by air passages for heat radiation as shown in Fig. 30. The type of radiator is generally defined by the type of core used.
The true honeycomb core consists of a series of six-sided or hexagonal-shaped tubes fastened into the water tanks in such a way as to allow of air space between the water passages. This type is frail and not to the writer's knowledge in use at present on commercial cars.
The more popular cellular core is often called a honeycomb. This consists of a series of square tubes placed either vertically or diagonally into the tank, forming a much more rigid structure. The vertical placing is more desirable, as this provides a continuous vertical tube from one tank to the other. It offers less resistance to circulation and is not so apt to clog up from dirt, rust and other substances. Radiating surfaces are approximately equal for either construction. This construction is clearly illustrated in Fig. 31.
The vertical tube type of core consists of a series of rectangular tubes fastened into the water tanks, with fins attached to them for heat radiation, as shown in Fig. 32. The fins also materially assist in strengthening the core, and in some cases they are made continuous so that the entire construction presents a very pleasing appearance, similar to the cellular type. This vertical tube type of radiator is very much in evidence on the popular-priced vehicles, as its first cost is considerably lower than the cellular type. During the past few years there has been a decided tendency to build up radiator cores from round copper tubes with separate round or square cooling fins and cast water tanks, the upper tank usually being provided with ribs to assist in beat radiation owing to thicker section of metal necessary in casting these tanks. This type of radiator is illustrated in Fig. 33 with a tubular core and cast columns between the tanks to relieve the core of the strains due to the weight of the tanks and water.
Another popular-priced truck uses this style of radiator. However, the tubes are fastened to the tanks by clamps in series of three. This construction presents an advantage in the simplicity of repair and the small cost of replacing these sections should they be damaged beyond repair. While this type of radiator presents some advantages in strength, the writer believes that it possesses a disadvantage in that it is not as efficient as the conventional style mentioned above. Only a certain portion of the tubes is exposed to the air currents, while the rear ones are naturally limited in cooling ability, owing to being obstructed by the forward ones. The volume of water is somewhat greater in these tubes for the same rate of circulation, so that the cooling effect is somewhat retarded. The writer has experimented with both types and has come to the conclusion that the conventional type is more efficient and will stand up under the most severe service when spring-mounted.
An unusual radiator construction is shown in Fig. 34, which represents the Mack radiator for heavy duty trucks. By the use of a large number of practically semi-circular sections of copper tubes, which are expanded into plates that in turn are bolted to the upper and lower tanks, a solderless radiator is obtained, which expands within itself and withstands severe vibration without failure. Upper and lower tanks are of aluminum alloy and the upper tank forms part of the cowl, while the lower is part of the frame that supports the entire unit. The tubes are expanded into the plates and no solder is used in the construction. As each tube is a unit in itself, one or more of the tubes can be blocked, or pinched off in case of emergency without interfering with others. Instead of being placed in front of the engine this radiator is placed back of it.
Fig. 30. Cutaway Section of Radiator.
Fig. 31. Section of Cellular Type Core.
Fig. 32. Section of Vertical Tube Type.
Fig. 33. Cast Water Tanks and Built-Up Core.
Fig. 34. Mark Model "AC" Built-up-Radiator.