Aircraft technical Basics: TM 1-405, Aircraft Aircraft Engines, 1941: VI - Engine Cooling

SECTION VI - ENGINE COOLING

48. General. - The . power developed by an engine is directly proportional to the heat of combustion ; however, it is essential that the operating temperatures of the engine are maintained within safe limits to prevent preignition, valve warpage, spark-plug failure, and other attendant disadvantages of a hot engine. The heat loss, charged directly to cooling, is approximately 30 percent of the total heat generated and cannot be appreciably reduced without decreasing the reliability of engine operation. Therefore, in order to utilize heat as power the engine must be adequately cooled. This is accomplished either by air or liquid cooling.

49. Fundamentals of air cooling. - a In cooling aircraft engines by air, the following factors are of importance:

(1) The rate of cooling is directly proportional to the area of the surface exposed to the cooling medium.

(2) The rate of cooling is dependent upon the thermal conductivity of the metal used, especially in the cylinder heads.

(3) The rate of cooling is dependent upon the volume of metal or cross section, consistent with conductivity, surface and the mass flow of air. Cylinder head fins taper from a comparatively heavy base cross section to a very thin cross section area at the tip.

(4) The rate of cooling varies almost directly with the mass of air flow over the heated surfaces. The use of pressure baffles around the cylinders increases the cooling efficiency in flight but often decreases it during ground operation.

(5) The rate of cooling varies directly with the difference in temperature between the metal surfaces exposed and the cooling air. Hence, a higher rate of cooling is obtained in cold weather than in hot weather.

b. As previously stated, air-cooled cylinders are made up of steel barrels and light alloy heads heavily finned for strength and adequate cooling. Improvement of the rate of cooling by means of fins can be readily observed by referring to figure 43. Exposed fin areas have been increased from approximately 600 square inches to 2.800 square inches per cylinder to adequately care for the tremendous increase in combustion temperatures obtained with the use of improved fuels. Special attention is given to the most effective distribution of the fins to provide uniform cooling over the entire combustion chamber, including the spark-plug bosses and exhaust valve seat.

c. Air deflectors or pressure baffles are used extensively in high performance aircraft engines to obtain a more effective use of the cooling air and to increase the velocity of the air flow over the cooling fins. Figure 44 illustrates a typical pressure baffle system for a twin-row engine. A disadvantage in the use of pressure baffles is that inadequate cooling during ground operation may result in engine overheating; therefore, ground operation must be restricted to a minimum before take-off and flight.

FIGURE 43. - Fin arrangement on air-cooled cylinders.

d. In addit ion to the heat dissipated from the engine cylinder fins, a certain amount is conducted to other parts of the engine and radiated into the cooling medium. The circulating oil also absorbs heat which is dissipated through the oil cooler.

c. The cowling surrounding an engine installed in an airplane must be considered as a vital element in the cooling system. Its design being dependent upon the design of the aircraft in which the engine is installed.

f. The operating temperature of an air-cooled engine is measured by a temperature indicator and thermocouple attached to the barrel or cylinder head of the hottest running cylinder, which is usually the one carrying the master connecting rod.

FIGURE 44.  - 'Typical  pressure baffles for a twin-row engine.

50. Fundamentals of liquid cooling. a. In cooling aircraft engines with liquid, the following factors are of importance:

(1) The rate of cooling is directly proportional to the amount of coolant brought into contact with the propeller slipstream. The large amount of surface required is obtained through the use of a satisfactory radiator made up of numerous tubes, arranged so as to produce a minimum of frontal resistance. Other surfaces, such as the cylinder jacket, manifolds, etc., assist in the rate of cooling.

(2) The rate of cooling is dependent upon the thermal conductivity of the metal used, especially in the construction of the radiator tubes or cores.

(3) The rate of cooling is dependent on the volume of metal or cross section in the radiator tubes or cores, consistent with their conductivity, their surface area, and the mass flow of air around them. In high output liquid-cooled engines, this factor becomes very important in the design of efficient radiators.

(4) The rate of cooling varies almost directly with the mass of air flow through the radiator. Provisions are usually made to control the air flow through the radiator by the use of shutters, either manually or automatically operated.

FIGURE 45.-Liquid-cooled cylinder jacket.

(5) The rate of cooling varies with the difference in temperature between the coolant and the cooling air, resulting in a higher heat transfer in cold weather.

(6) The rate of cooling varies with the,rate of liquid flow through the system, up to a certain critical velocity. Above this critical velocity turbulent flow or surface scouring occurs, and the rate of liquid flow does not increase the cooling effect, but does increase operating pressures in the system.

b. As previously stated, liquid-cooled cylinders are made up of a bank of several steel barrels inserted in a light alloy casting or jacket. This jacket (fig. 45) provides the recesses through which the cooling liquid is circulated by the pump (fig. 46), absorbing heat from the cylinder barrels and transferring it to the radiator for dissipation.

c. The use of ethylene glycol in its pure state or by dilution in water is preferred over water alone, as its high boiling point (350° F., l77° C.) will preclude evaporation of coolant liquid at normal engine operating temperatures which may be considerably higher (250° F.. 121° C.) than when using water alone. This permits the use of a comparatively small radiator, resulting in a low weight per horsepower and a minimum of head resistance.

FIGURE 46. - Coolant pump

d. As in air-cooled engines, a certain amount of heat is conducted from the cylinders to other parts of the engine and radiated into the air stream. The circulating oil also absorbs heat from the engine and dissipates it in circulating through the oil cooler.

c. The operating temperature of a liquid-cooled engine is measured by a thermometer installed in the coolant discharge outlet line to the radiator.

51. Coolants. - a. There are a large number of liquids which may be used as coolants for internal-combustion engines; however, in order to meet the desired requirements in liquid-cooled aircraft engines, this number is restricted to water and ethylene glycol.

(1) Water is universally used as an engine coolant largely because of its availability and high specific heat value. The disadvantages in its use in aircraft engines, compared to ethylene glycol, are its high rate of evaporation or low boiling point, its high freezing point accompanied by the fact that it expands when frozen, resulting in probable damage to the cooling system, and its impurity.

(a) Its comparatively low boiling point of 212° F. (100° C.), necessitates the use of a large radiator in the cooling system to maintain an operating temperature below 190° F. (88° C.), and a comparatively large amount of liquid. This results in a high head resistance and a high weight per horsepower in aircraft engines.

(b) Its high freezing point of 32° F. (0° C.), presents a problem in maintaining the water above that temperature when the power plant is inoperative in cold weather.

(e) Inasmuch as most of the water used in engine-cooling systems comes from wells, rivers, and lakes, even though filtered, it is some-what impure, and frequent draining and flushing of the system is necessary.

(2) Ethylene glycol ( Prestone) is a chemical substance made up of ethylene oxide (CH₂)₂O combined with H₂O (water) to the saturation point, forming a molecular structure C₂H₆O₂,. Ethylene is obtained from petroleum oil or by the destructive distillation of carbonaceous matter. The term glycol was designated by the chemist who originally combined the structure C₂H₆O₂,. Ethylene glycol is a colorless liquid having practically no odor, is nonpoisonous, moderately noninflammable, nontoxic, noncorrosive, and nonelectrolytic in action. The advantages of ethylene glycol over water as a coolant are its high boiling point and low freezing point, accompanied by the fact that when freezing it contracts instead of expands. An additional advantage is its comparative purity. The disadvantages are low specific heat, resulting in higher engine temperatures with a slight loss in power and its limited availability.

(a) The high boiling point of ethylene glycol, which is in excess of 350° F. (177° C.), permits the use of a smaller radiator than if water is used. Obviously, the smaller radiator reduces head resistance and weight per horsepower.

(b) Its low freezing point of approximately 0° F. (-18° C.), and the fact that it does not freeze solid until a temperature of -48° F. (-45° C.) is reached, protects the cooling system during extremely cold weather. However, no attempt should be made to start and operate an aircraft engine when the ethylene glycol is at a temperature below 0° F., as it will not circulate properly while in the form of slush. When it is known in advance that it will be exposed to temperatures below 0° F., for an extended period of time, it should be drained from the cooling system while hot and heated before it is replaced in the system.

(c) Although ethylene glycol is considered pure when first placed in the cooling system, its natural tendency to loosen rust, scale, etc., from certain parts, requires frequent draining and thorough straining before it is returned to the system.

(d) In military service, ethylene glycol conforming to current specifications is used as a coolant in aircraft engines.

b. In engines using water as a coolant, it is advisable in cold weather to add some kind of a soluble substance to prevent it from freezing and damaging the cooling system. Ethylene glycol, glycerin. and alcohol are satisfactory for this purpose. Of these. ethylene glycol is considered the best. It is soluble water in any proportion and when mixed with water, it freezes in the form of slush instead of solid.

(1) The prospective temperatures to be encountered govern the amount of ethylene glycol to be used with water. The percent. by volume, required to prevent freezing of the solution at different temperatures is indicated in figure 47. Ethylene glycol does not evaporate at the usual operating temperatures; therefore. under ordinary conditions only the evaporated water need be replaced. In the event that some of the solution has leaked out of the system through joints, connections. etc., it will be necessary to add more ethylene glycol ; the amount may be determined by a thermohydrometer ora  standard specific gravity hydrometer test.

(a) The thermohydrometer is of similar construction and is used in the same manner as the ordinary battery hydrometer. It has a small thermometer enclosed within a floating bulb and the readings are made from two scales and a chart enclosed within the hydrometer instead of one scale as with the ordinary hydrometer. This type of hydrometer gives a direct reading of the temperature at which the solution will freeze and is preferred to the specific gravity hydrometer which gives only the percentage of ethylene glycol in the solution. The specific gravity reading should be obtained with the solution at 60° F., and in addition, requires the use of a conversion table to find the actual temperature at which the solution will freeze. Detailed instructions for reading the various hydrometers are usually included in their containers.

(b) Ethylene glycol expands, when heated, considerably more than water at the same temperature ; therefore, care must be exercised to provide the proper amount of expansion space in the cooling system when it is used as an antifreeze solution. This is usually accomplished by draining off one gallon of the solution after filling the system to its normal water capacity.

FIGURE 47.-Ethylene glycol antifreeze solution.

(2) Glycerin is used to some extent as an antifreeze in water-cooled engines. Its use in aircraft engine cooling systems is not desirable, due to its gumming characteristics and difficulties encountered in preventing it from leaking out of the system.

(3) Alcohol is used extensively as an antifreeze in automotive engines. largely because it is inexpensive. The commercial alcohols furnished for this purpose are, known as denatured alcohols, made up principally of ethyl (grain) alcohol and methyl (wood) alcohol. The greater the percentage of ethyl alcohol in the mixture, the better it is for antifreeze purposes, as ethyl alcohol has a higher boiling point than methyl alcohol, therefore less is lost by evaporation. Due to the high operating temperatures of aircraft engines, the use of denatured alcohol as an antifreeze is prohibited.

52. Cooling systems. - Although the cooling fundamentals involved in the design and construction of an aircraft engine are highly efficient, the problem confronting the aircraft designer is in maintaining high efficiency with a minimum of head resistance. This problem applies to both air-cooled and liquid-cooled aircraft engines.

a. In air-cooled engine installations high cooling efficiency is maintained by mounting streamlined ring type cowling around the outside circumference of the cylinders. The cooling efficiency of a well-designed cowling ring varies somewhat with the speed of the aircraft or velocity of the air flow. As a general rule, cowling designed for high efficiency at high air velocities is somewhat inefficient at low air velocities, particularly when operating the engine on the ground. To overcome this difficulty, controllable flaps may be incorporated in the trailing edge of the ring cowling or by the development of a suitable low-pressure system.

(1) Some air-cooled engine installations in low-performance air-planes incorporate a cowling over the front crankcase section, providing a means of control by which the airflow can be circulated in and around, the crankcase in warm weather and partially closed off in cold weather.

(2) Present air-cooling systems are designed to maintain proper engine temperatures at maximum permissible power and rpm in level flight or at the best climbing speed when the ground temperature is 100° F. (38° C.) or lower. The cylinder-head temperature should not exceed 280° C. and the base temperature should not exceed 165° C. for safe engine operation. Normal head values range from 160° C. to 200° C. and normal base values from 120° C. to 140° C. Consult operating instructions for specific values of individual engines.

(3) The head temperature may be measured with a thermocouple mounted at the spark plug as illustrated in figure 48. This type of thermocouple is usually installed in place of the standard spark-plug gasket in the master connecting-rod cylinder. Where an attachable spark-plug shield is used, the thermocouple gasket is installed between the shield and cylinder head. The two thermocouple wires are of iron and iron constantin, each wire properly designated for correct installation on the cylinder temperature indicator in the aircraft cockpit. In lieu of the spark-plug type of thermocouple, the two thermocouple wires may be imbedded in the master-rod cylinder head or base flange and connected to the indicator in the same manner.

(4) Inasmuch as the cooling of the air-cooled engine is assisted by the circulating oil, an abnormal cylinder temperature indicator rise may be caused by an abnormal "oil in" temperature rise due to lack of oil, improper oil cooling, or an oil line restriction, hence the necessity for stopping or "throttling down" the engine when overheating is indicated.

b. While the cooling system of an air-cooled engine is more or less self-contained, such is not the case in a liquid-cooled aircraft engine.

Figure 48. - Spark-plug type of the thermocouple.

In addition to the cooling principles involved in the design and construction of liquid-cooled engines, there are a number of auxiliary units installed in the aircraft which form an integral part of the complete cooling system. These units include the radiator, auxiliary expansion tank, and the necessary plumbing connecting them to the engine. A thermometer and gage are incorporated in the system to indicate the operating temperature of the coolant. A controllable radiator shutter assembly and one or more centrifuge tanks may also be included.

( 1) The radiator performs the function of maintaining the temperature of the coolant within safe limits with a minimum of head resistance. The cowling installed around the radiator constitutes an important item, as its design controls the volume of air directed through the radiator. Satisfactory design and construction permits a maximum  amount of cooling with a minimum amount of weight. The radiator core is usually constructed of copper and the tanks or headers of brass.

(2) The cartridge core type of radiator as shown in figure 49 is a conventional type usually strapped or mounted in cradles with shock absorbing pads or cushions placed between the radiator and its main support.

(3) A shutter assembly, preferably of the single vane balanced type may be installed on the exit side of the radiator and may be controlled manually from the cockpit., or automatically through a thermostatically operated control assembly. When a combined manual and automatic control is used the automatic feature functions as a safety device in automatically opening the shutters when the coolant reaches a predetermined temperature.

FIGURE 49. - Cartridge core type radiator.

(4) The auxiliary expansion tank (fig. 50) is used in conjunction with the. radiator to provide for an expansion space for the coolant in the system as it becomes heated. It is installed at the highest point in the system and serves as the filler unit through which the entire system is serviced with coolant. The bottom of the expansion tank is connected to the top of the radiator through suitable plumbing. The capacity of the expansion tank and its outlet pipe to the radiator is 10 percent of the total capacity of the system plus 1 gallon. For example, if the total capacity of a cooling system is 10 gallons, the expansion tank and line to the radiator holds 2 gallons of that amount. A unique feature of the expansion tank is the installation of two valves in the filler unit (fig. 51). The large poppet valve operates and relieves internal pressure as the circulating coolant is heated and the small ball check valve permits admission of outside air into the system as the coolant temperature decreases. The automatic operation of both of these valves maintains a constant differential air pressure in the expansion tank.

FIGURE 50. - Auxilliary expansion tank

FIGURE 51. - Expansion tank filler unit.

(5) The plumbing in a liquid-cooled system includes piping, flexible hose connections, and necessary drain plugs. The piping is usually of brass or aluminum alloy tubing with a minimum wall thickness of 0.050 inch under the hose clamps. Aluminum alloy tubing is protected from possible corrosion by an anodic treatment. Rubber hose is used in flexible connections and is deterioration resistant. When installed, the hose should not have less than 1/4-inch or more than one pipe diameter exposed to the coolant. Standard hose clamps are used to hold the connections in place and prevent leakage of the coolant. In some instances it may be necessary to install additional clamps to prevent the leakage of ethylene glycol when it is used as a coolant. A large size drain plug is located in the lowest part of the system for draining the coolant.

(a) Water piping is marked with a white band near each union and on each side of every flexible connection. Ethylene glycol piping is marked with a black band bordered by two white bands.

(b) A thermometer well (fig. 52) is incorporated in the coolant outlet line from the engine to the radiator. The thermometer bulb is installed in the well and the outlet temperature of the coolant is indicated on the thermometer gage in the aircraft cockpit.

(6) In liquid-cooled systems, where leakage of the exhaust gases through cylinder gaskets or joints may occur, centrifuge chambers (fig. 53) are installed in the engine discharge. lines to the radiator. The turbulence of the coolant in circulating through the centrifuge chamber separates the gas vapors from the coolant and directs the vapors through the chamber vent line to the expansion tank. If gases were permitted to accumulate in the coolant circulating lines there would be a probability of gas-pocket formation which would interfere with proper circulation of the liquid. A general conception of the unit arrangement in a cooling system. with the exception of a radiator shutter, may be obtained by reference to figure 54.

(7) In the operation of water-cooled power plants care must be exercised to prevent water temperature from exceeding 85° C. Where ethylene glycol coolant. is utilized it must not exceed 150° C. With an adequate supply of coolant and proper functioning of the system, excessive temperatures may be prevented by opening the shutter control if this unit is installed. In case no shutter control is installed, high coolant. temperatures may be reduced by retarding the throttle or by enriching the mixture with the mixture control.

FIGURE 52. - Thermometer well.

At full throttle, approximately 100 gallons of coolant are circulated per minute through the entire system when the coolant is at a temperature of 60° C. In installations where no radiator shutters are employed, it is advisable to partially blanket the radiator cores in cold weather with fiberboard or other suitable material to maintain a minimum of approximately 60° C. outlet temperature.

FIGURE 53 . - Centrifuge chamber.

FIGURE 54. - Typical liquid cooling system.