1、The Engineering Society mAEFor Advancing Mobility -Land Sea Air and Space A IN TENATIONLw Principles of Engine Cooling Systems, Components and Maintenance SAE HS-40 Reprint COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesPrinciples of Engine
2、 Cooling Systems, Components and Maintenance SAE HS=40 Report of the Transportation Jl4, Radiator Caps and Filler Necks; J631A, Radiator Nomenclature; 58 14C, Engine Coolants; 5868, Large Size Radiator Filler Cap and Necks; 51004, Glos- sary of Engine Cooling System Tenns; J1148, Engine Charge Air C
3、ooler Nomenclature; and 51244, Oil Cooler Nomenclature And Glossary. Further reference to these numbers will be found throughout the text when relevant subjects are discussed. 1 COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling Services2. INTERRELATIO
4、N OF COOLING SYSTEM AND OTHER ENGINE SYSTEMS The cooling system, the fuel system, the lubrication system, the exhaust system, and many accessories are very interrelated from a performance standpoint. The, improper performance of any one of these systems will adversely affect the performance of the o
5、thers. A properly performing cooling system is one that has clean surfaces, both internally and externally, and has cooling system components such as the radiator, thermostat, pressure cap, and water pump, performing within the design require- ments of the equipment manufacturer. The enginehehicle m
6、anfacturer recognizes the interrelation of the cooling system with other engine systems. Consequently, recom- mendations of coolant type and operating temperature should be followed in order to protect the engine and related controls, as well as all heat exchangers in the cooling system. are cooled
7、by the cooling system and the lubrication system, and to some extent by the fuel system. The cooling system serves not only to prevent the heat created in the combustion chamber from damaging or melting pistons, valves, and cylinder heads, but also partially to control the operating temperature of t
8、he crankcase lubricant. In this chapter, the effect of a malfunctioning cooling system on the various engine or vehicle systems will be described. The operating parts of an internal combustion engine 2.1 ENGINE LUBRICATION SYSTEM coolant temperature on oil temperature. (Au engine conditions are simi
9、lar except for the change in coolant temperature.) In addition to lubricating, keeping parts clean, and acting as a sealant against extensive piston ring blowby, the engine lubricant must cool moving parts and remove waste heat. Any change in the lubricating system that detracts from its performance
10、 will be detrimental. If the cooling system overheats, the lubricant may rise to an excessive temperature and form varnish-like depos- its that can cause interference between closely fit parts. 2.1.1 Effect of Overheating - Fig. 1 shows the effect of 3 105 (221) 100 (212) 95 (203) 90 r o g (194) 8 8
11、5 3 (1%) - O c Lo (176) 75 (167) 70 (1W 65 70 80 90 100 110 (140) (158) (176) (194) (212) (230) Coolant Temp. Co(F) - Engine Outlet Fig. I Eflect of Coolant Temperature on Oil Temperature These deposits can also interfere with the transfer of heat so that metal parts may be subjected to excessive te
12、m- peratures, which will result in early failure. The increase of metal temperature caused by overheating of the cooling system may cause burned valves, scored pistons, and damage to bearings or other parts. Overheating will increase oil consumption, since the lower viscosity of the oil (caused by i
13、ncrease in oil tem- perature) will allow it to pass through smaller clearances. Increased oil consumption will materially affect the amount of combustion chamber deposits formed during a given operating period. Lower oil viscosity may also decrease oil film strength and bearing life. COPYRIGHT SAE I
14、nternational (Society of Automotive Engineers, Inc)Licensed by Information Handling Services2.1.2 Eflect of Overcooling - Too much cooling (over- cooling), resulting from (for example) a faulty thermostat, a lack of a thermostat, or a thermostat with too low a temperature setting, causes equally bad
15、 engine lubricating problems. Overcooling prevents the engine from quickly wm- ing up to proper operating temperature, thereby allowing water sludge formation and fuel dilution to occur in the oil. When fuel is bumed in the engine, water is formed as one of the by-products of combustion. Most of thi
16、s water vapor passes out the exhaust pipe but some may blow past the piston rings into the crankcase. Water is also formed as the crankcase ventilation system draws mois- ture bearing air through the crankcase. If the engine parts are cold, the water condenses and drips into the oil reservoir. There
17、, it is churned into the lubricating oil by the action of the crankshaft, forming a thick sludge. This may block the oil pump or plug oil lines, preventing proper lubrication of moving parts and causing them to wear rapidly or, in severe cases, to seize. Subnormal engine operating temperatures can c
18、ause poor fuel vaporization and combustion efficiency. Unburned fuel may blow past the piston rings into the crankcase and dilute the oil, thus reducing its lubricating properties. At a normal engine operating temperature, the water and unburned fuel are vaporized and passed out the road vent breath
19、er tube or are returned to the induction system in engines using positive crankcase ventilation. Tests have shown that the wear on cylinder wails is eight times greater with a coolant operating temperature of 38C (100F) compared with 82C (180F). Fuel con- sumption increases while power and efficienc
20、y decrease at subnormal temperatures. 2.1.3 Internal Leakage of Coolant into Engine - Coolant, either water or antifreeze solutions, leaking or seeping into the engine through leaky joints or through cracked or porous water jacket walls can produce varnish or sludge in the engine and impair proper l
21、ubrication. In severe cases, complete engine breakdown with seizure may result. It is essential that the cylinder head to engine bIock joints be tight and that the other joints and parts be maintained in a leaktight condition to avoid internal coolant leakage. gines with a low oil level or inadequat
22、e oil pressure permits rapid build-up of heat in the oil and in metal parts. The cooling system may not carry away the addi- tional heat load fast enough to maintain proper operating temperatures under these conditions. Extensive engine damage may result by a chain reaction when critical engine part
23、s are not adequately cooled because of im- 2.1.4 Low Oil Level or Pressure - Operation of en- proper lubrication. Therefore, always maintain oil level and pressure at the levels recommended by the manufacturer. 2.2 FUEL SYSTEM bum smoothly and evenly in the combustion chamber. The start of ignition
24、must be controlled. If the fuel is ignited prematurely during the burning cycle, the engine will suffer a power decrease and excessive noise and vibration. Such abnormal combustion can overheat and overstress parts, causing early failure. 23.2 Effect of Overheating - In engines, increasing the tempe
25、rature in the combustion chamber may produce abnormal combustion. By carrying away the waste heat, the cooling system plays an essential part in avoiding excessive heat build-up and damaged engine parts. In some gasoline engines, an abnormal rise in coolant temperature may affect engine knock, limit
26、 power and increase octane requirements. Thus, a dirty or defective cooling system with its less effective heat removal capacity can be the direct cause of engine “knock” with a fuel that would have otherwise been fully satisfactory for the engine. The year-round use of 45 to 70% coolant concentrate
27、 (antifreeze)/water mixture as coolant is now recom- mended by most engine and vehicle manufacturers. Its higher boiling point provides for increased heat transfer capacity at the radiator, i.e the boiling temperature of the coolant mixture is raised, allowing the engine to operate at a higher air t
28、emperature before boiling occurs. On the other hand, the lower specific heat of the coolant concen- trate/water mixture reduces the ability of the radiator to transfer heat, resulting in slightly higher engine and coolant temperatures, than if water alone had been used as the fluid. The net effect i
29、s still positive, in favor of the use of coolant concentrate/water mixture, because of its higher boiling temperature. Some vehicle manufacturers have had their cooling systems designed for and tested with coolant concentrate mixture, to determine the radiators cooling ability. Thus, the engine radi
30、ator is already sized for the use of approx- imately 50% coolant mixture on a year-round basis. The major advantage of the use of a coolant mixture other than boiling or freezing protection, is the control of internal corrosion on all metals in the engine and radiator. When using aluminum radiators,
31、 water pumps, thermostat housings, etc., this becomes extremely important. During the months when ambient air temperatures are highest, extreme under hood temperatures may cause vapor lock in the fuel system. However, it is wrong to run an engine “cool” by removing the thermostat. Modem To obtain op
32、timum performance from the fuel, it must 4 COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling Servicesengines are designed to operate with a specific thermostat opening temperature. The engine manufacturers thermo- stat specification must be adhered to
33、. 2.2.2 Induction System - Some engines have liquid cooled intake manifolds. If either overcooled or under- cooled, they will suffer a loss in performance (power output or fuel economy). Overheating will decrease the density of the air-fuel mixture thereby reducing the amount of oxygen available to
34、bum the fuel, which will result in loss of performance. Overcooling of the liquid cooled intake manifold may result in improper vaporiza- tion of the fuel with the result that some cylinders will run rich and some lean, resulting in a loss in power and fuel economy, and increased exhaust emissions.
35、2.3 IGNITION SYSTEM Late or retarded ignition timing may be the cause of engine overheating. When the fuel charge is fired late, a greater portion of the heat generated is wasted and must be carried away by the cooling system. This is particu- larly significant in engines with considerable exhaust p
36、assage area within the cylinder head. If the cooling system is already operating at near capacity, overheating may occur. When the ignition timing is late, power is lost and fuel economy is decreased. As stated previously, an excessively dirty cooling system may necessitate the use of higher octane
37、fuel. The use of lower octane number fuel may be tolerated if the ignition timing is retarded. This action, of course, results in a decrease in fuel economy. If the ignition is retarded, burning of the exhaust valves may result. Improper cooling of the combustion chamber caused by a severe loss of c
38、oolant, poor coolant circulation, or heavy cylinder head scale deposits may result in detona- tion or pre-ignition, with the result that spark plugs and/or injectors may be damaged and life drastically reduced. 2.4 EXHAUST SYSTEM Since the exhaust system normally removes as much waste heat as the co
39、oling system, any obstruction or defect that reduces the flow of exhaust gases (increased back pressure) will result in greater exhaust area tempera- res. The increased load on the cooling system may not be adequately carried away. The result will be overheat- ing and a loss of power and early exhau
40、st valve failure. 2.5 INDUCTION SYSTEM Some engines utilize turbochargers to increase engine power output. Some of these engines are also equipped with charge air coolers (CACs), whose purpose is to cool the induction air from the turbo before it is introduced into the engine for combustion, as a fu
41、rther means of increasing power. Some of these (CACs) are mounted in front of the radiator. CACs in this position usually reduce the cooling ability of the radiator. The factory installed radiator was designed to meet these conditions. It is essential that the cooling air passages be kept free of de
42、bris. Air to liquid charge air coolers are also used as a means of cooling the induction air. These are mounted directly to the engine for most applications, so their heat load is included in the engines heat load. It is important that the coolant passages be kept clear of products of corrosion depo
43、sited by the fluid circulating within the engine. (See Section 3.2 for more operating details and Section 3.2.1 for details of construction). 2.6 OIL COOLING SYSTEMS The automatic tmsmission fluid in some vehicles depends, for part of its cooling, on circulation through a cooler (heat exchanger) loc
44、ated in the cooling system. If the cooling system is operating near its limit and the automatic transmission fluid runs excessively hot because of low fluid level, mechanical maladjustment or failure, overheating of the cooling system may result. Con- versely, if the engine is overheating, the coola
45、nt cannot do a satisfactory job of cooling the transmission fluid, and high transmission fluid temperatures will be encountered. Varnishing of the precision parts of the automatic transmission, seal failure, or “chattering” with subsequent loss of power transmission (due to insufficient fluid viscos
46、ity) will result Iiom excessively high automatic transmission fluid temperatures. Corrosion deposits on the transmission cooler may cause the transmission fluid to operate at a higher temperature than normal. Leakage of the cooler or heat exchanger, which will allow mixing of the transmission fluid
47、and coolant, may be expected to result in severe damage to the transmission and/or the engine. Oil coolers for automotive engine transmissions may be water or air cooled, and sometimes used simultane- ously. The water cooled oil cooler is located in the return tank of the radiator, while the air coo
48、led type may be in the radiator air stream or remotely located where ram air assists the cooling. Air cooled oil coolers may be used when hauling a trailer, boat, etc. Larger engines used in trucks, farm machinery, on and off-highway and mining vehicles, etc., may require oil coolers of considerable
49、 size and may be of the coolant or air cooled type. Engine mounted coolant cooled oil coolers are usually of the shell and tube or the tube type and are used to cool automatic transmissions and/or retarders, power take-Offs and hydraulic powered systems. COPYRIGHT SAE International (Society of Automotive Engineers, Inc)Licensed by Information Handling ServicesAir cooled oil coolers cooling similar devices may have tube and fin coolers mounted ahead of the engine coolant radiator, perhaps even combined in parallel with a charge air cooler, compressed air interhfter cooler or A/C co
