Heating and Cooling Systems

Heating and Cooling Systems Figure 1. Typical Engine Cooling System Introduction Engines generate a great amount of heat. This heat is created when the fuel and air mixture is ignited and expands inside the engine s combustion chamber. Metal temperatures around the combustion chamber can run as high as 1,000 degrees Fahrenheit. To prevent the overheating of cylinder walls, pistons, valves, and other engine parts, it is necessary to dispose of this heat. This is the job of the cooling system (Figure 1). Two basic types of cooling systems are used by automotive manufacturers: liquid-cooled and aircooled systems. Few automotive engines are air-cooled. These engines have a fan that is driven by the engine. The fan moves outside air over the engine. As the air passes by, it absorbs some of the engine s heat. The heated air is then forced out of the engine compartment. 1

Liquid Cooling Systems The most commonly used system is the liquid-cooled system (Figure 2). In this system, heat is removed from around the combustion chambers by a heat-absorbing liquid (coolant) circulating inside the engine. The coolant moves through internal engine passages called water jackets. The coolant is pumped through the engine and, after absorbing the heat of combustion, is sent into the radiator where the heat is transferred to outside air. The cooled liquid is then returned to the engine to repeat the cycle. These systems are designed to keep engine temperatures within a range where they provide peak performance (typically between 195 to 210 degrees Fahrenheit). Figure 2. Flow of Coolant Through An Engine's Cooling System Most coolants are called permanent antifreeze. They have this name because they prevent freezing of the liquid and because they are permanent. They are permanent in the sense that they are used in all seasons. The word permanent doesn t mean that the antifreeze is good forever. Rather, the cooling system should be flushed, and refilled with new coolant every two years. General Motors and a growing number of other manufacturers have chosen to address the reality of cooling system neglect, with an orange colored antifreeze they call DexCool. DexCool is ethylene glycol based antifreeze, but its additive package uses a patented carboxylate corrosion protection package that's free of silicates, phosphates and chemicals traditionally used with ethylene. As a result, the recommended change interval for vehicles using DexCool falls more in line with real world practices by suggesting a five year/100,000 (or even 150,000) mile coolant change interval. Look for this type antifreeze under different labels from various manufacturers. All extended protection bets are off when the orange mix is diluted with traditional green antifreeze, and suggested service intervals drop back to the traditional 2 year/30,000 mile range. 2

The engine s thermostat (Figure 3) controls the temperature and amount of coolant entering the radiator. While the engine is cold, the thermostat is closed allowing coolant to circulate only inside the engine. This also allows the engine to warm up. When the coolant reaches the opening temperature of the thermostat, the thermostat begins to open and allows the flow of coolant to the radiator. The hotter the coolant gets, the more the thermostat opens, allowing more coolant to flow through the radiator. The opening and closing of a thermostat is controlled by a pellet, rubber, or silicone material that reacts to heat. Figure 3. Operation of Typical Thermostat Coolant is actually a mixture of water and ethylene glycol-based antifreeze/coolant. Water alone has a boiling point of 212 degrees Fahrenheit and a freezing point of 32 degrees Fahrenheit at sea level. A mixture of 67% antifreeze and 33% water will raise the boiling point of the mixture to 235 degrees Fahrenheit and lower the freezing point to -92 degrees Fahrenheit. The typical recommended mixture is a 50/50 solution of water and antifreeze/coolant. 3

Figure 4. Typical Location of a Recovery Tank Most cooling systems use an expansion or recovery tank (Figure 4). Cooling systems with expansion tanks are called closed-cooling systems. They are designed to catch and hold any coolant that passes through the pressure cap when the engine is hot. As the engine warms up, the coolant expands. This eventually causes the pressure cap to release. The coolant passes to an expansion tank. When the engine is off, the coolant begins to contract and its pressure decreases. The lower pressure allows the coolant in the expansion tank to be drawn back into the cooling system. The water pump (Figure 5) moves the coolant through the cooling system. Typically the water pump is driven by the crankshaft through pulleys and a drive V-belt. The pump housing usually includes the mounting point for the lower radiator hose. When the engine is started, the pump impeller pushes the water from its pumping cavity into the engine block. When the engine is cold, the thermostat is closed. This stops the coolant from reaching the top of the radiator. In order for the water pump to circulate the coolant through the engine during warm-up, a bypass passage is added below the thermostat, which leads back to the water pump. This passage must be kept free to eliminate hot spots in the engine during warm-up. It also allows hot coolant to pass through the thermostat s valve, which will open the thermostat when it reaches the proper temperature. 4

Figure 5. A Typical Water Pump Water pumps are often replaced. They face many elements that shorten their useful life. Water pumps are normally driven by a drive belt. The constant tug of the belt on the pump s driveshaft wears the pump s bearings. This results in noisy operation. Other common failures of water pumps include broken impeller vanes, worn impeller shaft, seized bearings, and leaking seals. The radiator, located in front of the car, transfers heat from the engine to the air passing through it. The radiator itself is a series of tubes and fins that expose the heat from the coolant to as much surface area as possible. This maximizes the potential of heat being transferred to the passing air. The radiator is usually based on one of two designs: cross flow or down flow. In a cross-flow radiator (Figure 6), coolant enters on one side, travels through tubes, and collects on the opposite side. In a down-flow radiator, coolant enters the top of the radiator and is drawn downward by gravity. Crossflow radiators are seen most often on large-engine or late-model cars because all the coolant flows through the fan air stream, which provides maximum cooling. 5

Figure 6. Cross-Flow Radiator There are two types of radiator core construction: honeycomb or cellular type and the tube and fin. There are also two radiator core construction materials used: the copper/brass, soft-solder coolers, and the vacuum-brazed, aluminum cores cinched to nylon tanks. Most radiators have petcocks or plugs that allow coolant to be drained from the system. Coolant is added to the system at the radiator cap or the recovery tank depending on the type of system being used. Radiator caps (Figure 7) are designed to keep the coolant from splashing out of the radiator. They are also designed as a two way valve to allow for an increase in pressure in the radiator, which raises the boiling point of the coolant. For every pound of pressure put on the coolant, the boiling point is raised about 3-1/4 degrees Fahrenheit. Today s caps normally are designed to hold between 14 and 17 pounds per square inch (psi). This pressure raises the boiling point of the coolant. When pressures are greater than this, the seal between the cap and the radiator filler neck opens. This lowers the pressure and allows the pressurized coolant to flow into the coolant recovery tank. As the coolant temperature decreases once the vehicle is turned off, suction is created on the system side and the coolant is dawn back into the coolant system from the recovery tank keeping the system full. 6

Figure 7. Radiator Cap Radiator caps are replaced quite often, however, probably not as often as they should be. The seals of the cap weaken from the change in pressure and temperature. With bad seals, the cap can no longer maintain the desired pressure in the radiator. NEVER open a radiator cap when the radiator is hot. Because the coolant is under pressure, it will rush out of the radiator. The hot coolant can cause serious burns. The water outlet (Figure 8) is the connection between the engine and the upper radiator hose through which hot coolant from the engine is pumped back into the radiator. The water outlet has been called a gooseneck, elbow, outlet, or thermostat housing. Generally, it covers and seals the thermostat and, in some cases, includes the thermostat bypass. Most water outlets are made of cast iron, cast aluminum, or stamped steel. The primary function of the cooling system hoses is to carry the coolant to other parts of the system. Nearly all cars have an upper and lower radiator hose and two heater hoses. Some may also have a bypass hose. The majority of cooling system hoses are made of butyl, neoprene rubber or EPDM (ethylene propylene diene m-class rubber). Some hoses maybe wire reinforced, and some are used to connect metal tubing between the engine and the radiator. Normally, radiator hoses are designed with expansion bends to protect radiator connections from excessive engine motion and vibration. 7

Figure 8. The Thermostat is Often Housed in the Upper Water Outlet. Nearly all original equipment and aftermarket radiator hoses are of the molded, curved design. The hoses today use reinforcing cords, usually Rayon or Aramid, that form a knit pattern over the intertube to provide burst strength of roughly six times the working pressure of the cooling system. The number one cause of hose failure is a result of electro chemical degradation (ECD). Because modern engines use a variety of metal material in the engine compartment, insufficient grounds often result in a small amount of electrical voltage being carried in the engine coolant. This voltage eventually eats up a hose from the inside. This type of degradation starts to eat away at the hose lining, forming cracks and fissures. Once the hose reinforcement is exposed to coolant, it too begins to degrade. The weakened hose may then develop pinholes or even rupture completely. Research has shown that ECD damage is most likely to occur about 2-4 inches from the ends of the hose, not in the middle of the hose. In addition, it attacks hot hoses like the upper radiator hose faster than cooler hoses like the lower radiator hose. ECD damage also occurs often in small diameter bypass hoses or any hose that is most likely to contain air. High coolant flow rates through small diameter hose and trapped air both accelerate ECD-related damage. It is also recommended that upper radiator hoses be replaced every four years. Hoses should always be replaced in pairs and the lower radiator hoses should be replaced when the upper hose is replaced or when the water pump or radiator is replaced. All cooling system hoses are basically installed the same way. The hose is clamped onto inlet-outlet nipples on the radiator, water pump, and heater core. There are many different types of hose clamps that can be used on cooling system hoses. Drive Belts Drive belts have been used for many years. V-belts and serpentine belts are used to drive water pumps, power-steering pumps, air-conditioning compressors, alternators, and emission-control pumps. Often individual V-belts are used to drive individual parts (Figure 9). Drive belts do a lot of work and face a lot of different elements, most of which can cause them to deteriorate. A glazed, cracked, damaged or worn belt should always be replaced. Belt tension is critical to the life of a belt and to the life of the 8

part driven by the belt. Remind your customers to always refer to the manufacturer s recommendations for belt tension and to use a NAPA belt tension gauge to check the tension. Figure 9. Normally the Water Pump's and Cooling Fan's Drive Belt Also Drives the Alternator. Serpentine belts (Figure 10) are long belts whose travel allows a single belt to drive all or most of the accessories. A serpentine belt is kept under constant tension by an idler pulley or tensioner. The automatic tensioner is designed to compensate for belt stretch and to keep a proper amount of tension on the belt throughout its duty cycle. Standard V-Belts should be replaced at least once every four years, regardless of their external appearance. Belt deterioration actually accelerates as the belt nears the end of its service life, and it has been found that four years is as long as a belt should be used to avoid unexpected failure. Newer EPDM serpentine belts are more reliable than V belts and last longer. Because serpentine belts are flatter and more flexible than V belts, they run cooler, and don t retain as much heat. They re also less likely to lose their grip at high rpm. Depending on the type of tensioner that s used, belt life can exceed 100,000 miles with an automatic tensioner, or up to 60,000 miles with a fixed adjustment. Always replace the tensioner with the belt. Replacing the tensioner will ensure the replacement belt will provide reliable service for another 60-100K miles. 9

Figure 10. The Routing of a Typical Dual Serpentine Drive Belt Fans and Fan Clutches The efficiency of the cooling system is based on the amount of heat that can be removed from the system and transferred to the air. The system needs air. At highway speeds, the ram air through the radiator is sufficient to maintain proper cooling. At low speeds and idle, the system needs additional air. This air is delivered by a fan. There are two basic types of cooling fans: mechanically driven and electrically driven. The design of fans varies with the air requirements of the engine s cooling system. Diameter, pitch, and the number of blades can be varied to attain the needed flow. Fans are usually shrouded to maintain efficiency by causing all of the flow to pass through the radiator. Mechanically driven fans are driven by the engine through a V-belt. They may be made of steel, nylon, or fiberglass, and are precisely balanced to prevent water pump bearing and seal damage. Some fans are fitted with a fan clutch. This unit connects the fan to the drive pulley (usually on the water pump shaft). Cooling fan clutches may be speed-controlled or temperature-controlled. The clutch slips at high speeds or low temperatures, therefore the fan does not turn a full engine speed. 10

Figure 11. Electric Cooling Fan Most late-model cars use an electrically drive fan (Figure 11). This fan and motor are mounted to the radiator shroud and are not connected mechanically or physically to the engine. The 12-volt, motordriven fan is electrically controlled by either, or both, of two methods: an engine coolant temperature switch or sensor, and the air-conditioner switch. Temperature Indicators Coolant temperature indicators are mounted in the dash to alert the driver of an overheating condition. These indicators may be a temperature gauge and/or a light. A temperature sensor is screwed into a threaded hole in the water jacket. The sensor sends electrical signals to the indicator. If a gauge is used, the temperature of the coolant will be displayed. If a warning light is used, the lamp will light when temperature increases beyond a certain point. Besides indicating coolant temperature to the driver, temperature sensors supply some important information to today s computer-controlled enginecontrol systems. 11

Heaters A hot liquid passenger compartment heater is part of the engine s cooling system. Heated coolant flows from the engine through heater hoses and a heater control valve to a smaller radiator, or heater core, located in the passenger compartment. A fan directs air over the hot heater core and the heated air flows into the passenger compartment. The heating system s primary job is to provide a comfortable passenger compartment temperature and to keep car windows clear of fog or frost. The main components of the heating system are the heater core, the heater control valve, the blower motor and the fan, and the heater and defroster duct hoses. In the cooling system, hot coolant from the engine is transferred by a heater hose to the heater control valve and then to the heater core inlet. As the coolant circulates through the heater core, heat is transferred from the coolant to the tubes and fins of the heater core. Air blown through the heater core by the blower motor and fan then picks up the heat from the surfaces of the heater core and transfers it into the passenger compartment of the car. After giving up its heat, the coolant is then returned to the engine to be recirculated by the water pump. The heater core is much like a miniature radiator. It features two tanks, with an inlet and outlet tube and a tube and fin core to facilitate coolant flow between them. The heater control valve (sometimes called the water flow valve) controls the flow of coolant into the heater core. In the closed position, the valve allows no flow of hot coolant to the heater core, keeping it cool. In an open position, the valve allows heated coolant to circulate through the heater core, providing heat. Heater control valves are operated in three basic ways: by cable, thermostat, or vacuum. Cableoperated valves are controlled directly from the heater control lever on the dashboard. Thermostatically controlled valves have a liquid-filled capillary tube located in the discharge airstream of the heater core. This tube senses air temperature, and the valve controls the flow of coolant to maintain a constant temperature, regardless of engine speed or temperature. Most heater valves are vacuum operated. These valves are normally located in the heater hose line or mounted directly in the engine block. A dash switch or lever controls the vacuum to the valve. Vacuum at the valve either opens or closes the valve, depending on the design. The blower motor is usually located in the heater housing assembly (Figure 12). It pushes air across the heater core into the passenger compartment. Its speed is controlled by a multi-position switch in the control panel. The switch works in conjunction with a resistor block that is usually located on the heater housing. 12

Figure 12. Location of the Blower Motor Transferring heated air from the heater core to passenger compartment heater and defroster outlets is the job of the heater and defroster duct hoses. These hoses vary in diameter from 1-1/2 inches to 6 inches. 13

Key Terms Coolant the heat absorbing liquid used in automotive cooling systems. Coolant is actually a mixture of water and ethylene glycol-based antifreeze. Expansion or recovery tank designed to catch and hold any coolant that passes through the pressure cap when the engine is hot. Fan clutch these units connect the fan to the drive pulley (usually on the water pump shaft) only when the engine is operating at certain speeds or temperatures. Heater core a small radiator, located in the passenger compartment, which transfers heat from the coolant to air inside the car. Radiator a large unit located in the front of the vehicle which contains many tubes, surrounded by cooling fins. Coolant from the engine moves through these tubes and the coolant s heat is transferred to outside air. Radiator cap Attached to the top of the radiator. They are designed to keep the coolant from splashing out of the radiator and to allow for an increase in pressure in the radiator, which raises the boiling point of the coolant. Serpentine belt long drive belts whose travel allows a single belt to drive all or most of the accessories. Thermostat a part of the cooling system that controls the engine s temperature and amount of coolant entering the radiator. Water jackets the internal engine passages that engine coolant moves through. Water outlet the connection between the engine and the upper radiator hose through which hot coolant from the engine is pumped into the radiator. Water pump moves the coolant through the cooling system and is typically driven by the crankshaft through pulleys and a drive V-belt. 14