Heating and Air Conditioning, Part 1

Transcription

1 1 Heating and Air Conditioning, Part 1 AUTOMOTIVE HEATING AND ENGINE COOLING SYSTEMS The Automotive Engine Creates Heat An automobile or truck engine is basically an air pump. The fuel that s fed to the engine is burned in a compressed form to maintain the rotation of the motor and the motion of air through the motor. Each time a cylinder ignites, the burning fuel and air mixture passes some of its heat to the engine block. The rest of the heat of the burning air/fuel mixture is passed on down the exhaust system to exit the vehicle. If this heat were allowed to accumulate, the engine would get so hot that the expansion of the cylinder block and the pistons would finally cause one or more pistons to seize in their bores. This often occurs when someone unknowingly drives a vehicle with a cooling system leak. Another cause of pistons seizing is when someone drives a vehicle with a broken fan belt. When this happens, the water pump can t circulate coolant through the cooling system components. (You should know that an engine is designed to operate with its built in metal-tometal surface tolerances when the engine is at about 195 F. A higher temperature will decrease the range of these tolerances, while too low a temperature will provide engine tolerances that are too high.) A second source of engine heat is friction. Many people think of engines as devices that turn easily. However, if you were to remove the spark plugs from a motor and try to turn the crankshaft, you would find the engine very difficult to turn. The many metal-to-metal contact points within the assembly cause this friction. As a result, many engines are now designed to run on low-viscosity oils to help decrease friction and increase fuel mileage. Methods of Removing Heat Figure 1 displays a modern vehicle s engine and engine cooling system. This system is called a water-cooled system, since water, mixed with a suitable antifreeze, is circulated through the system to lower engine heat. As you can see in this figure, a radiator and a series of cooling hoses are hooked to the engine.

2 2 Heating and Air Conditioning, Part 1 FIGURE 1 An engine s cooling system is comprised largely of the radiator and cooling hoses. A side view of the same type of system is shown in Figure 2. This view more clearly displays the flow of coolant through the water pump, the engine block, the thermostat, the upper radiator hose, the radiator, and finally the lower radiator hose, at which point it flows back to the engine block. This is known as a closed-loop system. FIGURE 2 This drawing illustrates the path of coolant through an engine s cooling system. At the engine block, the water/antifreeze mixture picks up engine heat and transfers the heat to the radiator. The airflow through the radiator then transfers this heat outward into the environment. During normal operation, the same coolant circulates from the engine block to the radiator again and again, first picking up, then releasing heat at various points of the system.

3 Heating and Air Conditioning, Part 1 3 A second method of cooling an engine is known as air-cooling. In this type of system, the cylinders of the engine are deeply finned; this allows a large part of the cylinders surface area to be exposed to the flow of air. The air-cooling system is very common in airplane engines and motorcycles, and it was the sole type of system used to cool early Volkswagen and General Motors Corvair engines. This system typically employs an engine-driven fan to blow air over the engine cylinders. But unlike water-cooled engines, air-cooled engines aren t able to regulate cylinder and engine temperature very easily. A third method of removing engine heat is by oil cooling. In all engines, a certain amount of heat is transferred from hot spots within the engine to the oil and then the oil pan. The main engine hot spots are the crankshaft and rod assemblies and the piston and wrist-pin assemblies. In some engines, a small stream of oil is pumped under the pistons to help cool the piston crowns and the wrist pins. Some motorcycle engines actually use two oil pumps. One pump delivers a low-volume, highpressure flow for standard engine lubrication. The second pump is a low-pressure, high-volume pump that s used to spray the pistons, especially the rear piston in a V-twin engine. In any type of cooling system, the idea is to transfer heat. Heat is transferred from the engine to the surrounding air to prevent the damage that can occur from overheating; this transferral also insures the engine is operating at its most efficient temperature. In almost all automobile and truck engines, the three methods of cooling are used together to cool the engine. However, the water cooling section is responsible for removing most of the engine heat and for precisely regulating the engine s temperature. A Basic Heating/Cooling System Let s take a more detailed look at the heating and cooling system within a typical engine. In this tour, we ll begin at the front of the vehicle and progress towards the rear. The Radiator, Pressure Cap, and Expansion Tank The radiator is mounted in the front of the vehicle where cool air can pass through it to pick up heat. The parts of a typical radiator are shown in Figure 3.

4 4 Heating and Air Conditioning, Part 1 FIGURE 3 Elements of an Engine Radiator All rights reserved.) (Reprinted with permission from Toyota Motor Corporation. Toyota Motor Corporation. The two basic types of radiators are down-flow and cross-flow radiators. Their names reflect how the coolant passes through the radiator on its way back to the engine. Most of today s radiators are of the cross-flow design. The main section of the radiator is normally made of aluminum tubes that are finned to offer more surface area of metal to the passing airflow. The top and bottom sections of the radiator are called the tanks. These tanks can be made up of many different materials. Older vehicles used copper and aluminum tanks that were soldered to the radiator s tubes. But many of today s vehicles use plastic tanks that are glued or pressed onto the tubes. Each tank contains a fitting that connects to a radiator hose. Usually, one fitting is located on the upper tank while another attaches to the lower tank. This configuration allows for the cross-flow radiator to work more efficiently and for the engine to pull in coolant of a reduced temperature. Radiators normally fail if they re leaking coolant. Leaks often start small but worsen as time passes.

5 Heating and Air Conditioning, Part 1 5 At the top of the upper tank is a fitting known as a pressure cap. A typical pressure cap is shown in Figure 4. FIGURE 4 A pressure cap, such as the one shown here, is used to keep the coolant under pressure and therefore from boiling in the radiator, hoses, and engine. (Photo courtesy of the Gates Rubber Company) The pressure cap is responsible for maintaining a pressure within the cooling system. This pressure can be as little as a few pounds per square inch, or psi, to a maximum of about 15 psi. The cooling system is pressurized because water typically boils at 212 F. If and when the engine s coolant reached that temperature, steam would be created; and steam is a very poor coolant. Instead, when the coolant is placed under pressure, the boiling point is raised and the coolant remains in a liquid form within the system. The pressure cap is simply a cap containing a spring-loaded plunger, or seal. As long as the pressure within the radiator remains at or below the rated pressure of the cap, the plunger will remain seated in the fitting. If the pressure exceeds the rating of the pressure cap, the pressure will lift the plunger from its seat and release some of the pressure in the form of pressurized coolant. A hose that connects to the bottom of the pressure cap fitting is used to collect this pressurized coolant, after which it s sent to an expansion tank. The expansion tank is responsible for collecting the coolant that has escaped from the radiator due to excess pressure. This tank is normally made of a semi-clear plastic and is mounted on one of the inside fender wells. The expansion tank often has markings on its side. These markings are used to define the upper and lower coolant levels for the heating and cooling system. The Thermostat The thermostat is a component of the heating and cooling system that s responsible for maintaining the engine s temperature. A typical thermostat is shown in Figure 5.

6 6 Heating and Air Conditioning, Part 1 FIGURE 5 A thermostat is used to control the temperature of the vehicle s coolant. courtesy of the Gates Rubber Company) (Photo The thermostat, mounted in the engine s water outlet, works as a temperature-activated fluid-control valve. When the temperature of the engine s coolant lies below the preset value of the thermostat, the thermostat remains closed and won t allow coolant to flow through the radiator. Instead, the coolant will bypass the radiator and circulate through the engine block. When the temperature of the coolant is equal to or exceeds the value of the thermostat, the thermostat opens and allows coolant to circulate through the radiator. Typical thermostat temperature values are 180, 185, 190, and 195 F. Most thermostats operate by means of the controlled expansion of a pellet. This pellet resides in the lower chamber of the thermostat and is normally made of wax. When cold, the pellet shrinks, pulling downward on the bellows and allowing the thermostat to close. When the passing coolant heats the thermostat, the wax pellet expands and presses upward on the bellows, which in turn opens the thermostat. A thermostat will normally fail in one of two ways. First, it can stick when closed and cause the engine to overheat. Secondly, the thermostat can stick when open; in a case such as this, the engine warms up too slowly and the engine never reaches its designed operating temperature since coolant is constantly flowing to the radiator. The Water Pump The water pump is mounted on the side of the engine where the beltdriven alternator, power steering pump, and air conditioning compressor are located. Like these other accessories, the water pump is belt-driven by the pulley that s located on the crankshaft. A side view of a typical water pump is shown in Figure 6.

7 Heating and Air Conditioning: Part 1 7 FIGURE 6 A side view of a typical water pump is shown here. There are four major components to a water pump the shaft, the seal, the bearing, and the impeller. The bearing at the front of the housing supports the pump s shaft. The seal is also present at this location to prevent coolant from leaking. The impeller is pressed onto this shaft and is located within the housing next to the engine. By turning the impeller within the pump housing, the water is pulled into the inlet and compressed before being pushed through the outlet. The coolant leaving the water pump flows through the engine, while the coolant entering the water pump comes from the radiator. The seal and bearing can both fail on a water pump, either individually or at the same time. In the case of either occurrence, the water pump should be replaced. The Heater Core Another major component of the heating and cooling system is the heater core. The heater core is mounted either inside the vehicle s dashboard or just behind the dashboard on the engine side. It s normally located near the glove compartment.

8 8 Heating and Air Conditioning, Part 1 The purpose of the heater core is to transfer heat from the circulating coolant to the passing air stream, which in turn warms the vehicle s passenger compartment. A typical heater core is shown in Figure 7. FIGURE 7 A heater core contains tubes and fins similar to a radiator. A heater core is basically just a small radiator. Like a radiator, it contains tubes and fins. Unlike a radiator, though, a heater core contains only two fittings and generally no side tanks. Heater cores normally fail when they become blocked, and coolant isn t able to flow through the radiator. A blocked heater core can often be repaired by back-flushing the core. In addition to blocking up, though, cores also leak. A leaky heater core produces a sticky-sweet smell within the vehicle and can fog the insides of the windows in colder climates. You may even notice drops or a puddle of coolant on the passenger floor when the core is leaky. Hoses There are two sets of hoses used in automotive vehicles radiator hoses and heater hoses. Both of these may appear to be simple rubber tubes. However, the typical radiator or heater hose is made up of a complex structure of layers. A typical automotive hose is shown in Figure 8. FIGURE 8 Section of Automotive Hose Replacement radiator hoses are specially made to fit particular vehicles. Heater hoses are easier to replace a new hose is simply matched to the length and diameter of the old hose.

9 Heating and Air Conditioning, Part 1 9 FIGURE 9A The Serpentine Belt Belts The automotive accessory drive belt is also a device of complex construction. A typical drive belt is made up of layers of cords that are covered by rubber and/or synthetic materials. When these layers form a flat loop, the belt is called a serpentine belt, while those forming a V shape create a V belt. The serpentine belt is shown in Figure 9A and the V belt is installed as shown in Figure 9B. FIGURE 9B The V Belt

10 10 Heating and Air Conditioning, Part 1 In either case, the belt is normally used to drive the water pump, the alternator, the air pump, the power steering pump, and the air conditioning compressor off of a pulley on the crankshaft. In the case of a serpentine belt, a single belt drives all of the accessories. In the case of a V belt, two or three belts are normally used to drive the accessories. Fan Assemblies There are two different methods of using a fan to pull cool air through a vehicle s radiator. Both methods involve the use of either an enginedriven or electrically-powered fan. When the vehicle is a rear-wheel drive vehicle with a longitudinally mounted engine, a fan can be connected to the front of the engine to pull outside air through the radiator. In most cases, the fan is mounted to a flange at the front of the water pump shaft and is driven by a belt off of the crankshaft. At low engine speeds, this arrangement allows the engine to turn the fan at the proper speed. However, at high engine speeds, the vehicle is usually traveling quickly enough to move enough air through the radiator naturally. At this time, the fan is typically stealing horsepower from the engine and providing no useful purpose. Two different types of fan designs are used to counteract these problems. The first design utilizes flexible fan blade tips that flatten out, or feather, at high speeds. The second design involves the placement of a clutch between the fan and its mounting flange, as shown in Figure 10. FIGURE 10 This type of engine-cooling fan clutch only drives the fan blades when it s necessary to circulate cooling air. (Courtesy of Chrysler Corporation)

11 Heating and Air Conditioning, Part 1 11 This clutch contains an internal fluid that s temperature-sensitive. Below about 170 F, the fluid passes the motion of the water pump shaft along to the fan. At higher temperatures, the fan is uncoupled from the water pump shaft and flange and freewheels, or rotates naturally. When the vehicle is a front-wheel drive vehicle containing a transversemounted engine, it s impossible to use an engine-driven fan. An electrically powered fan must therefore be used. This fan is mounted to a housing located behind the radiator. The electric motor, when energized, will rotate a fan assembly to pull air through the radiator (Figure 11). This motor is normally controlled by a relay. The relay is in turn controlled by various sensors on the engine. FIGURE 11 An electric fan motor is shown in (A),while its position in the fan housing is shown in (B). (A: Provided by JIDECO; B: Courtesy SCS/Frigette) An electrically operated fan offers many advantages. First, since sensors control the fan motor, the motor is only energized when the engine temperature exceeds a preset limit; this saves energy. Second, the fan doesn t require horsepower from the engine, saving further energy. The Blower A second type of electrically powered fan is used in vehicles to pull air through the under-dash ductwork and into the passenger compartment. This fan-and-motor assembly is often referred to as, simply, a blower. A typical blower motor assembly is shown in Figure 12. Many blower motors are mounted to the cowling under the hood of the vehicle. This location allows for simple replacement. However, many blower motors are located within the ductwork and are somewhat difficult to replace.

12 12 Heating and Air Conditioning, Part 1 FIGURE 12 A blower motor is used to force air through the heater core or evaporator to either warm or cool the vehicle. (Provided by JIDECO) Most blowers operate at three or more speeds. In order to provide these speeds, the switch on the dashboard of the vehicle connects the motor to the power source through series resistors of different values. These resistors are mounted inside the ductwork to keep them cool while they provide their function in dropping the voltage to the motor. Ductwork Below the top surface of the dashboard is a complex system of ductwork that s used to move outside, heated, or air-conditioned air into the passenger compartment. A typical ductwork system is shown in Figure 13. FIGURE 13 This illustration displays the paths air can take through the ductwork that resides under a vehicle s dash.

13 Heating and Air Conditioning, Part 1 13 Note that there are two components of the heating/air conditioning system mounted inside the engine compartment. These are the air conditioning system s evaporator and the blower motor assembly. The major component mounted in the ductwork within the vehicle is the heater core. The ductwork contains a series of doors that direct the fresh, heated, or air-conditioned air throughout the vehicle. These doors can be cable-operated from a dashboard control assembly. In addition, this control assembly usually contains a switch for the blower motor speed control and a switch that turns the air conditioning system on and off. In other cases, the doors are controlled by vacuum actuators, called pots, that move into one of two possible positions via computer controlled vacuum solenoids. The former system is a manually controlled environmental system, while the vacuum-controlled system is used more often as an automatic climate control system. Another type of modern ductwork door-control system uses small, DC-powered motors called servo motors. When these motors are energized, they re able to either open or close a particular door in the ductwork. In addition, the servo motor contains a feedback device that allows the computer-controlled system to position a door at any position from fully open to fully closed. Servo motors have become more common in recent years, as computers are now used to precisely control the interior climates of vehicles. Control Valves There are two main types of control valves in a typical automotive heating system. The first type, which we just discussed, is the ductwork door-control valve, or vacuum pot, shown in Figure 14. FIGURE 14 A vacuum actuator, or vacuum pot, can be used to open or close doors inside the vehicle s ductwork.

14 14 Heating and Air Conditioning, Part 1 The heart of this valve is the bellows. When vacuum action is applied from a solenoid, the bellows are pulled back to the left. This action in turn pulls on the attached rod, opening or closing the door within the ductwork. During normal operation, these vacuum pots rarely fail. When failures occur, it s usually due to a stuck door that the valve can t move, a leaky bellows, or a stuck rod at the exit of the pot. The second type of control valve is used to control the flow of coolant from the engine to the heater core. This valve is placed on one of the heater hoses and can be controlled manually, using a cable, or by way of a vacuum. Heater control valves are normally located under the hood on the passenger side of the vehicle, near the firewall. A heater control valve will sometimes fail by getting stuck in either an open or closed position after being set in one position for a long period of time. This type of valve can also get blocked if the coolant is circulating dirt and scale along with coolant. Power Check 1 In each section of Heating and Air Conditioning, Part 1, you ll be asked to check your understanding of what you ve just read by completing a Power Check. Writing the answers to these questions will help you review what you ve learned so far. Please complete Power Check 1 now. 1. A water pump is leaking coolant. This means that the has failed. a. bearing c. impeller b. seal d. belt 2. Which one of the following components has upper and lower tanks? a. The radiator c. The expansion tank b. The heater core d. The water pump 3. The radiator is the type most commonly used in today s vehicles. a. front-flow c. V-belt b. cross-flow d. serpentine-belt 4. What part of a thermostat responds to the temperature of the coolant by expanding and contracting? a. The bellows c. The tower b. The wax pellet d. The pressure cap Check your answers with those on page 65.