Anh văn chuyên ngành ô tô

11.1 INTERNAL COMBUSTION ENGINE An engine (Fig. 11-1) is a machine that converts heat energy into mechanical energy. The heat from burning a fuel produces power which moves the vehicle.Sometimes the engine is called the power plant. Automotive engines are internal-combustion(IC) engines because the fuel that runs them is burned internally, or inside the engine. There are two types: reciprocating and rotary (Fig. 11-2). Reciprocating means moving up and down, or back and forth. Most automotive engines are reciprocating. They have piston that move up and down, or reciprocate, in cylinder (Fig.11-3). These are piston engines. Rotary engines have rotors that spin, or rotate. The only such engine now used in automobiles is the Wankel engine (12-7). PISTON ENGINE BASICS 11-2 TWO KINDS OF PISTON ENGINES The two kinds of piston engines are the spark-ignition engine and the compression-ignition(diesel) engine. The differences between them are: The type of fuel used. The way the fuel gets into the cylinders. The way the fuel is ignited. The spark-ignition engine usually runs on a liquid fuel such as gasoline or alcohol blend. The fuel must be highly volatile so that it vaporizes quickly. The fuel vapor mixes with air before entering the engine cylinders. This forms the highly combustible air-fuel mixture that burns easily. The mixture then enters the cylinders and is compressed. Heat from an electric spark produced by the ignition system sets fire to, or ignites, the air-fuel mixture. As the mixture burns (combustion), high temperature and pressure are produced in the cylinder (9-9). This high pressure, applied to the top of the piston, forces it to move down the cylinder. The motion is carried by gears and shafts to the wheels that drive the car. The wheels turn and the car moves. In the diesel or compression-engine, the fuel mixes with air after it enters the engine cylinders. The piston compresses the air to as 1/22 of its original volume. Compressing the air this much raises its temperature to 1000°F (538°C) or higher. A light oil called diesel fuel is then sprayed or injected into the hot air. The hot air or heat of compression ignites the fuel. The method of ignition–by heat of compression–give the diesel engine the name compression-ignition engine. 11-15 Basic engine systems A spark-ignition engine requires four basic systems to run. A diesel engine requires three of these systems. They are : Fuel system Electric ignition system (except diesel) Lubricating system Cooling system Each system is described below. Later chapters cover their operation in detail. 11-16 Fuel system The fuel system supplies gasoline ( or similar fuel ) or diesel fuel to engine. The fuel mixes with air to form a combustible mixture. This is a mixture that readily burns. Each engine cylinder fills repeatedly with the mixture. Then the mixture is compressed, ignited, and burned. Figure 11-20 shows one type of fuel system used with spark-ignition engines. The fuel tank holds a supply of fuel. A fuel pump sends fuel from the tank to the fuel injectors. These are valves controlled by an electronic control module ( ECM ), or computer. Fuel tank: The fuel tank is made of sheet metal, fiberglass, or plastic. It has two main openings. Fuel is pumped in through one opening and out through the other Fuel pump: Figure 11-20 shows the fuel pump inside the fuel tank .This is the arrangement used in most vehicles with electronic fuel injection. An electric motor operates the fuel pump . Fuel injectors: fuel injectors, or fuel-injection valves are fluid-control valves. They are either open or closed .The fuel pump sends fuel under constant pressure to the injectors . On the system shown in Fig 11-20, each cylinder receives fuel from its own injector.This is a port injection system . At the proper time for fuel delivery, the ECM turns on each injector. This opens the valve in the end of the injector. The pressurized fuel then sprays out into the air entering the cylinder. Fuel delivery continues as long as the valve is open. The time is computed and controlled by the ECM. When the proper amount of fuel has sprayed out , the ECM turns off the injector . The valve closes and fuel delivery stops. Another fuel-injection system uses one or two injectors located above the throttle valve (Fig 1-13). They feed the proper amount of fuel to the air entering the intake manifold. This is throttle-body injection (TBI) In the past, carburetors (chap.21) were part of most fuel systems. Carburetors are mixing devices. Air passing through the carburetor picks up and mixes with the fuel to provide a combustible mixture. Most vehicles now have fuel-injection systems. 11-17 Electric ignition system The fuel system delivers a combustible mixture to each cylinder. The upward movement of the piston compresses the mixture. Then the ignition system (Fig 11-21) delivers an electric spark to the spark plug in that cylinder. The spark ignites the compressed air-fuel mixture and combustion follows The ignition system takes the low voltage of the battery (12 volts) and steps up the voltage as high as 47000 volts ( or higher ) in some systems. This high voltage produces sparks that jump the gaps in the spark plugs. The hot sparks ignite the compressed air-fuel mixture. 11-18 Lubricating system The engine has many moving metal parts. When metal parts rub against each other, they wear rapidly. To prevent this, engines have a lubricating system that floods moving parts with oil (Fig 11-22). The oil gets between the moving metal parts so they slide on the oil and not on each other. The lubricating system has an oil pan at the bottom of the engine that holds several quarts ( liters) of oil. An oil pump, driven by the engine, sends oil from this reservoir through the engine. After circulating through the engine, the oil drops back down in to the oil pan. The oil pump continues to circulate the oil as long as the engine runs 11-19 Cooling system Where there is the fire ( combustion ), there is heat. Burning the air-fuel mixture raises the temperature inside the engine cylinder several thousand degrees. Some of this heat produces the high pressure that cause the pistons to move. Some heat leaves the cylinder in their exhaust gas. This is the remains of the air-fuel mixture after it burns in the cylinders. The exhaust strokes clear out the exhaust gas. The lubricating oil also removes some heat. The oil gets hot as it flows through the engine. Then the oil drops into the oil pan and cools off . The engine cooling system ( Fig 11-23) removes the rest of the heat. The engine has open spaces or water jackets surrounding the cylinders. A mixture of water and antifreeze, called coolant, circulates through the water jackets. The coolant picks up heat and carries it to the radiator at the front of the car. Air passing through the radiator picks up the heat and carries it away. This action prevents the engine from getting too hot or overheating. 11-20 Other engine systems An engine will run with the four basic systems described above-fuel, ignition, lubricating, and cooling. For use in the car, the engine requires three other related systems. There are the exhaust system, the emission-control system, and the starting system. The exhaust system reduces the noise of the burned gases leaving the cylinders. Also, it carries the exhaust gases and excess heat safely away from the passenger compartment. The emission-control system reduces the air pollution from the vehicle and the engine. The starting system cranks and starts the engine. A battery provides the electric power to operate the starting motor and the ignition system during cranking. Later chapters describe these systems . 12-11 Firing order The firing order is the sequence in which the cylinders deliver their power strokes. It is designed into the engine. The crankpin and camshaft arrangement determine the firing other. In most engines, the firing order evenly distributes the power strokes along the crankshaft ( Fig 12-20). Most engine designs avoid firing two cylinders, one after the other , at the same end of the crankshaft . Firing orders for the same type of engine may differ. Two firing orders for in-line four-cylinder engines are 1-3-4-2 and 1-2-4-3. In-line six-cylinder engines use 1-5-3-6-2-4 (fig 12-20). A Chrysler V-6 and two General Motors V-6 engines (fig 12-19) all have the same firing order of 1-2-3-4-5-6. Ford V-6 engines have fired 1-4-2-5-3-6 and 1-4-2-3-5-6. A firing order used on V-8 engines by Chrysler and General Motors is 1-8-4-3-6-5-7-2 ( fig 12-20). Ford V-8 engines use 1-5-4-2-6-3-7-8 and 1-3-7-2-6-5-4-8 . Many engine service jobs require that you know the cylinder numbering and firing order. Some engines have cylinder numbering identification, firing order, and direction of ignition-distributor rotation cast into or imprinted on the intake manifold. The information is also in the manufacturer’s service manual. The complete firing order of a four-cycle engine represents two complete revolutions of the crankshaft. This is 720 degrees of crankshaft rotation. Most engines are “even firing “. This means, for example, that is an in-line six-cylinder engine a firing impulse occurs every 120 degrees of crankshaft rotation (720 ÷ 6 = 120). The firing order of this engine is 1-5-3-6-2-4. When piston number 1 is at TDC on the end of the compression stroke, piston number 6 is at TDC on the end of the exhaust stroke. To determine the two pistons that are moving up and down together ( piston pairs ), divide the firing order in half. Then place the second half under the first half : 1-5-3 6-2-4 The piston pairs for this inline six-cylinder engine are 1 and 6 , 5 and 2, 3 and 4 . 19-1 Introduction to gasoline fuel-injection systems Most 1980 and later cars have an electronic engine control (EEC) system. It controls the ignition and fuel-injection systems. The basic operation of electronic engine controls is described in chap 10. The fuel-injection system supplies the engine with a combustible air-fuel mixture. It varies the richness of the mixture to suit different operating conditions. When a cold engine is started, the fuel system delivers a very rich mixture. This has a high proportion of fuel. After the engine warms up, the fuel system “ leans out “ the mixture. It then has a lower proportion of fuel. For acceleration and high speed, the mixture is again enriched. There are two types of gasoline fuel-injection systems: Port fuel injection (PFI) which has an injection valve or fuel injector in each intake port (fig 19-1). Throttle-body fuel injection (TBI) in which one or two fuel injectors are located above the throttle valves ( fig 19-2). With either system, the electric fuel pump supplies the fuel injectors with fuel under pressure. As soon as the injector opens, fuel sprays out ( fig 19-3 ). An electric solenoid in the injection opens and closes the valve. The solenoid has a small coil of wire that becomes magnetized when the voltage is applied ( fig 19-4 ). The magnetism lifts the armature which raises the needle valve or pintle off its seat. Fuel sprays out as long as the pintle is raised. When the voltage stops, the coil loses its magnetism. The closing spring pushes the pintle back down onto its seat. This stops the fuel spray. Each opening and closing of the injector pintle is an injector pulse. Note : some injectors use a ball valve instead of a needle valve. Operation of the ball-type injector is basically the same as described above. 19-3 Electronic fuel injection Figure 10-19 shows the components of an electronic fuel injection (EFI) system. Most fuel-injection systems are electronically controlled. The controller is an electronic control module (ECM) or electronic control unit (ECU). It is also called an “ on-board computer“ because it is “on-board“ the car. Various components of the engine and fuel system send electric signals to the ECM (fig 19-5). The ECM continuously calculates how much fuel to inject. It then opens the fuel injectors so the proper amount of fuel sprays out to produce the desired air-fuel ratio. 19-6 Air and fuel metering The fuel system must accurately measure or meter the air and fuel entering the engine. This produces the proper air-fuel ratio to make a combustible mixture. A mixture that is too lean (not enough fuel in it) will not burn and produces excessive pollutants. A mixture that is too rich (excess fuel in it) will also produce excess pollutants. Figure 19-8 shows how mixture richness affects engine power. As the mixture becomes leaner, power falls off. The electronic engine control system includes the ECM and various sensing devices or sensors that report to it. A sensor is a device that receives and reacts to a signal. This may be a change in pressure, temperature, or voltage. Some sensors report the amount of air entering. The ECM then calculates for how long to open the injectors. 19-7 Operaion of fuel-injection systems Sensors that report to the ECM include ( fig 19-5) Engine speed. Throttle position. Intake-manifold vacuum or manifold-absolute pressure (MAP). Engine coolant temperature. Amount and temperature of air entering engine. Amount of oxygen in exhaust gas. Atmospheric pressure. The ECM continuously receives all this information or data. The ECM checks this data with other data stored in look-up tables in its memory. Then the ECM decides when to open the injectors and for how long (fig 19-9). For example, when the engine is idling, the ECM might hold the injectors open for only 0.003 second each time they open . The opening and closing of an injector is its duty cycle. How long the ECM signals the injector to remain open is the injector pulse width. Figure 19-9 shows how varying the pulse width varies the amount of fuel injected. Suppose more fuel is needed because the throttle has been opened for acceleration and more air is entering. Then the ECM increases the pulse width. This holds the injectors open longer each time they open to provide the additional fuel . Note: The system described above is a pulsed fuel-injection system. The injectors open and close (pulse). The continuous-injection system (CIS) is another type of fuel-injection system. It is used a few vehicles. The injectors are open continuously. Changing the pressure applied to the fuel varies the amount of fuel injected . 19-12 Indirect measurement of air flow Information about engine speed and engine load can be tell the ECM how much air is entering the engine. Using this information to regulate fuel feed is called speed-density metering. It is used in fuel-injection systems that do not directly measure mass air flow. The speed is the speed of the engine. The density is the density of the air or air-fuel mixture in the intake manifold. Throttle position (engine speed) and intake-manifold vacuum (engine load) measure air flow indirectly. Intake manifold vacuum is continuously measured by a sensor that changes vacuum (or absolute pressure) into a varying voltage signal. The ECM combines this with the TPS signal to determine how much air entering. Inputs from other sensor may cause the ECM to modify this calculation (fig 19-5 ). Engine speed (instead of throttle position) and intake-manifold vacuum can also tell the ECM how much air is entering the engine . 19-13 Measuring intake-manifold vacuum (manifold absolute pressure) Intake-manifold vacuum is measured in two ways ( fig 19-19 ): With a vacuum gauge. With a manifold absolute pressure (MAP) gauge. The two gauges are basically the same. Both have a flexible diaphragm that separates the two chambers in the gauge. The difference is that one chamber of the vacuum gauge is open to the atmosphere. One chamber of the absolute-pressure gauge contains a vacuum (fig 19-19). The vacuum gauge compares atmospheric pressure with intake-manifold pressure. In a naturally-aspirated engine, manifold pressure is less than atmospheric pressure. A vacuum gauge measures this partial vacuum in the intake-manifold . The manifold absolute-pressure (MAP) gauge compares the actual pressure in the intake manifold with a vacuum. This is more accurate than the vacuum gauge which compares intake manifold vacuum with atmospheric pressure. The vacuum gauge is less accurate because atmospheric pressure varies . Vacuum and pressure sensor are not constructed exactly like the gauges described above. But their operation is basically the same. Most electronic engine control systems include a manifold-absolute pressure (MAP) sensor (figs 10-19 and 19-20 ). It senses the pressure (vacuum) changes in the intake manifold. This information is sent as a varying voltage signal to the ECM . 19-14 Direct measurement of air flow Four methods of measuring air flow directly are vane, air-flow sensor plate, hot-wise induction, and heated film. Each continuously measures the actual amount of air flowing through the air-flow meter (fig 19-21). This information is then sent to the ECM . vane : The vane type air-flow meter is used in some pulsed fuel-injection systems such as the Bosch L system (fig 19-21). The spring-loaded vane is in the air-intake passage of the air-flow meter. Air flowing through forces the vane to swing. The more air, the farther the vane swings. A vane-position sensor works like the rotary throttle-position sensor. Depending on its position, it sends varying voltage signals to the ECM. This tells the ECM how much air is flowing through. The ECM then adjusts fuel flow to match . Air-flow sensor plate : The air flow sensor plate is used in mechanical continuous-injection systems (fig 19-14). The plate is in the intake-air passage of the air-flow meter. As air flow increases, the plate moves higher. This lifts a control plunger in the fuel distributor to allow more fuel flow to the injectors. The added fuel flow matches the additional air flow . Hot-wire induction : A platinum wire is in the path of the incoming air through the air-flow meter. The wire is kept hot by an electric current flowing through it. However, the air flow cools the wire. The more air that passes through the air-flow meter, the more heat that is lost from the wire . The system keeps the wire at a specific temperature by adjusting current flow. If more air flows through and takes more heat from the wire , the system sends more current through. This maintains the temperature. The amount of current required is therefore a measure of how much air is flowing through. The ECM reads this varying current as air flow . Heated film :The heated film consists of metal foil or nickel grid coated with a high-temperature material (fig19-22). Current flowing through the film heats it. Air flowing past the film cools it. Like the heated wire, the system maintains the film at a specific temperature. The amount of current required is a measure of air flow . 19-15 Atmospheric-pressure and air-temperature sensors Changing atmospheric pressure and air temperature change the density of the air. Air that is hot and at low atmospheric pressure is less dense. It contains less oxygen than an equal volume of cooler air under higher atmospheric pressure. When the amount of oxygen entering the engine varies, so does the amount of fuel that can be burned . Some systems include an atmospheric-pressure sensor. It is also called the barometric-pressure sensor or BARO sensor. It is similar to the MAP sensor. However, the barometric-pressure sensor reads atmospheric pressure. The air-temperature sensor (fig 19-23) is a thermistor. Its electrical resistance decreases as its temperature increases. Figure 19-21 shows its location in the vane-type air-flow meter. Both types of sensors send varying voltage signals to the ECM so it knows the atmosphe