Introduction of Diesel Electrical and Electronic Systems

Transcription

1 CDX Diesel Electrical and Electronic Systems Introduction of Diesel Electrical and Electronic Systems ÂÂ Basic Electrical Principles Basic Electronic Principles Sources of Electricity Effects of Electricity Basic Electrical Principles ÂÂ Electrical Measurement Electrical Components Electronic Components Batteries and Cells All matter is made up of atoms. Every atom has a nucleus, with positively charged protons, and neutrons with no charge. Moving around the nucleus are negatively charged electrons. Protons and electrons in equal numbers cancel each other out. An excess of electrons gives an atom a negative charge; a deficiency gives it a positive charge. Some materials have free electrons, which are loosely held by the nucleus. The more free electrons a material has, the better it can conduct electricity. Metals are an example. In insulators, electrons are bound tightly to the nucleus, making them poorer conductors. Semiconductors conduct electricity more easily than insulators but not as well as conductors. Free electrons, which are necessary for electrical current, require a complete pathway (circuit) and a force (such as a battery) to make them move. Negative electrons are repelled from the negative terminal and attracted toward the positive terminal. When charges flow in one direction only, it is called direct current (DC). The larger the charge at the positive terminal, the more strongly it attracts free electrons and makes them move. The greater this force called electromotive force (EMF) or voltage the stronger the electrical current. All materials have electrical resistance, measured in ohms, that affects current flow in a circuit. Four factors determine the level of resistance: Type of material Length of the conductor Size of the conductor Temperature of the conductor Higher temperature increases resistance. A resistor has a set resistance, usually marked or coded on its surface, and is designed to cause a particular voltage drop in a circuit. Before the discovery that electrical current was the flow of electrons, it was thought the natural way for electricity to move was from positive to negative. Current said to flow from positive to negative is called conventional current. Current said to flow from negative to positive is called electron current. Basic Electronic Principles Electronics usually refer to devices where electricity is conducted through a vacuum, gas, or semiconductors. The electrical resistance of semiconductors such as diodes, transistors, and power transistors is higher than that of most conductors, but lower than that of most insulators. A semiconductor s conducting ability depends on two kinds of charge carriers: Negative electrons Positive holes The number of charge carriers in a material can be altered by doping, or adding very small quantities of impurities. Electrons in excess make it an n-type semiconductor (n for negative). Holes in excess make it p-type (p for positive). When connected into a circuit, both the electrons and the holes move, and current can be thought of as moving in two directions. Both electron and conventional current are used in electronics. Most electronic components combine p-type and n-type semiconductors. Where they join is called the pn junction, an area where some electrons and holes cancel each other out. A thin layer of electrons forms that acts like an insulator. Since it has so few charge carriers, it is called a depletion layer. A semiconductor diode has a single pn junction. If it is connected to a current source, with the p region connected to a negative pole and the n region connected to a positive pole, the negative pole attracts the holes, and the positive pole attracts the electrons. This enlarges the depletion layer and the insulated space; thus current cannot flow across it Jones & Bartlett Learning 1

2 If the current source is connected in reverse, holes flow in large numbers across the junction toward the negative pole; the electrons move toward the positive pole. The pn junction floods with charge carriers. The depletion layer disappears, and with it the insulator effect. In this direction, the diode lets current flow. The conductivity of semiconductors can be manipulated and controlled by introducing impurities, an electrical field, or light. Semiconductor devices are replacing many other kinds of switching because they are small and light, use low operating voltages, need no maintenance, and are reliable and easy to manufacture. Some semiconductor materials include silicon, germanium, gallium-arsenide, and silicon carbide. Ground indicates connecting a component to the vehicle frame or chassis. Current flows from the positive terminal of the battery, through a controlling device such as a switch, then through the component. Return through the vehicle chassis or frame completes the circuit, allows the component to operate, and simplifies the circuit wiring. Sources of Electricity Rubbing two insulators together induces static electricity. One material loses electrons to the other and becomes positively charged. The other gains electrons to become negatively charged. Bringing two charged surfaces near each other may cause electrons to leap the gap to cancel out the charge imbalance. This can create an electrical shock. The spark can be dangerous, and if it occurs near fuel vapor, it can cause an explosion. Joining and heating two different metals can generate a small electrical current. The point that is heated is called a hot junction; the whole system is called a thermocouple. Engine manufacturers use thermocouples as hightemperature indicators to determine temperatures of components. When two dissimilar metals are immersed in a conducting liquid (electrolyte), the breakdown of chemicals into charged particles (ions) results in a flow of electricity (electrolysis). This principle is applied in the standard lead-acid battery used in most vehicles. Solar cells convert sunlight into electricity. The solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process is called the photovoltaic (PV) effect. If light energy strikes the surface of some semiconductor materials, they emit electrons. These freed electrons can then be made to flow in a circuit. The photovoltaic principle is applied in solar cells, some ignition systems, and vehicle speed sensors. Photovoltaic means of light and electricity, from the Greek photos, meaning light, and the name of the Italian physicist Volta. In piezoelectric energy, crystals of certain materials, such as quartz, are subjected to mechanical stress, and electrical potential is produced across the crystals. Electromagnetic induction is the creation of an electrical voltage or potential voltage difference across a conductor within a changing magnetic field. Alternating current (AC) flows one way when a conductor cuts across a magnetic field in one direction, then reverses as it cuts the field in the opposite direction. Moving a wire inside a magnetic field produces a current flow. Moving a magnet inside a stationary coil of wire produces the same effect. Electromagnetic induction is applied in alternators and ignition coils. Effects of Electricity As current flows through motor vehicle circuits, most of the electrical energy is transformed into other kinds of energy. Headlamps transform electrical energy into intense heat that makes the bulb filaments glow white hot and produce light. Electrical energy can demist a window or provide circuit protection from excessive current flow. Chemical effects of electricity depend on ions. Atoms become negatively charged if they gain electrons and positively charged if they lose electrons. When two different metals are immersed in an electrolyte, one loses electrons and becomes 2012 Jones & Bartlett Learning 2

3 positive. The other gains them and becomes negative. Negative ions in solution are attracted to the positive plate, and positive ions are attracted to the negative plate; thus a chemical reaction can occur. In a lead-acid battery, the electrical and chemical differences between the sets of plates create a potential difference that makes the current flow in a circuit. If the current continues for too long, there will be no difference between the plates and current stops, leaving a discharged battery. A battery can be recharged so that the difference between its sets of plates is restored. Magnetic forces occur due to the movement of an electrical charge. Magnetism causes materials to attract or repulse other materials. When current passes through a conductor, a magnetic field is created around it. When wire is wound into a coil, it produces a much stronger magnetic field. Turning current to the coil on and off can pull a switch open or closed. This is the principle behind electrical relays. An electrical field (and current) can be induced in a fixed conductor by changing magnetic fields. A current can be induced by moving a conductor in a fixed magnetic field. Electrical Measurement The current at any point in a circuit can be determined using Ohm s law. If resistance in a circuit stays the same but voltage rises, then the greater force pushes more current through the circuit. If resistance stays the same but voltage decreases, then less current will flow through the circuit. If voltage and amperage are recorded each time they change, and each voltage is put over each amperage, the resulting fraction always equals the same number. The total resistance of a circuit in ohms always equals the voltage divided by the amperage. R stands for resistance, V for voltage, and I for current in amperes. Therefore: R = V/I (if R = V divided by I...) I = V/R (then I = V divided by R...) V = I R (and V = I multiplied by R) Energy is the potential to do work. Work is done only when the energy is released. A disconnected battery has the potential to do work, so it is a source of energy. The difference in electron supply at the battery terminals is sometimes called the potential difference. In a standard charged automotive battery, the potential is 12 volts. Tapping this potential means turning one form of energy (the battery s electrochemical energy) into another. Turning one form of energy into another is called work. The amount of energy transformed is the amount of work done. Power is the rate at which work is done the rate of transforming energy. In an electrical circuit, power refers to the rate at which electrical energy is transformed into another kind of energy. The unit of power is the watt. When current flows in a circuit with a resistor in it, the resistor may become hot as it converts electrical energy into heat energy. The power equation can be simplified and transposed: Power equals voltage times current. Therefore, voltage equals power divided by current. And current equals power divided by voltage. Electrical power is a measurement of output, or the rate of work. Unlike EMF, current flow, and resistance, electrical power is not a direct measurement or a measure of power produced; rather, it is a measure of the amount of power consumed. The symbol for power is P and its unit of measurement is watts (W). To calculate power, the current flow in the circuit is multiplied by the EMF. So, P = I E, or Watts = Amps Volts. The most common electrical characteristics measured are current (amperage), EMF (voltage), and resistance. An ammeter measures the amount of electrical current in amperes. Current flows through a circuit; therefore, to be measured, it must flow through the ammeter. That means connecting the ammeter in series by breaking into the circuit. A voltmeter measures potential difference in volts across two points and is therefore connected in parallel. An ohmmeter measures resistance in ohms. The item to be tested must first be disconnected from its circuit so that any pressure in the circuit will not affect the readings of the meter. Separate measuring devices are now commonly combined into one digital measuring device, known as a digital multimeter (DMM) or a digital volt ohm meter (DVOM) Jones & Bartlett Learning 3

4 Electrical Components Every substance will conduct an electrical current if enough voltage is applied to it, but the word conductor normally is used for materials that allow current flow with little resistance. Most metals are good conductors. The most common conductor is copper. The heavier the current a conductor has to carry, the heavier the gauge, or thickness, of the wire. Materials that do not conduct current easily are insulators. Most plastics are good insulators. Fuses and circuit breakers are designed to break the circuit if current flow is excessive. The most common kinds are fuses, fusible links, and circuit breakers. Fuses are typically used in lighting and accessory circuits with moderate current flow. Typically, a fuse contains a metal strip that overheats and melts when subjected to an excessive level of current flow, breaking the circuit and stopping the current flow from damaging more valuable components. A fusible link is typically placed near the battery, and, except for the starter motor, it carries the current needed to power an individual circuit or a range of circuits. Circuit breakers are not destroyed by excess current. A bimetallic strip heats up and bends, opening a set of contacts and breaking the circuit. Relays are switches that are turned on and off by a small electrical current. Inside a relay is an electromagnet. When a small current energizes this electromagnet, it attracts an armature blade and closes contact points. Current can then flow across the points. Solid-state relays act like a mechanical relay but do not have any moving parts. Relays can be used to control the high levels of current in a circuit with a low-current signal, such as in a starter motor solenoid. Relays can also be protective switches, breaking circuits when faults are detected. When an electrical current passes through a wire, a small magnetic field is produced around the wire. If the wire is wound into a coil, the magnetic fields combine to create a stronger and larger magnetic field, with a north and a south pole, just like a permanent magnet. If the current flow is turned off, the magnetic field collapses and disappears. Therefore, by passing electrical current through a conductor coil, a magnetic field is produced, and turning it off causes the magnetic field to collapse. This is the operational theory behind an electromagnet. If a conductor is passed through a magnetic field, or a magnetic field is passed over a conductor, then a current flow is induced into that conductor. This is referred to as electromagnetic induction. Relays, solenoids, and motors use electromagnets to create movement. Ignition coils and transformers use electromagnetic induction to raise or lower voltage outputs. A relay uses an electromagnet to operate an electrical switch. Relays are used in circuits with small, low-current switches to control circuits that carry high-current flow. Most electrical components in a motor vehicle are controlled by relays. Relays are classified as normally open (NO) or normally closed (NC). NO relays are open circuited when de-energized and are closed by the electromagnetic action. NC relays are closed when de-energized and are open circuited when the electromagnet is energized. A solenoid uses a magnetic field to create lateral movement. The metal core, used by the electromagnet to strengthen the magnetic field, is referred to as an armature. Electric motors use magnetic fields to create rotary movement. Motors consist of two main components: the armature and the field, both of which contain electromagnets. The interaction between the stationary field coils and the moveable armature coil causes the armature to rotate. The armature is connected to the electrical supply by a set of carbon brushes that contact a commutator, which is a component part of the armature. The brushes allow the electrical connection even when the armature is turning. Ignition coils and transformers use electromagnetism to produce electricity, rather than movement. An ignition coil can be described as a step-up transformer because the output of 60 kv (or more) is higher than the input, nominally 12 V. Modern ignition coils are usually constructed using a heat-conducting hard resin and are cooled by their location on a heat sink, or by passing air. Step-down transformers operate under the same operating principles; however, the secondary coil has fewer turns than the primary, providing a lower induced output. Electrical wires commonly braided, multistranded copper core wrapped with plastic insulation are used to conduct electrical current around the motor vehicle Jones & Bartlett Learning 4

5 Twisted or shielded wires have the same construction but are harnessed in pairs and twisted to cancel the effect of electromagnetic interference. Ribbon cable is found inside computers and other electronic components. It is used for connecting between printed circuits. In certain locations where strong electromagnetic interference is present, wiring harnesses are subject to a situation where unwanted electromagnetic induction occurs. This interference is referred to as noise. To prevent noise, some vehicles use shielded wiring harnesses (twisted pair, Mylar tape, or drain lines). There are two scales used to measure the sizes of wires: metric, used by most countries, and AWG (American wire gauge). The metric scale indicates the cross-sectional area of the conductor in square millimeters. The AWG system uses a rating number; the larger the rating number, the smaller the wire and the lower its current-carrying capability. Copper is used to conduct electrical current because of its low resistance value. However, it does offer some resistance. To overcome the effect of resistance, the greater the length of the wire, the larger the cross-sectional area needs to be. Electronic Components By restricting the direction of movement of charge carriers, a diode allows an electrical current to flow in one direction, but essentially blocks it in the opposite direction. A semiconductor diode has a single pn junction. If it is connected to a current source, with the p region connected to a negative pole and the n region to a positive pole, the holes will be attracted to the negative pole and the electrons will be attracted to the positive pole. This enlarges the depletion layer, stopping current flow across the junction. If the current source is reversed, many holes flow across the junction toward the negative pole, and electrons travel in the opposite direction toward the positive pole. The pn junction floods with charge carriers, and the depletion layer, along with the insulator effect, disappears. In this direction, the diode lets current flow. Using conventional current flow, a diode lets a lowvoltage current flow through it if current flows from its p side to its n side, but the diode stops current flowing from its n side to its p side. A Zener diode is designed to block current flow, but if the voltage of the current source is large enough, it can force current to flow through the diode without damage. This is called breakdown. Because Zener diodes respond to certain voltage changes like switches, they are used in voltage regulators. Light-emitting diodes (LEDs) emit light when they are connected in a forward direction. Resistors are electrical components that resist a current running through them. Resistors are used to control the voltage that reaches various components. Each electrical component has its own resistance. Most resistors that can carry large currents contain a coil of high-resistance wire wound around a ceramic former to dissipate heat. Resistance is measured in ohms, so resistors are rated in ohms, indicating how strongly they will oppose any current flowing through them. Resistors have a wattage rating because resistors convert some of the electrical energy passing through them into heat. Some types of resistors include fixed resistors, variable resistors, thermistors, and metal oxide varistors. Thermistors are semiconductor resistors. Their electrical resistance varies according to temperature, making them suitable for temperature measurement and electronic control operations. There are two main types of thermistor: negative temperature coefficient (NTC) resistors and positive temperature coefficient (PTC) resistors. NTC resistors have lower resistance at high temperatures, which means they conduct current more readily when they are hot. PTC resistors have higher resistance at high temperature, which means they conduct current less readily when they are hot, making them useful as current limiting protective devices in circuits, instead of fuses. Resistors are used to control current flow in a circuit and are rated by their resistance value and power rating. Only the resistance value is marked. The resistor s power rating is determined by its size. To identify a resistor s value, each resistor is marked with four or five colored bands. Each color represents a number value. The color bands are placed close to each other and biased to the left. The last, or tolerance, band is spaced farther apart. Resistors found on circuit boards are normally fixed in value. Some resistors are variable. Variable resistors can have their value altered by movement of a slide or by temperature change Jones & Bartlett Learning 5

6 The three types of variable resistors are rheostats, potentiometers, and thermistors. Variable resistors can be linear (resistance value varies proportionally with movement or temperature change) or nonlinear (resistance change is not proportional with movement). A rheostat is a mechanical variable resistor with two connections. A resistance wire wrapped in a loose coil is connected to the supply at one end. A moveable wiper is connected to the other circuit connection. When the wiper is close to the beginning of the coil, the total resistance value is very small. As the wiper is positioned closer to the end, the resistance value increases. Rheostats alter the current flow in a circuit. Potentiometers are mechanical variable resistors with three connections: two fixed and one moveable. They act as voltage dividers and alter the voltage in a circuit. Thermistors are conductors whose resistance value is affected by temperature. There are two types: NTC and PTC. As temperature increases, the resistance value of PTC thermistors increases, while the resistance value of NTC thermistors decreases. NTC thermistors are the most common and are used in inverted circuits for ECU inputs. They are the sensing elements of devices such as coolant and air temperature sensors. Transistors are semiconductor devices used as switches and as current amplifiers. There are two kinds of transistor: npn and pnp. The npn transistor has a p-type semiconductor between two n-type semiconductors. A pnp transistor has an n-type semiconductor between two p-types. In a circuit, npn transistors can act as a switch. If the control switch is open, the depletion layer at one pn junction is blocking current from flowing through the transistor and driving the load. If the control switch is closed, a small current flows through the emitterbase pn junction. Extra charge carriers flow across the emitter-collector pn junction, letting current operate the load. The transistor is then said to be turned on. Batteries and Cells Electrochemical cells transform chemical energy into electrical energy. There are two types of cell: primary and secondary. In a primary cell, transformation is not reversible, and the cell is discarded at the end of its life. In the secondary cell, the transformation is reversible, and it can be recharged. There are two types of secondary cell: wet and dry. In automotive use, the usual main storage device is the wet cell of a lead-acid battery. It has two plates of dissimilar materials immersed in an electrolyte a solution that conducts electricity by using ions. The accepted, or nominal, voltage of a cell does not depend on the size of the cell. The surface area of the plates in a cell determines its current capacity. The wet-cell lead-acid battery is the main storage device in automotive use. An automotive battery can supply very high discharge currents while maintaining a high voltage useful for cold starting. It gives a high power output for its compact size and is rechargeable. Standard 12-volt car batteries consist of six cells, each of a nominal 2 volts. In a conventional open wet-cell battery, overcharging will generate hydrogen and oxygen gas, a highly explosive mix. The sulfuric acid in batteries can also be very harmful. Always handle batteries with care and wear protective clothing. In a discharged lead-acid cell, the active material of both plates is lead sulfate, and the electrolyte is mostly water a very weak sulfuric acid solution. The charging process increases the amount of acid in the electrolyte, making the electrolyte stronger. When further charging no longer makes the electrolyte stronger, charging is complete. Connecting a lead-acid battery to a load causes chemical changes as the battery discharges. A fuel cell is an electrochemical device that combines hydrogen and oxygen to produce water; in the process, it produces electricity and heat. Fuel cells operate without combustion, so they are virtually pollution free. In a lead-acid battery, all the chemicals are stored inside, and the battery converts those chemicals into electricity. The battery eventually becomes discharged until you recharge it. In a fuel cell, oxygen and hydrogen constantly flow through the cell, so it continues to produce electricity as long as fuel is available. There are four basic elements to a fuel cell: the anode, the cathode, the electrolyte, and the catalyst. A fuel cell stack is required for automotive applications. The number of fuel cells in the stack determines the total voltage, and the surface area of the cells determines the total current Jones & Bartlett Learning 6

7 Fuel cells use hydrogen and oxygen to produce electricity. The oxygen can come from the air. However, hydrogen is not readily available and is difficult to store and distribute; thus an additional device called a reformer is used. The reformer turns hydrocarbon or alcohol fuels into hydrogen. Hydrogen is the simplest and most plentiful element in the universe. It is a colorless, odorless, and tasteless gas. Hydrogen is the lightest element, yet it has the highest energy content per unit weight/mass of all energy-based fuels. Hydrogen can be extracted from virtually any hydrogen compound. It is safe to manufacture, and its energy can be harnessed in many pollution-free ways. Hydrogen can be burned directly as a fuel in a slightly modified internal combustion engine. Alternatively, hydrogen can power fuel cells, which power an electric motor, which in turn powers the vehicle. A fuel cell consists of two electrodes sandwiched around an electrolyte. Oxygen passes over one electrode and hydrogen over the other, generating electricity, water, and heat. It will produce energy in the form of electricity and heat as long as hydrogen fuel is supplied. Fuel cells are more reliable than internal combustion engines. They are compact, lightweight, and have no moving parts. Also, there is no explosive combustion. Fuel cell power-generating systems have separate hydrogen gas fuel and air intake ports and an outlet for the waste water. Electrical current is generated by the fuel cell and is recovered through the anode and the cathode. Anti-freeze is circulated through the system as a coolant. The electrochemical reaction that creates the electrical power takes place inside the individual fuel cells Jones & Bartlett Learning 7