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Note that some throttle position sensors. When a force is applied to the centre of a beam see Figure 1. The pump will usually have a permanent earth connection. The inductive sensors can usually be classified as self-generating. Because accurate timing is so important to engine efficiency, performance, emissions and economy, improvements in all of these factors are achieved with a greater number of timing reference points. The signal is therefore an analogue signal and is similar in appearance to the signal produced by a throttle position sensor Figure 1. The petrol or gasoline engine spark ignition has traditionally been the most popular due to its superior overall performance.
Additional applications include air conditioning systems. As with almost all modern ECU controlled systems. The value of this resistance changes with temperature. This is achieved by passing a ferrous metal object iron or steel close to or through the magnetic field. These sensors are often referred to as inductive or magnetic variable reluctance sensors.
For an engine system. Because it is common practice to use NTC sensors. The rotational speed sensor uses an adaptation of this principle. More specific values are quoted in Chapter 3. In both cases.
As shown in Figure 1. For wheel speed sensors.
As each tooth passes the sensor. Rotational speed sensor analogue signal When each reluctor tooth passes the sensor. When a metal component reluctor passes close to the sensor. Note that because the signal is analogue. Book 2 The analogue signal voltage produced by sensors with a thermistor progressively increases or decreases with changes in temperature.
In most cases. The typical signal voltage from a temperature sensor circuit ranges from approximately 4. The wheel speed information is used to enable calculations for anti-lock braking. The wheel speed information can of course also be used to calculate road speed or distance travelled. The strength of the magnetic field or flux increases or decreases when the metal object is moved close to or away from the magnetic field.
Rotational speed sensors are often constructed with a permanent magnet located inside or adjacent to a coil of wire. By adapting the previously described rotational speed sensor system.
The signal voltage progressively increases and decreases with the rotation. Note that the voltage progressively increases and decreases as the reluctor tooth approaches and leaves the pole of the sensor magnet the voltage increases and decreases. If there were 60 reluctor teeth. It is. Control of ignition timing. Whichever method was used. Other types use a Hall effect system to produce a signal.
With a possible 60 reference points or more in some cases. It should be noted that there are variations in the way in which some rotational position sensors operate. Rotational angular position sensor In some cases.
The voltage increase and decrease is shown in Figure 1. If there is a means by which the ECU can determine the position of the crankshaft during its rotation.
Assuming there were 60 teeth or reference points on the crankshaft reluctor disc. If there is no movement of the reluctor tooth. The camshaft sensor is included because a crankshaft TDC position reference usually relates to more than one cylinder. The rotor disc has a number of vanes and cut outs which. A permanent magnet is located close to the Hall chip. When a small input electrical current is passed across chip terminals A to B input current.
This pulsed signal can provide a speed reference signal to an ECU. Hall effect principle Figure 1. The rotor disc was mounted on the distributor shaft and. Therefore a camshaft position sensor can indicate to the ECU the position of cylinder 1 only or any other cylinder chosen to be the master reference cylinder. It is also necessary to have a cylinder reference signal for the modern generation of ignition systems that use individual ignition coils for each cylinder there is no distributor rotor arm to distribute the high tension HT to each spark plug.
The rotor disc had the same number of cut outs and vanes as cylinders. Hall effect ignition trigger On some earlier generations of electronic ignition systems. The result is that the flow of current across the chip terminals C to D will be switched on and off in pulses.
On the example shown in Figure 1. Almost all modern throttle position sensors see Figure 1. As the wiper moves along the track. If the rotor had four vanes and cut outs for a four-cylinder engine. Note that some throttle position sensors.
The pulsed digital signal would be passed to an ignition amplifier or to an ECU. Although there are variations in the construction of throttle position sensors and the signal voltages.
Throttle position sensor analogue signal The throttle position sensor provides a progressively increasing and decreasing voltage when the throttle is opened and closed. On engine management systems and on older fuel and ignition systems. Because the resistance along the track increases from a low value possibly as low as zero ohms to a high value. The wiper moves with the movement of the throttle. The potentiometer provides a signal voltage that increases and decreases when the throttle is opened and closed.
The wiper is then connected back to the ECU.
Information about rate of change of throttle position enables the ECU to provide more accurate fuel and ignition timing control. As with many other sensors. The throttle butterfly is located on a spindle and may rotate through less than 90 degrees.
A very common example is a throttle butterfly or throttle plate. In these sensors one Figure 1. The voltage is applied to the potentiometer resistance track. There are other mechanical methods for converting pressure change into an electrical signal.
Because engine intake depression varies with engine load and throttle position and other factors. Book 2 set of contacts is arranged so that they close when the throttle is fully closed. The change in strain causes a minor change in length or shape of the crystal. A more widely used type in the past was the capsule type. Mechanical type One simple mechanical type makes use of either a diaphragm or capsule. Note that on the diaphragm type sensor with a potentiometer.
Some throttle sensors have a combination of contacts and a potentiometer. Note that. The change in shape or length alters the resistance of the chip. There are other components fitted to ECU controlled vehicle systems that also use position sensors similar to the throttle position sensor. As a result. The signal voltage from the potentiometer is passed to the ECU. When the upper chamber pressure alters with engine operating conditions it will cause the diaphragm to deflect or move within the casing.
The diaphragm type sensor is in most cases too simple and inaccurate to be used for modern vehicle systems such as an engine management system. If the diaphragm type sensor Figure 1. A solid state component or silicon chip is exposed to the pressure or depression. A second set is also used to indicate when the throttle reaches a certain opening point. Electronic type Electronic pressure sensors are much more reliable and accurate than mechanical sensors and have no moving parts Figure 1.
The diaphragm can be connected to a lever. Digital pressure sensors generally provide a digital pulse. Pressure sensor analogue or digital signal Electronic type sensors can produce an analogue or a digital signal.
For this reason we should not refer to engine intake depression as being a vacuum. MAP sensors are generally of the electronic type and may still provide either an analogue or digital signal.
Therefore a gauge pressure of zero indicates a pressure of around 1 bar. MAP sensors It is general practice to refer to the atmospheric pressure as being zero. Some sensors may provide a high voltage when the depression is strong. These sensors are therefore referred to as manifold absolute pressure sensors MAP sensors.
The same applies to a pressure that is lower than atmospheric pressure. The intake pressure is dependent on a number of factors including: The ECU is then able to calculate the engine load and provide the required amount of fuel. Note that not all analogue pressure sensors operate in the same way. Therefore changes in throttle position and engine speed or load will affect the airflow. Absolute pressure is therefore the true pressure value as opposed to the traditional gauge pressure.
Intake manifold depression is a low pressure but it is not a true vacuum. Sensing manifold absolute pressure Pressure sensors that are used to sense engine intake depression generally now measure absolute intake pressure. Therefore the absolute pressure value provides a more accurate indication of engine operating conditions.
The analogue signals are generally simple voltage changes that increase and decrease according to changes in pressure. Engine condition affects the intake pressure. When the pressure changes.
Mechanical Mechanical airflow sensors are usually referred to as flap or vane type airflow meters. There are two types of commonly used airflow sensors: If the gauge pressure reading were lower than zero. We therefore refer to this as gauge pressure. When a complete vacuum is formed i. The flap is connected to a sophisticated potentiometer. If a gauge reading indicates 2 bar. Refer to section 1. A hinged flap is exposed to the airflow. A more detailed explanation is provided in Chapter 2. When the throttle is initially opened this allows the intake pressure to rise almost no depression.
When an engine draws in increasing volumes of air on the induction strokes. Through measuring only the volume. On mass airflow sensors. These airflow sensors are often called mass airflow sensors.
The greater the mass of air. The inaccuracies are quite small. For a given volume of air. Hot wire sensors are affected by air density and can therefore provide an indication of airflow. The signal is therefore an analogue signal and is similar in appearance to the signal produced by a throttle position sensor Figure 1. The principle of operation relies on the fact that. Measuring air volume not air mass It is important to note that the flap type airflow sensor measures air volume but not air mass.
As the temperature of the wire changes. When changes in airflow cause a change in the temperature and therefore changes in the resistance of the wire. The sensor uses a natural process that.
The 1. A variation on the hot wire system is a hot film sensor. This circuitry explained in Chapter 3 compensates for the change in sensing wire resistance and applies increased or decreased current to the wire to maintain the desired temperature.
On some types of hot wire system. The catalytic converter plays the major part in reducing the pollutants contained within the exhaust emissions. The operation is much the same as for the hot wire sensor but an integrated film type heated sensing element is used instead of the heated wire.
Although the previous explanation provides a brief understanding of the purpose of the oxygen sensor. Whilst the oxygen sensor is not critical to the direct efficiency of the engine.
The oxygen sensor is used to measure the oxygen content and provide a signal to the ECU which will in turn control fuelling to ensure that the exhaust gas has the correct oxygen level. On a modern engine.
The unburned or partially burned gases within the exhaust contain unburned or partially burned petrol. Zirconium oxide is one commonly used material for an oxygen sensor element.
Although the air: Monitoring the oxygen in the exhaust gas In reality. The oxygen sensor therefore senses the oxygen content of the exhaust gas and passes a signal back to the ECU. The change in required current flow is converted to a voltage signal that can be monitored by the ECU. When the sensor is located in the exhaust pipe. Around The theoretically correct mixture is approximately Oxygen measurement Refer to Chapter 3 for additional information.
For a catalytic converter to work efficiently. These topics are therefore explained in greater detail in Chapter 2.
A typical oxygen sensor is illustrated in Figure 1. Some examples of typical analogue signals produced by some sensors are shown and discussed in this section. In such cases the ECU will establish that the catalytic converter is not working and will illuminate the dashboard warning light. The position of the lambda sensor in front of the catalytic converter is referred to as pre-cat control because the combinaton of lambda sensor and ECU controls the oxygen content before it reaches the catalytic converter.
Because the oxygen sensor is effectively monitoring what is now referred to as the lambda value. Post-cat monitoring European legislation and legislation in other continents demands that an additional function is now incorporated into emission control systems. A fault related code or message would also be accessible from the ECU using appropriate diagnostic equipment.
Book 2 exact voltage will depend on the amount of oxygen in the exhaust gas. Analogue signals produced by sensors vary quite considerably. This kind of process is often referred to as a closed loop operation. The second lambda sensor signal post-cat will therefore be identical to the pre-cat lambda sensor signal.
This arrangement is shown in Figure 1. As discussed in section 1. The voltage produced by the sensor is then passed to the ECU. One aspect of OBD is that some form of monitoring should take place to ensure that the catalytic converter is performing efficiently. This function is part of a broad range of on-board diagnostic OBD functions. If the catalytic converter is not working. The exhaust gas will provide heat but some sensors have electrical heating elements built in to the sensor body to speed up and stabilise the heating process.
The process is almost continuous: The lambda sensor is located upstream in front of the catalytic converter and is therefore able to measure the oxygen level in the exhaust gas passing into the converter.
Pre-cat control As detailed above. Note that for the sensors to operate efficiently. It is also of interest to note that if the typical sensor signal voltage is between 0.
The voltage levels on a temperature sensor circuit generally range from a maximum of approximately 5 volts to a minimum of zero volts although for normal operation a typical range is approximately 4. Although some earlier throttle position sensors relied on switches and contacts. In converting the analogue signal into a digital signal.
When the temperature changes and the voltage consequently decreases or increases. Therefore when the throttle is opened and closed. Assuming that the progressive or analogue increase and decrease in voltage is converted to a digital or stepped signal in the same way as a temperature sensor analogue signal is converted into voltage steps. An incorrect voltage is most likely to occur as a result of a faulty component sensor or wiring fault.
The ECU can count the up or down steps in voltage to calculate the angle of opening. The form of the output signal from a potentiometer is very similar to that from a temperature sensor. As with temperature and throttle position sensors. Reluctor teeth are located on a rotating component such as a crankshaft or camshaft. The ECU can therefore adjust the fuelling. The analogue signals and the subsequent converted digital signals are therefore similar to those created by the throttle position sensor Figure 1.
The ECU can then provide the appropriate adjustments to fuelling. The ECU could therefore be programmed to illuminate a fault light on the dashboard and furthermore to provide some form of coded message. If each step of 0. The ECU can therefore switch off the fuel injector for that cylinder. The ECU is able to calculate how many degrees the crankshaft has rotated from the master position. The ECU counts the pulses generated by the teeth. The ECU is able to count the number of pulses.
This could enable the ECU to implement other control functions that are crankshaft position dependent. An ECU on an ABS system is therefore able to establish whether a wheel is accelerating or decelerating at a different rate from the other wheels. The main difference is that. As with other analogue signals. To achieve this speed calculation.
Many other vehicle systems use the information from the wheel speed sensors: Note that the ECU will also have information from the oxygen sensor. It is also possible for the ECU to assess the speed of the crankshaft as each tooth passes the sensor. The ECU can use this information. The voltage changes form the analogue signal that is then passed to the ECU. When a cylinder is on the power stroke. Note that some knock sensors must be tightened to the correct torque setting when fitted to the engine.
A solid state component or silicon chip usually referred to as a piezo chip or crystal can be used to sense pressure changes section 1. Knock sensors are discussed in detail in Chapter 2.
This is not possible with a crankshaft sensor. The signal provided by the knock sensor is analogue but it is very irregular because there is not a consistent rotation or movement of a component to create the signal. Hall effect sensors can therefore be used to provide speed or position related information to the ECU.
Such sensors are used on some ignition systems. The knock sensor detects the knock and passes a signal to the ECU. Because the sensor is mounted on the camshaft. Ignition knock is caused when isolated pockets of spontaneous combustion occur within the combustion chamber. Because modern engines operate very close to the limits at which combustion knock can occur. If this type of chip is built into a sensor that is attached to the engine cylinder head or cylinder block. The sensor signal therefore contains voltage spikes caused by all vibrations.
TDC of one of the cylinders on a multi-cylinder engine. Hall effect sensors are also used as camshaft position sensors. Although the engine does produce regular vibrations. Once the ECU has established a reference to one of the cylinders. Book 2 The signal produced by a Hall effect ignition trigger on older systems needs only to provide a trigger signal for spark timing.
The signal is used as a master reference for ignition or sequential injection timing Note that injection system control can also rely on a camshaft located Hall effect trigger. On a four-cylinder engine. Therefore one pulse of the signal corresponds to the ignition timing point for each cylinder. The ECU therefore needs to be given information regarding the position of one of the cylinders. The ECU control signal that is passed to the actuator causes some form of movement of a component.
If the injectors are operated in sequence. Remember that the ECU will be receiving speed and angular position information from a crankshaft sensor. Mechanical and non-mechanical actuators The term actuation is generally assumed to mean that something is moved or actuated.
On later ignition systems usually integrated into an engine management system. The camshaft rotates once for every engine cycle. Solenoid type actuators In a simple solenoid Figure 1. The engine management ECU can then momentarily reduce the engine power. The magnetic field can then be used to create movement.
Different designs and Figure 1.
The engine management ECU is able to calculate engine load conditions because it receives information from sensors such as the airflow sensor. When an electric current is passed through the coil of wire and the magnetic field is created. The engine management ECU can achieve this by slightly retarding the ignition timing or slightly reducing the amount of fuel injected.
Communication signals between different ECUs Another example where an ECU provides a control signal that does not result in mechanical movement is the communication of one ECU with another. Another example of non-mechanical actuation is where the ECU provides a signal to a digital dashboard display to enable the driver to view engine and vehicle speed as well as other information.
There are a number of variations in solenoids and electric motors. The operation of mechanical actuators solenoid and electric motor types relies on magnetism. Each of these actions would result in a momentary reduction in engine power. When the current is switched off. It is also possible for the ECU to regulate or control the average current flow and voltage passing through the coil of wire by altering the duty cycle and frequency of the control signal pulses see section 1.
The simple electric motor in Figure 1. This means that the electric current will be flowing from connection B to connection A Figure 1.
Book 2 constructions of solenoids allow many different tasks to be performed. Reducing the magnetic field will result in the plunger moving back slightly.
Simple examples include fuel or air pumps. When the electromagnetic field is created. Solenoid plungers can be connected to a number of different types of mechanisms or devices that will perform different tasks or functions.
Electric motor type actuators A simple electric motor operates on similar principles to the solenoid. If the plunger is moving against a physical resistance such as a spring. These north and south poles will either be attracted to or repelled from the north and south poles of the permanent magnet. This allows an ECU to move and position the plunger with reasonable accuracy. Note that the primary and secondary windings are wound around a soft iron core to concentrate and intensify the magnetic field a Current passes from A to B creating north and south poles on the electromagnet.
With this control process. One coil creates a magnetic field. Remember that like poles repel each other and unlike poles attract each other.
A single loop of wire. The result is that the north pole of the electromagnet is now a south pole. The electric current passes from the power supply to contact brushes which rub against the segments as the shaft rotates. When the current is initially passed through the wire loop. A principle that is used in electrical transformers is also used for ignition coils: Whilst the magnetic field is collapsing.
In an ignition coil a secondary coil can typically have times more windings than the primary coil see Figure 1. The build up of the magnetic field is relatively slow.
The partial rotation can be progressive from one position to another. Because the secondary winding may have times the number of turns or windings. An ignition coil relies on both processes.
The faster the magnetic field moves relative to the wire. The secondary winding can be adjacent to the primary winding. In addition. Other examples of ECU controlled motors are dealt with individually in the following sections and in other chapters of this book. Because the speed of collapse of the magnetic field is very rapid. Magnetism and non-mechanical actuators There is one main actuator used on motor vehicles that uses the effects of a magnetic field but does not produce mechanical movement — this is the ignition coil.
On most vehicles. When a current is passed thorough a coil of wire. The capacity to control the rotation of motors accurately allows them to be used for a variety of tasks such as opening and closing air valves in small increments used for idle speed control. To achieve the desired voltage necessary to create the spark.
Many of the motors do not in fact perform a complete rotation. The ignition coil must provide a way to increase the voltage from 12 volts to many thousands of volts. If volts could be produced in the primary winding owing to the rapid speed of collapse of the magnetic field.
When the magnetic field is created. These types of motors are controlled by using different types of wire loops usually coils of wire and using different designs of commutator. It is sufficient here to highlight the basic principles of ignition coil operation. The second example is the use of a solenoid as an air valve. The following examples deal with two types that are used for totally different tasks.
Key Points Actuators convert electrical signals into actions Common actuators. Solenoid air valve The example shown in Figure 1. Book 2 For most petrol engines. The earth circuit for the injector passes through the ECU. The opening and closing time can often occur in around three thousandths of a second 3 milliseconds or 3 ms.
There is therefore sufficient additional voltage available to overcome many minor faults such as a plug gap that is too large or contaminated. The pressure is produced by a turbocharger. The injector must open and close very rapidly and at high frequency.
Modern diesel engines that now use electronic control for the fuel system also use electronically controlled solenoid injectors that inject fuel directly into the combustion chamber. The valve does not have to operate at the same speed as the injector. The injectors are usually located in the intake manifold and therefore inject fuel in the region of the intake valves. On some modern petrol engines. Fuel injector Fuel injectors are high precision components used to control the flow of fuel into the engine.
The first example is a fuel injector. The example used in this section is a motor that is used to control an air valve. The pump will usually have a permanent earth connection. Continuous rotation fuel pump motor The example shown in Figure 1.
To switch the air valve. Some stepper motors used for idle speed control operate on this principle. Full rotation motors with controlled positioning are used to position a mechanism or device such as an air valve or a throttle butterfly. This will allow pressure from the intake manifold to act on the diaphragm in the wastegate. Partial rotation motors use the same principles of operation as a normal motor but the angle of rotation is limited. The pump will receive a power supply.
The solenoid air valve does not have to operate at the same speed and frequency as the fuel injector. Book 2 Idle speed stepper motor with full rotation and controlled positioning In the example shown in Figure 1. The ECU controls the average current flowing in the circuit by altering the duty cycle of the control signal. Idle speed partial rotation motor rotary idle valve In this design Figure 1. Connected to the end of the armature is an air flap or air valve assembly which.
This construction enables the ECU to switch on and off each electromagnet. The greater the average current. The ECU is therefore acting as a sophisticated switch that makes or breaks switches on or off the actuator circuit Figures 1. By continuously altering the duty cycle it is then possible to alter the angular position of the armature. But because the power transistor is the switch within the actuator circuit. In the simplest type. The motor contains more than one set of magnets and electromagnets.
The frequency is therefore still 1 hertz but the on and off times are different. Fuses and relays are also generally connected into the positive side of the circuit. Although the durations and frequencies of the two signals are different. If the durations of the on and off times are the same.
The completion of the on and off process is one complete cycle of operation or 1 cycle. In this case the frequency is 1 hertz. When examining the control signal. The positive path from the power supply the battery can be directly connected to the actuator or it may contain a switch such as an ignition switch.
If there are 10 cycles within one second. It is however general practice that the pulse width refers to the on time only. Because the light bulb has a 2 ohm resistance. Important note: In the control signal examples illustrated. When the circuit is switched on. It is important to note when using test equipment.
The control signals affect how the actuator operates in different ways because the actuator is altering the current flow in the circuit. This means that the injector can be opened for longer or shorter time periods. When a switch in this case the ECU is connected into the earth or negative path of an actuator circuit.
If the light switch could be switched on and off very rapidly. Altering the control signal duty cycle Altering the duty cycle or pulse width has the effect of altering the average current flow and applied voltage in a circuit. The on time is therefore shown as zero volts. The frequency of the control signal also affects how an actuator behaves. The solenoid would in fact adopt a half open position i.
Altering the control signal timing and frequency If the control signal consists of simple on and off pulses. In this example where the bulb is rapidly switched on and off. A sensor usually the camshaft position sensor is used by the ECU as a timing reference to calculate when the intake stroke is about to start.
Although this would regulate the flow of fuel in the pipe. For instance. This means that the average voltage. If this same process of altering the duty cycle is applied to a control signal that is being used on an actuator such as an electric motor.
It is therefore possible to provide the on and off pulses at a specified time. The same applies to any actuator control signal.
The injectors on some modern systems will open just before. Book 2 Web links Engine systems information www. One of the most famous is London. This process really started towards the end of the s. Legislation forced vehicle manufacturers to develop and fit electronic systems to engines.
This section relates to systems covered in chapters 3. It is generally accepted that the USA was the leading country in introducing legislation that forced a reduction in emissions levels produced by engines and vehicles in general. Smog is one particular problem Figure 2. Probably the single biggest factor in the increasing use of electronic systems was the introduction of emissions legislation.
Smog is a term that became commonly used with reference to fog that was not naturally formed in the atmosphere: As noted in Chapter 1. What did make things slightly difficult was that different states in the USA had different problems. At some stage therefore. Although motor vehicles must have started to contribute to the problem.
The smog in London had been present for too many years for it to be blamed entirely on motor vehicles. Burning fossil based fuels such as coal. Engines in the USA at that time were usually of V8 configuration with typical capacities of 4 litres to 7 litres. In the UK. The cost of petrol was and remains low compared to most countries and the petrol engine was more acceptable as a means of powering large American cars.
In isolation. The acids contained within vehicle related smog cause damage to buildings as well as people. Athens in Greece is a particular case where acids are destroying many of the ancient buildings. Emissions legislation and technical developments in the USA were therefore focused on the petrol engine rather than the diesel engine.
Although improvements had been made. In the USA. Los Angeles had a high concentration of motor vehicles that. Therefore a combination of location. Motor vehicles were a contributing factor to smog problems so they became a focus of attention for legislators.
It is perhaps coincidental or fortunate that at a time when emissions problems were a major focus of attention for legislators. Engine maintenance Engine design in the s had not really changed too much for many years. The vehicle manufacturers were therefore tasked by legislators to reduce the level of vehicle emissions. Serious illness.
For those readers who have never worked on older vehicles with carburettors and contact breaker points ignition systems. Emissions levels from these large engines were very high. Whilst smog can be quite thick and therefore makes driving hazardous. The Los Angeles problem. However it was recognised that one major factor that could help to reduce emissions was to ensure that regular maintenance was performed correctly.
It was therefore obviously going to be very difficult to change vehicle and engine design suddenly. Legislation dealing with factories and fires in houses has also been brought in. Vehicle emissions are a broad subject and smog is only one of a number of problems influenced or created by such emissions. Therefore any reasonable added cost for the vehicle became relatively acceptable. Europe and other regions were therefore able to take advantage of legislation and technical changes made in the USA.
Loss of efficiency of fuel and ignition systems Carburettors would progressively accumulate a build up of deposits in the airways and petrol jets small holes through which fuel passed. Older engine designs included many components and systems that required regular maintenance. A second problem with worn contact breakers is that. Progressive wear on the contact breakers was an acceptable part of the design.
A third problem with contact breakers is that. Ignition or fuel systems would not suddenly become inefficient at a particular interval. Another major factor that made it necessary to adjust the idle mixture setting was the deterioration in the way the engine was performing.
Other systems requiring maintenance It was more than just simple carburettor and contact breaker maintenance that resulted in increasing emissions. The opening time of the contact breakers causes the ignition coil to provide the electrical energy that in turn causes a spark at the plug. When ignition timing is incorrect usually retarded or later than specified. The net result was that the engine would be operating very inefficiently and excessive emissions would be produced.
This is generally achieved by making the mixture richer than normal. Incorrect ignition spark timing will reduce combustion efficiency. In turn. Those areas requiring regular maintenance included: Each of the above listed items would progressively wear.
One particular problem related to wear on the contact breakers is that it can reduce the quality or strength of the spark.
The result is that the combustion of the air—fuel mixture will not be efficient. Contact breakers on very old vehicles may have required replacement after as little as 5 miles 8 km but even the improved versions still required regular adjustment and were traditionally replaced at main service intervals. This results in weak electrical output from the ignition coil and therefore a weak spark. Although this problem was very much reduced by the introduction of a condenser or capacitor in the contact breaker circuit.
Ideally therefore. This in turn can also create higher emissions. It was also common for the carburettor to require slight adjustment of the idle mixture setting. Without regular and correct maintenance. The electronic module therefore needed a reference or trigger signal so it would be able to switch off the ignition coil circuit at the appropriate time. Early designs of electronic ignition fitted as original equipment were designed to eliminate the contact breakers see section 2.
Designers had made dramatic improvements to the capability and accuracy of carburettors. The advance and retard mechanisms were linked to the inductive sensor base plate and to the rotor shaft as on contact breaker systems enabling relative angular movement between the inductive sensor magnet and the reluctor teeth.
There was therefore a growing demand to find an alternative method of delivering fuel to the engine. With the contact breakers eliminated as a means of switching the ignition coil circuit. The ignition system delivered the high voltage to a rotor arm as was the case with contact breaker systems. Book 2 trying to create much improved reliability and durability of engine components and systems so that emissions did not increase significantly between maintenance intervals.
When each reluctor tooth passed the sensor magnet. Fuel systems. What we would now regard as simple electronics allowed considerable improvements to be made in the ignition system as described above but mechanical devices were still relied on to alter the ignition timing when engine speed and load changed. The carburettor actually developed into a complex and often unreliable device. The electrical pulses were used by the electronic module as a reference point for each cylinder.
On the early generations of electronic ignition. Figure 2. Ignition system developments A number of design changes were made over a period of time to various engine components and systems. Ignition timing is now controlled electronically instead of mechanically. The above is just one instance where engine design can be improved through the application of electronics.
Using electronically controlled systems allows engine designers to change certain design features so that there are fewer compromises.
Because the old mechanical timing controls were relatively inaccurate. The net result is that engine efficiencies are improved so that power and economy as well as emissions are all substantially better than was the case with older non-. In the end. Although some fuel injection systems used in the late s through to the early s were mechanically based. Europe and many other countries was becoming increasingly tough for vehicle manufacturers to comply with. High compression can result in combustion knock or preignition in the engine especially if the fuel quality is poor.
The remaining sections within this chapter detail more modern ignition and petrol systems. Other benefits Although emissions reductions are often regarded as the only motivation for using electronics on engine systems. While the early use of electronics improved reliability and durability. Diesel engines have of course traditionally always had direct injection into the cylinder or more accurately: This section deals initially with those ignition systems that were not necessarily integrated into engine management systems.
The continuous pace of development Many electronically controlled engine systems are covered within the rest of this chapter. Whilst it is often the case that existing mechanical systems are improved through electronic control. Book 2 electronic systems. Whilst the fundamental difference between petrol and diesel engines remains the way in which ignition occurs spark for petrol and heat generated from high compressions for diesel.
By studying many of these components and design features independently. Although we do tend to focus on ignition and petrol injection. There are many disadvantages associated with mechanical ignition systems. A good example is electronic diesel injection which is not an evolution of mechanical diesel injection.
Contact breaker systems are therefore not suited to the high engine speeds that are now typical for the modern engine weaker fuel mixtures can be more easily ignited with larger spark plug gaps but this requires more energy to be produced by the ignition coil. Plug gap — with a gap in the electrical HT circuit high tension or high voltage circuit. It was necessary therefore to replace the mechanical contact breaker with an electronic switch for the ignition coil primary circuit.
What has changed however is the quality of spark and the standards of performance reliability and timing accuracy. Many thousands of volts kilovolts. We can therefore look at the overall requirements of an ignition system at this stage. The primary requirements are to provide a spark or arc at a spark plug that is strong enough to ignite the air—fuel mixture. It is also true that the plug gap can become fouled or contaminated. The spark or arc produced at the spark plug must produce sufficient heat to cause ignition of the mixture.
There have been other requirements introduced over the years such as suitable interference suppression. The larger the gap. Voltage requirement firing voltage A typical voltage requirement for a modern engine is in the region of 7 kV to 12 kV or slightly higher assuming all components are good. The major factors affecting ignition systems are listed below.
High voltage A fundamental requirement of the ignition system is that it should produce a sufficiently high voltage from the ignition coil at all speeds to enable the air—fuel mixture to be initially ignited under cylinder pressure compression pressure.
Remember that electricity will take the easiest path to earth. Vaporised petrol ionises much more readily than air. To help overcome these problems. If however. Maintaining the spark spark duration Since the introduction of emissions regulations. Because electricity will always attempt to reach earth. A worn engine that has a low cylinder pressure will therefore require a lower voltage.
Cylinder pressures — with modern high compression engines the high pressure at the plug gap makes it more difficult for electricity to reach earth and a higher voltage is again required. Resistance — if we add resistance to the plug leads and possibly also to the spark plug to provide suppression. It is also interesting to note that if the ignition timing spark timing is slightly retarded so that the spark occurs at TDC instead of slightly before.
When a spark is created. If the voltage requirement is low. The rest of the voltage levels are detailed in the following paragraphs. Coil charge time dwell period If the coil is visualised as a container that stores electrical energy. Because the mechanical setting and operation of the contact breakers dictated this percentage.
Because the coil energy is now used to initiate and then maintain the spark for longer periods. If the percentage were any larger. When almost all of the coil energy has been used to initiate and maintain the spark. Like this presentation? Why not share!
An annual anal Embed Size px. Start on. Show related SlideShares at end. WordPress Shortcode. Published in: Full Name Comment goes here. Are you sure you want to Yes No. Be the first to like this. No Downloads. Views Total views. Actions Shares. Embeds 0 No embeds. No notes for slide. Book Details Author: Hillier ,Peter Coombes Pages: Nelson Thornes Ltd Brand: