Design Of Electronic Control Unit (ECU) For Automobiles .

Charging system demands and solutionsThe loads on the alternator can be classified as continuous, prolonged and intermittent,which keep increasing, thus increasing the power supply demand of the alternator. Ironlosses, copper losses and mechanical friction lead to inefficiency. There are varioussolutions for the high power demand [8].Larger alternator is the easiest, or alternators with higher maximum speed, thus allowinggreater drive ratio.Power management technique is used wherein certain loads like headlights, fog lights areswitched off when vehicle not moving [19].Two-speed drive technique, which uses a drive ratio of 5:1 for engine speeds under 1200rev/min and 2.5 at higher speeds [4].Increase Idle speed but this increases fuel consumption and emissions.Dual power supply technique is used wherein, each of two 12 V supplies are used forsmaller loads, while both 12 V, i.e. 24 V is used for heavier loads [18].Water-cooled and Integrated alternators / starters called ‘dynastart’ are also being used.2.3 Electronic Engine Starting circuit and systemIn order to start the engine a minimum starting speed (typically about 100 rev/min) shouldbe achieved. This is achieved with the help of an engine starter. Once this is achieved theengine would use the combustible mixture, compression stroke and ignition to continuerunning. The starting torque should be more than the engine torque at the time of starting.As the speed increases, and after the cranking speed, the engine torque takes over, and thestarter is disengaged mechanically. The cranking speed decreases with increase intemperature [5].11

Fig.3. Starting, engine torque and Starter Motor Characteristics [4]A typical cranking current of 150A to 500A is required to provide the initial stalledtorque. In a motor, when current is passed through the armature coil rotor (typically wavewound) placed in magnetic field stator (typically four-pole four-brush), a force is createdacting on rotor coils, causing it to rotate. A series wound DC motor, wherein the fieldwinding (electromagnet) is in series with the armature, is used for starting, because it hasa high initial torque (produced due to high current, low resistance and no back EMF),which is required to overcome the stall torque. Sometime, shunt wound, compoundwound or permanent magnet DC motors can also be used [1].Different types of starters are used like Inertia, Pre-engaged, Permanent magnet (smallsize) starters. Owing to the very high speeds at no load (0A), there is possibility ofdamage. Maximum power is at mid range speed and maximum torque at zero speed.Stall torque T BilrZ, where, B magnetic field in Wb/m2, I Current (V- EMF)/R, l length of conductor in field in m, r armature radius in m, Z number of armatureconductors [2].Power consumed P Torque x angular velocity. Typical efficiency is 60 %.The starter motor is the main load on the battery, thus to prevent power loss heavyconductors are used, with less voltage drop.Electronic engine starting circuit diagramThe circuit diagram is same as shown in the charging system. When the starter relayswitch closes, the hold-on and pull-in windings of the relay are energized. The current12

through pull-in winding flows through the starter motor, which slowly engages itself withthe engine, and at same, time the main contact to the starter motor closes. Thus the motoris now supplied by battery voltage, thus rotating and starting the engine. The pull inwinding switches off as equipotential. When the engine starts and start relay switch isreleased, the main contact opens and starter motor stops rotating and the starter isdisengaged from engine. This is the control circuit for the start relay switch.Fig.5. Circuit for controlling the start relay switch (used by Ford Motor Company Ltd) [8]13

The start relay switch (i.e. the starter) turns on only when the Ignition switch is at the startposition and either the Automatic transmission switch is at Park, Neutral position or theClutch pedal switch is at depressed position. Only under the above conditions will therelay coil activating the start relay switch is connected to earth through the Electronictransistorized Power control module (PCM). To prevent start operation when engine is on,the PCM does not complete the earth path.ECU controlled starter will have features such as starter torque evaluated in real time totell the precise instant of starting, so as to shut starter off after cranking speed, so as toreduce unnecessary wear and tear. The ECU will provide thermal and short circuitprotection.14

Chapter 3ELECTRONIC IGNITION SYSTEM’S DESIGNREQUIREMENTSHere it has been tried to propose a block diagram (which would be the basis on which theelectronic circuit would be designed) and flow chart, and hence the requirements, for thedesign of Ignition system section of Engine Management ECU. The fundamental purposeof ignition systems is to supply a spark inside the cylinder , near the end of thecompression stroke to ignite the compressed charge of air- fuel vapour.3.1 Basic terms and types of Ignition systemsFollowing are the four main factors considered for broadly classifying types of Ignitionsystems.Ignition coil and generation of High Tension (HT)For a spark to jump across an air gap of 0.6 mm in an engine cylinder under compression,a voltage of 2 to 3 kV is required. For higher compression ratios and weaker mixtures,voltage up to 20 kV or even 40 kV is required. Thus the ignition system has to transformthe normal battery voltage of 12 V to approximately 8 to 20 kV and in addition, has todeliver this high voltage to the right cylinder at the right time. By transformer action,primary winding of a coil is switched on and off causing a high voltage to be induced inthe secondary winding with more number of turns than the primary coil. The value of thismutually induced voltage depends on the primary current, turns ratio and rate of changeof magnetism [4].Advance angle (timing)The ignition timing (or the time at which the spark occurs) has a significant effect on fuelconsumption, torque, drivability and exhaust emissions. For optimum efficiency theignition advance angle should be such as to cause the maximum combustion pressure tooccur just after the Top Dead Centre (TDC) i.e. when the engine piston is at maximumcompression point [2]. The graph shows the effect of timing changes on emissions,15

performance and consumption. The quality of the spark determines its ability to ignite themixture. The stronger the spark, the less the likelihood of a misfire, which can causemassive increase in production of hydrocarbons. It is clear from the graph that acompromise on fuel consumption and emissions has to be done while choosing theadvance timing. The ideal ignition timing is dependent on factors to be discussed later.Fig.6. Effect of changes in ignition timing.DwellThe energy storage takes place in the ignition coil in the form of magnetic field. Thecharge on the coil before ignition point depend on dwell period. Ignition spark occurs is atthe end of dwell period. The term dwell is a measure of the tike during which the ignitioncoil is charging i.e. the primary current is flowing. It is often expressed as a percentage ofone charge – discharge cycle.DistributionDirects the spark from the secondary coil to each cylinder in a pre-set sequence. In a 4stroke 4 cylinder engine, it will distribute four sparks, one to each cylinder in tworevolutions.Thus depending on these factors the ignition system can be broadly classified as16

allyProgrammedProgrammed –ElectronicallyDistributedInductive /Inductive /Inductive cIgnitionInductiveDwell controlAdvancecontrolDistribution3.2 Proposed Block diagram and Design requirements ofElectronically programmed and distributed ignition systemFor obtaining the design requirements of ECU – Engine Management, we will considerthe Electronically Programmed and Distributed Ignition systems. The following is theproposed block diagram for designing the Ignition system part of the Engine ManagementECU. This diagram would be the basis on which the electronic circuit would be designed.Fig.7. Proposed Block diagram of Electronically Programmed and Distributed IgnitionSystem17

3.2.1 Ignition switchProvides driver control of the ignition system and is also used for starting the engine instarting phase.3.2.2Ignition or Spark generation - Inductive / Capacitor DischargetypeTypes of spark generation1. Inductive Charging of Ignition coilThe ignition coil stores energy in the form of magnetism. Te instantaneous value ofprimary current is given byi V (1 – e –Rt/L)RWhere i instantaneous primary current, R total primary resistance, L inductance ofprimary winding, t instantaneous time.The energy stored in the magnetic field of the ignition coil is given byE 1 (L x i2)2where E energy, L inductance of primary winding, I instantaneous primary current.The rate of increase of primary current determines the value of current when the circuit isbroken in order to produce the collapse of the magnetic field, thereby producing a highvoltage spike at the secondary.2. Capacitor discharge ignition (CDI)The CDI works by first stepping up the battery voltage to about 400 V DC, using an DCto AC converter (oscillator), amplifying it with a transformer, followed by a rectifier toobtain high voltage DC. This high voltage charges the capacitor, which is discharged atthe point of ignition through the primary coil by a thyristor, producing a high voltagespike at the secondary. The disadvantage is that since the spark duration is short, it cancause problems during starting, overcomed by multi-sparking [9].18

3.2.3 Electronic Voltage RegulatorThis circuit is used to provide a constant voltage supply to the ECU irrespective of thechanges in the supply and load, so as to provide accurate ignition control.3.2.4 Pulse Generator – Crankshaft / Hall effect / optical typeA pulse generator is used to provide the timing signal in correspondence to the speed andposition of the engine, so as to accurately control the turning on and off of the primarycoil, thereby providing spark at the desired time. Types of Pulse generators are1. Inductive pulse generator (Engine speed and position – crankshaft sensor)This device consists of a magnet, a winding and a soft iron core. It is mounted inproximity to the reluctor disc, which has 34 teeth, spaced 10o intervals around theperiphery of the disc as shown in the block diagram. It has two missing teeth , 180o apartat known positions before TDC and BTDC. As a tooth from the reluctor coupled with theengine passes the core of the sensor, it induces a voltage in the winding, the frequency ofthe waveform being proportional to the engine speed. The missing tooth causes a no pulseat those positions, so that the ECU can determine the engine positions.Fig.8. Typical inductive pulse generator (Engine speed and position crankshaft sensor)output [10]2. Hall effect pulse generatorThe principle of working of a Hall effect transducer is tat if a strip of conducting materialcarries a current in the presence of a transverse magnetic field, a difference of potential isproduced between the opposite edges of the conductor.19

EH KH IB/t , where KH Hall effect coefficient; Vm3/Awb, I current, B magneticfield, t thickness of hall stripAs the engine rotates, the vanes attached under the rotor arm alternately covers anduncovers the hall chip. Thus the chip produces almost a square wave output (0 to 7 V),which can easily be used to switch further electronic circuits.Fig.9. Hall effect pulse output [10]3. Optical pulse generatorThis involves a focused beam of light from an LED and sensed by a phototransistor. Therotating vane coupled with the engine interrupts the light, thereby forming a square waveoutput3.2.5 Pulse Shaping circuitA circuit within the ECU ( Schmitt trigger) converts the pulse generator signal into a puresquare wave.3.2.6 Electronic Dwell control - Constant / Open loop / ConstantEnergy closed loop typeTypes of Dwell control1. Constant Dwell systemsIn this type, the dwell percentage or dwell angle (ratio of the time for which the primaryis on to the time for which it is off) remains constant. Now as engine speed increases, the20

actual on and off times reduce, thus the time available to charge the coil reduces, therebyproducing a lower power spark.2.Open loop Dwell controlIn this type the dwell increases with engine speed. This will only be benefit, if the ignitioncoil can charge up to its full capacity in very short time. This figure shows the circuitdiagram of a transistorized ignition module.Fig.10. Circuit diagram of a transistorized ignition ECU module [9]Diode D1 acts a reverse polarity protection. The first part is a voltage stabilizer. Zenerdiode ZD1 provides a constant known voltage for the rest of the circuit. The pulsegenerator output is given as the trigger input to the Schmitt Trigger pulse shaping circuit.The diode D4, Transistors T1 and T2 act as a Schmitt trigger providing a square wave atthe collector of T2 with pulse position and frequency same as the pulse generated input,thus providing information of the position and speed of the engine. When T2 is off,capacitor C5 charges via R9 and T3. At low engine speed the capacitor has sufficient timeto charge. T3, T4, T5, T6 turn on, so is the ignition coil. T5, T6 is a Darlington pair outputswitching stage. At ignition point, T2 switches on, and C5 discharges and ignition coil is21

switched off suddenly thereby inducing a spike at secondary. As engine speed increases,the charge time available for C5 decreases, thus reaching a lower voltage and dischargingquickly, thus increasing dwell. ZD4 and D6 protect against back EMF of coil.3. Constant energy Closed loop systemThis is the type shown in the block diagram. Constant energy means that, within limits,the energy available to the spark plug remains constant under all operating conditions.Stationary engine primary cut-offDue to the high-energy nature of constant energy ignition coils, the coil cannot beallowed to remain switched on for more than a certain time, as when the ignition isswitched on but the engine is not running. This is known as ‘Stationary engine primarycurrent cut-off’ achieved by a timer. The sensing of current causes the feedback [14].Current limitingA very low resistance, high power precision resistor is connected in series with the powertransistor and primary of ignition coil, that causes the output stage to hold the primarycurrent at a constant value after the current exceeds as preset value.Battery voltage correctionCorrection to dwell settings, stored in the memory, is required if battery voltage falls, aslarger dwell is required. A three dimensional graph of the dwell against speed (forconstant energy output) and battery voltage is stored.3.2.7Electronics Spark Advance (ESA) timing control - speed, load,temperature, knock, idle, phase, anti-jerk, warm-up conditionsThe problems of mechanical centrifugal advance and vacuum advance are overcome byusing Digital electronics programmed timing advance. The ideal ignition timing to ensuremaximum pressure in the cylinder just after TDC, depend on two main factors, enginespeed and engine load followed by many other corrections.Engine speed and positionAt higher engine speeds the time taken for the piston to travel the same distance (i.e. pointof ignition) reduces. Advancing the time of the spark ensures full burning is achieved.22

Similarly at lower speeds spark timing is retarded. The ECU uses the pulse generator(crankshaft sensor) to obtain engine speed and position inputsEngine load measurement– manifold absolute pressure (MAP) sensorWeaker mixture is used on low load conditions, which burn at slower rate and requiresfurther ignition advance. Similarly for lower loads, richer mixtures, retardation is requiredas the mixture burns rapidly. Engine load is proportional to manifold absolute pressure.Thus Load / Pressure sensor connected to the ECU, is a silicon diaphragm withpiezoresistors connected in a Wheatstone Bridge and filter arrangement, to sense pressure.A lot of information is held in the ROM. The basic timing three-dimensional graphconsists of the correct ignition advance for 16 engine speeds and 16 engine loadconditions.Engine temperature measurement – coolant sensorCoolant temperature measurement is carried out by a thermistor, which is also used by theElectronic fuel system. Timing may be retarded when the engine is cold to assist in morerapid warm up. A three dimensional graph is used that has 8 speed and 8 temperature tothe timing settings.Detonation detection – knock sensorDetonation is caused due to the pressure wave traveling at high velocity causing impacton the cylinder walls setting them into vibration or ringing knock [8]. These knock aresensed by the knock sensor, which generate piezoelectric voltage when the seismic massvibrates. The resonant frequency is given asF 1/(2π)(k/m)1/2, where f frequency, k spring constant, m seismic massThe vehicle knock sensor has a frequency response of 15 kHz and a sensitivity of 20mV/g. The engine will run at its most efficient when the timing is advanced as far aspossible, but just below the knock range. To achieve this, the ECU responds to signalsfrom the knock sensor (accelerometer) in the engines knock window for each

Mar 02, 2010 · 1.1 The Electronic Systems design approach Electronic Systems can be defined as a group of electronic and electrical devices working to perform a specific task, with optimum and efficient results. The steps in the design of an Electronic System are surveying the Design Requirements, Analysis and Conceptualization of design i.e. proposing ideas .