The Motor Vehicle 2010 Part 7 doc

444 (a) The Motor Vehicle (b) Fig 12.19 Metering slit and diaphragm (a) At high rate of fuel flow, (b) at low rate of fuel flow The control plunger is that on the right in each diaphragm of 0.1 bar– with 4.6 bar in the upper chamber – is still maintained since a higher pressure would open the port wider, allowing the fuel to flow through at an even higher rate, and a lower one would close it The deflection of the diaphragm is in fact only a few hundredths of a millimetre From this it can be seen that the injection valve has no metering function It is closed by a spring and opens automatically when the pressure in the delivery pipe rises above 3.3 bar At this pressure the fuel is finely atomised as it passes through the discharge nozzle into the engine inlet valve port The only adjustments that can be made to this system in service are those of the engine idling speed and mixture For adjustment of idling speed, there is a screw that restricts the flow of air through a passage that by-passes the throttle valve when it is closed The greater the degree of restriction of course, the slower is the idling speed To increase idling speed, therefore, the screw should be turned anti-clockwise The idling mixture strength is adjusted by another screw, which acts on the arm by means of which the motion of the air sensor plate is transmitted to the control plunger Access can be gained for this adjustment, using a screwdriver, without any dismantling of the mixture-control unit As can be seen from Fig 12.11, the idling mixture screw is on the end of a lever swinging about the same pivot as, and approximately parallel to, the arm that carries the air sensor plate The end of the screw seats on that arm so, when it is screwed clockwise, it increases the angle subtended between the smaller lever and the arm This raises the control plunger slightly, thus supplying more fuel to the injectors and therefore enriching the mixture 12.14 Bosch KE-Jetronic system The KE-Jetronic system, Fig 12.20, is similar to the K-Jetronic except that it has a simple diaphragm-type fuel pressure regulator, in place of the warmup regulator, to maintain a constant primary pressure above the control plunger in the fuel distributor of Fig 12.17 Another change is the flange-mounting of an electro-hydraulic pressure actuator on the fuel distributor, to regulate the pressure in the supply to the lower chambers of the differential pressure valves Thus, whereas in the K-Jetronic the mixture enrichment is effected Petrol injection systems 445 15 12 14 11 13 10 Injector Cold start injector Fuel distributor Electro-hydraulic pressure actuator Fuel pressure regulator Air-flow meter Filter Fuel accumulator 9 10 11 12 13 14 15 Electric fuel pump Electronic control unit Idle by-pass valve actuator Throttle position switch Lambda sensor Engine temperature sensor Thermo time switch Fig 12.20 Bosch KE-Jetronic system by regulating the pressure above the control plunger, this function is performed in the KE by regulating the pressure input to the lower chambers and, moreover, not only for cold starting and warm-up but also for all other situations Yet another addition is a potentiometer on the air-flow sensor lever Its function is to signal to the electronic control the rate of air flow into the engine Additional input signals to the electronic control include engine temperature, engine speed (from the ignition system), idle, overrun and full-throttle signals (from the throttle position switch), exhaust content (from the lambda sensor), atmospheric pressure, and an engine starting signal from the ignition switch The output from the electronic control goes to the electro-hydraulic pressure actuator Fuel is drawn from the tank by the pump and delivered through the hydraulic accumulator to the filter and thence to the electro-hydraulic pressure actuator, in which it is directed through a nozzle on to a plate The clearance between the mouth of the nozzle and the plate is varied by an axial force exerted by an electro-magnet on a pole-piece on the plate This clearance is therefore determined by the magnitude of the electric current passing through the windings of the electro-magnet, which in turn is regulated by the electronic control During overrun, it can totally cut off the supply of fuel 446 12.15 The Motor Vehicle Bosch L-Jetronic system In the L-Jetronic system, Fig 12.21, the electronic control unit performs the same function as the mixture control unit of the K system It does this, however, by controlling the duration of opening of the solenoid-actuated valves in the injectors The advantage of electronic control is that there are fewer mechanical components liable to wear or to stick and thus to malfunction Moreover, ultimately, more accurate control is possible because the system can be made more easily to respond to a wider range of variables than when a mechanical system is used Because the injectors are solenoid actuated, Fig 12.22, lower delivery pressures are possible than those needed to open the pressure-actuated delivery valves of the K system Another advantage is that delivery can be made through all the valves simultaneously, which means that the injection system can be simpler than if each had to be opened individually Actually, to ensure that the distribution of fuel is uniform, to all cylinders, half the required amount of fuel is injected into each port twice, over two separate intervals, during each four-stroke cycle – that is, for each 360° rotation of the camshaft, Fig 12.23 The start of each injection pulse is signalled to the electronic control unit by the contacts in the ignition distributor However, the control unit has to 12 11 10 Pressure in intake manifold (p1) Atmospheric pressure (p0) Electronic control unit Injection valve Air-flow sensor Temperature sensor Fuel Thermo-time switch Start valve Electric pump Fuel filter Fig 12.21 Bosch L-Jetronic system Coolant 10 11 12 Pressure regulator valve Auxiliary-air device Throttle-valve switch Relay set Petrol injection systems 447 5 Nozzle valve Solenoid armature Solenoid winding Electrical connection Filter Cylinder Fig 12.22 Cross-section of the injection valve Operating cycle Suction stroke Instant of ignition Distributor contact points Engine speed Pulse shaper Engine speed Frequency divider Injection cycle n Charging operation Discharging operation Influence of compensation factors Basic quantity (Air-flow sensor) Temperature Operating condition 123 Injection pulse duration of injection Amount of fuel injected ti 0° 180° 360° 540° 720° CS Fig 12.23 Pulse diagram respond to only every second signal from the contact breaker in a fourcylinder engine and every third in a six-cylinder unit – since they have four and six sparks per cycle whereas only two injections per cylinder are required in each case 448 The Motor Vehicle Although the basic signal determining the duration of injection is that from the swinging gate type air flow sensor, Section 12.9, it has to be modified by a number of other signals received by the electronic control One is engine speed, which is signalled by the frequency of operation of the ignition contact breaker A throttle valve-actuated switch indicates whether enrichment is needed, for either full load or idling As in the K system, there is a temperature sensor, but it influences the duration of injection instead of the pressure It is necessary because the density and therefore mass of air drawn into the cylinders is greater in cold than in hot conditions An enrichment device for acceleration is unnecessary in either system since the air flow sensor gives its signal in advance of acceleration A solenoid-actuated start valve comes into operation for cold starting This is the same as in the K system, as also is the auxiliary air device for bypassing the throttle valve to compensate for the high friction losses Here, however, there is a relay set When the ignition is switched on, this relay set switches the battery voltage to the electric fuel pump, start valve, thermotime switch for switching off the start valve, and auxiliary air device When the engine starts, the power supply for the pump and auxiliary air device is maintained through a contact actuated by the air sensor If, on the other hand, the engine fails to start, a thermo-time switch interrupts the circuit to the solenoid-actuated start valve, to avoid flooding the cylinders With no control-valve unit, the fuel supply is simpler than before: it passes from the pump through a filter to a pressure-regulating valve and thence directly to the injector and start valve With such a simple and direct supply a fuel accumulator is unnecessary As can be seen from Fig 12.25, the fuel pressure regulator valve is of conventional design The fuel flows radially in one side and out the other, while fuel in excess of requirements passes out through the connection at the top of the unit and thence back to the tank With the L-Jetronic system, the fuel delivery pressure is either 2.5 or bar, according to the type of engine It is maintained at its set value by a spring-loaded diaphragm, which causes a valve to tend to seat on the port for the return line to the tank Any increase in pressure pushes the diaphragm down and opens this port To avoid variations in the back-pressure on the nozzles due to changing induction manifold Fig 12.24 Cross-section of the air-flow sensor Mixture adjustment screw for the idle range Air-flow sensor flap Non-return valve Air-temperature sensor Electrical connections Damping chamber Compensation flap Petrol injection systems 449 Fuel through-flow Return line to fuel tank Valve support Diaphragm Pressure spring Connection to intake manifold Valve Fig 12.25 Cross-section of the fuel-pressure regulator pressure, a pipeline connection is taken from the manifold to the chamber below the diaphragm Bosch also produce a petrol injection system in which the engine load signal is obtained by sensing the depression in the inlet manifold This is the D-Jetronic system It will not be described here, however, since the L system is the more advanced one and therefore of greater importance In any case, wear of the engine causes manifold depression characteristics to change 12.16 Bosch LH-Jetronic system All components of the LH are virtually identical to those of the L-Jetronic system, except for the electronic control unit and the substitution of a hotwire, air-mass-flow meter (Section 12.10) for the volume-flow meter A major advance made more easily feasible by the use of the hot-wire system is the use of a digital instead of analogue electronic control system, the former being potentially much more flexible Other advantages of the LH system are negligible resistance to air flow and absence of moving parts The incoming air flows past an electrically heated platinum wire, the temperature of which is maintained constant by using the wire as one arm of a Wheatstone bridge circuit and varying a resistance to balance the bridge Since the quantity of heat removed from the wire is a function of not only the velocity but also the density of the air flowing past it, the increase in current needed to maintain a constant temperature is a measure of the mass flow of the air A voltage signal taken from across a resistance through which this current is passed is transmitted to the control unit Among the other advantages is that the hot wire compensates automatically for changes in altitude Other signals required include engine speed, throttle position, and ambient air temperature An engine load–speed map is stored in the memory of the electronic control system which, taking into account all the other input signals, can in all circumstances accurately regulate the air : fuel ratio for optimum power output, fuel consumption, and exhaust emissions During idling the air mass flow is small so, in this condition, the air : fuel ratio is set by a potentiometer Another factor is that the surface of the wire 450 The Motor Vehicle can become contaminated when it is cold and when the engine is idling Therefore, to cleanse it and avoid subsequent inaccuracy of the signals output, the wire is automatically heated to a high temperature for one minute every time the engine is switched off 12.17 Bosch Motronic system Given that a microcomputer is used for regulating fuel injection, it can be even more cost-effective to employ it for other control functions too Primarily the Bosch Mono-Jetronic system integrates injection and ignition controls, but it can also be adapted for controlling other parameters such as exhaust gas recirculation and evaporative emission canister purging, which are explained in Sections 14.16 and 14.17 Comprehensive information on the electronic components of the Motronic and other injection systems is given in the Bosch publication Automotive Electric/Electronic Systems, and in their yellow book entitled Motronic Engine Management Its intermittent injection system is basically identical to that of the LJetronic, but all its signal-processing functions are done digitally Among the advantages of digital systems is that all the data processing can be done directly by the computer, and much of the electronics can be common to a wide range of applications Also, operating data can be stored on maps, which can be updated automatically by the computer to take into account changes such as might occur as a result of, for example, wear of the engine in service 12.18 The electronic ignition control Control of the ignition system is based on a spark advance characteristic map stored in the memory of the Motronic control unit The spark advance is continuously changed to correspond to the setting on the map, taking into account throttle position, and engine-coolant and air-intake temperatures When a spark is required, the electronic controller momentarily opens the circuit to earth, whereupon the collapse of the field around the primary generates the spark voltage in the secondary coil The resultant high-voltage current is passed through the distributor to the sparking plug Generally, there is neither a mechanical contact breaker nor a centrifugal and pneumatic advance and retard system, though some high-speed six-cylinder engines retain the centrifugal mechanism Obviously, a conventional mechanically actuated system could not vary the ignition advance to satisfy the complex requirements that are registered on the map, Fig 12.26, and stored in the memory of the ECU To obtain the data points on the map, the engine is run on a dynamometer, the ignition advance being optimised in respect of fuel consumption, emissions and driveability The data thus obtained are recorded electronically and transferred into the memory of the ECU By virtue of digital recording, the ignition point for each condition of operation can be set independently of all the others In operation, the microcomputer first reads from the map the point at which, on the basis of the instantaneous engine speed and load, the next spark should be triggered and then modifies it in relation to throttle position and coolant and air temperatures An inductive engine speed sensor signals Petrol injection systems 451 Ignition advance Load Rev/min (a) Ignition advance Load Rev/min (b) Fig 12.26 At (a) is a three-dimensional map showing the degree of accuracy with which the ignition timing can be controlled electronically as compared with, at (b), the best that can be obtained with a mechanical and vacuum advance and retard mechanism directly from the crankshaft This is more precise than using a Hall-effect sender in the distributor Consequently, the spark advance can be optimised while avoiding all risk of detonation, and both fuel utilisation and torque are therefore improved Actually, there are two inductive pulse senders on the flywheel One senses the passage of teeth past its permanent magnet core, for translation into engine speed The other, for indicating crank angle, senses the passage of either a pin or hole in the flywheel These two signals are processed in the control unit to make them compatible with the computer The parameters on the basis of which the ignition points are set include fuel consumption, torque, exhaust emissions, tendency to knock and driveability, the weighting given to each differing according to the type of operation For example, for idling, the priorities are low emissions, smoothness and fuel economy; for part-load operation, they are driveability and economy; and for full-load they are maximum torque and absence of detonation 452 The Motor Vehicle For all types of operation other than part-throttle light load, correction factors are applied to the map values Also included in the control unit is a switch which is actuated automatically during operation in the high-load range, to cater for different fuels and grades of fuels For starting, there is even a correction routine for adjusting spark timing in relation to cranking speed After the generation of each spark, a finite time is required for the reestablishment of the current in the coil to its nominal value, ready for the next firing The higher the engine speed, and therefore frequency of sparking, the longer is the dwell time needed to allow the current to build up in the coil Consequently, the relationship between current flow time in the coil and supply voltage has to be regulated, by reference to a dwell angle characteristic map similar to that in Fig 12.27 As soon as the current has risen to the appropriate level ready for the next ignition point, it is held there by the output stage so that, as the dwell time shortens during acceleration from low engine speeds, the appropriate current can be maintained throughout An indication of how the electronic control unit regulates injection and ignition simultaneously can be gleaned from Fig 12.28 To keep the breakaway and release times of the injection valves as short as possible without using current-limiting resistors, the current to them is limited by a special integrated circuit in the electronic control unit For a six-cylinder engine, for instance, the valve opening current is 7.5 amp and, at the end of the injection period reduced to a holding current of 3.5 amp 12.19 Fuel supply As can be seen from Fig 12.29, a roller-cell type pump delivers fuel, at a pressure of 2.5 to bar, through a filter removing particles down to 10 µm, directly to one end of a fuel rail At the other end is the pressure regulator, Fig 12.30, from which the return flow to the tank passes through a pulsation damper, Fig 12.31, to the tank This, by reducing fluctuations in the pressure in the return line, suppresses noises arising from both the operation of the pressure regulator and the injector valves Dwell angle Rev/min Battery voltage Fig 12.27 Three-dimensional plot showing how the dwell angle has to be varied relative to the supply voltage and engine speed, to allow the current enough time to build up in the primary winding Petrol injection systems 453 (a) (b) (c) (d) (e) (f) (g) 0° 120° 240° 360° Fig 12.28 Stages in the production of ignition sparks for a six-cylinder engine, by means of an electronic control such as that in the Bosch Motronic system (a) The reference signal for the crankshaft angle; (b) an indication of the degrees of rotation following the occurrence of the pulse signal; (c) the saw-tooth-shaped signal of the angle counter; (d) characteristic of the instantaneous operating condition, as calculated from the ignition and dwell angle signals and entered in the intermediate memory; (e) when the values of the signals from the counter and the intermediate memory are identical, signals are sent to the ignition output stage to switch the ignition coil on or off; (f) low-tension signal for ignition; (g) current through the coil By virture of its large volume relative to the quantities of fuel injected per cycle, the fuel rail acts as a hydraulic accumulator and ensures that all the injectors connected to it are equally supplied with fuel Injection occurs once per revolution (twice per cycle) and is directed into the ports 12.20 Overall principle of operation A swinging-gate-type air flow sensor, described in Section 12.9, is employed in the Motronic system, Figs 12.12 and 12.29 The duration of injection required for maintaining the λ value at 0.85 to 0.95, as needed for engines equipped with three-way catalytic converters, is assessed per piston stroke and in relation to engine speed, instead of per unit of time Corrections are applied, as required, in response to signals received from detonation, temperature, time and other sensors, and in accordance with plotted values on engine performance maps The sensors are as previously described for the Induction manifold design 499 the air is elastic and has mass, it responds by surging forward to restore the pressure, thus initiating an alternating set of depression and compression pulses, which are in fact sound waves travelling along the pipe at the speed of sound At low speeds and with valve overlap, there can also be a slight puff-back of gas from the combustion chamber into the inlet port, but this comes after the initiation of the sound wave It is less forceful and does not necessarily significantly interfere with the resonance, though the larger the overlap, the longer is the period available for such pulses and others from the exhaust system to have an effect The kinetic energy in the waves and momentum of the flow increase with engine speed This is because of the consequently increasing depression in the cylinders, and therefore the pressure ratio across the valve throat, with speed Resonant vibration phenomena are associated with mass–spring systems The mass is that of the column of air in the pipe and the spring element the compressibility of that air One wavelength λ is a complete cycle, or 2π radians, and therefore is equal to L in the top diagrams in Figs 13.20 and 13.22, and 4L/3 in Fig 13.21 The phase difference between the displacement and pressure waves is always π/4, or 90° At this point, some clarification as to what exactly happens at the open ends of a pipe is necessary On reaching the open end remote from the valve, a negative pressure wave sucks a slug of air in, and a positive pressure wave propels a slug out In both instances these effects take place against the influence of atmospheric pressure, so there is an inertia-driven over-swing followed by a bounce-back accompanied by a phase change If we plot the axial vibrations in the pipe to a scale such that the maximum amplitude of displacement in each direction equals the radius of the pipe section, they can be illustrated as shown in the top and bottom diagrams in these illustrations In each, the upper diagram represents the second-order, and the lower one the first-order, or fundamental, mode of vibration λ = 4/3 L A B A C B λ = 4L D Fig 13.21 Fundamental and first overtone modes of vibration of air in a pipe one end of which is closed and the other open 500 The Motor Vehicle λ=L A B C D E A B C D E λ = 2L Fig 13.22 Fundamental and first overtone modes of vibration of air in a pipe both ends of which are open From the upper diagram in Fig 13.20, it can be seen that axial motion of the air is positively stopped by the closed ends, A and E, of the pipe These ends are therefore displacement nodes Mid-way between them is a third displacement node, while B and D are displacement anti-nodes Because the air alternately moves towards and is bounced back from the displacement nodes, A and C and E are pressure anti-nodes In other words, while the pressure remains constant at B and D, it fluctuates cyclically at A, C and E This condition can occur in an induction pipe only when both a throttle and inlet valve are closed so, as regards manifold tuning, it is not of practical significance but it is relevant for automotive engineers concerned with body, cab or saloon noise If one end of the pipe is open, Fig 13.21, the air at that end is free to be displaced, so it becomes a displacement anti-node, which accounts for the different arrangement of the displacement curves for the fundamental mode of vibration and the overtones This condition can arise when the inlet valve is closed and the opposite end of the inlet pipe open For a pipe open at both ends, the fundamental and first overtone harmonics are shown in Fig 13.22 The third harmonic is illustrated in Fig 13.23 Since this is a condition that arises only when both the inlet valve and pipe end are open, it is of significance in relation to resonance effects initiated by the sudden pening of the inlet valve Clearly there must be some displacement beyond the open end before a reflection can occur, so a correction factor has to be applied to the length of the pipe In fact, the effective length of an open end is L plus about 0.6 times its radius r so, for one open and one closed end, the correction factor is L (1 + 0.6r), and for a pipe with both ends open it is L (1 + 1.2r) The time t taken for the completion of one wavelength is called the periodic time, or the period of the vibration, and the time required for the pulse to return to the inlet valve is 2L/c, where c is velocity of sound in the induction pipe For Induction manifold design 501 λ = 2/3L A B C D E F G Fig 13.23 Third harmonic mode of vibration of air in a pipe with both ends open several reasons, however, this is rarely directly applicable in the context of induction-system tuning First, the configurations of the ends of the passages are not those of a plain pipe end; secondly, there are other influencing factors such as air temperature and diameter of pipe; thirdly, and perhaps more important, c is not constant for large-amplitude waves such as occur in induction pipes More accurate results can be obtained if a cyclical mean value of c is used 13.15 Pipe end-effects Movement of the air into a pipe in general, and its displacement due to the vibrations, tend to cause turbulence around its open end, reducing the efficiency of flow This adverse effect can be considerably reduced by flaring the open end of the pipe to form a trumpet of approximately hyperbolic section, so that it guides the air flow smoothly in and thus increases the coefficient of inflow by up to about 2% The effective length of a pipe with such an end fitting is that of the parallel portion plus about 0.3 to 0.5 of the length of the flare If the outer ends of the pipes terminate in apertures in a flat plate, or in the wall of a plenum chamber, their flares should not only extend well clear of the flat surfaces but also be clear of any adjacent walls, to ensure that the approach velocity is well below that within the pipe Tapering the pipe, increasing its diameter from the inlet port to its open end, also reduces the end-effect This is sometimes done on very high-speed engines, for example in racing cars The aim is to reduce the velocity of flow into the open end, and therefore the tendency for turbulence to be generated there However, it is not conducive to the generation of powerful standing waves Incidentally, any reduction in the velocity of flow will also reduce the viscous drag between the air stream and the walls of the tube 13.16 Frequencies, wavelengths and lengths of pipes From the four illustrations, it is easy to see that the harmonic frequencies for pipes closed or open at both ends are f1, f2, f3, f4,…, fn, while those of the pipe closed at one end and open at the other are the odd numbers, f1, f3, f5, f7,…, fn The formula from which these frequencies can be obtained is f = c /λ, 502 The Motor Vehicle where c is the velocity of sound in air and λ is the wavelength The frequencies of the first three modes of vibration in each case therefore are as follows— Pipes with closed ends One end open Both ends open f1 = c/2L f1 = c/4L f1 = c/2L f2 = C/L f2 = 3c/4 L f2 = c/L f3 = 3c/L f3 = 5c/4L f3 = 3c/2L For waves of small amplitude the velocity of sound in dry air is √γp/ρ, where p is the gas pressure, ρ is the density, and γ is the ratio of the specific heats of the gas At the standard temperature and pressure in free air, this velocity becomes 331.4 m/s Standard temperature and pressure is 298.15 K and 105Pa (1 bar) Potential for some slight confusion arises, however, when referring back to data predating the universal introduction of SI units because, at the latter point, it became 273.15 K (0°C) and 101.325 Pa At velocities of more than Mach 0.25, viscous friction losses impair performance Whichever version of the speed of sound in free air is taken, it is independent of frequency and, because pressure divided by density is constant, it can be considered also to be independent of variations of pressure, certainly of the magnitudes experienced in inlet manifolds The velocity of sound varies with temperature according to the following relationship— cθ = c0 √(1 + αθ) where cθ and c0 are the velocities of sound at θ and 0°C respectively, and α is the coefficient of expansion of the gas While the local velocity of sound is dependent only on the temperature and composition of the gas, in induction pipes it is influenced also by diameter, Fig 13.24 This is because of the effect of viscous friction between the gas and the walls of the pipe Frequency is also affected, but relatively slightly, by the length: diameter ratio and internal smoothness of the pipe, both of which influence the degree of damping of the flow Since γ is dependent on the nature of the gas, the presence of fuel vapour, as in carburetted or throttle body injected spark ignition engines, will also affect the speed of sound in the manifold Even so, because extreme accuracy of calculation is generally unattainable, except possibly where the system comprises a set of straight tubes, this is not of much practical significance Indeed, induction systems have to be optimised experimentally, for example by the use of telescopic elements, during development The amplitudes of the resonant pressure pulsations too are modified by damping This can be due to roughness of the inner faces of the walls of the induction tract, the presence of bends, and obstructions such as throttle valves and inlet valve stems and guide ends From damped and undamped resonance curves in Fig 13.25, it can be seen that the effect of damping is not only a reduction in maximum amplitude but also it rounds off the peak, and spreads the resonance over a significantly wider range of frequencies In general, any bends in the pipes should be as close as practicable to the inlet valve ports, blended smoothly into the straight sections, and their radii should not be less than four times that of the bore of the pipe This arrangement leads to a minimum of both viscous losses and interference with the tendency for the air in the pipe to resonate freely Induction manifold design 20°C 40°C 60°C 0.75 (19.05) 1.0 (25.40) 0°C Pipe bore, in (mm) 1.5 (32.00) 2.0 (50.80) 3.0 (76.20) 5.0 (12.70) 20 1000 (304.8) 40 60 80 20 40 60 80 1100 1200 (335.28) (365.76) Velocity of sound in pipe, ft/s (m/s) Fig 13.24 Variation of the velocity of sound with diameter of pipe Undamped Amplitude Damped Resonant frequency Lightly damped Frequency Fig 13.25 Curves showing the effect of two different degrees of damping on the amplitudes of vibration around the resonant frequency Without any damping the curve rises to a sharp peak at the point of resonance 503 504 13.17 The Motor Vehicle Tuning the pipe to optimise standing-wave effects The time δt, expressed in terms of degrees rotation, required for a single standing wave to be reflected back to its point of origin (the inlet valve) is twice the length of the pipe divided by the velocity of sound (2L/c) From the lower diagram in Fig 13.21, it can be seen that the wavelength of the fundamental frequency is 4L, so δt is in fact half the periodic time During the time δt, the crankshaft rotates through an angle θt = 360Nδt/ 60 If we substitute for δt, this becomes θt = NL/c, where the suffix t refers to the time of the reflection, to distinguish it from θd, which is the time the valve is open, again expressed in degrees It follows that if it were practicable for the single wave to be an exact fit in the induction period, it would occur when θt = θd = 720/2n, where n is the number of the harmonic or overtone If, in our calculations, we substitute the actual velocity of sound in the pipe for that of sound in free air, we have what might be termed an induction wavefront velocity Then perhaps the simplest way to exemplify the time for the wavefront to travel one pipe length is to assume a wavefront velocity of 330 m/s and a pipe length of 330 mm which, of course, will give a time of ms 13.18 Harmonics of standing waves In addition to the standing wave at the fundamental frequency, harmonics are generated too by the initial impulse, Fig 13.26, so a number of modes of vibration, superimposed on each other, occur simultaneously Consequently, the initial reflection at the fundamental frequency is accompanied by a ripple of reflections at the smaller wavelengths of the overtone frequencies The successive reflected pulses are of progressively smaller amplitudes owing to attenuation by viscous friction and out-of-phase reflections from bends and other obstructions in their paths Consequently, no more than one, or possibly two, of the overtones are of significance, depending on whether the valve timing is late or early Long pipes and high speeds of flow increase both the flow losses and the degree of attenuation of pulses The actual timing, relative to the depression wave, of the appearance of the succession of waves at the inlet port can be adjusted by advancing or retarding the opening of the valve Neither the timing of valve opening nor the duration of overlap, however, have any significant effect on inertia ram, as distinct from resonance (or standing wave) ram, but they affect exhaust assisted scavenge Clearly, considerable advantage could be gained by combining induction system tuning with variable valve timing, Section 3.36 To fit the waves due to resonance into the valve-open period, the following condition must be met— n = θt = θd = 720/2n where n is the periodic time of the fundamental standing wave If θt is less than 720/2n, ripples will be superimposed on the depression pulse; if it is more, they may or may not affect it at all Clearly, the inertia effect will be predominant at high speeds This is because not only the magnitudes of these pulses increase with speed, but also, as the speed falls, the time available to fit more harmonics into the valve open period increases and, as previously mentioned, each successive Induction manifold design 505 Overtones Bmep (a) Engine speed Bmep (b) Engine speed Bmep (c) Engine speed Inertia wave Resonance wave Fig 13.26 The combined effects of the inertia and resonant standing waves At (a) the system is tuned for maximum power, at (b) to obtain a flat torque curve; and at (c) for good torque back-up wave reflection is weaker than its predecessor Maximum amplitude of the standing wave occurs when the pipe length is such as to contain a single wave, which occurs when L = θt = θd = 120°, and maximum overall amplitude is obtained when both the inertia and the standing-wave effects coincide Only the basic information has been given here In practice, the situation is further complicated by end-effects due to the presence of throttle valves, bends and the progressive motion of the closure of valves and by other factors For more comprehensive and detailed information, the reader is advised to refer to articles by D Broome, of Ricardo Consulting Engineers Ltd, and papers by K G Hall of Bruntel Ltd The former is a series in Automobile Engineer, Vol 59, pp 130, 180 and 262, while the latter were papers presented to the IMechE and AutoTech 89, Ref C399/20 The last mentioned contains a design chart presenting the graphical parameters in a manner such as to facilitate conceptualisation to an optimum inductionsystem geometry 13.19 Some practical applications of pipe tuning The obvious way to vary the length of the induction pipes to vary their resonant frequencies and the timing of the arrival of the reflection of the inertia wave back at the inlet valve is to have telescopic pipes, the lengths of 506 The Motor Vehicle which are controlled by the engine electronic management system This was in fact done by Mazda on their le Mans winning, Wankel powered racing car However, whether infinitely variable or a two-position pipe control is used, as in the le Mans car, the mechanism is complex and the whole system bulky and awkward to accommodate in a car A more practicable alternative is the Tickford rotary manifold, Fig 13.27, in which the central portion rotates to vary the effective length of the inlet pipe A commendably simple system was introduced in 1990 for some of the GM Vauxhall Carlton/Opel Senator models, and a similar principle has been applied to the Toyota 7M-GE engine The GM system will be described here As previously stated, the larger the number of cylinders that have to fire during the two revolutions of the Otto cycle, the more difficult it is to avoid overlap of valve open periods, and therefore inter-cylinder robbery This problem has been avoided in the GM system, called Dual Ram, by controlling the flow through the induction manifold so that at low speeds it has long pipes functioning like those in an in-line six and, at high speeds, it becomes in effect two integrated three-cylinder engines with short induction pipes How this is accomplished can be seen from Fig 13.28 Two tuned pipes take the air from throttle body and inlet plenum to a second, or intermediate, plenum chamber This chamber is divided by a flap valve so that, when the valve is open it is in effect one, and when closed, two chambers From the intermediate chamber, the incoming air passes through three short pipes to the six inlet ports in the cylinder head When the flap valve is closed, which is at the lower end of the speed range, each of the two sets of one long and three short pipes, together with the half plenum between them, form a single tuned duct At higher speeds, however, the flap valve is open, so that the intermediate plenum, now double the volume, isolates the six short inlet pipes, which of course resonate at a higher frequency The flap valve is opened at the speed corresponding to the cross-over point of the two torque curves in Fig 13.29 This valve is actuated by manifold depression and controlled by the ECU The six 60-mm diameter short pipes are 400 mm long and the length from the inlet valves to the plenum chamber next to the throttle barrel is 700 mm A smooth transition between the resonant speeds of 4400 and 3300 rev/min respectively is the outcome of this arrangement Fig 13.27 In the Tickford manifold, a central casting, distinguished by closer hatching, can be rotated to vary the effective length of the induction pipes This portion, extending the whole length of the cylinder block, serves also as a plenum chamber Induction manifold design 507 2-position valve Plenum Cylinder head casting chamber Throttle body Induction manifold branch pipes Fig 13.28 The Dual Ram system, with the two-position valve closed for operation in the × three-cylinder mode When it is open, the plenum chamber, then unobstructed from end to end, breaks the continuity of the tracts so that only the six short pipes resonate 170 150 kW @ 6000 rev/min 160 140 340 130 320 170 Nm @ 3600 rev/min Torque, Nm 300 120 280 110 260 Power, kW 150 100 240 220 200 6-cylinder mode 180 2000 × 3-cylinder mode 4000 Rev/min 6000 Fig 13.29 Power and torque curves for the Carlton GSi 300 24V engine equipped with the Dual Ram system 508 The Motor Vehicle Another tuned induction pipe system of interest is that of the Volvo litre 850 GLT engine, Fig 13.30 Each induction pipe comprises a pair of siamesed ducts, a section through the top of the pair resembling a figure-of-eight The diameter of the upper loop of the eight is slightly smaller and its length about twice that of the lower one over which, at its end nearest the head, is a steel flap valve Under the control of the ECU, this valve is initially held fully open by its return spring, but moves towards the fully closed position as the manifold depression increases Each valve is fitted with a rubber seal to obviate the need for machined seats and, when open, is parked in a recess in the pipe so that it does not interfere with the air flow At speeds below 1800 rev/min, both ducts are open, providing capacity for acceleration, though whether this adversely affects transient response is open to question Between 1800 and 4200 rev/min, but only so long as the throttle is 80° or more open, the shorter duct is closed Above 4200 rev/min, both ducts are open again to afford maximum flow potential In this condition, because one pipe is half the length of the other, the air is both resonate simultaneously but in different modes Calculated volumetric efficiencies are shown in Fig 13.31 and the actual power and torque curves in Fig 13.32 13.20 The Helmholtz resonator Another device that is being applied increasingly to induction systems is the evolve Fig 13.30 A sectioned V-VIS induction pipe of the Volvo 850 GLT engine The pipe (1) is about twice the length of pipe (2), and (3) is a plenum chamber Flap valves (4), one in each pair of pipes, are all moved simultaneously by a single manifold depression actuator (5) (Right) The complete system with, inset, a diagram showing how, by thickening one edge of the throttle valve, two-stage opening is obtained to provide a smooth take up of drive from the closed throttle condition See also Fig 11.10 Induction manifold design Closed control valve 100 Volumetric efficiency, % 509 90 80 Open control valve 70 1000 2000 3000 4000 5000 Engine speed, rev/min 6000 Fig 13.31 Estimated volumetric efficiencies obtained with the Volvo V-VIS system hp 180 170 kW Nm kpm 130 280 160 120 260 240 220 26 24 22 120 90 110 80 200 20 70 180 18 60 160 16 100 90 80 70 140 14 50 60 50 Torque Power 150 110 140 100 130 28 120 12 40 20 1000 40 2000 60 3000 4000 80 5000 rev/sec 100 100 10 6000 rev/min Fig 13.32 Power and torque curves of the Volvo 850 GLT engine Helmholtz resonator, Fig 13.33, which, because a larger mass of air may be displaced by it, can be more powerful in its effect than pipe tuning Because it is effective over only a very narrow band of frequencies, its use has been confined in the past to generating what has now become known as antisound, to eliminate induction pipe roar and exhaust boom Anti-sound is of course a sound of the same frequency but opposite in phase to that which has to be eliminated More recently, the principle of its application to boost the 510 The Motor Vehicle S = cross sectional area of neck L = length of neck L V = volume of cavity Fig 13.33 Diagrammatic representation of the Helmholtz resonator low speed performance of turbocharged engines has been described in two papers presented before the IMechE in May 1990, at the Fourth International Conference on Turbocharging and Turbochargers One is Paper C405/013, by G Cser, of Autokut, Budapest, and the other is Paper C405/034, by K Bsanisoleiman and L Smith, of Lloyd’s Register, and B A French, of the Ford Motor Company An earlier and equally interesting paper on this subject by Cser was C64/78, presented at the 1978 conference In general, Helmholtz resonators have been used also to detect extremely faint noise signals Another application is the damping of resonant vibrations, the damping effect being increased by, for example, placing porous material in the neck of the resonator Also, it can be used to increase the sound pressure in an acoustic field at a particular frequency This is of interest because of its potential for enhancing the effectiveness of a tuned manifold By the late 1980s, Helmholtz resonator principle began to be widely applied also as a primary engine-tuning device Although it is effective at only one frequency, it is particularly useful for improving volumetric efficiency at relatively low engine speeds For influencing induction-pipe resonances, either of two locations for the open end of the neck of the resonator are effective If it is positioned at a displacement anti-node in the induction tract, it is in phase and therefore increases the amplitude of displacement of air in the tract On the other hand, if placed at a displacement node, it tends to counteract the resonant vibration of the air in the tract, because it is π/2 out of phase The Helmholtz resonator generally comprises a short tube connected to an otherwise totally enclosed cavity This cavity can be of any shape, though a bulbous form may be preferred because it is less likely than almost any other to have natural modes of vibration that could influence the system as a whole The air in the neck is assumed to act like a piston, alternately Induction manifold design 511 compressing and expanding that in the cavity In other words, the air in the neck constitutes the mass, while the compressibility of that in the cavity forms the spring of a spring–mass system The wavelength of the vibrations it generates is large relative to the dimensions of the cavity Its natural frequency f corresponds to the value of the angular frequency at which the reactance term disappears, and is therefore given by— 2πf = c√ (S/LV) i.e f = (c/2π)√(S/LV) where c is the speed of sound, L the length of the neck, S the area of the neck and V the volume of the cavity From the last term in the equation, it can be seen that the natural, or resonant, frequency increases as the square root of the area of the neck, and decreases as both the square root of the length of the neck and of the volume of the cavity, or resonant chamber, are increased Incidentally, provided the length of the tube is small relative to the wavelength of the sound at the resonant frquency, the effective length of the neck numerically is approximately the actual length plus 0.8 times S As the crosssectional area of the neck is increased, the mass of air in it increases, but the relative viscous damping effect falls rapidly Clearly, however, both the mass of the air and the viscous friction in the neck increase linearly, with its length, so the main consideration is the area : length ratio As regards the volume of the resonator cavity, the smaller it is the higher is its spring rate and therefore also both the amplitude and frequency of oscillation An important consideration is the energy content, or what might be termed the ‘power’ of the resonator, which is a function of the mass of the air in the neck Therefore, the larger the volume of the neck, the greater is the effectiveness of the system In acoustical applications, the Helmholtz resonator is most effective at the lower end of the audible frequency range, down to about 20 Hz which, expressed in terms of incidence of inlet valve closure, is from about 600 rev/min upwards 13.21 Helmholtz resonators in automotive practice In automotive applications, however, things are not at all simple For instance, it has been suggested that the Helmholtz system may comprise the induction pipes with their cylinders acting as resonant cavities, but the volumes of the cylinders are of course varying continuously The suggestion is that the resonator volume can be taken to be that when the piston is at mid-stroke, which is half the piston displacement plus the clearance volume At this point, when the downward velocity of the piston is at its maximum, a rarefaction wave transmitted from the inlet valve to the open end of the pipe is reflected back as a pressure wave into the cylinder Optimum tuning is obtained when this wave arrives in the cylinder just before the inlet valve closes Since the resonance does not continue after valve closure, this type of resonator acts independently of engine speed and therefore can be effective over the whole speed range but, as previously indicated, decreasing in effectiveness as frequency increases Peak effectiveness occurs when the resonator frequency is approximately double that of the piston reciprocation Application of the Helmholtz resonator has been investigated in detail and reported by Thompson 512 The Motor Vehicle and Engleman, in ASME publication 69-GDP-11, and a good summary of the situation is presented by Tabaczinski, in SAE Paper 821577 13.22 Alternative Helmholtz arrangements In some instances, though mainly in the past, plenum chambers have been designed into the system simply to smooth out pulsations in the flow, or as a means of terminating, or isolating, the ends of tuned inlet pipes However, as a Helmholtz cavity, it may be a separate component introduced into almost any part of the induction system For instance, a plenum chamber or the filter housing with its inlet, or zip, tube may be utilised for this purpose In most instances, the pressures and densities (and therefore the masses) of air in the pipes will be lower than that of the air in the plenum, and this will affect the resonant frequency Moreover, the effective volume of the plenum and therefore the resonant frequency and effectiveness of the system may vary according to whether the throttle valve is open or closed In the latter condition, the incoming air will be passing the edges of the throttle at or near sonic velocity Other factors come into play too, such as the damping effect of various features of the induction system, including the throttle valve Damping can be actually helpful, in that it reduces the peakiness of the resonance curve and spreads the response of the resonator over a broader speed, or frequency, range With the advent of computer modelling, the introduction of Helmholtz resonators into induction systems no longer involves tedious and repetitive calculations One such model is the Merlin Model for the Diesel Engine Cycle, information on which is available from Dr Les Smith, Performance Technology Department, Lloyd’s Register, Croydon CR0 2AJ 13.23 Examples of the application of the Helmholtz principle Perhaps the most common practical application of the Helmholtz principle in the 1960s and 1970s was the suppression of unwanted frequencies in the noise spectrum issuing from the air intake For this purpose, the air passes through a tuned length of pipe into the filter housing, which serves as the resonator Any damping provided by the presence of the filter element broadens the band of frequencies over which the noise suppression system is effective More recently, it has been used on, for example, turbocharged diesel engines As the engine speed falls, so also does the torque but at a disproportionately high rate, owing to the square-law performance characteristic of the turbocharger There is also a tendency for black smoke to be generated In these circumstances, the low frequency effectiveness of the Helmholtz resonator can be put to good use A paper on this subject, No 790069, by M.C Brands, was presented at the February 1979 SAE Congress Similar conditions can arise in naturally aspirated engines with tuned induction pipes The energy content, or power, of the inertia wave of a tuned induction pipe falls with engine speed More significantly, however, not only is the tuning of the pipes invariably optimised for the upper speed range, but also, at some speeds, the pulse can actually be in a negative phase when the inlet valve is open, thus reducing the mass of air entering the cylinders below that which would occur even without a tuned system Consequently, a Helmholtz Induction manifold design 513 resonator tuned to oppose the negative inertia wave can significantly increase volumetric efficiency at low speeds Although it may tend to detract from the volumetric efficiency at high speeds, this effect is not necessarily of much significance owing to the characteristically weak performance of such resonators in the high-frequency range Incidentally, the latter phenomenon is accentuated by the fact that the spring rate of the resonator is inherently low, while the pressure and therefore the mass of the air in the pipe tends to increase with load, and the resistance to flow increases as the square of its velocity 13.24 Application to Vengines An objection to the Helmholtz resonator is that it tends to be bulky and therefore difficult to accommodate beside the cylinder head On the other hand, it can be fitted conveniently between the banks of cylinders in an engine of Vlayout, especially a V-8 unit The alternative of fitting long induction pipes to improve volumetric efficiency at low speeds is, by comparison, relatively unattractive Application to six-cylinder and V-8 engines has been discussed by Watson, in IMechE Paper C40/82 Induction-system layout for a six-cylinder engine is relatively simple, since resonator cavities can be allocated to groups of three cylinders, each group being isolated by a plenum chamber, as in Fig 13.34 For a four- or Firing orders 1.6 5.2 3.4 (a) Resonant chambers 1.4 8.3 5.2 7.6 (b) Resonant pipes Fig 13.34 Diagrammatic representation of induction-system layouts embodying Helmholtz resonator chambers P1: Dia cm, L 100 cm P3: Dia cm, L 20 cm P2: Dia and L varied V P2 P1 Single cylinder engine P3 Fig 13.35 Diagram used by Shimamoto to explain the effects of varying the geometry of a Helmholtz-tuned induction system ... senses the passage of either a pin or hole in the flywheel These two signals are processed in the control unit to make them compatible with the computer The parameters on the basis of which the. .. the temperature of the incoming air rises above 35°C, the Thermac valve opens, to vent the depression to the clean side of the air filter A restrictor in the connection from the manifold to the. .. delivers the fuel to a filter mounted in the engine compartment, and thence to the throttle body unit, in which is housed the injector A proportion of the fuel is then metered by the injector into the