Automotive Engineering Powertrain, Chassis System and Vehicle Body

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1.1.1.1 Engine components and terms

  tion end of the cylinder block and houses both the inletThe main problem in understanding the construction of and exhaust poppet-valves and their ports to admit air–the reciprocating piston engine is being able to identify fuel mixture and to exhaust the combustion products.and name the various parts making up the power unit. Most of the burnt gases will be expelled by the existing pressure energy of thegas, but the returning piston will push the last of the spent gases out of the cylinder through the exhaust-valveport and to the atmosphere.

1.1.1.3 Valve timing diagrams

  r ¼ effective crank-arm radius (m) 1.1.6.3 Engine torque This is the turning-effort about the crankshaft’s axis of rotation and is equal to the product of the force actingalong the connecting-rod and the perpendicular distance between this force and the centre of rotation of thecrankshaft. T ¼ Fr where T ¼ engine torque (N m) F ¼ force applied to crank (N) and During the 180 crankshaft movement on the power stroke from TDC to BDC, the effective radius of thecrank-arm will increase from zero at the top of its stroke to a maximum in the region of mid-stroke andthen decrease to zero again at the end of its downward movement ( Fig.

2 L

  L ¼ piston stroke (m) and N ¼ crankshaft speed (rev/min) v ¼ piston speed (m/min) N ¼ v This description of compression, burning, and expan- sion of the gas charge shows the importance of utilisinga high degree of compression before burning takes place, to improve the efficiency of combustion. This is because the crank-throw length determines the leverage on the crankshaft, and the piston speed dividedby twice the stroke is equal to the crankshaft speed; i:e: )The length of the piston stroke influences both the turning-effort and the angular speed of the crankshaft.

1.1.7 Compression-ratio

  In this chapter the principles of torque measurement The trunnion bearings are either a combination of are reviewed and then the types of dynamometer area ball bearing (for axial location) and a roller bearing reviewed in order to assist the purchaser in the selectionor hydrostatic type. 2.1.2 Measurement of torque: trunnion-mounted (cradle)machines The essential feature of trunnion-mounted or cradled dynamometers is that the power absorbing element ofthe machine is mounted on bearings coaxial with the machine shaft and the torque is restrained and measured T by some kind of transducer acting tangentially at a known radius from the machine axis.

2 N m

  ¼ Iu ¼2p NI Rotational speed of the dynamometer is measured either by a system using a toothed wheel and a pulse sensorwithin its associated electronics and display or, more recently, by use of an optical encoder system. While thepulse pick-up system is robust and, providing the wheel to transducer gap is correctly set and maintained, reli-able, the optical encoders, which use the sensing of very fine lines etched on a small disk, need more care inmounting and operation.

2.1.7 Choice of dynamometer

  In this machine, torque is The machines are of two kinds, depending on the means by which the resisting torque is varied.1(a) Constant fill machines: the classical Froude or A forced vortex of toroidal form is generated as a consequence of this motion, leading to high rates ofturbulent shear in the water and the dissipation of power in the form of heat to the water. These ‘wet gap’ machines are liable toEnergy is transferred in the form of heat to cooling water corrosion if left static for any length of time, have higher circulating through passages in the loss plates, while some inertia, and have a high level of minimum torque, arisingcooling is achieved by the radial flow of air in the gaps be- from drag of the cooling water in the gap.tween rotor and plates.

2.1.8.2 One, two or four quadrant? torque reversals

  Most engine testing takes place in the first A useful feature of such a machine is its ability also to quadrant, the engine running anticlockwise when viewedstart the engine. On occasions it is necessary formance of machines in this respect.a test installation using a unidirectional water brake to accept engines running in either direction; one solution isto fit the dynamometer with couplings at both ends mounted on a turntable.

2.1.9 Matching engine and marine engines are usually reversible

  Performance limited by maximum permitted shaftWhen it is required to operate in the third and fourth torque.quadrants (i.e. for the dynamometer to produce power as Performance limited by maximum permitted power, well as to absorb it) the choice is effectively limited towhich is a function of cooling water throughput andd.c. 1Measurement of torque, power, speed and fuel consumption The sizing of the motor must take into account the maximum break-away torque expected, usually es-timated as twice the average cranking torque, while the normal running speed of the motor should correspond tothe desired cranking speed.

2.1.10 Engine starting and cranking

  An alternative solution is to use a standard vehicle engine starter motor in conjunction with a gear ring car-ried by a ‘dummy flywheel’ carried on a shaft with sep- arate bearings incorporated in the drive line, but thismay have the disadvantage of complicating the torsional behaviour of the system. The engine starter will be presented with a situation not encountered in normal service: it will be required toaccelerate the whole dynamometer system in addition to the engine while a ‘green’ engine may exhibit a very highbreakaway torque and require prolonged cranking at high speed to prime the fuel system before it fires.

2.1.10.3 Non-electrical starting systems

  The shaft connecting engine and dynamometer must be designed with a suitable stiffness C to ensure that s the critical frequency lies outside the normal operating range of the engine, and also with a suitable degree ofdamping to ensure that the unit may be run through the Fig. 2.1a-3 .synthesized from a series of harmonic components, each c At low frequencies, the combined amplitude of a pure sine wave of a different amplitude having a fre-the two masses is equal to the static deflection of the quency corresponding to a multiple or submultiple ofshaft under the influence of the exciting torque, the engine speed and Fig.

1 The starting point is

0.08 Measurement of torque, power, speed and fuel consumption

  Values are given for a so- called ‘ p factor’, by which M Mmean the value of the mean turning moment developed by the cylinder, In the case of large multicylinder engines, the ‘wind- up’ of the crankshaft as a result of torsional oscillationscan be very significant and the two-mass approximation is inadequate; in particular, the critical speed may beoverestimated by as much as 20 per cent and more elaborate analysis is necessary. 1 2 W W This may be regarded as the ratio of the energy dis- sipated by hysteresis in a single cycle to the elastic energycorresponding to the wind-up of the coupling at mean deflection: W ¼ 1 2 Tq ¼ 1 2 T = C The damping capacity of a component such as a rubber coupling is described by the damping energy ratio:j c The damping energy ratio is a property of the rubber.

2 The engine or vehicle test engineer would normally

  2.1a.13 Notation The most common hazard of test rig mounted fly- wheels is caused by bearing or clutch failure whereconsequential damage is exacerbated by the considerable energy available to fracture connected devices or becauseof the time that the flywheel and connected devices will rotate before the stored energy is dissipated and move-ment is stopped. Any perceived shortcoming in the speed of re-sponse or accuracy of the simulation is usually considered to be of less concern than the mechanical simplicity ofthe electric dynamometer system and the reduction in required cell space.

2 Pa)

  Other factors are overcooling, largequench areas in the combustion chamber, the un- avoidable presence of a quench layer of gas a fewhundredths of a millimetre thick clinging to the walls of the combustion chamber, and quenching in crevicessuch as the clearance between the top land of the piston and the cylinder bore. The toxicity of hydrocarbons and oxides of nitrogen, on the other hand,arises indirectly as a result of photochemical reactions between the two in sunlight, leading to the production ofother chemicals.

2 O. Most of the undesirable ex-

  By 1968, weakening the air:fuel ratio, retarding the spark timing, preheatingthe air passing into the engine intake, and, on some models, the installation of a pump to inject air foroxidising the HC and CO in the exhaust system re- duced the total emissions about 39–41% by compari-son with the 1960 cars. By applying weighting factors to alter the relative Conditions that encourage the generation of NO in x effects of the three bag analyses on the totals, the results the combustion chamber are principally temperatures of the test are easily adjusted to represent different above about 1350 C in the gas at high pressures andcharacteristic types of operation.

3.1.3 Catalytic conversion

  Sandwiched between top halves of the outer and inner shells is heat insulation material. The perforations in the lower half of the outer shell, termed the grass shield, facilitate local cooling.

3.1.7 Metallic monoliths

  are prone to deterioration owing to attack by the sulphuric acid formed by combustion of impurities in the for catalytic converters fuel. A typical two-way converter for an American car contains about 1.6 g of noble metals in the Pt:P1 ratioAnother important aim is of course durability at both of 5:2.very high and rapidly changing temperatures.

3.1.6 Catalyst support

  The advantages obtained with the monoliths made ofEmicat include: rapid warm-up; resistance to both ther- mal shock and rapid cyclical temperature changes up towell over 1300 C (both due to the good thermal con- ductivity of the material and low heat capacity of theassembly); minimal back-pressure, by virtue of the thin sections of the catalyst carrier foil ( Fig. 3.1-5 ); com-pactness due to thinness of the sections and the absence of the mat needed around a ceramic monolith (to absorbits thermal expansion); large area of the catalyst exposed to the gas flow (owing to the high surface:volume ratio of Fig.

3.1.9 Three-way conversion

  By 1980 it became necessary for meeting the stringent requirements for the control of NO þ H 2 ¼ H 2 O þ CO These two reactions alternate in the combustion cham- ber which is why, with the rich mixture, there is alwayssome hydrogen in the exhaust, especially since some of the products of combustion are frozen by the cold wallsof the chamber. By 1978 GM had developed a three-way converter, the term implying the conversion of a third component,namely the NO x x With three-way conversion, a closed-loop control system is essential, for regulating the supply of fuel ac-curately in relation to the mass air-flow into the engine.

3.1.11 Warm-air intake systems

  Thus, air that has passed through a filter is drawn past the rocker gear into the crankcaseand thence to the manifold, whence it is delivered into the cylinders, where any hydrocarbon fumes picked upfrom the crankcase are burnt. There are three requirements for such a system: first, the flow must be restricted, to avoid upsetting the slowrunning condition; secondly, there must be some safe- guard to prevent blow-back in the event of a backfire and,thirdly, the suction in the crankcase has to be limited.

3.1.13 Crankcase emission control

  At the junction between the delivery pipe and the manifold, there isa check valve, to prevent back-flow of exhaust gas into the pump and thence to the engine compartment: this couldhappen in the event of failure of the pump or its drive. The sudden depression produced by the overrun conditionlifts the valve, venting the air delivery to atmosphere, and at the same time seating the valve on the port throughwhich the air was formerly being delivered to the manifold.

3.1.14 Air injection and gulp valve

  An orificein the central plate of the diaphragm assembly allows the depression to bleed away from the diaphragm chamber,the duration of diversion of the air to the cleaner being therefore a function of the sizes of the bleed and de-pression signal orifices and the volumes of the upper and lower parts of the diaphragm chamber. However, when the engine is warm and the computer control switches to closed-loop operation, the solenoid-actuated valve diverts the output of air from the exhaust manifold to the second catalytic bed ( Fig. 3.1-11 ), toenable the reducing catalyst in the first bed to function efficiently while, at the same time, to further the oxi-dation process in the second bed.

3.1.18 Diesel engine emissions

  from the diesel engine was, on average, between 5 and 10 timesthat of an equivalent gasoline power unit with a catalytic x In the early 1990s, the overall output of NO is an illusion: what happens in reality is that the weight of fuel injected is reduced, and the engine istherefore de-rated. The net result of the two effects isrelatively little or even no change in NO x x Increasing the cetane number reduces the delay period, so the fuel starts to bum earlier, so higher tem-peratures and therefore more NO output certain basic facts must be borne in mind.

3.1.20 Oxides of nitrogen, NO x

  It increases the time required ducing the output of NO x for the fuel to mix with the air, and therefore reduces the concentration of oxygen around the fuel droplets. Efforts are creasing the proportion of fuel sprayed on to the com-being made to develop catalysts suitable for diesel bustion chamber walls, the holes in the injector mustapplication, but at the time of writing no satisfactory again be smaller in diameter and larger in number.solution has been found.

3.1.21 Unburnt hydrocarbons

  Other means of reducing con-tamination by lubricating oil include improving the sealing around the inlet valve stems, the use of pistonrings designed to exercise better control over the thick- ness of the oil film on the cylinder walls and, if the engineis turbocharged, reduction of leakage of oil from the turbocharger bearings into the incoming air. Finally, the crevice areas, for example between the piston andcylinder walls above the top ring, also contain unburnt or quenched fractions of semi-burnt mixture, Expandingunder the influence of the high temperatures due to combustion and falling pressures during the expansionstroke, and forced out by the motions of the piston and rings, these vapours and gases find their way into theexhaust.

3.1.24 Particle traps

  The honeycomb ce- quality and particulate and NO output has been dem- x ramic monoliths generally differ from those used for onstrated by Volvo ( Fig. 3.1-24 ).catalytic conversion of the exhaust in gasoline engines, in A small proportion of the particulates is ash, most of that the passages through the honeycomb section arewhich comes from burning the lubricating oil. White smoke is a mixture of partially vaporised droplets of water and fuel, the former being products of com-bustion and the latter arising because the temperature of the droplets fails to rise to that needed for ignition.

3.1.26 Black smoke

3.1.27 White smoke

  However, this chapter describes rep-resentative control systems that are not necessarily based on the system of any given manufacturer, thereby givingthe reader an understanding of the configuration and operating principles of a generic representative system. When the injector valve is opened, fuel flows at a rate R f (in gal/sec) that is determined by the (con- stant) regulated pressure and by the geometry of thefuel injector valve.

4.1.3 Digital engine control features

  any cylinder is proportional to the time T that this valve is opened: F ¼ Rf T The engine control system, then, determines the correct quantity of fuel to be delivered to each cylinder(for a given operating condition) via measurement ofMAF rate. (Alternatively, the low air/fuel ratio may be maintained for a fixed time interval following start,depending on start-up engine temperature.) When the CT rises sufficiently, the mode control logic directs the system to operate in the open-loop controlmode until the EGO sensor warms up enough to provide accurate readings.

4.1.4 Control modes for fuel control

  In this mode, the engine speed is controlled to reduce engine roughness and stalling thatmight occur because the idle load has changed due to air conditioner compressor operation, alternator operation,or gearshift positioning from park/neutral to drive, although stoichiometric mixture is used if the engine iswarm. Similarly, the I/Osubsystem provides the output signals to drive the fuel injectors (shown as the fuel metering block of Fig. 4.1-2 )as well as to trigger pulses to the ignition system (de- scribed later in this chapter).

CT POS/RPM CYCLE

  The quantity of fuel to be injectedduring the intake stroke of any given cylinder (which we call F ) is determined by the mass of air (A) drawn intothat cylinder (i.e., the air charge) during that intake There is always the possibility of a CT failure. The larger fuel droplets tend to increase the ap- parent air/fuel ratio, because the amount of usable fuel(on the surface of the droplets) in the air is reduced; therefore, the fuel metering system must providea decreased air/fuel ratio to provide the engine with a more combustible air/fuel mixture.

A/F

  If thenumber of cylinders is N then the air charge (mass) in each cylinder during one revolution is: ð A=FÞd The quantity of air drawn into the cylinder, A, is computed from the MAF rate and the RPM. If the engine speed isRPM, then the number of revolutions/second (which we call r) is: r ¼ RPM A ¼ f MAF rðN=2Þ In this case, the mass of fuel delivered to each cylinder is: F ¼ MAF rðN=2ÞðA=FÞd This computation is carried out by the controller continuously so that the fuel quantity can be variedquickly to accommodate rapid changes in engine oper- ating condition.

4.1.4.3 Open-loop control

  In the open-loop mode the accuracy of the fuel delivery is dependent on the accuracy of themeasurements of the important variables. Whenever the EGO sensor indicates a rich mixture I(n) is the integral part of the closed-loop correction P(n) is the proportional part of the closed-loop The time-average EGO sensor output voltage provides the feedback signal for fuel control in the closed-loopmode.

IDLE AIR

  The control mode selection logic determines whenEGR is turned off or on. EGR is turned off during cranking, cold engine temperature (engine warm-up), idling, accel-eration, or other conditions demanding high torque.

4.1.6 Variable valve timing control

  In the open-loop case, the correct cam-shaft phasing depends on the relationship between axial position of the helical spline gear and the oil pressure/return spring relationship. In such a system the controller selects the ap-propriate coil and delivers a trigger pulse to ignition control circuitry at the correct time for each cylinder.( Note: In some cases the coil is on the spark plug as an integral unit.) An engine must be provided with fuel and air in correct proportions and the means to ignite this mixture in theform of an electric spark.

4.1.7 Electronic ignition control

  Of course, an SA control scheme based on limiting the levels of knocking requires a knock are beyond the scope of this book, it is generally a result of a portion of the air–fuel mixture autoigniting, as op-posed to being normally ignited by the advancing flame front that occurs in normal combustion following sparkignition. Although the details of what causes knocking When the spark is advanced too far, an abnormal combustion phenomenon occurs that is known as One scheme for closed-loop ignition timing is based on the improvement in performance that is achieved byadvancing the ignition timing relative to TDC.

4.1.7.1 Closed-loop ignition timing

  Whenever the knock level is less than the tation is integration with respect to time; this can bethreshold, the spark is advanced; whenever it exceeds the accomplished using an operational amplifier. The output voltage V at the end of 1 2 KT is given by: at the time for which the knock amplitude is largest the gate interval ð T (i.e., shortly after TDC).

4.1.7.2 SA correction scheme

  In the fast correction scheme, the SA is V ( T ) is greater than the threshold value, the compar- decreased for the next engine cycle by a fixed amountK ator output is high, indicating knock. However, a fullyintegrated electronic engine control system can include these subsystems and provide additional functions.(Usually the flexibility of the digital control system allows such expansion quite easily because the computerprogram can be changed to accomplish the expanded functions.) Several of these additional functions arediscussed in the following.

4.1.8.1 Secondary air management

  The next time the engine is started, the new lookup tablevalues will be used in the open-loop mode and will provide more accurate control of the air/fuel ratio. If the computer detects the loss of a primary control sensor or actuator, it may choose to operate in a differentmode until the problem is repaired.

4.1.9 Summary of control modes

  4.1.9.4 Closed-loop control For the closest control of emissions and fuel economy under various driving conditions, the electronic enginecontrol system is in a closed loop. engine control systems restricted the ability of the con-troller to continuously maintain the air/fuel ratio at stoichiometry under such changing operating conditions.

4.1.10 Improvements in electronic

  The newer, more capable digital engine control systems engine control are more precise than the earlier versions at maintaining stoichiometry and therefore operate more of the timeThe digital engine control system in this chapter has been within the optimum window for the three-way catalyticmade possible by a rapid evolution of the state of tech- converter.nology. It is worthwhile to review some ofquires, in some cases, data from the same sensor set, it is the technological improvements that have occurred inadvantageous to have a single integrated system for fuel digital engine control in greater detail to fully appreciateand ignition timing control.

4.1.10.1 Integrated engine control

  4.1.10.2 Oxygen sensor improvements system Improvements have also been made in the EGOOne of the developments that has occurred since the sensor, which remains today as the primary sensor forintroduction of digital engine control technology is the Fig. The fuel injector for each cylinder is typically mounted in the intake manifold such that fuel is sprayeddirectly into the intake port of the corresponding cylin- der during the intake stroke.

4.1.10.3 Fuel injection timing

  There are numerous issues and considerations in- volved in HV powertrain control, including the efficien-cies of the ICE and EM as a function of operating condition; the size of the vehicle and the power capacityof the ICE and EM; the storage capacity and state of charge (SOC) of the battery pack; accessory load char-acteristics of the vehicle; and finally, the driving charac- teristics. Vehicle speed and 0 applied to the primary results in an AC voltage in the secondary that is essentially N ICE RPM and loadEM voltage and current The system outputs include control signals to: ICE throttle positionEM motor control inputsClutch engage/disengageSwitch ICE ignition on/off In this vehicle, there is no direct mechanical link from the accelerator pedal to the throttle.

1 The validity of this simple model for a transformer

  Conversion of DC electrical power from one voltage to another can be accomplished using a transformer onlyif the DC power is converted to AC power. 4.1-23 is a greatly simplified schematic of a DC to DC converter inwhich a transistor is used to convert an input DC signal toAC that is sent to a transformer for conversion to a dif- ferent voltage.

C. The secondary voltage is fed back to the

  It is beyond the scope of this book to energy is absorbed by vehicle deceleration), the re- attempt to cover all possible operating modes for all HVcovered energy appears as increased SOC. SectionFive TransmissionsSection Five Section Five Section Five Section Five Section Five 5.1 Chapter 5.1Transmissions and driveline Julian Happian-Smith The aims of this chapter are to: demonstrate the need for transmission design andmatching; give examples of common gearboxes and transmis-sions available for vehicle design; indicate the terminology and methods for transmis-sion design; aid the designer to understand the elements of theanalysis of transmission systems.

5.1.1.1 Definitions

  Effects include:the space available for the powertrain and how it is packaged within the vehicle including the locationof ancillary components; the weight distribution, since the powertrain compo-nents are relatively heavy; the structure to support the powertrain and reactagainst the driving torques; vehicle handling and ride both from weight distribu-tion and the location of the driven wheel set; safety structure and passenger protection. These include the opportunity for the used in ATs to disconnect or connect particular gearsdifferential to be included in the same casing as the and hence allow the gear change required; these appli-transmission and eliminate the need for an additional cations do not have the capacity of starting the vehiclehousing.

5.1.2.3 The vehicle requirement – what

  These also have greater complication in the powertrain has to deliver ancillary and cooling system layouts that are discussed in more detail in the environmental considerations inIf we consider the torque requirements (on the engine Section 5.1.6. There is also a particular fuel economy and driveline), there are a number of forces acting on theadvantage for transverse layouts that do not have to turn body of the vehicle that have to be overcome:the drive direction through a right angle. This eliminates The rolling resistance of the tyres.a bevel gear set that is less efficient than parallel shaft The aerodynamic drag of the vehicle body.transfer gears.

5.1.2.2 Starting from rest accelerating

  All of these factors will influence the selection of the gear the lines of constant fuel flow to indicate the max ratios in practice and possibly cause a compromise torque line.)between the calculated, ‘ideal’ ratio set for a given car Taking some of the vehicle transmission details as: and what can be used on an existing vehicle. Final drive ratio 4.2 Example of the considerations in matching a transmission Fifth gear 0.75 to a vehicle Fourth 1.0For this example, we will look at some of the factors Third 1.4 which would need to be considered when designingPlotting the engine conditions for 120 km/h (motorway) a gearbox for a road car, in this case a large 4 4.and assuming a loss of 5% in the transmission system Consider the rolling resistance of the 4 4 vehicle in Fig.

5.1.3 The manual gearbox

  The outer part of the release bearing is held The spring pressure clamps the pressure plate onto the driven plate and the flywheel; with the assembly likethis, the drive is passed from the engine to transmission(see Fig. 5.1-13 ). When the driver depresses the clutch pedal, the movement is passed to the release bearing byeither hydraulics or cable and the release bearing then pushes or pulls the diaphragm spring (depending onwhether the clutch is a ‘push’ or ‘pull’ design), see Clutches are associated with manual gearboxes and are normally operated by the driver.

5.1.3.5 The clutch

  There istransmissions are gaining in popularity in the perfor- often significant development in this area as the loads andmance car market, probably because of the links to movement of the pedal have to be as low as possible forFormula 1 racing and as a result of clever marketing! The success of this combination lies in the simplicity of the torque converter as a device that inherently has idealcharacteristics to start a vehicle from rest, and the op- portunity that epicyclic gear sets provide to give rela-tively easy and controllable changes between ratios.

5.1.4 The AT

  This demand difficulty to control precisely.comes principally from the accelerator pedal position The term torque converter is used here to describe the but is modified by brake application and both the gear converter coupling as most frequently used in automotiveselector (D, 4, 3, 2) and a pattern selector (drive, sport applications. It is so and snow).called because, in a part of its operating range, it gives a torque multiplication (behaving as a converter) and in theremainder, it behaves as a coupling with a 1:1 torque ratio.

5.1.4.2 The hydrokinetic torque

  Planet shaftSchematic representation Annulus shaft Sun shaftSun gear Planet gear Annulus gearPlanet carrier where t is number of teeth and subscripts a and s refer to annulus and sun respectivelyConsideration of the torques acting on the gear teeth at the two meshing diameters indicates that these will begiven by the inverse of the speed relationship. Thus, in Ds Da ¼ ¼ 5.1.4.3 The epicyclic gear set – the key component in the AT a u s u i ¼ An example of how the device works can be envisaged with the carrier shaft locked, leaving the sun and annulusfree to rotate connected via the rotation of the planets about their now fixed centres.

5.1.4.5 Shift strategy

  The strategy is funda- mentally a function of vehicle speed and the driver’saccelerator demand and a typical example is shown in Fig. However, it is also obvious that for a given vehicle speed in manysections of the map a heavy-footed driver can easily invoke up and down shifts by moving the acceleratorpedal.

5 Fig..1-23 Shift strategy (courtesy of Jatco)

  If an ideal speedratio is defined for a gear pair then the output torque will be less than the ideal torque as a result of these losses,but the output speed will be the same as the ideal speed. Since gears * The idea of ‘infinitely’ can be easily accommodated when it is appreciated that trasmission ratios are usually exressed in the form of input to output as a SPEED ratio, and that an infinite speed ratio is obtained when the output speed is zero.

5.1.5.1 The rationale for the CVT

  This gives a delay between a driverpressing the accelerator and the vehicle responding, this is also accompanied by the sound of the engine speeding upbut nothing apparently happening. In this category, the performance margins are smaller and most manufacturershave not controlled the engine to rigidly follow the economy line and have compromised this to allow a largertorque margin for acceleration.

5.1.5.2 Hydraulic transmissions

  Hydrostatic drives also rely on fluid flow to transmit power but it is the pressure level in the fluid that is sig-nificant rather than the flow velocity. The first commercially produced vehicletransmission (1958) was the DAF Variomatic and here the load was provided by pre-loaded springs in thepulleys, and the ratio change by a centrifugal effect working against these springs.

5.1.5.3 Variable pulley variator designs

  Since the bands are free to slide relative to the segments and it is not possible for the segments totransfer load in tension, the only remaining mechanism is for the segments to transfer load in compression. When the bands have been placed in tension by the pulley clamping forcesthey will stretch and gaps open up between the segments on the non-compression side of the belt.

5.1.5.4 Variable pulley transmissions

  5.1-31(a) the axis of rotation of the roller is angled to the left and contacts the input disc at a smaller radius than the output disc, hence giving a reduced outputspeed relative to input. In these traction drives both the materials used in the contact surfaces and the lubricating or traction fluid areimportant in giving good reliability and efficiency, ulti- Full mately the effectiveness of the transmission.

5.1.5.6 Toroidal transmissions

  One path from the engine is via thevariator and the sun of the epicyclic gear, and the second transferred from the engine directly to the planet carrierof the epicyclic gear via the low regime clutch. From the above discussion, it can be seen that the control of the lubricant temperature is not a trivialproblem and many companies are working towards management of the transmission in much the same wayas the engine.

5.1.6.2 Efficiency

  The problem with developing these is finding a posi- tion on the transmission and a design of breather,which allows the air to move in and out of the transmission without allowing water in or oil out. In practice, the difficultyof linking the controls (gearlever) with the transmission unit is much to do with the relative position of the twoand the interface at the gearbox end.

5.1.6.3 Other transmission components

  As many established automotive engineers, brought up in the IC-engine era, now face the real possibility offuel-cell driven production vehicles, the fundamentals of electric traction and the experience gained by pastEV builders are now of real interest to those contem- plating a move to that sector. 6.1.2 Electric batteries According to battery maker, Exide, the state of de- velopment of different battery systems by differentsuppliers puts the foreseeable time availability for the principal battery contenders, relative to the company’sparticular sphere of interest, lead–acid – as in Fig.

6.1.2.1 Advanced lead–acid

  A number of special high energy versions have been devised such as thatshown at (b), due to researchers at the University ofIdaho. The table at (c) lists the main parameters of the battery.

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