Ajodynamiikka Koe PDF

AJODYNAMIIKKA KOE RENGASTEORIA Basic terms for tires The most important external forces and moments which act on a vehicle occur during the transmission of force between tire and road surface. Added to this are the wind forces which act on the vehicle special situations. Footprint Force is transmitt...

AJODYNAMIIKKA KOE RENGASTEORIA Basic terms for tires The most important external forces and moments which act on a vehicle occur during the transmission of force between tire and road surface. Added to this are the wind forces which act on the vehicle special situations. Footprint Force is transmitted between tire and road surface by friction in the contact area, also known as the footprint. The two most important types of friction are adhesive friction (intermolecular adhesive force) and hysteretic friction (gearing force). Lateral force and longitudinal force. Forces can be generated in the road plane by friction. Lateral force FS is the force perpendicular to the wheel center plane; longitudinal force FU runs in the direction of the wheel center plane (Figure 15). The forces are generally not applied exactly at the wheel contact point such that moments are likewise generated in relation to the wheel contact point (tire aligning torque MR). Slip angle When lateral and longitudinal forces occur simultaneously, the forces may be mutually influenced. The situation where lateral and longitudinal forces occur but not combined is considered in the following. Force transmission The four tire contact patches (footprint) are the direct and sole interface between the road surface and the vehicle. Slip angle and slip Only the tires rolling under an angle to the wheel rolling plane (slip angle α, Figure 7) and in the process deforming and constantly more or less slipping (see slip) transmit simultaneously and within physical limits the forces requested by the driver through steering, braking and accelerating. Conversely: A tire that does not roll or slip at an angle does not transmit any forces. The tire transmits ever higher forces as the slip angle and slip increase. This relationship however is not linear. The effect is reversed after a relevant maximum value is reached (Figure 8). For passenger- car tires this reversal point for the slip angle is roughly 4…7°, corresponding to a pronounced steering-wheel angle. The reversal point for slip is 10…15% (on snow up to 30%). Excessive slip is a consequence of excessive acceleration or excessively heavy braking. If the steering angle or brake pressure is increased further, the wheels lock if the vehicle is not fitted with an antilock braking system. The slip is then –100%. Longitudinal and lateral forces If a force Fx in the peripheral direction and a side force Fy occur simultaneously (e.g. when braking while cornering), the resulting transmitted horizontal force 𝐹ℎ = √𝐹𝑥2 + 𝐹𝑦2 cannot exceed the value µhFz. This situation can be explained by reference to the Kamm (friction) circle (Figure 9). The radius of the Kamm circle is equal to the maximum horizontal force µhFz that can be transmitted via the tire. The maximum side force Fy is therefore smaller is at the same time a force Fx occurs in the peripheral direction. With the forces Fx and Fy marked in Figure 9 the wheel is exactly at the limit of the maximum horizontal force that can be transmitted. Tire grip Generation of grip Tires must transmit all the dynamic forces to only four roughly postcard-sized areas. The grip required for this purpose between the tire contact patches and the road surface is generated by several simultaneously occurring phenomena. These are essentially positive locking (also referred to in this context as the interlocking effect) and adhesion by molecular forces of attraction. When a vehicle driving past at a constant speed is considered from the outside, as the tire rolls the continuously changing ground contact patch remains apparently fixed in relation to the vehicle (Figure 10) – while each individual rubber block of the tire tread runs into this forcibly flattening contact patch, deforms, and is “ejected” again at the other end. Relative movements and thus slip are generated in this contact patch: Each individual rubber block more or less slips during the dwell time in the contact patch. Viscoelasticity Viscoelasticity describes the time-, temperature- and frequency-dependent elasticity and the viscosity of polymer and elastomer substances (e.g. of plastics, rubber). Internal damping, molecular interlocking and creeping processes prevent the two extreme states “fully elastic” (e.g. like an elastic spring) and “highly viscous” (like a solid body). Deformation and the force that causes it as well as the mechanical strain pass off at different times. Interlocking effect The interlocking effect is created by the direct and intensive contact of the tire with the road, depending on the micro-and macro-roughness of the road surface (Figure 11). In the passage of the contact patch the tread block under consideration comes up against a bump in the asphalt, is upset, and slips off again on the other side of the bump at accelerated speed. Only when it produces slip in the process can it build up in the tangential direction a counterforce contrary to the rolling direction which counteracts the sliding and thus permits the transmission of steering, drive or braking forces. Rolling resistance Definition of terminology Tractive resistances inhibit the forward movement of the vehicle and must be overcome by motive means. In addition to the aerodynamic drag, the frictional resistances in moving engine, transmission and chassis/suspension components, the climbing resistance, and inertia forces, the rolling resistance of tires is classed as one of the main tractive resistances. It contributes to approx. 20% of the fuel consumption on expressways/interstates, approx. 25% on orbital roads and approx. 30% on urban and ordinary roads. Rolling-resistance decreases thus result directly in consumption and emission reductions. Rolling resistance (RR) corresponds to the energy loss per unit of distance and is given like every force in N (newtons). The dimensionless rollingresistance coefficient cRR denotes the ratio of rolling-resistance force to vehicle weight. Example: Assuming a rolling-resistance force FRR = 120 N and a vehicle weight G = 10,000 N (G = mg; vehicle mass m in kg, gravitational acceleration g ≈ 9.81 m/s2 ) cRR amounts to 0.012 = 1,2%. Standard values for cRR for passenger-car tires on asphalt amount to 0.006 to 0.012, the (lower) value for truck tires to 0.004 to 0.008. The “unit” kg/t (kilogram per ton) is occasionally used. In the example above the result 0.012 is 12 kg/t. This means in the case of a wheel load of 1 t = 1,000 kg that the rolling-resistance force FRR assumes a value of 120 N. Rolling-resistance-optimized tires have additional designations such as “Eco”, “Green” or “Energy”. Tire designers could indeed immediately and quite significantly reduce the rolling resistance by choosing rubber grades with low hysteresis and thus a low energy loss, but, as already explained, this would reduce the grip values to unacceptable levels. In other words, lower fuel consumption comes at the expense of less grip. Generation of rolling resistance With each wheel rotation the tire is deformed during the forcible flattening in the contact patch by flexion, upsetting and shearing of the rubber blocks and of the proportional tire casing (Figure 16). The fabric plies of the tire rub against each other (flexion), during which the tire performs flexing work. This produces a viscoelastically caused energy loss in the form of non-utilizable heat. This heat loss makes up 90 % of the rolling resistance. Narrow tires and higher tire inflation pressure do increase the rolling resistance because contact patch and flexing work are reduced. However, designers are subject to very tight limits in terms of their scope of action in that the catalog of requirements with regard to handling performance, grip level and comfort are in direct conflict. Nevertheless, the technical specifications of the tire industry for future tire generations already feature tire dimensions such as 115/65 R 15 for subcompact-size cars and 205/50 R 21 for medium-size cars. Further drastic rolling-resistance reductions are not achievable with the requirements still standard today with regard to vehicle size, weight, maximum speed, sportiness, and comfort. Even increasing the tire inflation pressure by 1 bar above the recommended value will only deliver a rolling-resistance reduction of 15%. In a vehicle with an assumed fuel consumption of 10 l / 100 km this would only produce a saving of 1.6%. But because drivers still neglect to check tire pressure, the rolling-resistance reductions achieved in the most recent development cycles are not always implemented in real driving conditions. In addition, there is a direct conflict of aims between rollingresistance optimization and the wet grip essential to road safety. Conflict of aims between rolling resistance and grip Frequency ranges The deformations of the tread and rubber blocks caused by micro- and macroroughness in the contact area between road surface and tire surface which generate the grip potential due to viscoelasticity occur in the very high frequency range of 103 …1010 Hz. The energy loss caused by hysteresis is high here, resulting in the creation of high grip values. However, the frequency spectrum which is important to rolling resistance is much lower at 1…100 Hz – precisely the range in which the inner tire structure is excited with each wheel rotation. At a driving speed of 100 km/h a passenger-car tire is deformed roughly 15 times per second, equating to a load frequency of 15 Hz. Tire designers nonetheless refer here to lowfrequency excitation of the tire structure, particularly the casing. Optimization incompatibility These greatly differing frequency ranges explain the basic optimization incompatibility of simultaneously high values for grip and low rolling resistance. In conventional tires using industrial soot as the primary filler (up until the mid-1990s) a rubber compound with high hysteresis in the high-frequency grip range automatically results in a high energy loss in the tire components subjected to low-frequency load and thus in high rolling resistance. Silica to resolve the conflict of aims The solution in the late 1990s involved the introduction of silica (trade name for refined silicate) as a gray-powdered filler, which increasingly replaced the standard industry soot used up that point. Together with auxiliary binding materials, or silanes, the conflict of interests between rolling resistance, grip and abrasion resistance can be elevated to a high level of compromise. Silica-based rubber compounds exhibit low energy losses in the low-frequency range relevant to rolling resistance, but high energy losses in the high-frequency rubber-grip range (Figure 17). The curve for energy absorption has this rise steeply and thus advance in frequencies of 102 …104 Hz. This results in tires which have low rolling resistance but nevertheless very good grip. Rolling resistance and air pressure Reduced tire pressure means increased flexing work and thus higher rolling resistance and impaired steering precision and braking stability. Safety checks carried out in 2009 by the tire industry (Goodyear, Dunlop, Fulda) on 52,400 vehicles in 15 EU countries established that 81% of all car drivers drive with excessively low tire pressure. Of these, 26.5% were driving with clearly (down to 0.3 bar) and 7.5% with significantly reduced pressure (0.75 bar and higher). The upshot of excessively low tire pressure is that fuel is wasted. 1. Rullausvastus: Tämä viittaa vastukseen, jonka rullaava esine kokee pyöriessään tasaisella pinnalla. Esimerkkejä rullausvastuksesta ovat auton renkaat, potkupyörät tai rullaluistimet. Rullausvastus riippuu renkaan materiaalista, pinnan karheudesta, ilmanpaineesta ja muista tekijöistä. Sitä voidaan pienentää esimerkiksi käyttämällä sileämpiä renkaita tai lisäämällä renkaiden ilmanpainetta. 2. Vierintävastus: Tämä on vastus, jonka esine kokee liikkuessaan pinnan yli. Se voi johtua esimerkiksi kitkasta tai maaston epätasaisuuksista. Vierintävastus voi olla suurempi tai pienempi riippuen pinnan laadusta ja esineen massasta. Esimerkiksi hiekkainen tai epätasainen pinta aiheuttaa suuremman vierintävastuksen kuin sileä asfaltti. Polttoaineen kulutus Energy demand for drives Vehicles which draw the energy for propulsion from fossil fuels and consequently emit CO2 while driving are subject to limits on these emissions. CO2 emissions are proportional to fuel consumption. This also applies to mild- and full-hybrid vehicles which can run their internal-combustion engines at more fuel-efficient operating points and thus exhibit lower fuel consumption. The batteries of externally rechargeable hybrid vehicles can be charged from the power socket of the mains-supply network or from a charging post; this supplied electrical energy stored in the traction battery must therefore be taken into account in the energy demand. Electric vehicles ultimately draw all the energy required for drive purposes from the mains-supply network and thus do not emit any CO2 locally. Fuel consumption of internalcombustion engines Specific fuel consumption The most common way of presenting engine fuel-consumption maps is the “graph diagram”, in which the effective mean pressure pme and, as family parameters, lines of constant specific consumption (fuel throughput referred to power output in g/kWh) are plotted against the engine speed (Figure 1). This makes it possible to compare the efficiency of engines of different sizes and types. Another way of presentation is, as a family parameter, the fuel throughput or mass flow (e.g., in kg/h). This form of presentation is particularly suitable as an input variable for CAE programs (computer-aided engineering), which can be used to simulate fuel consumption (corresponding to CO2 emissions). Entered in both presentations as additional information are the full-load torque curve as the upper limiting line, the idle and breakaway speeds as limiting speeds, and often also lines of constant power P (power hyperbolas on account of P~pme n, with the engine speed n). Consumption as sum of all losses and running resistances If one disregards the influence of the driver on fuel consumption, which can run to 30 %, there are, based on the consumption formula (Figure 2), three distinct groups of influencing factors: – the engine (including belt drive and ancillaries), – the internal running resistances of the drivetrain (e.g., transmission, differentials), and – the external running resistances. External running resistances The external running resistances determine a vehicle’s minimum energy demand in a specified driving profile. They can be reduced by measures such as reduced vehicle mass, lower tire rolling resistance, and improved aerodynamics. On an average production vehicle without regenerative braking, 10 % reductions in mass, drag and rolling resistance result in fuel-consumption reductions of roughly 6 %, 3 %, and 2 %, respectively. The formula in Figure 2 distinguishes between acceleration resistance and braking resistance. This illustrates clearly that consumption per unit of distance increases above all when the brakes are subsequently applied and not the overrun fuel cutoff of the engine or even a hybrid system is used to recover some of the brake energy. Internal running resistances The internal running resistances are made up of the losses in the drivetrain from the crankshaft to the wheels. In Figure 1, the sum total of the internal and external running resistances are shown by curves c and d, where vehicle A shows significantly lower resistances than vehicle B. As well as the transmission losses in the drivetrain, the overall ratio also affects fuel consumption. This is calculated from the product of the transmission and differential ratios. The choice of overall ratio results in different operating points in the engine fuel-consumption map for a specific driving speed. A “longer” ratio, i.e., a smaller overall ratio, will generally move the operating points to map areas corresponding to lower fuel consumption. At the same time, it must be noted that the accelerating performance is reduced and the NVH behavior worsens (“Noise Vibration Harshness”, impacting on driving comfort). A useful choice of ratio is therefore only possible within limits. Determining the distance-specific fuel consumption of conventional drives The official data for standard fuel consumption are determined on the basis of dynamometer tests conducted on exhaust-emission test benches in which statutory test cycles (e.g., NEDC for Europe, FTP75 and Highway for the USA and JC08 for Japan) are run. WLTC (World Light-duty Test Cycles), intended to deliver more realistic consumption figures, have been applied since 2017 and 2018 in Europe and Japan. The exhaust gas is collected in sample bags and its constituents HC, CO and CO2 are subsequently analyzed for the purpose of determining consumption (see Exhaust-gas measuring techniques). The CO2 content of the exhaust gas is proportional to the fuel consumption. The following reference values apply to Europe: Diesel: 1l/100km ≈ 26.3 g CO2/km. Gasoline: (Euro 4): 1l/100km ≈ 24.0 g CO2/km. (Euro 5, E5): 1l/100km ≈ 23.4 g CO2/km. (Euro 6, E10):1l/100km ≈ 22.8 g CO2/km. The change from Euro 4 to Euro 5 and Euro 6 is based on a 5 % admixture of ethanol in gasoline (E5) and a 10 % admixture for Euro 6 (E10). Vehicle masses are simulated on the test bench as an alternative by test masses. In today's certification cycles the test mass is, depending on the country and the curb weight of the vehicle, graded in increments of 55 to 120 kg. When the vehicle mass is allocated to the corresponding the ready-for-operation weight of the vehicle (including all the fillers, tool kit and the fuel tank filled to 90%) plus 100 kg as substitute for a driver and luggage is applied. The difference in consumption in the jump to an adjacent inertia weight class is – depending on the vehicle – between 0.15 and 0.25 l/100 km. In the WLTC the actual vehicle mass is applied as the test mass. Units of fuel consumption Standard fuel consumption is given in different units, depending on the country and the test cycle. In Europe, the figure is given in g CO2/km or l/100 km, in the USA in mpg (miles per gallon), and in Japan in km/l. Conversion examples: 30 mpg → 235.21/30 → 7.8 l/100 km, 22.2 km/l → 100/22.2 → 4.5 l/100 km. The fixed value of 235.21 used for conversion is obtained from the conversion of gallons to liters and the conversion of miles to kilometers. Jarrutus Definitions of brake systems relating to vehicle combinations Single-line brake system Assembly in which the brake systems of the individual vehicles act in such a way that the single line is used both for the energy supply to, and for the control of, the brake system of the trailer. Dual- or multi-line brake systems Assembly in which the brake systems of the individual vehicles act in such a way that several lines are used separately and simultaneously for the energy supply to, and for the control of, the brake system of the trailer. Continuous brake system Combination of brake systems for vehicles forming a road train. Characteristics: – From the driving seat, the driver can operate a directly operated control device on the tractor vehicle and an indirectly operated control device on the trailer by a single operation and with a variable degree of force. – The energy used for the braking of each of the vehicles forming the combination is supplied by the same energy source (which may be the muscular effort of the driver). – Simultaneous or suitably phased braking of the individual units of a road train. Semi-continuous brake system Combination of brake systems for vehicles forming a road train. Characteristics: – The driver, from his driving seat, can gradually operate a directly operated control device on the tractor vehicle and an indirectly operated control device on the trailer by a single operation. – The energy used for the braking of each of the vehicles forming the road train is supplied by at least two different energy sources (one of which may be the muscular effort of the driver). – Simultaneous or suitably phased braking of the individual units of a road train. Non-continuous brake system Combinations of the brake systems of the vehicles forming a road train which is neither continuous nor semi-continuous. Brake-system control lines Wiring and conductors: These are employed to conduct electrical energy. Tubular lines: Rigid, semi-rigid or flexible tubes or pipes used to transfer hydraulic or pneumatic energy. Lines connecting the brake equipment of vehicles in a road train Supply line: A supply line is a special feed line transmitting energy from the tractor vehicle to the energy accumulator of the trailer. Brake line: A control line is a special control line by which the energy essential for control is transmitted from the tractor vehicle to the trailer. Common brake and supply line: Line serving equally as brake line and as supply line (single-line brake system). Secondary-brake line: Special actuating line transmitting the energy from the tractor vehicle to the trailer essential for secondary braking of the trailer. Braking mechanics Mechanical phenomena occurring between the start of actuation of the control device and the end of the braking action. Gradual braking Braking which, within the normal range of operation of the control device, permits the driver, at any moment, to increase or reduce, to a sufficiently fine degree, the braking force by operating the control device. When an increase in braking force is obtained by the increased action of the control device, an inverse action must lead to a reduction in that force. Brake-system hysteresis: Difference in control forces between application and release at the same braking torque. Brake hysteresis: Difference in application force between application and release at the same braking torque. Forces and torques Control force F_c: Force exerted on the control device. Application force F_s: On friction brakes, the total force applied to a brake lining and which causes the braking force by the effect of friction. Braking torque: Product of frictional forces resulting from the application force and the distance between the points of application of these forces and the axis of rotation of the wheels. Total braking force Ff: Sum of the braking forces at the tire contact patches of all the wheels and the ground, produced by the effect of the brake system, and which oppose the movement or the tendency of the vehicle to move. Braking-force distribution: Specification of braking force according to axle, given in % of the total braking force Ff. Example: front axle 60%, rear axle 40%. Brake coefficient C*: Defines the relationship between the total peripheral force of a given brake and the brake’s application force. F_u Total peripheral force, F_s Application force. The mean is employed when there are variations in application forces at individual brake shoes (i number of brake shoes): Time periods Reaction time (see Figure 1): The time that elapses between perception of the state or object which induces the response, and the point at which the control device is actuated (t0). Actuating time of the control device: Elapsed time between the moment when the component of the control device (t0) on which the control force acts starts to move, and the moment when it reaches its final position corresponding to the applied control force (or its travel). (This is equally true for application and release of the brakes). Initial response time t1−t0: Elapsed time between the moment when the component of the control device on which the control force acts starts to move and the moment when the braking force takes effect. Pressure build-up time t19−t1: Period that elapses between the point at which the braking force starts to take effect and the point at which a certain level is reached (75 % of asymptotic pressure in the wheel-brake cylinder as per EU Directive 71/320/ EEC , Annex III/2.4). Initial response and pressure build-up time: The sum of the initial response and pressure build-up times is used to assess how the brake system behaves over time until the moment at which full braking effect is reached. Active braking time t4−t1: Elapsed time between the moment when the braking force starts to take effect and the moment when the braking force ceases. If the vehicle stops before the braking force ceases, the time when motion ceases is the end of the active braking time. Release time: Elapsed time between the moment when the control device starts to release and the moment when the braking force ceases. Total braking time t4−t0: Elapsed time between the moment when the control device on which the control force acts starts to move and the moment when the braking force ceases. If the vehicle stops before the braking force ceases, the time when motion ceases is the end of the active braking time. Braking distance s Distance traveled by the vehicle during the total braking time. If the time when motion ceases constitutes the end of the total braking time, this distance is called the “stopping distance”. Braking work W Integral of the product of the instantaneous braking force Ff and the elementary movement ds over the braking distance s: Instantaneous braking power P Product of the instantaneous total braking force Ff and the vehicle’s road speed Braking deceleration Reduction of speed obtained by the brake system within the considered time t. A distinction is made between the following: Instantaneous braking deceleration Mean braking deceleration over a period of time The mean braking deceleration between two points in time tB and tE is This means that: where υB and υE are the vehicle speeds at the times tB and tE. Mean braking deceleration over a specific distance The mean braking deceleration over the distance between two points sB and sE is: This means that: where υB and υE are the vehicle speeds up to the points sB and sE. Mean braking deceleration over the total braking distance The mean braking deceleration is calculated according to the equation: where υ0 relates to the time t0 (special instance of ams where sE = s0). Mean fully developed deceleration dm Mean fully developed deceleration over the distance determined by the conditions υB = 0.8 υ0 and υE = 0.1 υ0 thus: The mean fully developed deceleration is used in ECE Regulation 13 as a measure of the effectiveness of a brake system. Since positive values for dm are used here, the mathematic sign has been reversed in this case. (In order to establish a relationship between braking distance and braking deceleration, braking deceleration must be expressed as a function of the distance traveled.) Braking factor z Ratio between the total braking force, Ff, and the permissible total static weight, Gs, exerted on the axle or axles of the vehicle: CHASSIS CONTROL Wheel-slip control systems Function and requirements Slip When starting off, accelerating and braking, the efficiency required to transfer forces to the road depends on the traction available between the tires and the road surface. Slip occurs when the speed υR at which the wheel center moves in the longitudinal direction (vehicle speed) differs from the speed υU at which circumference rolls.Brake slip λB and drive slip λA are calculated as follows: When a wheel is locked, according to this definition brake slip λB = −1; when the vehicle is stationary with spinning wheels drive slip λA = 1 (see Slip, Fundamentals of automotive engineering). Adhesion/slip curves The tire must roll with slip so that it can transmit force to the road. A tire without slip would not deform on the wheel contact area and therefore could not transmit either a longitudinal force or a lateral force. The transmittable forces are dependent on the slip. The adhesion/slip curves (Figure 2) illustrate this dependence. They progress identically for braking and propulsion/traction. Wheel-slip control systems ensure the optimum transmission of forces between tires and road to keep the vehicle directionally stable and more easily controllable for the driver. For this purpose the longitudinal slip of the individual wheels – i.e., the wheel speed referred to the velocity of the wheel center in the longitudinal direction – is regulated by modulating the braking or drive torque. There is a fundamental distinction between the antilock braking function (ABS, antilock braking system) and the traction control system (TCS). The vast majority of acceleration and braking operations involve only limited amounts of slip, allowing response to remain within the stable range in the adhesion/slip curves. Any rise in slip (brake slip when braking and drive slip when accelerating) is accompanied initially by a corresponding increase in adhesion. Beyond this point, any further increases in slip take the curves through the maxima and into the unstable range (Figure 2) where any further increase in slip generally results in a reduction in adhesion. When braking, this results in wheel lock within a few tenths of a second. When accelerating, one or both of the driven wheels start to spin more and more as the drive torque exceeds the adhesion by an ever increasing amount. Effect of ABS and TCS As brake slip increases the ABS function becomes active and prevents the wheels from locking; as drive slip increases TCS prevents the wheels from spinning. Thanks to ABS the vehicle retains its directional stability and steerability even under emergency braking on a slippery road surface. The dangerous phenomenon of jackknifing is also prevented in commercial-vehicle combinations. The TCS function optimizes the transmission of forces of the drive wheels when accelerating and thereby improves both traction and stability. Figure 1 shows an ABS system with its components for a passenger car with a hydraulic brake system. In contrast to passenger cars, commercial vehicles have pneumatic power-brake systems (air brakes). Nevertheless, the functional description of an ABS or TCS control process for passenger cars also applies in principle to commercial vehicles. The functions of ABS and TCS have in the meantime been integrated into the driving-dynamics control. ABS control Basic closed-loop control process The wheel-speed sensor senses the state of motion of the wheel (Figure 3). If one of the wheels shows signs of incipient lock, there is a sharp rise in peripheral wheel deceleration and in wheel slip. If these exceed defined critical levels, the ABS controller sends commands to the solenoid-valve unit (hydraulic unit) to stop increasing or to reduce wheel brake pressure until the danger of wheel lock is averted. The braking pressure must then rise again to ensure that the wheel is not underbraked. During automatic brake control, the stability or instability of wheel motion must be detected constantly, and kept within the slip range at maximum braking force by a sequence of pressure-rise, pressure-retention and pressure-drop phases. Typical control cycle The control cycle depicted in Figure 4 shows automatic brake control in the case of a high friction coefficient. The change in wheel speed (braking deceleration) is calculated in the ECU. After the value falls below the (–a) threshold, the hydraulic-unit valve unit is switched to pressure-holding mode. If the wheel speed then also drops below the slip-switching threshold λ1, the valve unit is switched to pressure drop; this continues as long as the (–a) signal is applied. During the following pressure- holding phase, peripheral wheel acceleration increases until the (+a) threshold is exceeded; the braking pressure is then kept at a constant level. After the relatively high (+A) threshold has been exceeded, the braking pressure is increased, so that the wheel is not accelerating excessively as it enters the stable range of the adhesion/slip curve. After the (+a) signal has dropped off, the braking pressure is slowly raised until, when the wheel acceleration again falls below the (–a) threshold, the second control cycle is initiated, this time with an immediate pressure drop. In the first control cycle, a short pressure-holding phase was initially necessary to filter out any faults. In the case of high wheel moments of inertia, low friction coefficient and slow pressure rise in the wheel-brake cylinder (cautious initial braking, e.g., on black ice), the wheel might lock without any response from the deceleration switching threshold. In this case, therefore, the wheel slip is also included as a parameter in the brake-control system. Under certain road-surface conditions, passenger cars with all-wheel drive and with differential locks engaged pose problems when the ABS system is in operation; this calls for special measures to take into account the reference speed during the control process, lower the wheel-deceleration thresholds, and reduce the engine-drag torque. Control cycle with yaw-moment build-up delay When the brakes are applied on a road surface with uneven grip (µ split: left-hand wheels on dry asphalt, right-hand wheels on ice), vastly different braking forces at the front wheels result and induce a turning force (yaw moment) about the vehicle’s vertical axis (Figure 5). On smaller cars, ABS must be supplemented by an additional yaw-moment build-up delay device to ensure that control is maintained during panic braking on asymmetrical road surfaces. Yaw-moment build-up delay holds back the pressure rise in the wheel-brake cylinder on the front wheel with the higher coefficient of friction at the road surface (“high wheel”). The yaw-moment build-up delay concept is demonstrated in Figure 6: Curve 1 represents the brake- master-cylinder pressure pMC. Without yaw-moment build-up delay, the braking pressure at the wheel running on asphalt quickly reaches phigh (Curve 2), while the braking pressure at the wheel running on ice rises only to plow (Curve 5); each wheel brakes with the maximum transferable braking force (see Individual control). The yaw-moment build-up delay 1 system (Curve 3) is designed for use on vehicles with less critical handling characteristics, while yaw-moment build-up delay 2 is designed for cars which display an especially marked tendency toward yaw-induced instability (Curve 4). In all cases in which yaw- moment build-up delay comes into effect, the high wheel is under-braked at first. This means that the yaw-moment build-up delay must always be very carefully adapted to the vehicle in question in order to limit increases in stopping distances. Curve 6 in Figure 6 shows that for an ABS system without yaw-moment build-up delay a significantly higher steering angle is required when countersteering. ABS control methods The axle-based ABS control methods differ essentially in the number of control channels and the behavior when braking at µ split. Individual control Individual control, whereby each wheel is individually slip-controlled, produces the shortest braking distances. The drawback, however, is the yaw moment occurring under µ-split conditions, which must be compensated for by appropriate countersteering. This method is used exclusively on the rear axle since the steering and yaw moments occurring at the front axle would not be controllable for the driver when braking at µ split. Select-low control Select-low control (SL) is used to avoid yaw and steering moments entirely. Here, single-channel wheel-slip control is effected at the wheel with the lowest friction coefficient (select low), as a result of which both wheels on one axle receive the same brake pressure. Therefore only one single pressure-control channel per axle is required. Under µ-split conditions this produces optimum steerability and directional stability at the expense of braking distance. In the case of homogenous friction coefficients, braking distance, steerability, and directional stability are similar to those of the other methods. Individual control, modified “Individual control, modified” (IRM) has proven to be a good compromise between steerability, stability, and braking distance. This two-channel control method necessitates a pressure-control channel at each wheel on the axle. By appropriately limiting the brake-pressure difference between the right and left sides, the yaw and steering moments are restricted to a controllable extent. This results in a braking distance which is only a little longer than that for individual control, but it does ensure that vehicles with critical handling characteristics remain controllable. TCS control The traction control system has two fundamental tasks: – optimizing traction by utilizing the available friction coefficient in the best possible way, – ensuring vehicle stability (directional stability) by preventing the drive wheels from spinning. To optimize traction all the driven wheels must utilize their individual friction coefficients to the maximum to the best possible extent. To this end, the wheel speeds are synchronized by active braking of the spinning wheels (brake controller or electronic differential-lock function). In this way, the braking torque exerted on the spinning wheel is available through transmission by the differential to the non-spinning wheel as drive torque. To ensure directional stability, the wheel slip is controlled with the aid of the drive torque by the engine controller in such a way as to achieve the best possible compromise between traction and lateral stability. Using the brake-control function described above, the driving wheels can also be synchronized so that a mechanical differential lock, if fitted, can be activated automatically, e.g., with the aid of a pneumatic cylinder. The ABS/TCS ECU calculates the correct point and conditions for releasing the differential lock. In contrast to mechanical differential locks, the tires do not scrub on tight corners. A fundamental observation about this type of system (when it assumes an electronic brake-control function) is that it is not intended for continuous use on difficult offroad terrain. Since the brake-control function is achieved by braking the relevant wheel, brake heating is an inevitable consequence. For multi-axle vehicles with complex drive configurations and a number of differentials (e.g., 6×4 or 8×6 with three or five differentials) the TCS function can control up to six wheels individually. Engine drag-torque control Particularly at low load with a low friction coefficient or very powerful engines the wheels on the powered axle can lock due to high engine-drag torques (e.g., when downshifting), which results in unstable handling. In this case, engine drag-torque control increases the wheel speeds by increasing the drive torque and thereby prevents the impending instability. The actively exerted drive torque is limited for safety reasons. Electronic load-dependent braking-force regulation Load differences and the dynamic axle-load shift that occurs during sharp braking call for an adaptation of the braking forces. This was originally performed by ALB valves (automatic load- dependent braking-force regulator), which reduce the brake pressure usually on the rear axle depending on the axle load. In current ABS systems this function is assumed by electronic load- dependent braking-force regulation. Here, under minimal deceleration conditions, the differential slip between the front and rear axles is minimized, whereby the brake pressure on the rear axle is electronically reduced. This results, assuming the same friction conditions on both axles, in identical braking and consequently optimum braking under driving-dynamics considerations. This function dispenses with the need for the additional ALB valve. AJOHALLINTA Operating principle Driving-dynamics control (ESC) is a system which uses a vehicle’s brake system and drivetrain to deliberately influence the vehicle’s longitudinal and lateral motion in critical situations. When the stability-control function assumes operation, it shifts the priorities that govern the brake system. The basic function of the wheel brakes – to decelerate and/or stop the vehicle – assumes secondary importance as ESC intervenes to keep the vehicle stable and on course. ESC can also accelerate the drive wheels by means of engine interventions to contribute to the vehicle’s stability. Both mechanisms act on the vehicle’s intrinsic motion. During steady-state circular-course driving, there is a defined connection between the driver’s steering input and the resulting vehicle lateral acceleration and thus the tire forces in the lateral direction (self-steering effect). The forces acting on a tire in the longitudinal and lateral directions are dependent on the tire slip. It follows from this that the vehicle’s intrinsic motion can be influenced by the tire slip. The specific braking of individual wheels, e.g. of the rear wheel on the inside of the bend in the case of understeering or of the front wheel on the outside of the bend in the case of oversteering, helps the vehicle to remain on the course determined by the steering angle as precisely as possible. Typical driving maneuver To compare how a vehicle handles at its operating limits with and without ESC, the following example is given. The driving maneuver reflects actual operating conditions, and is based on simulation programs designed using data from vehicle testing. The results have been confirmed in subsequent road tests. Rapid steering and countersteering Figure 2 demonstrates the handling response of a vehicle without ESC and of a vehicle with ESC negotiating a series of S-bends with rapid steering and countersteering inputs on a high-grip road surface (coefficient of friction µ = 1), without the driver braking and at an initial speed of 144 km/h. Figure 3 shows the curves for dynamic-response parameters. Initially, as they approach the S-bend, the conditions for both vehicles, and their reactions, are identical. Then come the first steering inputs from the drivers (phase 1). Vehicle without ESC As can be seen, in the period following the initial, abrupt steering input the vehicle without ESC is already threatening to become unstable (Figure 2a, Phase 2). Whereas the steering input has quickly generated substantial lateral forces at the front wheels, there is a delay before the rear wheels start to generate similar forces. The vehicle reacts with a clockwise movement around its vertical axis (inward yaw). The vehicle barely responds to the driver’s attempt to countersteer (second steering input, Phase 3), because it is no longer under control. The yaw velocity and the side-slip angle rise radically, and the vehicle breaks into a skid (phase 4). Vehicle with ESC The vehicle with ESC is stabilized after the initial steering input by active braking of the front left wheel to counter the threat of instability (Figure 2b, Phase 2): This occurs without any intervention on the driver’s part. This action limits the inward yaw with the result that the yaw velocity is reduced and the float angle is not subject to an uncontrolled increase. Following the change of steering direction, first the yaw moment and then the yaw velocity reverse their directions (between Phases 3 and 4). In Phase 4, a second brief brake application – this time at the right front wheel – restores complete stability. The vehicle remains on the course defined by the steering-wheel angle. Structure of the overall system Objective of driving-dynamics control The control of the handling characteristics at the vehicle’s physical driving limits is intended to keep the vehicle’s three degrees of freedom in the plane of the road – linear velocity υx, lateral velocity υy and yaw velocity ψ˙ about the vertical axis – within the controllable limits. Assuming appropriate operator inputs, driver demand is translated into dynamic vehicular response that is adapted to the characteristics of the road in an optimization process designed to ensure maximum safety. System and control structure The ESC system comprises the vehicle as a controlled system, the sensors for determining the controller input variables, the actuators for influencing the braking, motive and lateral forces, as well as the hierarchically structured controller, comprising a higher-level transverse-dynamics controller and lower-level wheel controllers (Figure 4). The higher-level controller determines the setpoint values for the lower-level controller in the form of moments or slip, or their changes. Internal system variables that are not directly measured, such as the float angle β for example, are determined in the drivingcondition estimation (“observer”). In order to determine the nominal behavior, the signals defining driver command are evaluated. These comprise the signals from the steering-wheel-angle sensor (driver’s steering input), the brakepressure sensor (desired deceleration input, obtained from the brake pressure measured in the hydraulic unit) and the accelerator-pedal position (desired drive torque). The calculation of the nominal behavior also takes into account the utilized friction-coefficient potential and the vehicle speed. These are calculated in the observer from the signals sent by the wheel-speed sensors, the lateral-acceleration sensor, the yaw sensor, and the brake-pressure sensor. Depending on the control deviation, the yaw moment, which is necessary to make the actual-state variables approach the desired-state variables, is then calculated. In order to generate the required yaw moment, it is necessary for the changes in desired braking torque and slip at the wheels to be determined by the transverse-dynamics controller. These are then set by means of the lower-level brake-slip and traction controllers together with the brake- hydraulics actuator and the engine management actuator.