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Authors:Johannes Liebl, Elmar Frickenstein,Manfred Wier, Marcus Hafkemeyer,Fathi El-Dwaik and Elmar Hockgeier

Intelligente Generatorregelung – Ein Weg zur effizienten Dynamik

You will find the figures mentioned in this article in the German issue of ATZ elektronik 04I2006 beginning on page 6.

Intelligent Alternator Control SystemA Path to Efficient Dynamics

In automotive engineering, it wouldseem – at first glance at least – diffi-cult to simultaneously achieve bothefficiency and dynamics in the senseof improved driving performance andfuel economy. The BMW Group hasset itself the following goal: to driveinnovation in automotive and power-train engineering for the benefit of itscustomers. An example is the specificcontrol of the alternator in such a wayas to improve driving dynamics andfuel economy.

1 Introduction

Mobility is fundamental to all economic sys-tems based on the division of labour. It is,therefore, a key factor behind growth andemployment. Moreover, mobility is an ex-pression of personal freedom and, therefore,contributes significantly to quality of life.Taking into account the needs of man andthe environment, it is important, therefore,

that mobility be preserved for future gener-ations. The growing global demand for ener-gy, the finite nature of fossil fuel resourcesand technical limits to annual output willcontinue to drive the price of oil in themedium and long term to levels well abovethe average level of the last century. There-fore, it is imperative that we find new solu-tions to reduce our dependence on this re-source.

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Combustion engines will in the longterm continue to be the dominant means ofpowering vehicles thanks to their generallyfavourable characteristics and ongoing de-velopment work. In this respect, an enor-mous amount of effort is going into the con-tinuous reduction of fuel consumption andexhaust emission levels. However, this is re-sulting in ever-increasing development costsand hence rising vehicle prices. At the sametime, customers expect affordable automo-biles with improved performance, highersafety standards, more spacious interiors,enhanced comfort and quality.

The development tasks which derivefrom these needs result in what initially ap-pears to be a conflicting set of objectives.However, maintenance and enhancement ofdriving enjoyment are vital to the success ofthe BMW Group's products in the market-place. And that is why it is so important todeliver a maximum of driving enjoymentwhile at the same time utilizing energy re-sources as efficiently as possible. The furtherdevelopment of efficient dynamics is, there-fore, one of the key areas of emphasis.

2 Previous Achievements

A comparison of two typical BMW 3 Seriesmodels with 6-cylinder engines exemplifieshow the driving experience has changed sig-nificantly for our customers over the last 20years, Figure 1. We have witnessed not onlysignificant improvements in driving per-formance, but also marked reductions in fu-el consumption levels. In addition, exhaustemissions have been reduced using increas-ingly sophisticated exhaust aftertreatmenttechnology to such an extent that certainemissions can only be measured using preci-sion instruments.

The efficiency of engine has been in-creased through improved combustion,more efficient charge cycles and reducedfriction. Transmission ratio spreads have in-creased and the number of gears has beenadapted accordingly. Drag values have infact decreased, despite enlarged frontal sur-face areas, thanks to the outstanding effortsof the aerodynamics engineers in the windtunnel. In spite of the use of lightweight ma-terials, such as plastics, aluminium andmagnesium, weight has, however, increasedon account of higher safety standards, morespacious interiors and enhanced comfort.

3 Energy Flow Management

To achieve further marked improvements infuel efficiency while enhancing vehicle dy-namics, the complete energy system of the

input at the wheel to power output to thewheel.

The amount of energy available for drivingthe vehicle, i.e. the actual potential, is there-fore approximately half the storable amount,i.e. 700 kJ. This amount of energy representsthe maximum possible reduction in fuel con-sumption in this cycle (approx. 10 %).

However, the requisite technology is notonly highly complex, but also takes up addi-tional space and increases the weight of thevehicle. In this case, only a small circle ofcustomers would be addressed.

The solution for BMW involves the use ofexisting and proven technology for the re-covery of braking energy.

Although it is only possible to exploit apart of the energy-saving potential with thissystem, this can be achieved at reasonablecost, with little additional weight and withpractically no additional space require-ments.

Therefore, it will be possible in future toprovide this function to every BMW or Miniowner, and thereby reduce significantly to-tal fleet emissions.

5 Intelligent Alternator Control

Since the series launch of the current BMW7 Series, the BMW Group has implementedits proprietary electrical energy manage-ment system in all model series down to thecompact class. Until recently, the focus wason cost-oriented improvements in robust-ness and optimized partitioning of the actu-ators, sensors and the associated softwarefunctions needed for energy management.

Based on its experience with this provenelectrical energy management concept, theBMW Group will in future integrate a newoperating strategy which, in addition to theprevious task of function maintenance byintelligent electrical energy distribution,represents a first step towards the recoveryof overrun/braking energy.

Whereas previously electrical energy wasconverted to a large extent independently ofthe current driving phase (acceleration oroverrun/braking phase), it will in future bepossible to recover part of the overrun/brak-ing energy through intelligent alternatorcontrol in combination with selective bat-tery charge control, Figure 4.

The centrepiece of this control system isan extended battery charging strategy. Ac-cording to this strategy, the battery is nolonger unconditionally charged to full ca-pacity, rather a battery state of charge suffi-cient to ensure startability and availabilityof consumers when the vehicle is stationary(Cstarting/stationary) will in future be deter-

vehicle must be analyzed. A classic analysisof the engine, transmission and drag aloneis no longer enough to make furtherprogress. New solutions cannot be devel-oped until the relationships between thevarious energy flows have been fully andproperly understood, Figure 2.

The internal combustion engine convertsthe energy contained in the fuel into me-chanical energy and heat energy. Dependingon operating state, no more than 20 to 30percent of the primary energy used is trans-mitted to the driven wheels. Further opti-mization of the thermodynamic primaryenergy conversion process in internal com-bustion engines is, therefore, fundamentalto achieving efficient dynamics. The impor-tance of this is highlighted by the fact thateven small improvements in efficiency havean immediate effect. Even incremental im-provements in efficiency have a major im-pact on aggregate CO2 emissions due to themultiplication factor of the internal com-bustion engine in car fleets.

Given that the goal is to improve the over-all efficiency of the energy conversionprocess, the intelligent management of en-ergy flows in the vehicle will in future be-come an increasingly important considera-tion. New solutions for the avoidance, reduc-tion and utilization of lost energy can be de-veloped through systematic control and reg-ulation of energy flows. If the individualcomponents are redesigned within the con-text of a general energy architecture, then itwill also be possible to achieve significantadvantages in terms of fuel efficiency anddynamics.

4 Use of Lost Energy

If energy loss cannot be avoided, then itmakes logical sense to recover the lost ener-gy, where possible. Today, the kinetic energyof a vehicle is usually converted to heatthrough the brake discs while the vehicle isdecelerating. This heat energy is dissipatedto the environment, and therefore is lost. Inthe European registration cycle, the maxi-mum potential amount of storable brakingenergy in the case of a vehicle in the 1500 kgweight class is approx. 1400 kJ, Figure 3. Thisestimate allows for energy loss due to tirefriction and aerodynamic drag. Braking en-ergy is best recovered by converting it toelectrical current. To achieve this end, a suit-able electrical machine and a large electricalstorage device must be installed.

Assuming that this electrical machinehas an average efficiency of approx. 70 % inalternator and engine operation, overall effi-ciency is approx. 0.7 x 0.7 ≈ 50 % from power

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mined and set depending on the relevantambient conditions. The battery is onlycharged beyond this value during energeti-cally favourable driving phases in which nofuel is consumed. If the battery is chargedbeyond Cstarting/stationary during these recu-peration phases, this will make availableelectrical energy which can be returned tothe vehicle electrical system during accelera-tion phases and, therefore, does not have tobe generated by the alternator. This form of

micro hybridization allows fuel savingsof up to 4 % to be achieved, depending ondriving profile and ambient conditions.

6 Recuperation in the Energy Balance of the Vehicle

The energetic and economical basis for recu-peration capacity consists in the use of ki-netic energy of the vehicle during the over-run/braking phases. In these phases, the ve-hicle drives the alternator via the enginewithout consuming fuel, and the alternatorgenerates more power than is currently con-sumed via the vehicle electrical system. Theconversion of mechanical energy to electri-cal energy during these phases must not beaccomplished through additional fuel con-sumption, but by using the vehicle's inertia.To make use of this 'prepaid' surplus electri-cal energy, this energy must be storable.

At the present stage of development ofproduction technology, only the availablevehicle battery is suitable for use as an ener-gy buffer due cost and packaging reasons.Therefore, it is necessary to have free capaci-ty in addition to the energy available forstarting the vehicle and for the availabilityof electrical consumers when the vehicle isstationary. In this case, the vehicle electricalsystem can be supplied temporarily fromthis surplus capacity during the fuel-con-suming phase. No (or less) power is takenfrom the alternator while the battery is sup-plying the vehicle electrical system, and theengine only has to deliver the correspond-ingly lower alternator load through the beltdrive. Specifically in acceleration phases ofthe vehicle, high priority is given to reduc-ing the driving power of the alternator in or-der to maximize the engine power availablefor forward drive.

The fuel saving is the result of the totaldifference between normal alternator loadduring engine operation and this reducedload. The possible saving is, therefore, deter-mined by the driving phases (sequence of ac-celeration and overrun/braking phases), thesurplus power of the alternator, the capacityfor the storage of recuperated energy andthe rate of storage (charging current level).

To make recuperation possible, a centralcontroller module is required in which therecuperation strategy is implemented. Up-grades to the vehicle electrical system in arecuperation context include an improvedinterface in the alternator for the purposesof voltage control (BSD controller) and amodified intelligent sensor for the purposesof battery charge detection. In addition, abattery which meets the requirements forrecuperation is used. The components par-ticipating in the recuperation process are in-tegrated according to the topology shown inFigure 5. If one of the components requiredfor recuperation fails or if the recuperationlogic recognizes an implausibility, the sys-tem switches over to the standard energymanagement system as a fail-safe strategy.

7 Integration of the Intelligent Alternator Control in the Vehicle Electrical System Architecture

7.1 Alternator with Communication InterfaceToday's alternators are generally rated formanufacturer-specific, simulated and em-pirically determined charge balance cycles,and operate at electrical currents from 90 Ato 210 A. Electrical power demand is risingsteadily due to the increasing electrificationof functions in modern motor vehicles. Con-sequently, peak torques of up to 60 Nm areproduced at the crankshaft specifically tomeet the power demand of the alternatorwhen the engine is idling. To prevent suchloads from arising suddenly and in an un-controlled manner, and to stabilize engineidling and improve fault diagnostics, theBMW Group has been using alternator con-trol units with a communication interface(BSD-Bus) since the year 2000.

This interface is used for communicationbetween Digital Engine Electronics or Digi-tal Diesel electronics (DEE or DDE) and alter-nator control units and allows the enginemanagement system to counteract suddenengine-incompatible loads. This is accom-plished by the interchange of actual statevariables and set-point inputs, such as alter-nator voltage or alternator temperature, andby incorporating these into appropriatetorque-based models. Using the torque bal-ance calculated in these models, the enginemanagement system is able to stabilize en-gine idling by, among other things, adapt-ing the alternator control dynamics (LoadResponse). It is also possible to control theactual battery voltage via this interface insuch a way that drops in voltage along thesupply leads can be compensated for. As a re-sult of the gradual introduction of this con-

trol technology with each new engine gener-ation, all new vehicles of the BMW Groupnow have this facility for communicationbetween the engine management systemand the alternator.

To exploit the available braking energyrecovery potential, it was necessary to cou-ple the driving situation to the state of volt-age of the vehicle electrical system. In vehi-cle acceleration phases, the alternator termi-nal voltage is set by means of the BSD bus toor slightly below battery voltage level. As aconsequence of this, the power demand ofthe vehicle electrical system is covered bythe battery (the SoC of battery decreases)and the alternator does not place any loadon the powertrain.

During overrun and braking phases, thealternator terminal voltage is transmitted tothe alternator control unit with a defaultvalue higher than the actual battery voltage.As a result, the alternator not only coversthe complete power demand of the vehicleelectrical system, but also charges the bat-tery (the SoC increases). The alternator nowplaces load on the powertrain and effective-ly brakes the vehicle. With the introductionof the second generation of interface controlunits for motor vehicle alternators, it is nowpossible to control the power and torquecharacteristics of the alternator in a contin-uously variable manner. This is a prerequi-site for engine and vehicle-compatible gener-ation of additional electrical power in over-run/braking phases. The alternator is gener-ally controlled in a much more dynamicmanner because it makes allowance for thedriving situation, as well as engine and elec-trical power supply requirements.

7.2 Intelligent Battery Sensor (IBS)In the context of electrical energy manage-ment, and specifically in recuperationmode, it is essential that precise informa-tion on battery charge and wear state beavailable at all times. For this purpose, it isnecessary to measure the battery chargingand discharge currents, battery terminalvoltage and electrolyte temperature.

The wide range of battery currents to bemeasured represents the greatest challenge.The battery currents range from -200 A dur-ing charging to discharge currents of 1000 Aat vehicle start-up. To be able to measure thestatic current with sufficient accuracy, themeasurement resolution lies within therange 0 to 10 A, and is 10 mA over the com-plete temperature range. To accomplish thismeasurement task, the BMW Group has de-veloped a mechatronic component whichintegrates an efficient electronic device inthe battery negative terminal (the Intelli-

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gent Battery Sensor – IBS). The IBS covers anextensive measurement range spanning sixpowers of ten using three precision-matchedcurrent measurement ranges in differentfrequency bands. The actual current meas-urement is obtained using a shunt resistor.To ensure the IBS fits into the standardizedbattery terminal niche and to be able tomeasure the battery terminal temperature(which is an indicator of battery electrolytetemperature), a fingernail-sized board ac-commodating a full-fledged control unitwas integrated. The IBS is also connected viaa BSD interface to the engine managementsystem in BMW Group models.

As the IBS has to perform a multitude oftasks especially when the vehicle is parked,a cyclical Sleep mode was introduced in or-der to reduce the power demand of the IBS,during which measurement preparation iscompletely deactivated. The IBS can be acti-vated by a higher-order control unit throughthe BSD interface. In addition to monitoringthe battery state, the IBS measures the staticcurrent, and therefore is responsible for di-agnosis of the electrical power supply sys-tem. For use in recuperation mode, the al-ready tough demands on the performanceof the IBS are increased by a factor of four bythe use of a 32-bit processor.

As a result, it is possible to determine theState of Health (SoH) of the battery with suf-ficient accuracy. The State of Charge (SoC)and the State of Health give the State ofFunction (SoF) of the battery, which allowsreliable conclusions to be drawn as regardsits recuperation capacity. To further opti-mize the use of the IBS in production in acost-effective manner, the BMW Group is co-operating in this project with other OEMsand component suppliers.

7.3 AGM BatteryIn the iGR (intelligent alternator control)systems of the current generation, the recu-perated energy is stored in the vehicle bat-tery. To be able to store recuperated energy,the battery must be operated in a partiallydischarged state. The storage capacity of thebattery increases with decreasing state ofcharge, whereby both the chargeable energyamount and the maximum power consump-tion increase.

The conventional charging strategy todaygenerally attempts to charge the battery toits available maximum capacity Cmax. Thisensures both reliable starting of the vehicleand the availability of all electrical con-sumers when the vehicle is at a standstill. Ifthe battery is selectively discharged, then itmust be ensured that all present require-ments are still met. One possibility is to use

a battery with a higher capacity. However,this involves substantial drawbacks in termsof weight and packaging. A better alterna-tive is to use an optimized battery manage-ment system which allows a reduced state ofcharge depending on the ambient condi-tions and the vehicle state. From a technicalpoint of view, the total amount of energy orenergy capacity available in the battery doesnot have to be fully available at all times.During the summer, for instance, less ener-gy and less battery power are required forstarting the internal combustion enginethan during the winter. The challenge fac-ing battery management in the iGR operat-ing strategy involves striking an optimumbalance between the present-day demandson the battery and the new demands relat-ing to recuperation. The usable recupera-tion capacity CRecup is the difference be-tween Cmax and Cstarting/stationary. On coldwinter days, in accordance with the prioriti-zation strategy, CRecup is reduced in order toensure engine startability, Figure 6.

In its role as a recuperated energy storagedevice, the battery is subjected to additionalloads: the average battery state of charge de-creases, thus increasing the sulphate level inthe electrodes. There is an increased likeli-hood of parts of the electrode becoming irre-versibly sulphatized, with the result thatthey can no longer be used for electricalcharge storage. The temporarily highercharging voltage causes increased electroly-sis, which, in turn, results in a higher loss ofwater (whereby the higher charging voltagecan partially reverse any sulphatizationwhich may have occurred). The high charg-ing current results in the production ofhighly concentrated sulphuric acid at theelectrodes which, in wet cell batteries, dropsto the bottom of the battery due to its highdensity and forms a layer below the remain-ing electrolyte. A phenomenon known aselectrolyte stratification occurs in the bat-tery. The feedback of recuperated energyfrom the battery into the vehicle electricalsystem increases the cycling rate of the bat-tery.

To counteract these additional loads, theBMW Group will in future make greater useof AGM batteries in lieu of conventionallead-acid batteries following the introduc-tion of iGR.

In the case of AGM batteries (AbsorbentGlass Mats), the electrolyte is suspended in anon-woven fabric made of micro-fine fibre-glass. This serves to keep electrolyte stratifi-cation to a minimum. The active material ofthe electrodes is also stabilized mechanical-ly by the non-woven material. The cyclabilityof the battery increases to three times the

value of a normal lead-acid battery and wa-ter consumption is reduced significantly bythe AGM-typical recombination circuit.

An AGM battery is also subject to aging inrecuperation mode, however it ages muchmore slowly than a conventional battery. De-pending on the extent of battery aging,startability is ensured by shifting the valueCstarting/stationary towards Cmax according todemand.

8 Vehicle Electrical System with Voltage Measurement and Central Control of the Voltage Supply

In iGR mode, the vehicle electrical systemoperates within a voltage range of 12 to 15 V.The task of the electrical energy manage-ment functions consists in supplying allelectrical components with enough electri-cal power at any time and ensuring thatchanges in voltage do not compromise theoperational safety of the vehicle and are notperceptible by the driver.

For instance, during trailer operation ona hill at high ambient temperatures, it mustbe ensured that the full power output of theengine cooling fan is available in order toprevent the engine from overheating. Thehuman eye is, in certain conditions, able toperceive even minimal variation in thebrightness of light, which can occur if thevoltage changes within the range of only afew 100 mV. Even a small change in thespeed of the air conditioner blower may beacoustically perceived as unpleasant by thehuman ear. Electrical drives such as the fuelpumps or engine cooling fans generally re-act sensitively to the absolute state of voltageat operating points where load demand ishigh, but not to the voltage change gradi-ents, whereas both factors have to be consid-ered in the case of lighting. However, tomaximize the recuperation potential of abattery, it must be possible to change thevoltage level quickly.

To implement the iGR function, all elec-trical consumers connected to the vehicleelectrical system had to be analyzed with re-gard to their voltage sensitivity. To this end,several hundred components were exam-ined, whereby the safety-critical compo-nents of the suspension electronics were an-alyzed very closely.

It was found that many components arealready equipped with pulsed voltage con-trol devices and only a few key componentsin the extended voltage range exhibit slight-ly impaired performance in response tochanges in voltage. This is the reason why al-lowance is made of these functional require-ments in the operating strategy. Figure 7.

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Normal halogen headlights exhibit smallperceptible differences in brightness even ifsmall changes in voltage occur, whereasxenon lights are supplied through a ballastwhich provides a constant light efficiency atinput voltages from 9 to 16 V. For years now,BMW has been equipping its vehicles withpulsed outputs on the central light switchingunit which provide a constant lamp voltageof 13 V. Input voltages below 13 V result in areduced light efficiency. In vehicles withoutxenon headlights, therefore, the onboardvoltage may only be adjusted below the 13 Vlevel in such a way that this is imperceptibleto customers while the lights are on.

In entry-level models, the speed of theheater and air conditioner blower is oftencontrolled by ballast resistors. Changes involtage are perceptible upwards of a certainblower speed. By using a suitable signal pick-up, it is possible to detect the blower speedselected by the customer and avoid exces-sively large changes in voltage at high blow-er speeds. Alternatively, the speed of theblower motor can be controlled independ-ently of the onboard voltage by using of arelatively expensive voltage control unit. Toimplement the iGR function, the voltage re-quirements dependent on the driving situa-tion are prioritized over requirements,which depend on the state of the vehicleelectrical system and the battery, and areused to control the alternator voltage. Ensur-ing the availability of electrical power is al-ways accomplished at the expense of recu-peration requirements. The iGR function isimplemented in the digital engine electron-ics/digital diesel electronics (DEE / DDE) be-cause it is used to control the alternator andthe engine electronics are networked viaCAN with the electrical components in-stalled in the vehicle. Figure 7.

9 Summary and Outlook

The holistic energy management systemshown in Figure 2 along with the differentforms of energy which have evolved overtime will become simpler, not least due tothe concept of intelligent alternator control,Figure 8.

Hydraulic and pneumatic power willgradually be replaced by electrical power.

Systems which facilitate demand-basedcontrol and the relatively simple recovery oflost kinetic and thermal energy will sub-stantially improve both fuel efficiency andpower availability to the drive wheels.

This will also lead to a continuous im-provement in the efficiency of automotivepower generating components (alternator),as well as electrical consumers. An example

is the use of power saving lighting elements(LED) which significantly reduce the powerrequirements of lighting functions.

In order to meet even greater future de-mands on the vehicle electrical system dueto the ongoing electrification of vehicles,new storage systems must be developed fortemporary energy supply and new electro-chemical accumulators for long-term ener-gy supply across a wide product spectrum.

A scalable module with different powerratings would be advantageous for electricalmachines. Depending on vehicle require-ments, it makes energetic and economicalsense to implement this solution on asbroad a scale as possible.

The development of an independent, par-allel energy path circumventing the inter-nal combustion engine, e.g. using fuel celltechnology, will open up further consump-tion-reducing potentials not considered inthis analysis. ■