25MTZ 04|2006 Volume 67

Authors:Marco Jentges, Dirk van der Weem,Hans Kemper and Martin Pischinger

Optimierte Ansteuerung eines Downsizing-Konzepts mit elektrischem Boost

You will find the figures mentioned in this article in the German issue of MTZ 04|2006 beginning on page 290.

Optimized Activation of a DownsizingConcept with Electrical Boost

Everyone is speaking about hybrids in vehicle drive trains. A numberof hybrid concepts have been designed over the past few years, rightup to the start of production. One of the most relevant elements is totap the full potential of consumption whilst giving the guarantee of thedrivability aspects is to be found in the area of control strategy. In thiscontext FEV Motorentechnik GmbH designed an operating strategy forthe activation of a “downsized”-engine with electrical boost support.The development has been verified in a vehicle.

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26 MTZ 04|2006 Volume 67

1 Introduction

The target in the development of new trans-mission systems is based on three main ob-jectives, the interests of the legislation, theautomotive manufacturer and the end-con-sumer. The requirements, with respect tothe legal framework, are defined by the leg-islator. The automotive manufacturer is anx-ious to satisfy the interest of customers aswell as to new technology to improve costand economic efficiency. The interaction ofthe respective market and brand lies in theiremphasis. SI- and diesel engines are meas-ured, taking the above criteria in considera-tion, by the balanced resolution of the con-flict of different objectives.

For further reduction of the fleet fuelconsumption in respect to the agreed ACEAfinal values, further activities in the engineand power train development up to the year2008 or 2012 need to take place. The disad-vantages of the SI-engine compared to thediesel engine means that more effort is es-sential.

The further development of diesel tech-nology has improved the driving perform-ance whilst decreasing the fuel consump-tion. Owing to this development in the pastfew years the range of diesel engines has im-proved dramatically. The significant area ofthis development lies in the direct injectionand exhaust-gas turbo charging which hasalso improved the power density. The down-sizing which has been adopted within thediesel technology should be a signal for theexternally ignited engine technology. By re-locating the operating point, by constantpeak performance, and reducing the dis-placement of up to 40 % a fuel reduction ofup to 18 % can be achieved.

A disadvantage of highly charged inter-nal combustion engines, compared to thenatural aspirated engines with a bigger dis-placement at the same peak performance, isthe unfavorable torque at lower enginespeed. Owing to the combination of com-bustion engines with an electric machinethe dynamic torque has clearly improved. Asan electric machine has a high torque at lowengine speed compared to a combustion en-gine, it can ideally compensate the deficien-cy of the turbo engine.

2 Mild Hybrid Concept

The original reference vehicle is an Audi A6,six cylinder V-engine, a displacement ofthree liters, five gear manual transmission.At a rated speed of 6200 rpm the SI enginereaches a power rating of 162 kW. This serialvehicle is used as a reference value for the

aprox. 400 kJ. The technical energy amountwhich can be used is aprox. 230 kJ. The max-imum torque of the asynchronous machinecan be called upon up to a lower energy con-centration of 180 kJ. Below that value, due toa limitation of the electric current, a reduc-tion of power will be inevitable.

The EPB concept does not require any fur-ther construction space as, the extension ofthe power train through the installation ofthe electric machine, the reduction of thedisplacement within the combustion en-gine compensates this space. The totalweight of the vehicle is about the same com-pared to that of the reference vehicle.

3 Operational Strategy

The superior hybrid power train controlcommunicates in a Master-Slave structurewith the other components control units.These contain important information onthe condition of the separate componentsand generates, dependent of the operationalstrategy, the set value. The data exchange tothe supercap controller (SCMU) and the con-troller of the electric machine (EMCU) takesplace via CAN; however the engine manage-ment system is connected via a serial bypasswith the hybrid controller, Figure 3.

Through the hybridization of the powertrain it results in an added degree of free-dom, a new source of torque is added, whichcan be positive or negative. The requirementof the hybrid controller is the distribution ofthe drive input torque to the available chaindrive.

A negative torque requirement by thedriver can, if possible, be fielded by the elec-tric machine and used to recover the brak-ing energy. A part of the braking energy, dueto limited power or the energy storagereaching its maximum, the braking systemhas to convert into heat. A braking distancesensor supports the controller with addi-tional information so the process of recuper-ation can work to optimal conditions.

Optimizing the split up of the positivedrive input torque is not trivial. The con-stant conversion of the required torquethrough the electric machine, may be atfirst without emissions (electric drive), buton the middle term it drains the energysource. A shift of the operating point of thecombustion engine through torque compen-sation by the electric machine leads inmiddle term to a full electric energy storage.In conventional vehicles the operatingmethod of the components primarily de-pends on the driver. Compared to the hybridpower train, where the algorithms arestored in the power train management

analysis of power consumption and bench-marking.

The FEV-EPB (Electric Power Boost) con-cept combines the same vehicle and samegear box with a combustion engine with re-duced displacement and an additional ener-gy converter. The downsized combustion en-gine, with about the equal power rating, hasa 1.8-l-displacement turbocharged engine,extracted from an Audi TT. With a displace-ment of 1.8 l and a power rating of 165 kWat an engine speed of 5900 rpm the enginereaches a maximum torque of 280Nm in thearea of 2200 rpm to 5500 rpm, Table 1.

In Figure 1 are the torque characteristicsof the basis and turbocharged engine. Belowan engine speed of 2500 rpm the torque de-ficiency for the turbocharged engine com-pared to the natural aspirated engine can beseen. To adjust the drive system perform-ance features a backup, through an electricengine is essential.

The resulting system structure is that of aparallel hybrid system, Figure 2. The electricmachine from Valeo is fully integrated inthe drive chain between the combustion en-gine and the standard transmission. It iscontrolled through power electronics fromSaft provided by a double layer capacitormodule, for a short period of time, at low en-gine speed, an additional torque can be ap-plied. On account of the thermal sluggishbehavior it is capable of handling a short ex-cessive load. Concept conditions, result inrestricted energy, a requirement for an inter-mittently torque request to the electric ma-chine for a demand of acceleration, a doublelayer capacitor module is used as an ade-quate electric energy store. A superior hy-brid control unit coordinates the powertrain elements, combustion engine, electricmachine and electric energy storage.

The layout of the electric machine andsupercap module required necessary specifi-cation within the system. As the NEDC test isdriven without boost support and in the testresults the fuel consumption, adds upthrough downsizing, start/stop and brakingenergy recovery, shows the layout of thecomponents is not adequate for the worstcase scenario. The torque backup only worksat a certain state of charge, for these an ownworst case cycle has been developed. Thiscontains full-load acceleration with gearchanges at low engine speed (~2500 rpm).The cycle comprehends an accelerationphase up to the speed of 100 km/h, changingthrough all gears. After reaching the resid-ual speed a phase of delay and recharging ofthe supercap module takes place. The elec-trical storage leads to a system with a maxi-mum voltage of 90 V with a total energy of

27MTZ 04|2006 Volume 67

shows the greater importance of the system.There is a degree of freedom in the new op-erating system; this can be used, dependenton the arrangements, to influence the ener-gy consumption or the drivability.

The aim of the operating strategy is toachieve an optimal operating vehicle, re-garding the fact that the final criteriaguidelines have to be met. The assignmentto the appointment to reach to optimal op-erating point of the hybrid electric powertrain can furthermore be seen as criteria forthe optimization problems. The definitionfor the optimization criteria is physicallybased on the quality function combined ina scalar function of time. The weighting oc-curs regardless of the driving situation, theoperational conditions and the type of driv-er. The subjective purpose dimensions com-bined into several higher ranking cate-gories: – warranty for the drivability– minimize the fuel consumption– minimize the emissions– warranty to the endurance of the compo-

nents– minimize the noise emissions.The different categories for the various setvalues can be allocated to a range of objec-tive criteria. Often the objective criteria canonly be qualitatively characterized, as manyare subjectively observed. It also occurs thatthe objective criteria reciprocative to oneanother, so an allocation is necessary. Thesubjective objection criteria must be pro-jected as equation between quantities. Anyconflicts of objectives are uncoupled till alater point when they are looked upon un-der the perspective of the multiple valueoptimizations. The objectivity takes place inthe formulation of quality functions thatare weighted depending on chosen strate-gies.

4 Function Development Process

The complexity of the hybrid control andfeedback algorithms requires a modernmethod of system design. From the conceptup to the serial product defined proceduresneed to accompany development process.

The influence of the operating strategyon the behavior of the power train can onlygive a reasonable statement in an adequateclosed cycle. An important difference needsto be taken in account for, the Model-in-the-Loop (MiL) at the early stages of develop-ment and the Hardware-in-the-Loop (HiL) ata later stage when real power train compo-nents are available, Figure 4.

The development of hybrid functions aswell as the function test for the entire con-

trol unit combination is completed in theHiL-test cell. The Bosch engine managementsystem, from the reference vehicle, and theETAS ASCET-MD hybrid controller are physi-cally available. Furthermore a couple of ac-tuators from the reference engine and thecockpit module are connected to the Bread-Board via the vehicle harness. All powertrain components which are not physicallyavailable are simulated in real time via aHiL-Board. These are connected over a pro-tective circuit and a Breakout Box to the con-nection ports of the control unit. The HiLsystem allows a cost efficient optimizationof the algorithms.

The two major criteria of objectives, re-duction of consumption and drivability,were tested in the test runs. As a result of thedifferent requirements to the objective, onthe one hand the NEDC-driving cycle for anassessment on the reduction of fuel con-sumption and on the other hand the “worstcase” cycle for performance simulation.

5 Result of Measurement

Important for a successful downsizing con-cept is, apart from the reduction of fuelconsumption, adequate drivetrain sover-eignty. Acceleration, elasticity as well asspontaneity are part of this. The weaknessat low engine speeds of the drivetrain fromthe 1.8 l turbocharged engine has beencompensated with an electric machine.With help of the electric motor, responsebehaviour of the drivetrain is achievedwith an equal power rating to a naturallyaspirated engine. To compare the drivingperformance and fuel consumption thegearbox and the final drive have not beenchanged. In elasticity measurements thedrivetrain sovereignty of the EPB vehicle isimpressively proved, Figure 5.

The acceleration from 30 km/h to 80km/h in third gear corresponds to a real ac-celeration driving condition. From a con-stant driving speed of 1250 rpm full load ac-celeration up to 3300 rpm is performed. The

1.8 l turbocharged engine reacts firstly witha torque of approximately 140 Nm. Onlywith support of the electric machine withapprox. 120 Nm (18 kW at 1500 rpm) it cancompensate the static and special dynamiclow torque from the turbocharged engine.The results of measurement impressivelyprove how clear the contrast is between theEPB-vehicle and the vehicle with the 1.8l tur-bocharged engine. 8.4 seconds is the timefor the turbocharged vehicle, 6.4 seconds forthe EPB-vehicle and approx. 7 seconds forthe reference vehicle.

Likewise the acceleration in fifth gearfrom 80 km/h to 120 km/h and from 60km/h to 100 km/h in fourth gear both withan engine launch speed of ~2000 rpm theEPB-vehicle has a better elasticity than the1.8 l vehicle are lower due to a higher driv-ing speed of ~2000 rpm. The EPB-vehicleneeds approx. 0.7 seconds less compared tothe vehicle with the 1.8 l turbocharged en-gine. The vehicle with the 3.0 l naturally as-pirated engine shows at the higher dynamicacceleration between 60 km/h and 100 km/hits better acceleration behaviour than the1.8 l turbocharged vehicle, in comparison tothe EPB-vehicle which is to the same stan-dard. Overall the EPB-vehicle with supportof a 18 kW electrical engine is slightly supe-rior to the 3.0 l engine.

The electric machine allowed in particu-lar spontaneous response with homogenoustorque behaviour.

Looking at the economic and ecologic as-pects the fuel consumption is of essentialimportance. The three consumption reduc-tion measures: Downsizing, start/stop andrecuperation were all separately measured.To do so the start/stop and recuperation theyeither switched on or off. This was possiblebecause the features are in the same vehicle,Figure 6.

The capability of downsizing has beenconfirmed: the exchange of the 3.0 l natural-ly aspirated engine with a 1.8 l turbochargedengine has an outcome with 17 % reductionin the NEDC (urban area: 22 % and rural

AcknowledgementParts of this report are a scientific result of a European research project. For this reasonwe want to thank the European Commission for the funding support for this forward-look-ing project (EU project no.: Contract No. ENK6-CT-2001-00531). Besides we want to thank our project partners Valeo VES (electric machine, inverter mod-ule, DC/DC converter), SAFT (supercap module), the University of Sheffield, Renault andthe institute for combustion engines of the university of Aachen.

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area: 11 %). Also the theoretical potential ofstart/stop and recuperation has been ap-proved. The start/stop mode reduces the fuelconsumption by 5 % (refer to the 3.0 l en-gine) and through recuperation by another2 %. Overall a reduction of 24 % in the NEDCwas achieved (urban area: 30 %, rural area:17 %), Figure 7.

As there is only small energy storage andan electric machine adapted to compensatethe turbo lag only a part of the braking ener-gy can be used for recuperation. By enlarg-ing the storage capacity, a bigger electricalmachine and by optimizing the recupera-tion strategy the potential of fuel economycould be raised from 2 % up to 5 %. Another5 % to 7 % of less consumption can beachieved by using a direct injection enginewith a fully variable valve train (VVT). Due toa clearly retarded heating-up of the engineof a hybrid vehicle the fuel economy poten-tial of thermomanagement measures are 2 % to 3 %.

By tapping the full potential while stillhaving the same drivetrain sovereignty theEPB-Vehicle has the potential to reduce thefuel consumption in NEDC by approx. 35 %so that CO2 values (CO2: ~165g/km) areachievable, which lie under that of a similarpowered conventional diesel vehicle.

Vehicles with a strong torque and manu-al transmission are driven by the customersconsistently at low engine speed. In order tomake fuel economic load point shifts it isnecessary to have relatively high torque ex-cess. The EPB concept from FEV with its fastenergy storage provides this torque excessand thus gives further fuel economy.

6 Summary

To reduce the CO2 emissions and to achievethe goal of the ACEA further developmentsin the engine and power train technologyare necessary. An effective and well knowmethod is the recharge technology in com-bination with a downsizing concept. Low en-gine speed has a negative influence on theresponding behaviour and leads to a drasticreduction of the displacement and the vehi-cles acceleration behaviour has also radical-ly declined. With the hybridization of thepower train the negative effect can not onlybe compensated but also, depending on thesize of the electric machine, it can be used toimprove the acceleration behaviour. This ispossible due to the features of electric ma-chines, at a spontaneous response, to give ahigh torque at low engine speed. At thesame time hybrid features like start/stop andrecovery of braking energy are still madeavailable.

The application of a double layer capaci-tor makes a high Energy flux to the electri-cal storage possible that can be used for therecovery of the braking energy. A specificenergy management is needed due to thelimited energy storage. In a conventional ve-hicle the torque request on the drivetrain isdependent on the driver where as with thehybrid controller, the algorithms are storedwithin, the functions are split betweencombustion engine and electric machine.The operating strategy shows a new degreeof freedom and influences the vehicle char-acteristics, like energy consumption anddriveability. With modern methods of sys-tem development the appropriate develop-ment tools a target-orientated optimizationof the complex background can beachieved.

The capability of downsizing has been ap-proved with the measurements results ofstart/stop and recuperation. The results,through the exchange of the 3.0 l naturallyaspirated engine with a highly charged 1,8 lturbocharged engine in the NEDC test, is areduction of consumption by 17 %. The fuelconsumption has also been decreased by 2 %through recuperation and by another 5 % us-ing the start/stop mode. The total sum of re-duction in the NEDC test comes up to 24 %. Aclient specific big advantage of the combina-tion through downsizing and hybrid tech-nology can be seen: a clear reduction of con-sumption under all relevant driving termswith excellent driving characteristics. ■