Micro-hybrid System for Inte gration

in Timing Chain Drives

The micro-hybrid system developed by Iwis Motorsysteme GmbH & Co. KG provides the installing of a compact electric machine on the combustion engine with power transfer via timing chain drive. While keeping the standard 12 V vehicle power supply, a specially developed high-power electric machine replaces the functions of serial starter and alternator and implements additionally hybrid functions.

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The Authors

Dr.-Ing. Volker Hirschmann is Head of Advanced

Engineering at iwis

motorsysteme GmbH

Co. KG in Munich

(Germany).

Dipl.-Ing. (FH) Bernhard Schachtner is responsible for

hybrid systems at

Advanced Engineering

at iwis motorsysteme

GmbH Co. KG in

Munich (Germany).

1 Introduction

In recent years, hybrid drive concepts have been increasingly developed and put into mass production by automo-tive manufacturers in the interest of greater fuel efficiency in vehicles. The new powertrains stand out with an elec-tric motor alongside the internal com-bustion engine in the vehicle power-train to avoid unefficient operating ar-eas of the combustion engine and to integrate additional functions, such as start-stop, power boosting and braking energy recuperation. A superordinated control strategy that harmonizes the potential of the different drive sources and energy accumulators with the ac-tual driving status of the vehicle and the driver’s power request can achieve clear savings in energy consumption and emission levels.

Micro-, mild- and full-hybrid systems are differentiated according to the out-put of the electric machine integrated in the powertrain. Micro-hybrid systems are extremely compact in design and constitute the smallest intervention in the existing powertrain architecture.

The micro-hybrid system developed by Iwis Motorsysteme provides the install-ing of a compact electric machine on the combustion engine with power transfer via timing chain drive. While keeping the standard 12V vehicle power supply, a specially developed high-power electric

machine replaces the functions of serial starter and alternator and implements additionally hybrid functions.

2 System Description

2.1 Overall ConceptThe timing chain drive of combustion engines offers ideal conditions for inte-grating a micro-hybrid system. To verify this concept, Iwis Motorsysteme has produced a functional test setup based on a standard combustion engine and carried out successful tests. An electric machine can be easily integrated in the existing available space, because chain drive systems offer powerful traction el-ements. In contrast to belt drives, they can transmit higher power and torques as occurring in the start-stop, generator or boost functions in typical hybrid op-erating phases, without needing any major modifications.

In the present concept, Figure 1, a high-power electric machine is integrat-ed on the intermediate shaft in a two-part timing chain drive of a four-cylin-der diesel engine with 2 l displacement. Thanks to the package conditions on the engine block, the diesel high-pressure pump connected to the intermediate shaft can be moved in longitudinal di-rection so that the hybrid components can be integrated without affecting the space available.

Figure 1: Micro-hybrid system

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The structure of the hybrid system lets the chain drive perform the cold start, generator function, boost function and vibration absorption functions within resonance ranges of the chain drive. To-gether with the existing chain drive which is slightly modified, the system consists of the high-power electric ma-chine with integrated power supply unit and an upstream transmission unit with gear shifting actuators. The new micro-hy-brid system can perform both the engine start and the generator function so that the standard components starter and al-ternator can be omitted, together with some of the connected transmission me-dia (starter ring gear, belt drives, etc.).

2.2 Timing Chain DriveThe combustion engine chosen for test-ing is equipped with a two-part timing chain drive which offers an ideal trac-tion element for optimal connection of the hybrid system in the chain drive. If there is a sufficient wrap angle of the chain around the sprocket, the concept could also be implemented with a one-part chain drive. To test the system on the test bench, the chain drive between intermediate shaft and camshaft was retained; the chain drive between crank-shaft and intermediate shaft (primary chain drive) was adapted to the needs of the hybrid system. There was no need for any changes to the package for the

timing chain drive to ensure reliable transmission of the high torques of up to 200 Nm occurring during cold start conditions of the chosen combustion engine. The tensioning system of the primary chain drive could be adapted to prevent any loose strand of the chain during start procedure. When the com-bustion engine is operating, the torque is transferred by the crankshaft via the tight span of the primary chain to the intermediate shaft and on to the cam-shaft. The section of the chain not sub-ject to tension is called slack span. The chain tensioner presses on the tension-ing rail to prevent the sagging of the slack span of the chain. During the start procedure, the electric machine gener-ates torque on the intermediate shaft which is transferred as tensile force via the chain and generates a torque again at the crankshaft, retaining the crank-shaft’s direction of rotation. This chain tensioner was modified to prevent any loose strand in the chain and the retrac-tion of the chain tensioner. During gen-erator operation, the force conditions in the separate chain sections are the same as during normal operation of the com-bustion engine without hybrid system.

2.3 Gear Transmission Unit and Shifting Actuator SystemFor start/cold start and generator func-tion, a transmission unit is to be integrat-

ed upstream of the electric machine. A cold start of the combustion engine re-quires a torque of approximately 200 Nm at the crankshaft. As the electric machine cannot provide such a high level of torque to start the combustion engine, an on-de-mand gear transmission stage is required. The gear transmission stage is switched on before the start by means of an actua-tor; after starting successfully, the actua-tor is switched back to neutral setting (gear not engaged) and then to the gen-erator mode. With an upstream transmis-sion stage, the electric machine can be used for a cold start of the combustion engine without needing an additional starter. Micro-hybrid systems currently on the market normally have an addi-tional starter for cold start conditions.

In the generator mode, the electric machine is connected directly to the in-termediate shaft. The diesel high-pres-sure pump required to operate the diesel engine is mounted to the electric ma-chine and coupled directly to the inter-mediate shaft.

To avoid accelerating the electric ma-chine during high acceleration phases of the car, it can be decoupled from the intermediate shaft by setting the shift-ing actuator to the neutral position. The total power output of the combustion engine is available for propulsion of the vehicle, with a noticeable increase in dy-namic performance for the driver. The generator is subsequently coupled to the intermediate shaft again. Necessary syn-chronization processes are carried out by accelerating or decelerating the elec-tric machine.

2.4 Electric Motor and Power Supply UnitThe efficiency of the hybrid system is es-sentially determined by the electric sys-tem, consisting of electric machine and power supply unit. To replace the starter and alternator used in production vehi-cles by an electric machine, the compo-nent must be capable of both a high torque during the start at low engine speeds and also a generator output of ap-proximately 2 kW at engine speeds of up to 5000 rpm while achieving good effi-ciency levels. A concept study was carried out to evaluate the possible machine types in terms of technology, production and cost aspects. The Table shows the

Table: Evaluation matrix electric machines

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various electric machine concepts, main-ly comparing machines based on perma-nent magnet machines with asynchro-nous machines. Other machine types such as switched reluctance machines, electrically excited synchronous ma-chines, claw-pole machines and DC ma-chines were not considered in view of their clear disadvantages in several crite-ria compared to the stated electric ma-chines.

Basically, all permanent magnet ma-chines create more torque at a given cur-rent than asynchronous machines. Input for the power electronics and the power demand from the battery is therefore less during the start; however, at high en-gine speeds the magnet field has to be permanently reduced by actively control-led current. This can only be achieved with a special design for the rotor lami-nations which in turn reduce the maxi-mum possible torque. The disadvantages of the conventional asynchronous ma-chine with round wire (ASM (wire)) can be widely leveled by using an asynchro-nous machine with bar winding (ASM (bar)). A special design of the permanent magnet synchronous machine (PSM) con-sists of the axial flux machine, better known as the transverse flux machine. Particularly high torques can be gener-ated by a large number of poles, a ring winding with low resistance and in the version as outside-rotor motor. But this

has drawbacks for the generator because of the very high frequencies (up to 2 kHz) which cause high iron losses and a more complicated control of the power supply unit. This problem consists in principle in all permanent magnet machines and increases the more pairs of poles there are. This means that on the one hand, very good efficiency levels are achieved at low to moderate engine speeds and medium to high loads in generator mode. On the other hand, the values are poor for part loads and particularly at high engine speeds. In view of the fact that in real driving conditions, the oper-ating points are distributed over a wide range of the torque-speed field, the dif-ferences in efficiency to the asynchro-nous machine are not as pronounced as the values in the best point would tend to indicate.

Altogether, this resulted in choosing the machine for the prototype that pro-vides the best basis for reliable imple-mentation of the start function. Mass production could also consider other machines, which have to be optimized carefully to the specific application.

The current required by the electric machine is provided by the power sup-ply. The vehicle system voltage of 12 V means that relatively high currents (up to 500 A) are being switched. The power switches (MOS FET) are mounted directly on the metal carrier of the cooling ele-

ment, Figure 2. The power supply unit is connected to the electric machine by short bus bars, with the power supply unit attached solidly to the electric ma-chine. The power supply unit is activated by CAN bus. The cooling water is sup-plied by the coolant circuit of the com-bustion engine, taking the required cool-ant from the combustion engine feed line. The cooling water outlet from the power supply unit is connected with the water cooling of the electric machine and is then returned to the coolant cir-cuit of the combustion engine.

3 Functional Verification

3.1 Test SetupA specialized test bench with correspond-ing software was developed to perform function and endurance tests on the mi-cro-hybrid system, Figure 3. The timing chain drive of the chosen four-cylinder diesel engine is geometrically replicated on the test bench, with the hydraulic chain tensioners supplied by a separate hydraulic unit. The chain drives are lu-bricated similar to the combustion en-gine by the tensioning systems of the up-per and lower chain drives.

The camshaft in the upper chain drive is reproduced by using an adapted inertia mass. As the hybrid functions in-tervene particularly in the lower chain

Figure 3: Test bench setupFigure 2: Electric machine with power supply for vehicle application

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drive, the test bench operation ignores the changing torque levels occurring at the camshaft in the combustion en-gine.

The modified timing chain drive was tested thoroughly before integration of the high-power electric machine. Both the crankshaft and the intermediate shaft were connected to commercially available industrial electric motors each with outputs of 13.5 kW, and starts were carried out with torques of up to more than 200 Nm, as well as endurance runs with changing operating points to simu-late the generator mode.

After testing the timing drive compo-nents, the industrial electric motor on the intermediate shaft was replaced by the water-cooled high-power electric ma-chine including power supply unit, Fig-

ure 4. While the power electronics in the prototype system is realized as a separate power supply unit connected by cables to the electric machine, for mass produc-tion it will be installed on the housing of the electric machine embedded in the water cooling system. The electric ma-chine will be provided with current and voltage from a 12 V starter battery. The power consumers in the vehicle are sim-

ulated by means of an electronic load that can simulate a power consumption of up to 3 kW.

3.2 Cold Start ModeFor the time being, commercially avail-able micro-hybrid systems cannot be used for cold start conditions of the com-bustion engine at low ambient tempera-tures (e.g. -25 °C). In many cases, the hy-brid system can only start the engine at ambient temperatures in positive double figures, whereas at low temperatures the additional starter fitted in the vehi-cle is activated. To enhance the competi-tiveness of the hybrid system developed by Iwis Motorsysteme, special attention has been paid to the cold start function. The cold start operating point was exam-ined on the test bench using a conven-tional 12 V starter battery which had been cooled in a cold chamber to -28 °C. Figure 5 shows the torque curves of the standard starter compared to the high-power electric machine. The losses of the integrated gear transmission unit with the ratio of i=5 are included in this measurement. In spite of the voltage drop from 12 V to approximately 10 V at the deep-frozen starter battery during the start procedure, the electric machine shows a torque profile that safeguards the start of the combustion engine even at this extreme operating point. Thanks to the additional torque potential at crankshaft engine speeds exceeding 200 rpm, it is even conceivable to motor the combustion engine to higher start-ing engine speeds in order to reduce the fuel enrichment range during the start procedure, which in turn helps to save energy.

In order to ensure that starting on the test bench reproduces real-life vehi-cle operation even better, the irregular-ity of crankshaft excitation in the 2nd engine order can be simulated by the drive engine on the test bench. The soft-ware which has been adapted specially for this purpose makes it possible to set the maximum starting torque, torque fluctuations and the duration of the start itself, within the performance range of the drive machine. Figure 6 shows an example of a start recorded at the test bench. All required measure-ment signals can be freely selected and scaled.

Figure 5: Starting torque curves

Figure 4: High-power electric machine with power supply

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3.3 Generator ModeBy far the largest operating contribution of the electric machine (> 98 %) will be made in the generator mode. The elec-tric machine was designed to cover the functionality of a standard alternator. The maximum power in generator mode is 2.2 kW, available from engine speeds of 2000 rpm, Figure 7. Together with the attainable output in generator mode, special attention was given to good effi-ciency levels.

Before adapting and integrating the electric/electronic system in the timing chain drive, the components were meas-ured separately. Working on the basis of combustion engine idle speeds of approx-imately 500 rpm up to maximum engine speeds of 5000 rpm, an efficiency map was produced for the electric motor in-cluding associated power electronics, Fig-

ure 8, working with generator outputs of up to 3000 W (nominal output 2200 W). Efficiency levels of up to 86 % can be achieved at high generator outputs, where load-independent losses are of mi-nor significance. Efficiency levels of ap-proximately 80 % are achieved over wide parts of the characteristic map, thus clearly higher than current standard claw-pole generators, whose maximum efficiency level is probably about 55 %.

4 Fuel Saving Potential

On the basis of the power and efficiency maps for the sample system measured at the test bench, an overall vehicle simula-

tion with integrated micro-hybrid system from Iwis Motorsysteme for a middle-class vehicle, driven by a 2-l diesel engine with 120 kW, was carried out. The New European Driving Cycle (NEDC) was used as comparison cycle. Fuel savings with the sample system explained above can be achieved preferably in the start/stop function and during recuperation while coasting.

The standstill time in which energy can be saved by switching off the com-bustion engine amounts to 254 s in NEDC. The time for detecting the stop status and the time for the engine start have to be subtracted from this theoreti-Figure 6: Measuring signals during starting procedure

Figure 7: Generator characteristic lines

Figure 8: Efficiency map electric machine with power supply

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cal value. It must also be considered that part of the vehicle power supply unit system has to remain active even when the combustion engine is switched off in order to react when the driver wants to drive on. In the end, the additional energy required to restart the engine has to be subtracted in the energy bal-ance, too. Recuperation during the brak-ing phase (212 s) and decoupling the electric machine (see chapter 2.3) in ac-celeration phases (214 s) were considered as further potential fuel savings. The simulations showed that decoupling the electric machine only had a very slight fuel saving effect of approximately 0.04 %, while on the other hand this measure gives the driver the feeling that the vehicle is far more agile. Potential fuel savings during recuperation in turn depend to a great extent on the power consumer levels in the vehicle power supply system. Working on the basis of vehicle power supply consumption of approximately 500 W, recuperation can produce further savings of up to 3.3 %, resulting in total potential savings of ap-proximately 6.9 % in NEDC, Figure 9. This value is definitely interesting enough to make it worthwhile conducting further studies of this system.

Opening up greater potential for fuel savings particularly in the recuperation phase would entail a far greater genera-tor output. Simulations with a generator output increased to 4 kW show addition-al potential reductions in consumption of nearly 2 %. In the end, tests will have to be carried out on concrete customer engines to see which generator output can be installed in the existing package.

5 Summary and Outlook

The prototype of the micro-hybrid system developed by Iwis Motorsysteme shows that an innovative drive concept and a specially developed high-power electric machine can be used in the timing drives of combustion engines to integrate the additional functions of start/stop, recu-peration and boost mode to reduce fuel consumption without requiring any ad-ditional physical space.

Compared to commercially available micro-hybrid systems, the developed con-cept stands out not only with a signifi-

cant efficiency improvement of the elec-tric machine but also in particular through its ability to start the combus-tion engine even at ambient tempera-tures below minus 25 °C (cold start capa-bility). The technical data of the electric machine (torque, generator output) were selected so that the operating range should be adequate for most engine ver-sions of middle-class vehicles. It is pre-sumed that the system has to be adapted to specific customer requirements. The high power density together with the good wear characteristics of chain drives therefore provide the customer with a system which safeguards the function over the entire service life of the combus-tion engine without any wear-related re-placement of components, as necessary for example in belt-driven electric motor transmissions.

The prior explained advantages of the prototype system are reflected in the simulation results, carried out for a middle-class vehicle. Based on the New European Driving Cycle (NEDC) a fuel reduction potential of 6.9 % could be shown. Further fuel savings can be de-veloped by a specific adoption of the hy-brid system to the combustion engine of the customer.

Close cooperation with the compo-nent manufacturers involved in this sys-tem ensures that the micro-hybrid con-cept can undergo customized develop-ment and delivery under the responsibil-ity of Iwis Motorsysteme. It is planned for the micro-hybrid system to go into mass-production use in the near future; initial

concrete applications have already been identified in this context. An extensive test program with the involvement of fu-ture suppliers is currently in progress as preparation for mass-production applica-tion in the near future.

Figure 9: Potential fuel savings

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