Hybrid Architectures for Automated Transmission Systems

1 / 5 Hybrid Architectures for Automated Transmission Systems - add-on and integrated solutions - Dierk REITZ, Uwe WAGNER, Reinhard BERGER LuK GmbH & Co. ohg Bussmatten 2, 77815 Bühl, Germany (E-Mail: dierk.reitz@schaeffler.com) In this report, a hybrid module will be presented which combines the drivelines currently in production with the full functionality of a hybrid system. This requires the special consideration of the specific properties of the different transmissions in the early concept phase in order to minimize the complexity of the different transmission types. In the following, the limits and new requirements of the hybrid clutch will be identified. A concept with an integrated wet clutch shall show solutions for the fulfillment of the aforementioned requirements and the possibility of further decreasing the required design envelope between the engine and transmission. Key words: Hybrid, dry clutch, wet clutch, ICE restart, torque capacity 1. INTRODUCTION Increasing oil prices lead to a continuously increasing relevance of hybrid drive systems for automotive applications. This is especially apparent in North America and Asia where the already available hybrid vehicles gain market shares at a very fast rate. Besides fuel economy, emissions are becoming a key factor. Especially the emission of CO2, which unlike other harmful exhausts cannot be eliminated by catalytic systems, limits the ongoing unrestricted consumption of fossil fuels. The self-imposed obligation of the automotive industry to reduce CO2 emissions creates new requirements to driveline technologies and will lead to new concepts. The introduction of new limits by 2012 which are even lower than those to be implemented by 2008 is already being discussed. Since these targets cannot be achieved by applying one single technology, several measures must be implemented in the driveline and engine in order to further reduce fuel consumption. Hybrid technology not only allows an ideal combination with several other measures, but also offers the greatest potential for a long-term reduction of emissions as a single system. Besides improvement of the fuel economy, the reduction of costs is the actual challenge to the currently known topologies. (1) 2. Requirements In the following, driveline configurations will be presented and different concepts will be analyzed independently of the corresponding vehicle classes. Within the individual vehicle classes, different structures are evaluated in accordance with the targets (fuel economy, vehicle dynamics). For driveline concepts implying transmission-integrated electric motors, the high investments required for the development of new transmissions must be considered with respect to the relatively low quantities to be expected at the introduction of the technology. The option of extending available transmissions by hybrid systems with comparatively low effort is hence a key development target. Fig. 1: Hybrid drive systematic First, the required customer benefit and the derived functionality of the vehicle must be defined. Depending on the realization of these functions, the systems are classified as micro, mild and full hybrids. With micro hybrids, the installed electric capacity is just sufficient to automatically restart the engine after its deactivation at standstill. The recuperation of brake energy is very limited. However, such systems will be available in future vehicles because of the relatively low additional costs for such micro-hybrid solutions. Fig. 1 shows the systematic of the drive systems and their potentials. If brake-energy recuperation is required, electric capacities between 10 and 15 kw must be installed. This is sufficient

2 / 5 to manage deceleration processes in urban traffic and to recuperate the greatest part of the total deceleration energy. A higher rate of installed electric capacity would facilitate the recuperation of even more deceleration energy. However, such severe braking occurs only rarely, and the installation of more electrical capacity does hence not appear to be economically feasible. A mild-hybrid system does not allow electric driving, but is rather designed to boost the performance of the combustion engine. This concept already yields the highest share of the total fuel-saving potential. The start/stop concept will yield a fuel saving of approx. 5 %, with another 10 % being achieved by optimized energy management and recuperation. an active brake system is installed. The mechanical brake system is then only required for higher deceleration rates (2). In order to completely separate the combustion engine from the drivetrain, parallel-hybrid vehicles are equipped with an additional clutch. This clutch is a dry clutch in almost all applications. Such dry clutches are a suitable solution because they are not used for taking off and are hence only subject to low friction energies. A low drag torque is another key requirement, and is best achieved with a dry clutch. A dry clutch is characterized by a high inertia on the flywheel and pressure plate side and a low inertia of the clutch-disc side. Fig. 3: Parallel hybrid with conventional clutch Fig. 2: Hybrid functionality of the different topologies A further reduction of fuel consumption can be achieved by switching off the combustion engine during operation in unfavorable map areas and providing the complete drive power from the electric motor. The installed electric capacity is rated at 30 to 50 kw and is significantly higher combined to a mild hybrid. In addition to the capacity required for electric driving, a short-term supplementary capacity must be provided for restarting the combustion engine. Here, the dimensioning of the battery as a highcapacity short-term accumulator is a key factor. Furthermore the Full-hybrid concepts are also used for downsizing combustion engines. The resulting driveline consists of a combustion engine which always requires the boost of a powerful electric motor to provide its full performance. 3. Conventional clutch for parallel hybrid 3.1. Solution with additional clutch For recuperation or electric driving, the combustion engine must be disengaged from the electric motor and the remaining drivetrain. This allows the combustion engine to be switched off. The deceleration energy available for recuperation can now be completely used to drive the generator. During electric driving, the friction loss of the combustion engine is eliminated. In addition, further braking energy can be provided by the electric drivetrain if The design of the dry clutch allows either the high-inertia or low-inertia side to be fixed to the crankshaft. If the highinertia side is fixed to the crankshaft, it works like a conventional flywheel. Accordingly, the engine will also run smoothly with the clutch disengaged. However, at restart, the clutch side with the high inertia must be accelerated. This causes a higher load on the clutch, a longer engine speeding up period, possibly a noticeable deceleration of the vehicle when the engine is started by the vehicle inertia (fig. 3). The reversed configuration is hence also being developed. Only a light-weight clutch disc is mounted onto the crankshaft. The inertia of this clutch together with that of crankshaft, connecting rods and pistons is not sufficient to keep the engine running with the clutch disengaged. However, this is not a requirement. The light-weight clutch configuration positively affects the engine restart. The low slip of the start-up element causes an instantaneous acceleration of the crankshaft by the inertia of the electric motor. This results in extremely fast engine starts. The clutch must then transmit without slip the static torque as well as the irregular engine torque, which with modern engines may be many times higher than the static engine torque. Such high requirements can most likely only be met by a dry clutch. Fig. 4 shows the design and a prototype with a clutch completely radially integrated into the electric motor s rotor, which results in minimal inertia on the combustionengine side.

3 / 5 Fig. 4: Impulse Clutch Fig. 5 shows the curve of the restart times relative to primary-side inertia. Due to these facts a solution which combines the advantages of both clutch types was searched. The target for an exeptable restart time of the enginewas defined to be less than 400ms. Starting time Engine in ms 600 400 200 field of optimization Fig. 5: Comparison of clutch inertias 1500 1000 500 The solution of this investigation is shown in Fig. 6. An inertia optimized flywheel and clutch cover are connected to the crankshaft and the damper function is integrated into the clutch disk. This damper is not a conventional dry spring system but a wet system known from dual mass flywheel technology which provides a damper rate of less the 20Nm/. As a result of the very good driveline isolation the internal clutch of the torque converter can be closed over the speed range from 1000rpm to 6000rpm. Clutch torque in Nm Fig. 6: Design with integrated damper 3.2.. Substitution of torque converter Especially front-wheel drive vehicles with transverse mounted engines do sometimes not allow the integration of the presented hybrid module because of packaging constraints. One possible approach is the elemination of the automatic transmission s torque converter in order to gain space for the integration of the electric drive system. Start-off would then have to be realized by the electric drive only, which would negatively affect full-load acceleration. Vehicles with a conventional drivetrain reach an acceleration of approx. 5 m/s² near the slip limit of the tires, whereas an electrically propelled vehicle may only accelerate at a rate of 3.3 m/s² at the maximum. The acceleration times from standstill to 60 km/h and 100 km/h of the reference vehicle will not be achieved despite of the started-up combustion engine cutting in and the boost provided by the electric motor. This comparison is even more dramatic when starting off repeatedly at an incline. Acceleration in electric mode at a 25 % incline is inacceptable. Also, when starting off repeatedly, the minimum battery SOC cannot be maintained. The vehicle must be driven in creep mode. In this case, the energy balance can only be compensated by an additional belt-driven starter-alternator. These simulations lead to the requirement that the combustion engine must always be ready to be started in situations of high load demands and that hence a start-off element is required as a replacement for the eleminated torque converter. One possible solution is the extension of the transmission-internal holding-clutches to work as startoff element. However, this would require a great effort. A new approach is the extension of the engine-side hybrid clutch into a takeup clutch. This allows the combustion engine to be engaged when a defined torque is exceeded so that takeup is performed by both the combustion engine and the electric motor. This results in acceleration figures comparable to those of a hybrid drive with torque converter.

4 / 5 Fig. 7 shows a comparison of full load takeup of different drivetrain configurations (simulation). oil flow is provided by a channel in the modified pump housing (see graphic) or, alternatively, by a second channel in the transmission input shaft. 4. Hybrid for double-clutch transmissions Fig. 7: Vehicle acceleration in comparison The requirement for an extremely compact takeup clutch that can be completely integrated into the packaging space of the rotor of an electric motor can best be fulfilled by a wet clutch. The heat generated by friction is reliably dissipated by the oil cooling of this clutch type. Solutions involving dry takeup clutches cannot be considered under such demanding packaging requirements. Fig. 8 shows a very compact design of an electric motor with a wet takeup clutch replacing a torque converter. By design, damper and electric motor are located in the dry area. Alternatively, the electric motor can be located in the wet area. However, the improved cooling of the electric motor has the disadvantage of drag losses caused by the oil in the gap between stator and rotor. As with the above-mentioned amendment of a combustion engine by an electric motor and conventional clutch, drivetrains with double-clutch transmissions can also be extended to hybrid systems. Here too, packaging space is usually not sufficient in front-wheel drive vehicles with transverse engines. A compact design is achieved by connecting the electric motor to the even gears. An additional clutch is no longer required as the already present double-clutch can perform the takeup function and also disengage the deactivated combustion engine. This setup is a hybrid variant named ESG and allows the realization of all hybrid functions with very little effort, but is only feasible in combination with double-clutch transmissions. Fig. 9 shows the layout of the ESG driveline. The electric motor starts the combustion engine via the clutch of the even gears and simultaneously transmits a part of its torque to the driveline via the engaged clutch of the first gear. The driver feels the instant response of the vehicle even before the combustion engine has picked up speed. This fast response is realized by electric actuators which completely engage the clutches while the vehicle is at standstill, independently of the driveline speed. Fig. 9: Layout of the ESG driveline Fig. 8: Wet-clutch design The clutch is supplied with oil via two separate channels. The required cooling-oil flow is supplied from a channel in the transmission input shaft. The supply of the actuation Since the air-conditioning compressor is directly coupled to the electric motor, the recuperated energy can either be supplied to the electric system or to the compressor. This layout also facilitates the operation of the A/C system with the engine off, because the electric motor can directly drive the compressor when the clutches and synchronization elements in the transmission are disengaged. With respect to drivetrain dynamics, the activation and deactivation of the combustion engine while driving is the most critical challenge. Especially the engine restart

5 / 5 following accelerator-pedal commands requires a very quick response. The disengaging of the combustion engine is performed after a defined period by a hand over of the clutch torque K2 to the electric motor. After the disengagement of the clutch, the combustion engine speed falls to zero and the electric motor, acting as an generator, handles the deceleration of the vehicle. Fig. 9 shows the details of the driveline layout with the geometries of even and uneven gears as is typical for double-clutch transmissions. The electric motor is located parallel to the axis of the transmission. Torque is transmitted from the fixed pinion of the 4 th gear at a ratio of approx. 1.2. LuK has built a prototype vehicle with said transmission and a 1.3-l Diesel engine. The electric motor of this prototype (fig. 10) has an output of 10 kw, which is absolutely sufficient for recuperation purposes in this performance class and for this vehicle, which is classified as a mild hybrid, accordingly. By upscaling the electric motor, a full hybrid can be realized with this layout. The A/C compressor is directly located below the electric motor and is driven by a poly-v belt. This connection facilitates air conditioning during recuperation as well as during standstill. 0% 5% 10% 15% 20% city 14,5 cycle: NEDC urban 6,5 combined 12,0 Basis: Vehicle with DCT (4% improvement to MT) Stop / Start 4% Recuperation 7% E - machine efficiency 1% Opel Astra 1,3 CDI; 170 Nm, 51 kw 1350 kg 5-speed ESG 10 kw E - machine; mild hybrid Fig. 11: Fuel consumption 5. REFERENCES (1) W. Reik; world of hybrids; 8 th LuK Symposium 2006 (2) D. Kraft; hybrid drivelines; Aachen Symposium 2006 Fig. 10: View of prototype After completion of the functional tests, the fuel-savings achieved with the prototype were measured, see fig. 11. The set-up is based on a double-clutch transmission with already highly optimized fuel-saving properties. The implemented hybrid function, consisting of start/stop function and brake-energy recuperation, yields a 12 % fuel-saving in the NEDC mixed-cycle test drives.