Dual Mode Vehicle with In-Wheel Motor: Regenerative Braking Optimization

Dual Mode Vehicle with In-Wheel Motor: Regenerative Braking Optimization — To meet the growing need for mobility of people and goods while massively reducing CO2 emissions, the electrification of vehicles is an essential solution. The variety of vehicles and their use results in innovative solutions for adapted architecture. This is especially true for light commercial vehicles where the objective is to promote full electric use in urban conditions (zero emission vehicle) while maintaining significant range autonomy on road. The project VelRoue, a partnership between Renault, Michelin and IFP Energies nouvelles, aims to develop a dedicated dual-mode vehicle using a conventional thermal powertrain on the front axle and in-wheel motors on the rear one each powertrain to its use and makes it possible to achieve a low level of homologation CO2 emissions. In addition to features that meet the specific use of a commercial vehicle, in this paper we will particularly demonstrate the benefit of such an architecture to optimize the regenerative braking while ensuring a safe dynamic behaviour. Oil & Gas Science and Technology – Rev. IFP Energies nouvelles Copyright © 2013, IFP Energies nouvelles DOI: 10.2516/ogst/2012013 RHEVE 2011: International Conference on Hybrid and Electric Vehicles RHEVE 2011: conférence internationale sur les véhicules hybrides et électriques IFP Energies nouvelles International Conference Rencontres Scientifiques d’IFP Energies nouvelles Oil & Gas Science and Technology – Rev. IFP Energies nouvelles 2 INTRODUCTION In the context of a massive reduction in CO2 emissions related to the mobility of persons and goods, vehicle electrification is one of the most effective solutions in combination with a production of “clean” electricity. This breakthrough technology can be deployed in different ways, from a micro hybrid with an electric start & stop system to full electric vehicles (Fig. 1). This variation of architectures is justified by a proper adaptation of vehicles to different societal uses as well as infrastructure needed for plug-in vehicles. The VelRoue project, a partnership between Renault, Michelin and IFP Energies nouvelles funded by the ADEME in the framework of the AMI program, aimed to develop an architecture tailored to the needs of a light commercial vehicle, such as the Renault Trafic or Kangoo. The choice was made for a dedicated dual mode architecture using in-wheel motors on the rear axle. This solution combines the advantages of an electric vehicle in urban conditions (no pollutants, CO2 emissions and noise), with a significant preservation of the range autonomy of a conventional thermal vehicle while offering a competitive price compared to full electric vehicles. Many technological issues related to the use of in-wheel motors had to be address, such as their integration and impact on vehicle dynamics. Control and supervision of both powertrains had to be developed taking into account the desired performance (energy management, CO2 emissions, dynamic) and safety operation. In the first section, this architecture and the impact of requirements on individual component design will be discussed. The second one will present the vehicle simulator used for development and validation of supervision and control strategies. We will then focus on one valuable feature of the use of in-wheel motors: namely the new prospects of regenerative braking optimization and wheel slip control due to the proximity of the electrical motors to the wheel. Section 3 will then present how the supervisor can integrate all the constraints to optimize the use and coordination of all the actuators and Section 4 how the use of in-wheel motors can improve estimation and control of road friction. 1 DUAL MODE ARCHITECTURE 1.1 Objectives With currently available technologies, the deployment of plug-in electric vehicles is widely being considered for urban use in order to reduce CO2 emissions. However, the limit of range imposed by the current battery capacity makes them less suitable for extra-urban use. The objective here is to develop a dual-mode architecture particularly suited to the needs of small commercial vehicles users. The vehicle must be comparable to a plug-in electric vehicle (ZEV) in urban use, to a combustion engine vehicle in suburban use and global optimization of the architecture should allow for very low CO2 emissions and a competitive price compared to electric vehicles. The dual-mode solution is based on the development and optimization of two separate powertrains dedicated to each use (Fig. 2): – a thermal driving mode with a conventional thermal powertrain on the front axle; – an electric driving mode with two in-wheel motors on rear wheels. (1) Agence De l’Environnement et de la Maîtrise de l’Energie. (2) Appel à Manifestation d’Intérêt. (3) Zero Emission Vehicle. Figure 1 Different architectures for electrified vehicles. Figure 2 Dual mode vehicle with in-wheel motors. G. Le Solliec et al. / Dual Mode Vehicle with In-Wheel Motor: Regenerative Braking Optimization 3 This innovative concept will be validated on a Renault Kangoo demonstration car. The simultaneous use of both powertrains will be limited to a minimum. When the driver chooses the electric mode, the engine will still be stopped. If the battery is discharged, the customer will be informed of the need to switch to thermal mode. In this mode, the driver will have a conventional thermal vehicle equipped with a robotized gearbox, except for certain operations where the use of both powertrains will be imposed for optimization. For example, during vehicle pull-away or acceleration requests, electric motors will assist the engine to prevent the gearbox from downshifting. Similarly, during deceleration and braking, the electric motors will be used for regenerative brake. Limiting ZEV mode to urban use allows for the adaptation of required battery capacity compared to a fully electric vehicle, with the ability to then reduce its volume and cost. On the other hand, the use of an in-wheel motor, less intrusive than a motor mounted on a powertrain as in a hybrid vehicle, can reduce the cost of platform adaptation and maintain relevant cargo volume, essential in the case of a commercial vehicle. This dual-mode architecture therefore represents an attractive economic alternative significantly reducing CO2 emissions, with a target at 40 gCO2/km on NEDC cycle, while maintaining a large cargo volume. 1.2 Thermal Powertrain The dedicated thermal powertrain on the front axle must be optimized for road and highway use with a limited cost. The purpose was not to design specific components, but to adapt existing ones. 1.2.1 Engine The candidate engine must have good efficiency at medium and high loads. A supercharged spark-ignition engine meets this requirement and the selected engine is a Renault D4Ft with a displace volume of 1.2 liters. Supercharging can reduce consumption without incurring significant additional costs but with two drawbacks: the need to manage the tradeoffs between high and low load performances and thermal limitations at high loads resulting in over consumption. At high loads, the thermodynamic conditions during combustion cause the emergence of knock phenomenon potentially destructive to the cylinders. This is usually prevented by delaying the combustion with spark advance. However, when this is done the combustion phasing is no longer optimal and reduces engine efficiency, increasing consumption and reducing engine performances. The use of E85 fuel (more than 85% of ethanol) has a main advantage for this application. Its octane number, representative of knock resistance, is much higher than a standard gasoline fuel. It is therefore possible to keep optimal combustion over the entire operating range, while retaining minimum consumption and improving engine performance (Fig. 3). In order to prevent damage on a three-way exhaust catalyst, one needs to control and limit its upstream temperature. Delaying combustion at high loads to prevent knock ignition will increase exhaust temperature. It is usually prevented by increasing injected mass more than needed to get stoichiometric conditions, with directly increased consumption at those loads. Using E85 to prevent knock will then naturally reduce the need of increasing injected fuel (Fig. 4). 6000 5000 4000 300