The Potential of a Hybrid Powertrain in Fuel Consumption and Thermal Drive-Off Element Load for Drive-Off Procedures Regarding Driving Styles

Hybrid powertrains derive fuel consumption benefits from using an electric motor. These benefits are more significant in city traffic than on the highway and depend on the vehicle and the driving style. Further detailed research on the fuel consumption of hybrid powertrains during drive-off procedures is rarely found in the literature. Therefore, this study focuses on analyzing the potential of a mild-hybrid powertrain, in which the electric motor is integrated with the transmission (P2.5 concept). The fuel consumption and thermal load in the drive-off element, a wet frictional clutch, are analyzed for a city cycle with a focus on the first drive-off procedure for different driving styles. Particular attention is paid to the influence of different driving styles on the torque demands of the electric motor. These simulations are realized with a so-called backward-forward model. The backward-facing part enables following a given driving cycle without considering a driver model. It calculates the required torque based on the driving cycle and the deviation between the target and current speed. The forward-facing part extends the simulation by dynamic effects, which are initially neglected but are very relevant for drive-off procedures. It also ensures that the limits of the drives and the actuators are not exceeded. As a control strategy, an ECMS (Equivalent Consumption Minimization Strategy) supplemented with a penalty costs function for a high engine torque jerk coordinates the power distribution between the internal combustion engine and the electric motor. Since the investigated powertrain is a mild hybrid, the used electrical energy in the drive-off procedure has to be recovered during further driving. This study shows that the fuel consumption of the internal combustion engine and the frictional thermal load in the clutch can be significantly reduced during drive-off procedures by shifting torque to the electric motor. Nevertheless, these potentials are limited by the maximum power of the electric motor and decrease with more sporty driving styles.