For the hybridization of price-sensitive small to compact vehicles with front-wheel drive, the greatest challenges are currently additional cost and development effort as well as limited installation space. “With eAMT, ZF has developed a full-fledged plug-in hybrid drive for front-transverse vehicles. This increases flexibility for vehicle manufacturers. They can use existing platforms to implement conventional drives or plug-in hybrids,” explains Norman Schmidt-Winkel, functional developer of electric drives at ZF. The ZF concept integrates an automated manual transmission and an electric axle drive system on the rear axle into one unit. In some vehicle classes, automatic transmissions are out of the question for reasons of weight, space or cost. In this scenario, the automation of manual transmissions is a great way to significantly increase comfort and efficiency for drivers, as they don’t need to actuate the clutch or change gears.
Thanks to the additional electric drive and intelligent drive management, eAMT’s shift comfort and performance are almost on par with more costly torque-converter or dual clutch transmissions. As soon as the AMT disengages in order to engage a new gear, there is a tractive force interruption. This is normal for automated manual transmissions due to their design. With its Traction Torque Support function, the new eAMT almost completely compensates for this short break in accelerative force. The electric drive on the rear axle precisely bridges this break with a perfectly timed insertion of torque. A ZF eAMT demonstration vehicle, based on a current compact SUV platform, underscores just how well this balance of force between the front combustion engine, the automated transmission and the rear electric drive works in reality: “The driver is absolutely unaware of the complex system sequences and control processes running in the background. When accelerating, only the benefits of completely jerk-free, powerful acceleration can be felt. Previously, these benefits would only be available in much more expensive hybrid vehicles with more complex transmissions,” says Schmidt-Winkel. “We also utilized the eAMT system´s potential for other features that increase efficiency and driving safety.”
Electric thrust and traction from the rear
The electric rear axle drive not only supports gear changes: It will also activate automatically and at lightning speed as soon as additional thrust is required when overtaking, for example, or when all-wheel drive is needed on slippery road sections. In addition, ZF has dimensioned the electric motor in the demonstration vehicle to be so powerful that it has the capacity to move under electric power alone. It then travels in all-electric mode and with zero local emissions. This eAMT operating mode is particularly suitable for urban driving, creep mode in traffic jams as well as maneuvering and parking in general. Conversely, eAMT also enables sailing, i.e., saving energy while gliding along with the combustion engine drive disengaged. This function benefits from the asynchronous machine (ASM) in the rear. Unlike permanent-field synchronous machines (PSM), the former turns without resistance as long as the hybrid manager does not actuate it. The plug-in hybrid also has familiar features such as automatic engine stop and recuperation. With eAMT, manufacturers can freely select the functional scope of future plug-in hybrids and determine how powerful their electrical motors are to be.