Expert knowledge for the electric lion: the driveline

A diesel truck uses a combustion engine that burns diesel as fuel. A battery-powered truck, on the other hand, uses an electric motor that is powered by batteries. The features that the MAN eTruck has in common with the diesel models from MAN are the cabs, frame geometry and rear axle. Apart from this, the eTruck has been completely redesigned.

In the battery-powered truck, much of what we know from the combustion engine has disappeared: combustion engine, diesel tank, complex gearbox, exhaust system, noise and exhaust emissions.

The battery-powered truck brings into play components that are not available in the diesel truck. These include: electric motor, battery packs, charging connections, recuperation system for energy generation, battery management system, complex thermal management, high-voltage components such as air conditioning compressor and air compressor.

In the MAN eTGX and MAN eTGS, a large electric motor is installed centrally in the vehicle. This motor converts the electrical energy from the batteries into mechanical energy. The power of the electric motor is transmitted to the drive shaft via a two- or four-speed gearbox. The gearbox provides the necessary ratio to adapt the engine torque to the respective driving requirements. The drive shaft transmits the power from the gearbox to the rear axles. Like on a diesel truck, the conventional rear axles then distribute the power uniformly to the wheels and thus enable the vehicle to be driven. With this drive concept, MAN can use the proven technology and weight reserves of conventional rear axles and combine these with the advantages of electric drive – such as its higher efficiency and lower emissions. At the same time, the central drive minimises the unsprung masses in the vehicle, which has a positive effect on driving characteristics.

The central drive unit (including electric motor, gearbox, and inverter) has a compact design and has been moved significantly further back to the centre of the vehicle compared with the diesel driveline. This positioning frees up the installation space under the vehicle for other electrical components. A relatively short propshaft transfers the drive torque to the tried-and-tested and highly efficient hypoid rear axle.

The electric motor and inverter are compactly arranged within a common housing. This optimises the electromagnetic compatibility of the vehicle. Other advantages of this design are: electrical robustness with high fault tolerance and vehicle availability, low electrical losses due to short connections between inverter and electric motor.

The synchronous motor with permanent magnet rotor is driven by the rotating magnetic field in the stator windings. For this, AC voltage must be applied to the motor. Permanent magnet synchronous motors have a higher power density (power to mass ratio), higher efficiency and higher accuracy compared with other motor types such as asynchronous motors. The electric motor and gearbox are integrated into a common oil circuit. This serves to cool the active electric machine components as well as to lubricate the gear wheels, anti-friction bearings and shift elements and cool the transmission parts. Cooling as part of the vehicle’s thermal management makes it possible to achieve a high continuous output and service life.

For the driver, the electric motor is particularly noticeable due to its powerful performance with high starting torque. Compared with the D26 diesel engine, the maximum torque of the electric motor of the new MAN eTruck is approx. 50 percent lower, but it is available over a significantly larger rpm range: from standstill to very high revs. As a result, the number of gears can be reduced to two or four – without any loss of driving performance. Due to the higher rpm level, the MAN electric motor achieves a performance level comparable to the D26 diesel engine.

The MAN eTruck needs a gearbox because the electric motor works most efficiently at low motor speeds and high torques. At higher driving speeds, the ratio between motor speed and torque would be unfavourable without gearshifts. The gears of the gearbox enable the best possible efficiency and performance over the entire speed range. This increases the maximum range of the vehicle with one battery charge. Engaging a suitable gear supports the highest possible energy recovery during recuperation.

The target rpm range for the economical design of the electric driveline is considerably wider than for a diesel engine. For this reason, only four gears are required for the electric drive. The gearshifts are barely noticeable when driving. The tractive power remains virtually uninterrupted over the entire speed range. A clutch for disconnecting the driveline during gear changes is not required, as the speed can be reliably synchronised during gear changes by means of an inverter. A clutch is also not required for moving off. The electric motor has a fixed connection to the gearbox. For reversing, the electric motor simply changes direction of rotation. An additional reverse gear wheel is not required for this.

The inverter is an important component in the drive system of an electric vehicle. Its main task is to convert the direct voltage supplied by the battery into an alternating voltage that can drive the electric motor. The inverter is therefore a kind of transformer that converts the energy provided by the battery into a form that can be used by the electric motor. In addition to converting the DC voltage into AC voltage, the inverter is also responsible for controlling the motor. It adjusts the current frequency and amplitude to the motor to achieve the desired power and speed. The inverter can also change the direction of the current to enable recuperation, i.e. the recovery of energy during braking or deceleration.

If the kinetic energy is used to charge the battery packs during deceleration, this is called recuperation. Advantage: Energy costs are reduced while the range is simultaneously increased. This also pays off in urban traffic with frequent stop-and-go situations. In addition, the load on the service brake system is reduced by recuperation. The system works like this: The electric motor usually converts the electrical energy into drive power for the vehicle. When driving downhill or to reduce the speed, the function can be reversed. The motor works as a generator and converts the kinetic energy of the vehicle back into electrical energy. This charges the batteries. The process of energy recovery is shown to the driver on the main instrument. The power meter shows the driver how much they can decelerate without using the service brake. In this way, the driver can control recuperation by pressing the brake pedal in such a way that the greatest possible range gain is achieved.