Plug-in hybrids are a key technology on the road to a locally emission-free future for the automobile. They offer customers the benefits of two worlds: in town they run in all-electric mode, while on long journeys they benefit from the range of the combustion engine. They make the vehicle more efficient overall because they can firstly recover energy and secondly allow the combustion engine to run in favourable operating ranges. Mercedes-Benz is advancing development of the third generation of its plug-in hybrids under the banner EQ Power and also combining hybrid drive with its sophisticated four-cylinder diesel engine for the first time. A demonstration of the possibilities of the Mercedes-Benz hybrid strategy, which is a central part of the EQ brand and has quickly led to a wide range of different plug-in hybrids.

As the strengths of plug-in hybrids come to the fore especially in larger vehicles and on mixed route profiles, Mercedes-Benz is now opting for this powertrain concept from the C-Class upwards. A key advantage in the strategic hybrid initiative is offered by Mercedes-Benz's intelligent, modular hybrid concept: designed to be scalable, it can be transferred to a large number of rear-wheel-drive model series and body styles as well as left- and right-hand-drive variants. Hybridisation makes the combustion engine more efficient while delivering a high degree of dynamic performance and driving pleasure. And all new plug-in hybrids from Mercedes-Benz now boast an all-electric, locally emission-free range of around 50 km (NEDC).

Hybrid traction head in torque-converter transmission: more powerful, more compact

At the heart of the mechanical system in all third-generation plug-in hybrids with combustion engine is the 9G-TRONIC 9-speed hybrid transmission. It augments the familiar nine-speed automatic transmission with torque converter by adding a hybrid traction head with integrated converter, a separating clutch and a powerful electric motor. All of the benefits of the basic transmission are retained, including the exceptional motoring comfort, barely perceptible gear shifts and, in the case of the E-Class, a high towing capacity. The most powerful version of the transmission with a transmissible torque of up to 700 Nm is used for hybrid operation to enable the combined power of the combustion engine and electric motor to be used if required. The exceptionally efficient 9G-TRONIC 9-speed hybrid transmission helps to improve the efficiency of the powertrain, especially in electric mode.

One of the main benefits of the new hybrid traction head is its compact design, which was achieved thanks to the innovative integration and connection of the separating clutch, torsional vibration damper and torque converter lockup clutch within the rotor of the electric motor. This results in the transmission being slightly longer than the 9G-Tronic basic transmission, but only by 108 mm.

In contrast to the second-generation traction head, in which the electric motor was connected directly with the transmission input and a wet start-off clutch was used as a starting and separating clutch, a torque converter between the electric motor and the transmission now takes care of starting off. The separating clutch has been improved with respect to drag torque, now that it no longer has to deal with starting off, in order to reduce losses during electric mode.

To optimise vibration decoupling, the hybrid transmission includes two torsional vibration dampers that damp the combustion engine excitations. The first vibration damper is installed between the engine and transmission, while the second is integrated in the torque converter.

90 kW electrical output for all plug-in hybrids

Developed together with Bosch as part of the EM-motive joint venture, the electric motor was newly designed for the 9G-TRONIC plug-in hybrid transmission and is based on the principle of a permanently excited synchronous motor as an internal rotor. The likewise new, significantly enhanced power electronics have allowed significant increases in power and torque density. A peak output of 90 kW and a start-off torque of 440 Nm make for an effortlessly superior driving feel even in all-electric mode and allow top speeds in excess of 130 km/h. The stator is permanently integrated in the traction head housing, while the rotor is between the powerflow of the separating clutch and transmission input. On-demand stator and rotor cooling allows use of the electric motor's peak and continuous output without any problems.

More energy stored in greater density, for a longer electric range

A key factor in increasing the electric range to around 50 kilometres is the nominal capacity of the new lithium-ion battery, which has increased to 13.5 kWh, while the size of the battery has been kept the same. The evolution of the cell chemistry from lithium-iron-phosphate (LiFePo) to lithium-nickel-manganese-cobalt (Li-NMC) made it possible for the cell capacity to be increased from 22 to 37 Ah. The highly efficient battery system comes from the Daimler subsidiary Deutsche ACCUMOTIVE. The power electronics are housed in the engine compartment.

On-board charger with 7.4 kW capacity

The new on-board charger more than doubles the charging capacity from 3.6 kW to 7.4 kW and strikes an ideal compromise between size, weight and charging capacity. A discharged battery can thus be fully recharged in 1.5 hours at a wallbox, for example in the convenience of one's own home. The same is possible in around five hours even at a conventional domestic power socket.

Electrical pre-entry climate control for the interior

The high-voltage on-board electrical system supplies not only the powertrain components and the vacuum pump of the regenerative braking system, but also the electric refrigerant compressor and the high-voltage heater booster. Both allow pre-entry climate control of the interior not only in summer but also in winter because they can also operate without the combustion engine.

Intelligent operating strategy

Third-generation hybrid technology supports the driver with further-improved, intelligent powertrain management. It comprises all processes that access the on-board energy supply and influence consumption, including the hybrid operating strategy, i.e. the interaction between electric motor and combustion engine, the transmission gear shift strategy, thermal management, i.e. the energy-efficient control of the cooling circuit of the combustion engine and electrical components to maximise the electric range, recuperation management and, in the case of the diesel hybrids, even particulate filter regeneration. Through the extended use of data from the navigation system and information from the camera and the radar sensors, third-generation hybrid vehicles can look ahead well beyond the driver's field of vision and adjust to the specific speed/route profile. Events such as negotiating towns on the way to the destination are taken into account when planning the available electrical energy, and during recuperation and thermal conditioning of the powertrain components.

ECO Assist helps to save energy

The new hybrids provide the driver with comprehensive assistance in terms of predictive driving and fuel-saving: the driver is prompted when appropriate to come off the accelerator, e.g. because there is a speed limit approaching, and assisted by innovative functions such as 'gliding' and energy recovery based on anticipatory data. For this purpose, navigation data, traffic sign recognition and information from the intelligent safety assistants (radar and stereo camera) are linked and processed.

ECO Assist takes the following traffic situations and information into account in its driving recommendations and efficiency strategy:

• Route profile (bends, junctions, roundabouts, gradients)
• Speed limits
• Distance from vehicles ahead.

ECO Assist continuously generates coasting simulations in the background: depending on the state of charge of the battery and the traffic situation, it computes whether the vehicle should ideally be allowed to coast with the lowest possible driving resistance with the driver's foot off the pedals, or whether it should be decelerated so that the battery can be efficiently charged (recuperation). (For details and examples, see "Under the microscope: ECO Assist")

The haptic accelerator helps the driver to achieve an economical and comfortable driving style. A variable pressure point in the pedal, for example, tells the driver that the maximum electric power is being delivered. If the driver continues to press the pedal beyond the pressure point, the combustion engine kicks in. A perceptible resistance in the haptic accelerator advises the driver to take their foot off the accelerator. If the driver follows this recommendation, the combustion engine is switched off and decoupled from the powertrain.

Within the limits of the system, ECO Assist controls the overrun mode according to the situation as soon as the driver takes their foot off the accelerator. The driver is also given a visual prompt to do this: by a "foot off accelerator" symbol in the central display (or, if installed, in the Head-up Display). At the same time, a diagram gives the driver the reason for the recommendation (e.g. "Junction ahead" or "Gradient ahead").

To increase the driver's motivation to follow the recommendations of ECO Assist, the on-board computer records how many kilometres/for how much time during a journey the car was driven with the engine off, and shows this in the central display. The reward not only takes the form of reduced fuel consumption, but also an increased electric range.

Performance to suit the driver's wishes: four hybrid modes

Depending on the driver's wishes, four operating modes can be preselected. They are:

• HYBRID: Default setting; all functions, such as electric driving, boost and energy recovery, are available and used according to the driving situation and route profile
• E-MODE: Electric driving, for example in the city centre. The accelerator signals the pressure point at which the combustion engine is started
• E-SAVE: The charged battery is held in reserve to allow driving in all-electric mode later
• CHARGE: The battery is charged while driving.