Under the microscope: The third-generation plug-in hybrids: More power, more range, more fun

Sep 14, 2019
Stuttgart/Frankfurt

The hybrid drive units of the models with a longitudinally installed engine, from the C to the S-Class and the GLC to the GLE, are already the third hybrid generation since the introduction of the S 400 Hybrid in 2009. The current electric engine has been redesigned for the 9G-TRONIC plug-in hybrid drives and is designed according to the principle of a permanently excited synchronous motor as an internal rotor. The likewise new, significantly enhanced power electronics have allowed considerable increases in power and torque density. One of the greatest innovations compared with the previous version is the use of a torque converter with integrated lockup clutch as a starting device, and an additional clutch between the combustion engine and electric motor for all-electric driving.

A peak output of 90 kW (and even 100 kW for the GLE 350 de 4MATIC) and a starting torque of 440 Nm ensure superior performance even under electric power alone, allowing top speeds above 130 km/h (GLE 350 de 4MATIC: up to 160 km/h). The stator is permanently integrated in the traction head housing, while the rotor is between the power flow of the separating clutch and transmission input. On-demand stator and rotor cooling allow use of the electric motor's peak and continuous output without any problems.

More energy with more density: longer electric range

The Mercedes-Benz plug-In hybrids in the current generation have a purely electric, locally emission-free range of around 50 km (NEDC). The GLE even manages a range of over 100 kilometres (NEDC). The rated capacity increased to 13.5 kWh is decisive for this increase in the electric range. The battery of the GLE even has a capacity of 31.2 kWh. 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.

Onboard charger with 7.4 kW output: faster charging

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 with alternating current (AC), for example in the convenience of one's own home (GLE: 3 hours 15 minutes). The same is possible in around five hours even at a conventional domestic power socket.

For direct-current charging (DC) the battery in the compact hybrids can be charged from 10 - 80 percent SoC in around 25 minutes. The GLE has a combined charging socket for AC and DC charging. It is located in the left side wall, symmetrical with the fuel flap on the right side of the vehicle. At corresponding DC charging stations, its battery can be charged in approx. 20 minutes (10-80 percent state of charge (SoC)) or in approx. 30 minutes
(10-100 percent SoC).

Electric pre-entry climatisation of the interior: pleasant temperatures even before starting off

The high-voltage on-board electrical system supplies not only the drive 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.

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

The mechanical centrepiece of all the third-generation plug-in hybrids with a longitudinally installed engine is the nine-speed 9G-TRONIC hybrid transmission. This adds a hybrid traction head with an integrated torque converter, a clutch and a powerful electric motor to the familiar automatic torque converter transmission. All of the benefits of the basic transmission are retained, including the exceptional drive comfort, barely perceptible gear shifts, and a high towing capacity. The most powerful version of the basic transmission with a transferable torque of up to 700 Nm is used for hybrid drive, so that the combined power of the combustion engine and electric motor can be used when required. The 9G-TRONIC nine-speed hybrid transmission excels with very high efficiency, and particularly contributes to improved efficiency while driving under electric power.

For models with a transversely installed engine and the 8G-DCT dual-clutch transmission, a compact hybrid traction head was developed along the same technical lines as the corresponding component for models with a longitudinally installed engine. See the next chapter for details.

The major benefit 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 lock-up clutch within the rotor of the electric motor. All in all the transmission is only 108 mm longer than the basic 9G-TRONIC transmission.

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.

High CO2-saving potential: using the plug-in hybrid correctly

In order to gauge a vehicle's environmental compatibility, the Daimler environmental experts consider the emissions and the use of resources over a vehicle’s entire life-cycle. This is achieved by means of a life cycle assessment (LCA), which records the key environmental impacts – from extraction of raw materials to production and use to recycling. This reveals the following: Already today, and despite the higher cost and effort in production, the eco-balance of plug-in hybrids in terms of CO2 emissions is certainly positive.

In production, a plug-in hybrid of the new Mercedes-Benz generation gives rise to approx. 20 percent higher CO2 emissions than a comparable car with conventional drive, owing to the technological components and especially the high-voltage battery. Consistent use of the plug-in function by regularly charging the battery from the mains, and more efficient operation, mean up to 40 percent lower CO2 emissions on the road, even with the current electricity mix. If the vehicle battery is charged solely using power from renewable energy sources, the reduction in CO2 emissions during normal operation is as high as 70 percent.

Despite the much higher energy use during production, the plug-in hybrid can therefore avoid a large share of the CO2 emissions over its entire life cycle and, in the best-case scenario, accounts for around 45 percent of the total emissions of a combustion engine. Thus, in this case more CO2 emissions during the manufacturing phase is an investment that more than works itself out while driving.

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