Drive Technology New Mild Hybrid System from Mercedes: The Trick With the Clutch

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The new Mercedes CLA uses a new 48-volt hybrid system. Thanks to a larger battery, e-drive in the transmission, and recuperation in all gears, it drives purely electrically in many situations.

Almost all manufacturers are electrifying their combustion engines because significant efficiency improvements can no longer be achieved with traditional measures like downsizing or friction optimization. Mild and full hybrids reduce consumption, make everyday driving smoother, and help meet future fleet regulations. The 48-volt technology is considered a particularly pragmatic approach: it costs significantly less than a high-voltage hybrid but performs almost as well in urban traffic.

Mild hybrids support the combustion engine with recuperated energy. Usually, they function as belt-driven starter-generators that briefly boost and handle start-stop operations. In the new CLA, Mercedes takes the mild hybrid concept to a new level with a larger battery, an e-drive in the transmission, and recuperation in all gears.

Three Clutches Aim to Improve System Efficiency

The Stuttgart-based company relies on an unusually sophisticated concept, where the electric motor operates directly within the eight-speed dual-clutch transmission. A third clutch completely disconnects the combustion engine from the drivetrain. "Combustion, mechanics, transmission, and hybridization—we wanted a system that is optimized as a whole," says engine developer Heiko Schilling. Thus, the CLA's concept not only serves as a start-stop aid or to assist the combustion engine during acceleration and starting. Rather, the CLA can travel longer distances purely electrically in urban traffic.

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This is made possible by an unusually large battery for 48-volt systems with 1.3 kWh of usable energy. The energy storage allows for maneuvering, quiet cruising, and is quickly recharged during recuperation with up to 25 kW of power, and this in all eight gears.

The hybrid transmission itself is one of the most intricate components of the system. The electrified eight-speed dual-clutch transmission combines a standard dual-clutch with a third clutch. This completely disconnects the gasoline engine from the drivetrain and enables pure electric driving. Additionally, the combustion engine can be entirely started via the electric motor and separation clutch. A conventional pinion starter is therefore no longer necessary.

All Gears Are Electrically Assisted

The electric motor itself is positioned between the two transmission shafts. Thanks to the triple clutch, the CLA can boost and recuperate in all gears—a technical advantage that neither BMW nor Audi offers. "Other transmission concepts only allow electric driving in either the even or odd gears," explains the engine expert. Additionally, the electric motor, at 22 kW power and 200 Nm, is relatively strong, meaning the combustion engine takes over less frequently. For example, the electric motor in the Audi A3 1.5 TSI delivers only 50 Nm of torque. The 1.6 quarts engine in the BMW 220 Gran Coupé is only supported electrically with 15 kW and 55 Nm. The batteries of BMW and Audi are also significantly smaller.

The downside of Mercedes' complex solution becomes evident in practice. During spontaneous power demand, the interaction of clutches and hydraulics does not always function smoothly. When accelerating, the four-cylinder audibly revs up, but the thrust initially comes exclusively from the electric motor. The combustion engine is running but is not yet connected to the transmission. Only when the clutch engages does it take over.

In terms of consumption, the effort pays off. On paper, the values of the 115 kW/156 hp and 135 kW/184 hp versions, at 48 mpg , are roughly on par with the BMW 220 Gran Coupé (1.4 gallons) and the Audi A3 1.5 TFSI Automatic (1.3 gallons ). In practice, however, the Mercedes is likely to be more efficient than the competition due to its higher electric share. During an initial test drive, the Mercedes behaved more like a small full hybrid. It frequently drove electrically, coasted repeatedly up to around 60 mph, and noticeably recovered energy, so that after a relaxed 60 miles, the onboard computer showed 1.1 gallons.