Explained: Mercedes-Benz EQ-boost

What is it?

A mild hybrid system for Mercedes’ latest-generation straight-six powertrains. Built around a 48-volt electrical architecture, a powerful integrated electric starter motor/generator provides an acceleration boost, teamed with an electric compressor in AMG applications that fleshes out the bottom of the torque curve while the conventional turbo spins up.

The electrically assisted six powers the Merc-AMG CLS53 four-door (p28 of Wheels Magazine) and E53 Coupe and Cabriolet as a ‘starter’ series of AMG’s larger cars, like the V6-powered 43 range, but packing far more tech. It also features in non-AMG Benzes such as the CLS450.

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How does it work?

The EQ Boost system’s key piece of hardware is an integrated starter-generator (ISG); a compact motor/generator sandwiched between the engine and the transmission that acts as both a starter motor and a hybrid assist motor, as well as allowing kinetic energy regeneration. Under hard acceleration the EQ Boost system augments the engine with up to 250Nm and 16kW. It also enables coasting, an extension of Merc’s ‘sailing’ mode, where the engine drops temporarily out of the equation to help save fuel.

In the CLS53, that engine is a 3.0-litre straight-six with an exhaust-driven turbo plus an auxiliary 48v electric compressor. The 48-volt power supply that makes the ISG and electric turbo viable already features in the S-Class, and will soon filter into other Mercs. These models, like the EQ Boosted CLS53, will supply a conventional 12-volt system for cabin accessories without a separate 12-volt battery.

Why does it matter?

Most importantly in an AMG, EQ Boost brings more immediate, harder-hitting performance than a turbocharged internal combustion engine of a given capacity could deliver on its own. With EQ Boost at work, the engine hits peak torque from idle within two seconds under full throttle. The 2999cc straight-six packs 320kW and 520Nm, and the ISG can bolster the bottom end by up to 250Nm. In addition, the electric auxiliary compressor can build charge pressure almost instantly, to fill a torque hole while the conventional turbocharger comes on stream. There are multiple side benefits.

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By recuperating kinetic energy, the system increases the car’s efficiency. The 48-volt system can also power the water pump and air-conditioning compressor, eliminating the need for belt-driven accessories, reducing drag losses and freeing space. Powering the motor on 48 volts requires a quarter the current it would with a 12-volt system, so the wiring can be thinner and lighter. Finally, the 48-volt power supply is less costly than plug-in hybrid technology, which will see it deployed widely to the benefit of a broad cross section of Benz and AMG buyers.