What is the Mercedes EQ Boost mild-hybrid system?

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Mercedes calls their Mild Hybrid technology “EQ Boost”, but don’t let the term ‘mild’ fool you — this hybrid system is built to perform.

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One of its applications is the Mercedes-Benz M256 powerplant: a 3-litre straight-six turbocharged gasoline designed specifically to be electrified. I’ve now logged about 6,000 testing kilometres with this engine in 3 different E-Class models over the past year or so. The most recent of these was the E-Class Sedan.

On its own, the configuration of M256 engine used by my tester develops 362 horsepower and 369 pound-feet of torque. Further power is added by the EQ Boost system, which uses electricity to spice things up by adding a 21-horsepower boost, cranking up low-end torque, and saving fuel in the process.

Here’s how it works.

First, the alternator (basically a generator), and the starter (basically a motor), are replaced with a single unit called the Integrated Starter Generator (ISG). This compact unit is both a motor and a generator, and it makes its home inside of the transmission. A vital part of the vehicle’s driveline, the ISG is connected to the engine’s rotating crankshaft.

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Since the ISG is (in part) an electric motor that can spin the engine’s crankshaft electrically, it also serves as the engine starter — making a conventional starter motor redundant. Drivers will notice a subtle audible clue about this every time they start the engine, which quietly breathes into action, free of any sound or sensation of a conventional engine starter.

So, to summarize so far, EQ Boost is a form of Mild Hybrid system, and the ISG, a powerful electric machine that’s connected to the engine, is the system’s heart.

When it’s working as a motor, the ISG can apply electric power directly to the engine, boosting output without burning more fuel.

When it’s working as a generator, the roles switch. Now, the ISG is a generator, driven by the engine’s spinning crankshaft. The electricity generated is stored inside of a powerful 48-volt battery.

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Press the throttle, and electricity moves back from the battery to the ISG, boosting performance.

Lift the throttle to coast or brake, and the spinning action of the engine is used to drive the ISG, generating power while you slow down. Speeding up uses electricity, and slowing down creates it. This is all seamless and automatic, with no driver decision-making required. Drivers can monitor the system via a small gauge in the instrument cluster that displays the current status of the system. In some situations, drivers may notice the EQ Boost system switch from POWER (the motor boost) to CHARGE (the generator), many times per minute. The effect is a continual stream of power from battery to ISG, or vice-versa.

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EQ Boost, with its ISG, is called a mild hybrid because the system is relatively more simple and complex than a ‘full’ hybrid. Rather than building an entire hybrid driveline, a mild hybrid setup like EQ Boost allows powertrains to be electrified by adding some additional hardware to an existing powertrain. 

In a mild hybrid like this, the electric system can’t drive the car directly. If you’re accelerating, the gasoline engine is running. This setup intends to give drivers stronger acceleration without using additional fuel, and smooths out power delivery by flattening (and fattening) the engine’s torque curve.

The ISG boosts hardest from very low revs, ie. the millisecond you touch the throttle. This is exactly the operating range where turbocharger lag lives. By strategically overlapping and blending the immediate snap of electric motor assist with the turbocharged power surge that arrives a few hundred revs later, drivers get a match made in heaven where throttle response and smoothness are concerned.

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Best of all, there’s no driver bandwidth required: you just get in and drive.