What is a mild hybrid?

The future for mild-hybrid vehicles rests in the hands of no less a brand than Mercedes-Benz.

While other companies are going 'full hybrid' or 'plug-in', Mercedes-Benz has developed a sophisticated mild-hybrid powertrain (including an in-line six-cylinder engine and 48-volt electrical system) for a range of models – from mild Mercedes-Benz C-Class variants through to the storming Mercedes-AMG E 53.

A mild hybrid set-up commonly combines an electric motor housed between the internal-combustion engine and the automatic transmission and a relatively small battery pack. The electric motor provides additional torque for performance applications (or perhaps even overtaking); it can also start the engine and it can produce electric current on the move – just like an alternator – to keep the battery charged.

Some mild-hybrid systems can also recover energy from braking or at least ‘recuperate’ electric energy when the need for extra engine power is not required. Some mild hybrid set-ups also see systems like air-conditioning and even oil pumps electrically powered, rather than mechanically powered.

But the electric motor in a mild-hybrid system does not turn the drive wheels independently of the petrol engine. And that's an important point; it's what technically distinguishes a mild-hybrid car from any other type of hybrid car, including a plug-in hybrid.

The mild-hybrid car has been around for a long time, but in the olden days no one distinguished between different types of hybrid, unless they were automotive engineers.

An early example of a mild hybrid was the first-generation Honda Insight from 2001. This was a quirky two-seat liftback coupe that lacked the practicality and affordable pricing to make much of an impact in the Australian market.

Honda persisted with the mild-hybrid approach in cars like the Civic Hybrid and second-generation Insight in the face of the full-hybrid Toyota juggernaut.

With Toyota bringing down the price of full-hybrid models – to the point where just $1500 currently separates a hybrid Corolla from a conventional model trimmed to the same specification – the prospects of mainstream, affordable cars migrating en masse to mild-hybrid drive systems look less likely. If the R&D budget's going to be blown out in the quest for emissions reduction, one might as well make it worthwhile.

It's popular wisdom these days that developing mild-hybrid technology involves the integration of a 48-volt electrical system to support an electric motor (a starter generator) for constant start/stop operation and for regenerative power recovery as well as supplying motive power. This is a combination of operating modes that demands a high-voltage system.

• Combines electric motor/generator with internal-combustion engine
• Torque from electric motor supplements output from internal-combustion engine
• Electric motor does not operate independently of the internal-combustion engine
• Electric motor can operate as a combined starter/alternator

What made the mild-hybrid idea attractive in the first place was low weight and low manufacturing cost. If the lithium-ion battery didn't need to be large, it could be relatively light, as could the downsized petrol engine and the electric motor – which didn't have to do all the work propelling the car. A lighter car should be more fuel-efficient.

The theory was somewhat flawed. Honda's second-generation Insight was a close competitor to the third-generation Toyota Prius. Shorter by 57mm and narrower by 50mm, the Honda was also 165kg lighter than the Toyota.

Unfortunately, that didn't translate to competitive fuel consumption or environmental sustainability. According to the Green Vehicle Guide, the fuel economy of the Prius tested in an urban cycle was 3.9L/100km. For the same test, the Honda could come no closer than 4.9 – a full litre more. In the real world, the difference was greater still: more like 1.5 to 2.0 litres.

Parallel hybrids:
Combustion engine and electric motor supplying torque to the drive wheels – jointly or independently

Mild hybrids ('Power-assist hybrids'):
A subset of 'parallel hybrids', but without an electric-only 'EV' driving mode.

Full hybrids ('Strong hybrids', 'series/parallel hybrids' and 'power-split hybrids'):
Both electric and combustion power sources drive to the wheels and recharge the battery – often at the same time.

Plug-in hybrids:
Full hybrids, but with the further option of recharging the (larger) battery from an electric powerpoint as well as from the internal-combustion engine

Range-extended electric vehicles ('Series hybrids'):
An on-board combustion engine keeps the battery charged for the electric motor to turn the drive wheels.