Volkswagen Golf Mild Hybrid Prototype

What's this?

It might look like a conventional Volkswagen Golf Estate, but what lies underneath its skin is what the German manufacturer calls its Mild Hybrid system. So what makes up a Mild Hybrid? Well, at the rear of the car is a small lithium-ion battery, roughly around the size of a shoebox, which stores and delivers energy in much the same way as a larger battery would in a typical hybrid or electric vehicle.

The big difference is that this energy store doesn't go towards propelling the car at low speeds - quite the opposite in fact. Under the bonnet is a conventionally fuelled turbocharged petrol engine (that's all we were told and no, we weren't allowed to look) linked up to the familiar seven-speed DSG automatic gearbox. Also fitted to the engine is a belt-driven starting generator (RSG), which works separately to the standard engine starter motor.

How does it drive?

From starting the engine to travelling at town speeds the Golf Mild Hybrid prototype drives just like a standard Golf model. Its party trick comes when, at higher speeds - say on a motorway for example - when the driver lifts off the throttle the car almost instantaneously engages a 'Coasting' mode whereby the petrol combustion engine shuts off and the car free-wheels.

During this coasting period with the engine off, the lithium-on battery powers all of the ancillary systems of the car, such as the power steering, braking and air conditioning. During this period a monitor in the dashboard's infotainment screen can demonstrate what power is being used. The amount of energy recovered during the regenerative braking process has been significantly increased in order to have the optimum levels of charge stored to be used during the next coasting cycle.

As the car's speed naturally decreases the DSG gearbox continues to have the ability to shift between ratios thanks to an electric oil pump, thus ensuring that, as soon as the combustion engine restarts, it has the correct gear ratio already selected. Unlike a conventional stop-start system, which requires the engine to be at zero revs before being able to re-start, the Mild Hybrid system can restart the engine at any speed, should the driver re-apply the throttle. This is made possible by the belt-driven starting generator that operates faster and quieter than a conventional starter motor.

Verdict

Although the system remains a work in progress it already felt very refined when driven, though engineers say there is more development work planned. Overall the system adds less than ten kilograms of additional weight, including the lithium-ion battery. When quizzed on the projected fuel savings in comparison to a conventional stop-start system of today, the engineers claim that a drop in fuel consumption of between five to ten per cent can easily be achieved. No exact timeline has been announced for when this system will be available, but it is expected to start appearing in cars within three years, probably first in the next generation Golf.