48-Volts to Freedom: How VW’s New Mild Hybrid System Works

[This feature is part of an ongoing series of tech explainers. If there are any automotive technologies or cool features you’d like explained, let us know in the comments.]

Last week, Volkswagen announced that it would be bringing a hybrid system to the next-generation Golf when the Mk 8 arrives next year. But this is a mild hybrid system like the one Audi has been expanding across its line, not the full hybrid system offered by cars like the Toyota Prius or the Volkswagen Jetta. So what is a mild hybrid system, and how does the Volkswagen Group system work?

The mild, or micro-hybrid uses a much smaller battery pack than a big hybrid, with lower voltage and little or no pure-electric range. That’s why it’s called a mild hybrid. It’s sort of a hybrid, but not all the way. Some of the electrification, none of the fat. Or cost.

When automakers started talking about electrifying their entire lineups, they were largely referring to this type of hybrid system, not an EV or full-hybrid system. This mild method is cheaper and easier, but it does still deliver fuel economy results.

The Volkswagen Group 48-volt mild hybrid system starts with a combined starter generator that replaces the separate starter and alternator on a conventional car. There is also a lithium-ion battery and a control system to operate everything.

So why 48-volts? It’s a compromise that allows for more power but keeps down costs. A 48-volt system can pass four times the power of a 12-volt system at the same current level. That means more electricity to turn to power by the motor, but also more to be recovered under regenerative braking. It’s important to note, though, that this isn’t the 48-volt electric turbo that Audi is fitting to vehicles like the SQ7 TDI. That’s different, and it boosts low-end power without adding to fuel economy.

Full-hybrid systems can use over 300-volts to allow for even more power, so why are mild hybrids going with 48? Safety. Current regulations state that if the voltage ever goes above 60-volts then heavier shielding for the high voltage wires is needed and that significantly raises the cost. So 48 is the highest voltage they can use without things getting expensive. The automaker can still use similarly sized wiring to that of conventional cars.

The starter generator is a large electric motor driven by a belt attached to the engine’s crankshaft. Think of it like a big electric supercharger, mounted where the alternator used to be on the front of the engine. And unlike the electric leaf blower turbochargers you’ve seen on late-night TV, these ones actually work. Because instead of adding air, they add horsepower directly by turning the belt. In the case of the Audi A8 hybrid, the system adds 16 hp to the engine’s output. This isn’t a tiny alternator belt, it’s a wide and rigid belt that can handle the power transfer. But that extra motive power isn’t really the point.

The large electric motor means that the engine can use start-stop more efficiently. It can spin the engine to idle rpm almost instantly, and without using fuel. Because it can start the engine so quickly and smoothly, the start-stop function can work at speeds up to 14 mph. So coasting slowly up to a light, or in stop and go traffic, can use zero gas.

On the highway, very little power is actually needed to propel the car. So if you’re gliding along at speeds from 34 to 100 mph, the car can shut the engine down. It restarts seamlessly thanks to the electric motor if you ask for more power than the motor can deliver.

Coast down a hill, or brake, and the motor becomes a generator and recharges the 48-volt battery. Some of that power is sent to the 12-volt battery that still powers features like the radio and lights.

Additionally, on top of the smooth engine re-start, the electric motor gives you a boost at low engine speeds. Because it makes peak torque as soon as it starts spinning, the electric motor can boost acceleration before the engine starts to rev. So step on the gas at a light, and the electric motor boosts acceleration until the engine moves into its power band.

So basically the mild hybrid system works by using stored electrical power to shut off the engine in situations where it isn’t really needed, but couldn’t turn off without the electrical boost. It also helps with low-speed pep to improve drivability. The question is, how much fuel does that save?

It doesn’t save a huge amount of fuel, but it isn’t insignificant either. Audi says the savings is about half a gallon per 100 miles. That’s enough to notice more of a difference at the pump than the technology costs to add to the car. Which is the point. Big fuel savings, like EV, full hybrid, or some fancy new tech yet to be developed is expensive. It can put the product too far out of reach. Small changes, like this mild hybrid system, help fill the gap while the other fuel-saving tech develops and gets cheaper.