Many cars will soon have 48-volt electrical systems. They’ll power stop-start motors, hybrid motors, and turbochargers, allowing for smaller engines with better fuel economy and performance. They’ll handle accessories ranging from mechanical or hydraulic power to electric power such as power steering, power brakes, water pump, radiator cooling, and air conditioning.
These will be combination 12- and 48-volt systems with 12 volts for traditional lighting and infotainment, 48 volts for more power-hungry components. But don’t freak out: The 12-volt system isn’t going away any time soon.
What 48 volts can do: low-cost mild hybrid
Bosch, Continental, Delphi, and Valeo are among the key components makers working to provide 48-volt systems to automakers.
Odds are your next car will have a start-stop system, probably 12 volts, and a slightly larger 12-volt battery. When you come to a stop at the light, the car shuts down after a couple seconds. It fires up again with a little hesitation, a couple tenths of a second, on some cars. At a light, the car feels poky. It makes you nervous if you’re at a stop sign trying to get across the street with crossing traffic getting closer. With 48-volt stop-start:
- The starter is replaced with a beefier 48-volt device called a motor generator unit (MGU), belt alternator starter, or belt-driven starter generator.
- A 48-volt lithium-ion battery pack, typically in the trunk.
- A DC-to-DC converter.
Effectively, you’ve got a mild hybrid powertrain, one that works in parallel with the combustion engine. Backers say it provides two-thirds the benefit of a full hybrid at a third the cost. Fuel economy increases by 15 to 20 percent.
What 48 volts can do: e-charger for faster acceleration
A four-cylinder car with a turbocharger has the performance of a V6 without a turbo. But there’s lag here, too, those tenths of a second while the turbo spools up to 100,000-200,000 rpm and forces more and more air into the engine. Virtually every review describes turbo lag as “barely noticeable,” which really means “noticeable” when you want to pass on a two-lane road, or get over the railroad tracks when the signal lights start blinking.
Enter the electric turbocharger / electric supercharger, or e-charger. Rather than wait for exhaust gases to eventually bring the impeller up to speed, an electric motor makes it happen more quickly so that the lag is truly barely noticeable. This, too, requires a 48-volt system.
Audi’s electric supercharger concept cars
Audi has electric supercharger in its Q8 Sport Concept vehicle. Earlier, it put one in the SQ7 TDI (diesel, schematic above). A traditional turbocharger is driven by exhaust gas and a supercharger is driven by an engine-powered belt; both force air under pressure into the car’s intake manifold. Some automakers called their e-charger an electric turbocharger, others an electric supercharger.
Delphi’s lower-cost hybrid system
Delphi is one of the automakers thinking about a multi-component solution to delivering a lighter, more efficient drivetrain. It starts by replacing the starter motor with a 48-volt motor-generator that starts and restarts the car, provides extra power beyond what the combustion engine offers, and recharges the 48-volt lithium-ion battery in the trunk. Delphi’s motor-generator is the same size as a regular starter motor (plus two cooling hoses; photo inset), so it can be added to an existing car without having to move other components such as the radiator.
A 48V e-charger is added to complement the car’s existing turbocharger. Finally, the engine is modified for cylinder deactivation, which Delphi grandly calls Dynamic Skip Fire. At cruise on level roads, a car with Dynamic Skip Fire might fire, on average, only one and a quarter cylinders per revolution.
More mpg for China, more horsepower for the US
I drove a Delphi prototype car at the 2017 CES show in January. Even with the e-charger not yet activated, the test Volkswagen Passat was quick in city driving. Best of all, it started up immediately from stops with zero hesitation.
Mary Gustanski, Delphi vice president of engineering and program management, said the prototype’s purpose is to show automakers what can be done, affordably. In the US, buyers are looking for more horsepower as well as economy. In the booming Chinese market, it’s fuel economy. In Europe, it’s a mix of fuel economy and reduced emissions. For diesel engines, automakers can trade off some of the savings in CO2 (essentially fuel economy) for a reduction in NOx; urea injection may still be necessary to clean up the exhaust, but in smaller amounts.
Total cost of a system including the e-charger? “For $ 1,500, you’re looking at a 20 percent improvement in fuel economy,” Gustanski says. Twenty percent better fuel economy puts a lot of otherwise traditional combustion-engine cars into the territory of full hybrids such as the Toyota Prius that can drive 1-2 miles on battery alone.
Why 48 volts? Didn’t 42 volts flop a decade ago?
This is the second go-round for 48-volt systems. Circa 2000, there was a proposal for 42-volt systems, which is effectively the same as 48 volts. (What matters is that peak power flow never hit 60 volts DC, which is the point where more heavily shielded wire is required.) It fizzled out over concerns about the cost of dual systems, as well as practical matters such as switch contacts arcing, requiring costlier spring-loaded switches to reduce wear.
Between then and now, there is more concern about the environment, and more car components have gone electrical. Cars with lane keep assist and lane centering assist, and self-driving cars shortly, need electric not hydraulic power assist.
At the high end, the $ 230,000 Bentley Bentayga SUV has added a 48-volt active anti-roll bar. Mechanical anti-roll bars reduce the car’s lean in a turn. This one reduces roll further, so passengers feel less discomfort when the driver takes a curve at high speed.
Air conditioning is more efficient with an electric motor instead of a driven belt, because the AC compressor is off more than it’s on when the AC is turned on. Also, it can cycle on even when the combustion engine is is stop-start mode.
Even as cars demand more power, 48 volts may allow for lighter wiring harnesses. A wire of a given size can carry four times as much power (amps) at 48 volts as a 12-volt system. In a DC system, power (watts) equals the volts times the amps. A 12-volt wire controlled (limited) by a 15-amp fuse could carry 180 watts. Step the power to 48 volts and the same wire could carry 720 watts.
In the late-1990s era attempting 42 volt systems, the focus was on delivering more power to the vehicle. Now, says Jason Schwanke, senior systems engineer at Bosch, “The main focus is improving the emissions and fuel economy of the vehicle. 48-volt is a system to augment the combustion engine, reduce fuel consumption, reduce [diesel] particulate emissions, and improve the driver experience by increasing the responsiveness of the vehicle.”
Schwanke envisions a second iteration of cars that go beyond a smallish belt-drive motor generator. Rather than 2 kilowatts, it could be 10, 15, or 20 kilowatts. “Decouple the combustion system [from the motor generator],” he says, “you can do some advanced features you can’t do from a belt system … things like going through a parking lot [on electric power] or low-speed urban driving.” In other words, while 48 volts is a step up from 12 volts, it’s also a step down in cost from 300-volt-plus systems in traditional hybrids.
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