Preparing for a 48-volt Revival

April 1, 2016

Dan Carney

More than a decade after the widely anticipated 42-volt electrical revolution died—the victim of increasingly efficient 12-volt components, wiring strategies, and digital controls—demand for greater on-board electrical capacity is again driving plans for higher-voltage systems. This time 48-V (or more accurately, dual-voltage 48-V/12-V) hybrid systems soon to enter the market will deliver performance and functionality at a fraction of the cost of high-voltage systems.

A key to the emergence of 48-V hybrids is the move by European Union regulators to replace the current NEDC test protocol with the new Worldwide Harmonized Light Vehicles Test Procedure in 2017. The WLTP cycle was devised to more accurately reflect real-world driving and vehicles’ expanding electronic feature content. Engineers say WLTP compliance is more challenging than the NEDC; the benefits of 12-V stop-start systems are diminished by nearly half, for example.

As a result, meeting the WLTP will require new powertrain technologies that can deliver greater electrical power than is available from current engine downsizing and down-speeding strategies. Combining integrated starter-generator “mild hybrid” systems with the rapid transient response of electrically-enhanced e-superchargers, the experts explain, can optimize energy recuperation, reduce vehicle NO_x and CO₂ emissions and even improve the fun-to-drive quotient.

Mild-hybrid technology is hot again because “We’re out of power. Four times the voltage means four times the power,” observed Mary Gustanski, Delphi Automotive Vice President of Engineering. She noted that mild hybrids present a better value than “strong” hybrids: “You get 70 percent of benefit of a full hybrid at 30 percent of the cost of a higher-voltage hybrid.”

Not surprisingly, momentum toward 48-V-equipped vehicles is rising. Last December, IHS Inc. revised upward its 2025 forecast to reflect 10 m global sales in 2025—exceeding sales of conventional strong hybrid by one million units that year. Activity is particularly intense among German OEMs, according to suppliers, as those companies want to use, first in Europe, 48-V technology to replace today’s 12-V start-stop components.

Audi recently announced a 48-V system in its new SQ7 sport SUV, to power electrified boosting and an active roll control chassis system (p. 34); both are power-hungry subsystems. Delphi is going into production with an unnamed OEM next year, according to Gustanski. Similarly, Robert Bosch has customers for its mild-hybrid module preparing to launch in the next two to three years, reports Jason Schwanke, Senior Engineer for Gasoline Systems. Continental soon will have a 48-V system in production, and Valeo also has a 48-V package with engine e-boosting under development.

General Motors has deployed three types of mild hybrid systems in production cars and trucks since 2004, using batteries from Hitachi Automotive and Continental. While its Gen-1 36/42-V belt alternator-starter (BAS) systems fell victim to unfavorable economics, said Larry Nitz, GM Executive Director of Electrification Powertrain Engineering, GM offers its latest-generation mild-hybrid system, eAssist, in the 2016 Buick LaCrosse and Regal. Company engineers continue to develop the technology, which GM will deploy next in its full-size pickups.

Cost and safety threshold

The 48-V systems operate near the 60-V upper threshold of mortal electrocution threat, providing the most power possible without the need for costly shielding or specialized wiring connectors that cost ten times more than lower-voltage components. “A 12-volt connector is about $2, but a high-voltage connector is about $20,” Gustanski noted.

“At 48 volts, you are leveraging the components we’ve been using for 60 or 70 years,” said Schwanke.

A lithium-ion battery is the preferred energy-storage medium for 48-V mild hybrids, because “their charge acceptance is well above what you can do with lead-acid batteries,” noted Daniel Kok, Manager for Advanced Electrification at Ford. That enables the mild hybrid to recover more energy under braking.

The previous obstacle to 48-V acceptance was its weak added-value proposition. GM’s eAssist equipment package cost consumers more than $2000. But the increasingly stringent regulatory environment has skewed those calculations in favor of mildly hybridized internal-combustion powertrains. According to Schwanke, Bosch can offer its 48-V mild hybrid systems to OEMs for less than $1000.

Also, the first wave of 42-V mania envisioned complete conversion of vehicles to the higher voltage, while to reduce costs, the current design calls for parallel 48/12-V systems. The object for engineers to is apply the higher voltage to systems that can benefit most from the higher available current, while retaining the majority of the 12-V system. Engineers expect the inexpensive, reliable (and heavy) lead-acid battery will remain to handle hotel loads.

Improved driveability

Some of the primary benefit of 48-V mild hybridization comes from its ability to stop and start the IC engine even more frequently than 12-V stop-start systems, including shutting off the engine while the vehicle is in motion or seamlessly restarting it on the fly if the driver slows but doesn’t come to a complete stop, then tips into the accelerator.

Additionally, drivers annoyed by the intrusive behavior of many of today’s lesser 12-V stop-start systems will be less inclined to switch off the systems, boosting their real-world efficiency. “Consumers are saying ‘thanks but no thanks’ on the stop-start,” said Gustanski. “[The systems’ performance] is a disgrace.”

Moreover, a stronger electric machine can provide some degree of torque assistance to the IC engine when the driver asks for maximum acceleration.

Experts note that using the 10- to 12-kW (13-16 hp) available from a 48-V belt alternator-starter system to power an e-supercharger not only entirely eliminates the lag of exhaust-driven turbochargers, it allows the booster to provide extra performance the instant the IC engine restarts.

The forced induction helps offset the downsized-displacement trend. “They went so small with the IC engine to save on fuel economy and emissions they just don’t have enough power,” explained Gustanski.

In addition to the functional improvements to the drivetrain, the benefit of running the high-load accessories at higher voltage enables use of lighter, smaller-gauge wiring, which contributes to vehicle mass reduction.

“Aircraft went to higher voltage years ago for the same reason: lightweighting and more cold-weather reliability,” remarked Mike O’Brien, Vice President of Product Planning at Hyundai Motor America.

The additional power enables all manner of appealing ancillary features and systems for Hyundai’s Genesis luxury models, O’Brien explained. “It is good for anything that requires a high-torque motor, like ABS pumps, stability-control systems with high-load actuators and adjustable stabilizer bars,” he said. “When you think about customer needs and where 48 volts can be most leveraged, our large luxury products are the best fit for it now,” he concluded.

What Ford likes about 48-V is that it is an “overlay technology,” said Kok. That means that the company can install such systems relatively easily in all kinds of vehicles, diesel- or gasoline-powered, with automatic or manual transmission, because of the system’s similarity to a conventional starter system. But it doesn’t mean there is no work involved in the conversion. Calibration is tricky and space must be found for the 48-V battery. “What may package in a Fusion may not package in a Fiesta,” he cautioned.

But while dedicated hybrid models may nor may not achieve high-volume sales, the popularity of mainstream conventional models - which serve as the base for the predicted boom in 48-V hybrid systems - is certain to continue.

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