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A 48-volt system could reduce belt-driven ancillaries to the starter/alternator and offer a fail-safe electrical architecture for autonomous driving.

Delphi says 48-volt mild-hybrid systems could offer 15% CO2 reductions

Delphi believes that 48-volt mild hybrids could offer significant CO2 reductions and driver benefits for cars in the future. Speaking at the IAA 2015 (Frankfurt Motor Show), Christian Schäfer, Global Director of Advanced Electrical and Electronic Architectures at Delphi, told Automotive Engineering that there are several drivers for the technology that were not present 15 years ago.

“The main driver is the carbon dioxide threshold of 95 g/km fleet average in 2021 across the European Union," he said. "The idea is that we have a mild hybrid, so we don’t need high-voltage on all cars because it is too expensive to implement. We would have 48-volts, especially in the low segment, up to C-segment cars. Then we could have a belt-driven starter/alternator and can use it as an e-boost if you don’t have a rear drive unit, so the alternator is an electrical machine to boost the combustion engine.”

Delphi envisages that the car would start with electric propulsion for the first 100 m (328 ft) of movement.

Regenerative braking would be the main path to reduce CO2 emissions, reckons Schäfer.

“From our experience and our discussions with European OEMs, we have seen between 7 and 10 percent reductions in CO2 emissions from the current test fleet with today’s 48-volt alternators, which have a nominal power of between 10 and 12 kW. For the next generation with up to 20 kW, we expect that we could increase the CO2 reduction to 15%.”

Besides the CO2 reduction potential of 48-volt systems, Delphi sees other reasons to switch to the higher voltage electrical architecture. Some systems, such as dynamic chassis control, would impose a high power load on the car. “You would have high current peaks, so you can’t do it with a 12-volt system,” said Schäfer.

In addition, the aim of many vehicle OEMs, according to Schäfer, is to remove all belt-driven ancillaries from the engine with the exception of the alternator. This would mean that all these systems—such as the HVAC compressor, power steering pump, and coolant pump—would be driven electrically from the 48-volt system.

“With the belt, you have a lot of friction; for instance on the highway, you have the highest speed on the power steering pump, but you don’t need it,” said Schäfer. Then power steering could be an “on-demand” electrically driven system. “That’s another way you can reduce CO2 emissions by a small amount.”

The penalty charges that would be imposed on manufacturers who might struggle to comply with the 2021 EU CO2 emissions average could at least be offset by fitting a 48-volt system. From the discussions that Delphi has already had, between 20 and 70 percent of all hybrids will be produced with a 48-volt mild-hybrid system. The company thinks that it is a reason why the growth in battery-electric vehicles is not as strong as was expected in 2009 and 2010. “You can reach about 60 to 70 percent of the targets with around 30 percent of the cost,” said Schäfer.

Delphi is already working on the electrical components that would be required for a 48-volt architecture. This includes a 3-kW DC/DC 48/12-volt converter.

“The important thing is that it is a bi-directional device, producing 3 kW from 48 volts where the alternator is, to the 12-volt side to support the battery and all the comfort functions, bulbs, and traditional equipment,” explained Schäfer, “But to make a jump start possible from a 12-volt car to a 48-volt car, we have a boost mode as well to transform 1 kW nominal power from 12- to 48-volts.” Delphi will produce the device with both air and liquid cooling options.

Other components include waterproof 48-volt electrical connectors that meet the highest waterproof and vibration-resistant standards. Delphi also plans to introduce color-coding for 48-volt systems to warn users that this is not a 12-volt system. The suggestion is that 48-volt systems should be coded blue.

Delphi expects to begin production of a 48-volt electrical distributor in 2016 that will provide a 40-amp output. A smart 48-volt distributor, incorporating a semi-conductor MOSFET fuse, is already in development, with production planned for 2017.

“Smart fusing is the only chance to detect all failure modes which could occur on a 48-volt and 12-volt board,” said Schäfer. This would include electrical arc detection, because it could be too dangerous to use conventional electrical melting fuses.

An algorithm is used to detect arcing. “We have to distinguish [between] 'good arcs' and 'bad arcs,'” said Schäfer, “The good arcs are the relays. If a relay is open, there is always an arc for around 2 ms. If we have more than 10 ms, we would switch off and then only the dedicated path and not the complete 48-volt system.”

A 48-volt architecture in combination with 12-volt system also offers a level of electrical redundancy for future autonomous driving systems, where a failsafe system would be required.

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