Moving to a 48V architecture in hybrid and electric vehicles offers plenty of promise. It provides about four times the energy recuperation – from regenerative braking – than is available on the traditional 12V system. Calum MacRae takes a closer look at OEM plans in this area and asks, what will 48V bring?

For a start, it will mean that there’ll be an increasing number of mild hybrid demonstrator vehicles at the world’s motor shows in the next 18 months or so. These mild hybrids will be cousins to those offered by Honda – a key differentiator will be the ability (albeit limited) for some to drive on electric power only at low speeds and the implementation of coasting/sailing strategies.

Together, the OEMs and the suppliers estimate that 48V mild hybrids will have the potential to reduce CO2 emissions by 10-20%, depending on the test cycle. As with many future powertrain developments at this time, the attractiveness of 48V mild hybrids to OEMs is enhanced by the forthcoming Worldwide Harmonized Light Vehicle Test Procedure & Cycle (WLTP/WLTC). In the WLTC, the proposed acceleration and deceleration phases are much more aggressive compared to the NEDC (New European Driving Cycle). Furthermore, the deceleration phases count for a higher proportion of the proposed cycle (43.1% compared to 31% for the NEDC, according to analysis by BMW). In most cases, the WLTC will suit the energy regeneration, sailing and torque boost functions that 48V hybrids can provide and give more CO2 saving benefits than would be apparent under NEDC. According to calculations by Valeo, on the NEDC cycle 48V mild hybrid savings are 10-15% but on the WLTC test the benefits are in the range of 15-20%.

Crucially, for the commercial proposition, 48V mild hybrids are 50-75% cheaper than full hybrids. The implementation of 48V will also reduce the weight of wiring harnesses – four times the voltage means the current is reduced by a magnitude of four, which will offer potential for the wiring harness wire diameter to be reduced.

In the medium-term, it can be expected that dual 12V and 48V electrical architectures will be maintained with higher power electric features running through the 48V architecture and lower power draws maintaining 12V usage.

Are 48V mild hybrids a Europe only initiative at the moment?

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While the North American OEMs were actively involved in the promotion of 42V plans in the 1990s, this time around it had seemed that 48V would be a Europe-only move pressured by CO2 regulations. Now, while many European OEMs have shown their hand, Mitsubishi, Hyundai and Kia have all recently demonstrated 48V concepts and Fiat-Chrysler has said it is planning to participate from 2016 according to reports by Wards Auto.

Presently, it looks like Audi will be one of the first OEMs with a 48V mild hybrid on the market. It’s targeting 2016 as an on-sale date while in 2015 it expects to demonstrate a proof of concept for the 48V architecture. Its electric supercharger technology (supplied by Valeo), shown as a concept in the RS5 TDI concept of mid-2014, is expected to be powered by a separate 48V circuit when it is released on the market in 2015 in the revamped Q7. A logical next step is a 48V mild hybrid.

The direction for Audi’s 48V mild hybrid was first outlined in the 2012 iHEV concept based on an Audi A7 and incorporating Audi’s Predictive Efficiency Assistant (PEA) that uses topographical information from the sat nav to make eco driving suggestions to the driver for the gear and/or also decides when to best implement the iHEV’s coasting function. In August 2014, Audi confirmed that its ICE vehicles would be moving to a dual 12V/48V architecture.

BMW, meanwhile, has stated it expects an on-sale date of between 2016 and 2018 for its 48V vehicles, although it has been reported that the 2015 G11 7-Series could be the first recipient.

PSA has been one of the more active companies in demonstrating 48V hybrid concepts. In May 2013, it demonstrated its “Hybrid Eco” 48V low-cost hybrid system, a mild hybrid coupled with a 10kW electric motor. The electric motor can move the vehicle independently at speeds of up to 20km/h and provide boost when accelerating. PSA says it will launch its first 48V mild hybrid system in 2017. Hybrid Eco was developed in conjunction with Valeo, Bosch and Continental.

At the 2013 Tokyo Motor Show, Mitsubishi displayed its Concept AR vehicle. As well as being a design study for a MPV/SUV crossover, the AR featured a mild hybrid BSG system with 48V lithium-ion battery and DC-DC converter allied with a 1.1L DI turbocharged petrol engine. Concept AR’s mild hybrid demonstrated three functions: torque assist; brake regeneration and start-stop.

Hyundai demonstrated a mild hybrid i40, with belt-driven starter generator (BSG) replacing the alternator, at the 2014 Paris Motor Show. Compared with the regular 1.7L diesel model, power is boosted by 15% and fuel economy is improved by 20% thanks to the 48V mild hybrid technology. Compared to Valeo’s estimate, Hyundai estimates that its system is a quarter of the cost of a full hybrid and at a weight penalty of 46kg. The i40 48V Hybrid can operate in electric-only mode at low speeds and when cruising. The lead-carbon battery pack recharges itself during deceleration and through regenerative braking, with the BSG working as a generator. During acceleration, the BSG supports the engine with its additional 10kw to reduce fuel consumption and CO2 emissions.

Hyundai’s brand stable mate, Kia, had already demonstrated a 48V hybrid concept at the 2014 Geneva Motor Show again featuring a lead-carbon battery. The chemistry was favoured by Kia over lithium-ion as it doesn’t require active cooling, is more recyclable and is more effective in sub-zero conditions. Kia claimed the system reduce CO2 emissions by up to 25% while bringing a 15-20% power increase. At the later Paris Show Kia demonstrated the mild hybrid technology in the Optima T-Hybrid and revealed that the system also allowed for the installation of an electric supercharger.

Volvo is another European manufacturer exploring 48V mild hybrid possibilities. In a 2013 presentation the company pointed out that back in 1990 it explored dual voltage architectures (24/12V and 48/12V at the time) combined with ISGs (integrated starter alternators) which provided a good base for developing mild hybrids and proved a good learning base for the later stop-start micro-hybrid introduction. Additionally, in 2001 it had developed an ISG hybrid on a 42V system that achieved a 15% fuel economy saving with a manual transmission.

Toyota also has one eye on 48V developments. As today’s acknowledged full hybrid market leader due to its success with Hybrid Synergy Drive, the full hybrid system that has seen build of over 3m in over 15 years of production, it’s perhaps a lesser known fact that back in 2002 the company developed a 42V mild hybrid system for its domestic market Toyota Crown sedan.

While North American OEMs do not seem to be actively involved in 48V development in the main, GM has had some similar experience with its BAS and eAssist mild hybrids, which operate on a similar principle to 48V mild hybrids. Its BAS (belt alternator starter) hybrids allowed start-stop, regeneration and torque assist with a 36V system operating the small electric motor. The BAS system was phased out in favour of eAssist with an air-cooled 115V battery and 15kW electric motor. However, while both systems promised impressive fuel economy benefits (up to 32% in the case of BAS) neither were successful in the market place and eAssist was withdrawn from the Chevy range for 2015MY. eAssist does continue with certain Buick models. GM’s travails with BAS and eAssist mild hybrids perhaps suggest a reason for the reluctance of North American OEMs – besides Chrysler – to join the 48V mild hybrid bandwagon, although Ford is part of the research team behind Ricardo’s ADEPT research vehicle.

This article was extracted from just-auto/Qube‘s engine technologies service: Global light vehicle engine technologies market- forecasts to 2029

See also: February 2015 management briefing: 48V mild hybrids (1)