At the 2^nd International Conference Automotive 48 V Power Supply Systems, held last week in Düsseldorf, Germany, Controlled Power Technologies (CPT), a major sponsor of the event, together with leading car makers and Tier 1 suppliers, argued the need for an internationally agreed 48V electrical standard.

Such a standard will be necessary for the global automotive industry to achieve the economies of scale demanded by original equipment manufacturers.CPT, together with other sponsors of the event, including AVL and Ricardo, is in the vanguard of 48V mild hybrid developments intended to address the ever-tightening European CO₂ regulations and potentially a new and more aggressive test cycle—i.e., the Worldwide harmonized Light vehicles Test Procedures (WLTP).

The EU goal is to introduce the WLTP by 2017 if possible and no later than 2020 to coincide with the 95 g/km requirement. The WLTP defines a global harmonized standard for determining the levels of pollutants and CO₂ emissions, fuel or energy consumption, and electric range from passenger cars and light-duty commercial vehicles—therefore it becomes even more important for the industry to agree beforehand a global electrical standard for the 48V mild hybridization of its vehicles.

One of the issues for the industry to confront in moving away from the NEDC test is that the WLTP typically halves the measurement of CO₂ reduction gained from first generation stop-start systems, which characteristically reduce CO₂ emissions on the NEDC by 5%. This reduces to 2.5% when measuring the same vehicle on the WLTP.

The WLTP cycle will be much more challenging than the NEDC and the CO₂ effect of a simple stop-start system will be halved. Car makers will have to further optimize their control system strategy, which can be aided by a B-ISG [belt-integrated starter generator] with a fast transient response to maximise the recuperation and boosting potential over the WLTP test cycle, or under real world driving conditions. Moreover, any electric boosting using energy recuperated, rather than lost in friction from the brakes, not only reduces CO₂ and NO_x emissions, but can also have a positive impact on vehicle performance and drivability. The WLTP is significantly more dynamic with faster acceleration rates. This means that in addition the power demand is more difficult to achieve with engine downsizing and transmission down-speeding alone.

Thermal management of the electrical machine is another critical factor, because the duration of the boosting and energy recovery operating modes directly impacts fuel economy and emissions. Hence the trend towards liquid cooling of electrical machines, which can be achieved by tapping into the engine coolant system, ensuring their performance including any integrated electronics is independent of ambient temperature.

Stable thermal characteristics achieved through liquid cooling allows better utilisation of torque assist and longer recuperation opportunities, since both modes generate heat. It is how we manage to deliver peak recuperation in excess of 12 kW and peak torque assist in excess of 8 kW for extended durations of typically 20 seconds or more. The recuperation potential in the NEDC test cycle allows use of electric assistance in accelerations up to 30 km/h [18.6 mph] and potentially to provide torque assist at other road speeds including during engine-off coasting.

The bottom line of course is that cost is the most critical factor when setting out to achieve CO₂ compliance, particularly in the high volume sector of the market. Until we have a really significant breakthrough with battery chemistry or fuel-cell technology, the high voltage approach to hybridization and pure battery electric vehicles will remain too expensive for universal application across high volume vehicle platforms. Low voltage hybridization on the other hand, for a new class of 48V hybrid vehicle, provides an affordable compromise between efficiency and cost. And it’s cost-effective because it’s an evolution of existing powertrain architecture rather than a completely new propulsion system.

The conference. The three-day conference showcased technological advances being achieved by the European industry in the development of 48V mild hybrids.

Keynote speaker Florian Kühnlenz, responsible for series development of low voltage energy systems at Volkswagen AG, set the scene with a presentation of the electric and electronic architecture requirements of dual voltage power supplies in vehicles at 12 volts and 48 volts; initial steps have already having been taken for the adoption of the proposed LV148 standard suggested by Audi, BMW, Daimler, Porsche and Volkswagen.

Kühnlenz’ outlook is that the introduction of a second, 48V system addresses new challenges for automotive electrical and electronic systems, but that most issues have now been identified with preliminary solutions already developed for introduction during the ramp up to the 95 g/km CO₂ requirement by 2020. The displacement of high wattage loads to a more efficient 48 volt network is expected to be the next step in the development of a new generation of low voltage mild hybrid vehicles.

Commenting on the need expressed by Tier 1 suppliers for an international standard when introducing low voltage hybrids and their note of caution should this not happen, Paul Bloore, product validation manager for CPT’s hybrid product group said:

It makes sense to have a common global standard, because 48V hybrids are currently the most cost-effective way of meeting stringent CO₂ emissions being introduced in 2020, compounded potentially by a shift from the current NEDC test to the more aggressive WLTP test, with further 25% reductions anticipated in 2025 and 2030. Moreover, a consensus of global forecasts suggests that 48V hybrids will soon come to dominate the market, so clearly that’s what the vehicle manufacturers are expecting to happen. A common international 48V standard would be a smart move.

Dr. Salah Benhassine, a specialist in electromagnetic compatibility (EMC) at PSA Peugeot Citroën, set out the French carmaker’s approach to 48V mild hybridization by focusing on one of the specific technical challenges. He similarly concluded that collaboration between automotive industry and suppliers is essential.

Robert Eriksson, senior technical leader for electric propulsion architecture at Volvo Cars, concluded that there were several opportunities for building scalable and modular solutions with different attributes in the mild hybrid category, commenting on the recuperation possibilities with expanded 48V architecture, thereby achieving the goal of less than 95 g/km of CO₂ levels after 2020.

Ulf Stenzel, lead engineer for new battery technologies, hybrid and electric powertrain systems at AVL Schrick GmbH, which is heavily involved with CPT and the European Advanced Lead-Acid Battery Consortium (EALABC) in the development of the ‘LC Super Hybrid’ gasoline powertrain, discussed the development by Hyundai and Kia of a 48V mild hybrid diesel powertrain, commenting that its combination with an electric supercharger offered “realistic potential in terms of performance increase and further fuel consumption reduction.”