Saving CO2: 48V hybrids versus plug-ins
Steve Doyle – Ricardo product group head, hybrid & electronic systems
The growth of the plug-in hybrid and battery electric vehicle (PHEV and EV) market can rightly in my view be seen as a resounding success over recent years. According to the Society of Motor Manufacturers and Traders, a total of 28,188 cars were eligible for the UK Plugin Car Grant scheme in 2015, compared with just 14,532 in the previous year.
However, this rapid market growth should be seen in the context of a very low base, in the case of the UK just 1.2 percent of the market in 2015, and some significant obstacles to growth remain. Range anxiety, safety concerns, the ready availability of charging stations and the time taken to recharge are all significant battery-related issues that impede the growth of the plug-in vehicle market. Perhaps more intractable than any of these, however, is battery cost.
Or at least, that’s how things stand at the moment. Looking ahead, a common goal in the R&D efforts of the major industrialized nations is to push battery costs to below $100 per kilowatt hour, as this is widely believed to be the tipping point at which EVs can compete headto-head with gasoline or diesel cars.
With much effort being focused on the development of new higher power and lower cost cell chemistries, Ricardo believes that the market will be dominated to at least the end of the current decade by developments of today’s lithium-based technologies. In the 2025-2030 timeframe, however, it may well be that the current problems preventing the commercialization of lithium-air will be resolved, offering at today’s prices a sub-$100/kWh technology capable of energy densities in excess of 0.7 kWh/kg.
48V hybrids: a new competitor
But if this tipping point at which battery electric vehicles become genuinely commercially attractive against diesel or gasoline cars is still some way off, what can be done in the medium term to deliver the further significant reductions in carbon dioxide emissions that governments and regulators are demanding in the US, the EU and elsewhere?
A potential innovation that many automakers are exploring is the 48V hybrid. At the heart of the system is a 48V battery enabling the application of a motor-generator capable of torque assist, ancillary electrification and, eventually, radical engine downsizing and down-speeding when combined with an e-supercharger. With batteries currently being developed to absorb regen energy and deliver in excess of 14 kW, there is already further demand for units to deliver 25 kW to support increased electrification.
A key attraction for this powertrain architecture is its performance in the new WLTC drive cycle, which includes more aggressive accelerations and decelerations than its NEDC predecessor. The new cycle – which is intended to be more representative of real-world driving – will suit the energy regeneration, sailing and torque boost functions that 48V hybrids can provide. Work by Ricardo has shown that 48V mild hybrid powertrains can deliver CO2 savings of 10-15 percent over the NEDC and 15-20 percent over the WLTC. In addition, these powertrains can also offer improved acceleration and NVH through engine downsizing and down-speeding, as well as silent pureelectric take-off, and stop-start. As such, they can offer most of the benefits of full hybridization but at a fraction of the cost.
ybridization but at a fraction of the cost. With the market demand that it is currently experiencing, Ricardo believes that the 48V mild hybrid technology may well contribute more to emissions reduction in the next five to 15 years than all plug-in vehicles combined. In this race, therefore, the 48V mild hybrid and the EV may both emerge as winners – it just depends on whether you’re looking at a marathon or a sprint.
This view point featured in RQ Q2 2016 - click here to download the full publication.