Series plug-in hybrids best bridge to EVs, says panel

  • 22-Oct-2010 02:19 EDT

The RAE panel favors using series hybrid technology, like that in the 2011 Chevrolet Volt extended-range electric vehicle, as a bridge to pure EVs.

When it came to formulating their recommendations for the U.K.’s automotive future in light of tough, new government restrictions on carbon dioxide emissions, the members of the Royal Academy of Engineering (RAE) panel who had been selected for the task had to face reality, said Roger Kemp, the panel chair and transportation researcher at the University of Lancaster.

After analyzing various ways to satisfy mandated limits on the rates of CO2 release that Britain’s cars will be permitted in the years 2025 and 2050, they realized that the prime candidate, electric vehicle (EV) technology, was for some time to come probably going to remain too costly and deliver insufficient range for many motorists, he reported. In addition, they concluded that installation of a widespread smart recharging system, even in a nation that is only about the size of Michigan, would be very costly and take considerable time to complete.

The panel of nine leading automotive industry consultants, academic researchers, and engineers from top technology companies, including Ricardo and Prodrive, decided that even one of the outwardly reasonable proposals made by EV proponents for step-by-step implementation and expansion of electrification within selected regions “seems a pretty inconceivable option,” Kemp stated. “We can’t see it happening without a massive amount of investment in plugs and sockets for recharging. And we can’t see anybody investing the required amount of money unless an EV customer base already exists to use it.”

“That’s why we came around to the view that plug-in hybrid technology is the way to go,” he continued. Plug-in hybrid technology is practical in the short term, promotes the development of a charging infrastructure, and “saves the driver from worrying about range.” Kemp described a range-extender configuration with a 10- to 15-kW·h battery pack and a small 10- to 15-kW engine to recharge the battery that could handle the one out of 10 days that an average British driver needs to travel 150 to 200 mi (240 to 320 km) rather than the usual daily commute and around town range of 40 to 60 mi (64 to 97 km).

The detailed conclusions and recommendations of the RAE panel, a report titled Electric Vehicles: Charged with Potential (, ended up highlighting the prospects of plug-in hybrids in a "far more favorable light most of us on the board expected," he noted.

Another board member, Richard Parry-Jones, principal of RPJ Consulting, who is a former Chief Technology Officer and product executive at Ford, said that he was pleased to be able “to get on the table for public discussion in an independent fashion the relative attractiveness of plug-in hybrids vs. very long-range EVs. An industry report would have been seen as biased.”

Mixed vehicle fleet

“We don’t see the U.K. fleet ever going totally electric,” he said. “It will be a mixed fleet for some time. EVs, with continuing range and affordability issues, will be used in ever greater numbers for short trips in cities and urban areas. Plug-in hybrids, with essentially no range limitations, will provide the versatile vehicles people are used to” until EV propulsion improves. “Essentially our conclusion is, if you want bigger range, don’t double the size of the battery, make it a plug-in hybrid,” he said.

Crucially, the government needs to push low-carbon electricity generation sources, a combination of renewables, and nuclear power. “Without that the whole effort is a bit of a waste of time,” Parry-Jones noted.

Parallel vs. series hybrids

Parry-Jones then spoke about the prospects for parallel vs. series plug-in hybrid propulsion technology. In a parallel hybrid, both the internal-combustion (IC) engine and battery-powered electric motor can drive the transmission that turns the wheels (at the same time). By contrast, the IC engine in a series hybrid drives a generator that can either charge the batteries or power an electric motor that drives the transmission. The engine never directly powers the vehicle.

Parallel hybrids, such as today’s Toyota Prius, are efficient overall mostly because of the efficient direct-drive mechanical transmission, he explained, but added that inherent “problems with battery efficiency, power electronics, generators, and motors are eating away at the parallel hybrid’s current advantage over series hybrids.”

“These developments should leave the series hybrid configuration engine with significant advantages at typical duty drive cycles,” the former Ford exec said. Engineers could optimize the small generator engine for two ultrahigh-efficiency operational states: normal and maximum charge. “Parallel engines have to compromise to run at a range of speeds and loads; we’re placing our money on the series hybrid as the long-term winner,” Parry-Jones said.

“We feel that it makes sense for carmakers to develop plug-in series hybrid vehicles such as Chevrolet’s Volt and Jaguar’s Limo Green prototype. The Volt, for example, has a cost penalty of about $1000 to go 500 miles when needed.” Sustainable, renewable biofuels will eventually have to replace fossil fuels to reach the mandated emissions goals.

Vehicle efficiency and lightweighting

During the transition to EVs, many IC engines will still be produced but will get increasingly more efficient. “I’d expect at least another 30% improvement in the efficiency of gasoline engines by 2025 using advanced combustion methods such as DI, special stratification techniques, HCCI variants, and the like,” said Parry-Jones. For diesels, he envisions maybe a 15 to 20% rise in efficiency but warned that the technology will be increasingly burdened by cost penalties that will be imposed by aftertreatment technologies to deal with sooty exhaust particulate and, especially, nitrogen oxides (NOx).

Don’t look to lightweighting vehicles for much further improvement in fuel efficiency, according to RPJ. “To me, lightweighting efforts only make sense if you can reduce the weight of the vehicle by enough to downsize the engine by one architectural step, thereby avoiding any degradation of performance.”

As a rule of thumb, he explained, taking out 400 kg (880 lb) of overall mass lets a carmaker get by with one-third less engine or roughly 500 cm³ of engine displacement. One might contemplate, for instance, replacing a six-cylinder with a four-cylinder engine or a four-cylinder with a three-cylinder unit—in doing so, offsetting the cost of lightweighting with the lower cost of the engine.

“That replacement will happen eventually, but I suspect that it will require the premium manufacturers to pave the way,” Parry-Jones predicted. “But I don’t see the mass-market manufacturers getting anywhere near that step even in 50 years.” Taking cylinders away from consumers constitutes a significant marketing risk, he emphasized. The reality is that “people are willing to pay a price premium for more cylinders and/or engine displacement in a car."

“You’re assuming that customers are willing to pay the same price for an equally performing vehicle with fewer cylinders or a smaller engine that gets better fuel economy,” he said. “That kind of venture will require significant market education risk and lead time, which makes such an undertaking less attractive at the moment.”

Government’s role in EVs

In the meantime, government will have to intervene strongly to incentivize the long-term transition of power generation to low-carbon sources and promote the adoption of EVs enough to achieve truly mass-market penetration, according to the RAE board. Several prerequisites must transpire including the development of an infrastructure for charging, functional standards for billing, a compelling consumer proposition, and a business case for stakeholders—perhaps new ownership models for personal transport vehicles.

Government can stimulate EVs by the early adoption of standards, ranging from agreement on plugs and connectors to protocols for billing both at home and en route. However, the major challenge is the provision of a charging infrastructure for EVs, which, in the U.K., would require the active collaboration of national and local governments, car-park operators, electricity distribution companies, bank-card issuers, and many businesses not usually concerned with transport energy supply.

In the meantime, electrochemical and battery researchers must continue their steady progress toward a longer-range EV battery, said Parry-Jones. “Researchers have been making some gradual innovations in the dosing and changing around the anode materials of lithium-ion batteries, but I don’t expect fundamental reductions in cost and weight."

“I do think that at some point there will be major innovations in battery materials,” he continued. “But even lots of technical enhancement will probably only halve the current $5000 to $10,000 cost penalty of using batteries, depending on the maximum range capability.”

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