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PHEVs on the Global Engineering Agenda; Defining and Refining the Problem

Although the field is still relatively young, engineering issues concerning plug-in hybrid electric vehicles (PHEVs) received a great deal of exposure at this year’s SAE 2007 World Congress—the theme of which is “Engineering for Global Sustainable Mobility—It’s Up to Us”.

The SAE World Congress is primarily a technical conference; marketing presentations are few. The bulk of the content is papers (about 1,500 at this year’s event) dealing with the essential engineering behind developing a new combustion regime, cutting emissions, or squeezing out an extra percentage point of fuel efficiency, for example.

In addition, both AVL and FEV sponsored some higher level symposia at which top engineering and research executives from OEMs and suppliers—as well as the EPA—had a chance to discuss their views of the technical and regulatory paths to meeting the sustainability challenge. That fuel economy, energy availability and climate change—i.e., lowering CO2—are the top concerns that will shape the nature of the industry appears not to be in doubt. That’s not to say that the shape itself is clear, only that those are driving forces.

[Carbon regulation] is no longer a question of whether, only a question of when, and what it looks like.

—Cristopher Grundler, Deputy Director, Office of Transportation and Air Quality, US EPA

Hybrids and plug-in hybrids are here to stay. They will permeate the [global] fleet. They are enablers for fuel economy.

—Jeremy Holt, President, Ricardo, Inc.

Fuel economy will become more important than emissions.

—Nigel Gale, VP, Engine, Emissions & Vehicle Research Division, SwRI

This is all about reducing CO2 and displacing petroleum. This is not about wants, this is about needs. This has to happen.

—J. Gary Smyth, Director Powertrain Systems Research, GM

That doesn’t mean that PHEVs are seen as a default dominant outcome. There are other engineering pathways to reducing petroleum use (multiple technologies to enhance the efficiency of gasoline engines, alternative fuels, and next-generation diesel), and all are being considered in the context of feasibility, cost and customer desires.

This has to be an “and” not an “either or”.

—J. Gary Smyth, GM

While attendance at the PHEV sessions was not as packed as, for example, the overview of diesel emissions technologies by Timothy Johnson from Corning, this year’s conference provided a platform for the presentation and discussion of some of the more detailed PHEV testing and modelling work being done by Argonne National Laboratory and the National Renewable Energy Laboratory. 

The Department of Energy has designated Argonne as the lead national laboratory for the simulation, validation and laboratory evaluation of plug-in hybrid electric vehicles, as well as advanced technologies required for PHEVs.

Testing Plug-in conversions. Richard Carlson from Argonne presented the results of Argonne’s testing of three current PHEVs: the CLEANOVA series plug-in hybrid, and two converted Prius PHEVs, one with a Hymotion battery pack, the other with an Energy CS battery pack.

While the testing found that the converted Prius hybrids “showed remarkable petroleum displacement and energy efficiency for being modified production vehicles,” it also surfaced a problem: emissions.

The Hymotion conversion has one operating mode, while the Energy CS conversion has two. One is designed to maximize battery depletion, the other “California” mode to improve tailpipe emissions over maximum-depletion mode while still displacing a significant amount of petroleum. The ANL team tested all of the modes.

The two Prius PHEV variants operate in EV mode at high states of charge (SOCs) to maximize the benefit of the charge-depletion control strategy. By maximizing the charge-depletion rate for maximum petroleum displacement, other characteristics are compromised, including emissions, as a result of infrequent engine operation and diminished engine operating efficiency because the battery is not recharged while the vehicle is driven, reducing engine load.

This maximum charge-depletion operation in a power-split hybrid, although not optimized, can serve as a benchmark for future research on PHEVs to study the effects of a charge-depletion control strategy on battery sizing and life, real-world cost, charging, controls optimization and powertrain configurations.

The Argonne team found that NOx and THC emissions from both the Hymotion (5 kWh pack) and Energy CS (9 kWh pack) exceeded those of the production Prius in the UDDS cycle (city cycle) testing. The problem stems from the lower frequency of engine use. The short-term regulatory can of worms aside (you’re not supposed to modify a rated engine and produce a worse emissions outcome), the problem is certainly surmountable, but it requires work.

The Argonne team also concluded that the trade-off of battery cost to petroleum displacement requires more research for production feasibility. Related to that is the trade-off of engine efficiency with charge depletion rate. The impact of this trade-off needs to be studied in terms of the overall fuel and energy use by such vehicles, according to the researchers. 

Summary Comparison of Prius Models on UDDS Cycle
Hymotion UDDS #2Energy CS
Max Depletion
Energy CS
Production Prius
Hot Start
Fuel economy [mpg] 200 221 114 66
Grid Elec consumption
[DC Wh/mi]
128 147 88.9 0.0
Operating cost [$/mi] $0.031 $0.031 $0.036 $0.042

Simulation results for PHEV component requirements. Another Argonne paper, presented by Phillip Sharer, reported on the development of a process to define the requirements of energy storage systems for plug-in applications. The paper describes the impact of All-Electric range, drive cycle and control strategy on battery requirements for both a midsize sedan and SUV classes of vehicles.

Among the conclusions of the research are:

  • Both classes of vehicles exhibit similar energy consumption trends during charge depletion mode. The SUV has the greater overall energy and power needs.

  • The battery energy is approximately a linear function of the All-Electric Range.

  • Power requirements are not significantly influenced by the AER as a result of the high specific energy of the li-ion battery used in the model.

  • The high specific power of li-ion technologies does not have a significant influence on vehicle mass. Specific energy has the greatest affect on vehicle mass.

  • At high AER—after about 30 miles—the pack voltage needs to increase due to capacity limitations. Higher capacities or battery packs in parallel might need to be used to avoid an increase in bus voltage.

The result of these simulations will be used to define the component requirements of PHEV vehicles in the DOE R&D plan.

Energy management strategies for PHEVs. Jeffrey Gonder from the National Renewable Energy Laboratory (NREL) presented work summarizing three potential energy management strategies for plug-in hybrids: a strategy to maximize the all-electric range; an engine-dominant blended strategy; and an electric-dominant blended strategy. 

The AER-focused strategy requires larger and more expensive electric components, but offers more benefits, including receiving greater credits towards satisfying CARB’s Zero Emission Vehicle (ZEV) regulation.

The two blended strategies do not deliver as many benefits as all-electric operation, but can use smaller and less-expensive electric components.

The AER strategy is particularly sensitive to driving aggressiveness, because it will be unable to satisfy significant power demands during CD mode as designed. (An earlier paper provided more detail on the sensitivity of conventional hybrids to aggressive driving.)

By contrast, the engine-dominant blended strategy is sensitive to driving distance—the vehicle must exceed the charge depletion distance in order to benefit from the efficiency maximization approach. In other words, not driving far enough wastes the stored electricity and results in a significant fuel penalty.

The greatest fuel savings, according to the research, would result if the vehicle could make intelligent predictions about the upcoming cycle, and switch adaptively between the two blended approaches. If travelling a long-distance, the controller would select the engine-dominant strategy, which would maximize use of the stored grid power.

Absent such intelligent route-based control strategies, however, and assuming an effective emissions control strategy can be developed, the electric-dominant blended strategy delivers effective utilization of the stored electric energy during charge depletion mode, while minimizing the fuel penalty.

A PHEV manufacturer designing such a vehicle for electric-dominant blended CD operation over real-world driving could still size the electric drive large enough to meet the peak power requirement on the UDDS. Although cost remains a major challenge for PHEVs...the additional cost incremental for this extra power capability could be worthwhile, particularly since the increased electric power would improve the vehicle’s acceleration capability, which in turn increases its consumer appeal.

Platform engineering for PHEVs. Tony Markel from NREL described work in using platform engineering to optimize PHEV design.

Platform engineering consists of enhancements that are not dependent on powertrain technology, including the use of lightweight materials, aerodynamic drag reduction, rolling resistance reduction, combustion engine efficiency improvement and the relaxation of performance constraints.

The application of platform engineering to PHEVs reduced energy storage system requirements by more than 12%, offering potential for more widespread use of PHEV technology in an energy battery supply-limited market. Results also suggest that platform engineering may be a more cost-effective way to reduce petroleum consumption than increasing the energy storage capacity of a PHEV.

As part of the study, NREL also considered platform engineering for a conventional combustion engine vehicle and a conventional hybrid. The results, in terms of reduced retail cost and lowered petroleum consumption, were relatively larger than those obtained for PHEVs, with the conventional platform showing the greatest percentage improvement in fuel consumption (although in absolute terms still far below the PHEVs).


  • Testing and Analysis of Three Plug-in Hybrid Electric Vehicles (SAE 2007-01-0283)

  • Midsize and SUV Vehicle Simulation Requirements for Plug-in HEV Component Requirements (SAE 2007-01-0295)

  • Platform Engineering for Plug-in Hybrid Electric Vehicles (SAE 2007-01-0292)

  • Energy Management Strategies for Plug-in Hybrid Electric Vehicles (SAE 2007-01-0290)



Production Prius can not be modified to become effective PHEV. Electric propulsion motor is too weak to assure proper acceleration during some city modes, and to highway speed. And it can not be scaled-up because it is incorporated into electromechanical transmission and will wreck havoc with gearing if it will be beefed-up. Hence gasoline engine of modified Prius is kicking in quite frequently, significantly diminishing fuel efficiency and increasing emissions. Pure PHEV would not start ICE until battery is depleted, under any driving conditions, like sharp acceleration or high-speed driving.

Interesting thing could be done when Toyota Matrix will be offered with Toyota synergy drive (exactly the Prius forward wheel drive unit). Chassis of Matrix allows 4WD configuration, so addition of separate electric RWD (like in previous news post) will automatically make it perfect PHEV with terrific performance and handling. Proper battery is still critical to such vehicle to be economical.


No results for the Cleanova?

Harvey D.


It is being said that the Prius III will have (much) more powerful electric motor(s), higher capacity-lighter advanced battery pack + improved lighter control system. Toyota has not confirmed that the early version (2008/09) will have PHEV capabilities but latter versions (2010?) will most likely be PHEVs.


It seems that the SVE-Cleanova II, built on relatively low cost Renault Kangoo 2007 and/or Fiat's equivalent, will be the first 1000 utility BEV units produced under SVE. A later version may use the new updated Kangoo 2008 or other similar affordable utility vehicle body.

The Cleanova III would use the larger 5-door Renault Scénic MPV and Saft batteries. It too would become a utility BEV.

Peugeot is working on a limited production Model 308 HDI diesel hybrid for 2008/09. Will it materialize?



Agree that Prius is not optimized to take advantage of plug in feature. In any event, these modifications are way too expensive for the average or even above average consumer. The increase in mileage and reduced operating costs cannot in any way justify the exorbitant costs to modify these vehicles. The only way these vehicles will make sense is on a mass production basis that beefs up the electric motor and otherwise optimizes the system from the ground up.

Eventually, it might be shown that 2nd or 3rd EVs to be used mostly in the city only might make more sense than the complicated and expensive PHEV approach. Besides available low power/range NEVs, there is reportedly a Chinese vehicle on the near horizon which will provide highway driving capability and a Corolla type body for around $30,000.


The results of the government research is telegraphed in the title -- "refining the problem". What I have trouble understanding is that these are intelligent people, unable to dismiss easily the growing concern expressed by a large body of science, so how are they able to sleep at night?

Lee Dekker

Nothing happens overnight. Things always take longer than we expected. But when we are barking up the wrong tree, more time will not help.

General Motors screwed up when they produced the EV-1. The EV-1 proved that an all-electric vehicle (using very old technology) “had” a viable place in the mix of American cars. That mistake forced GM to crush the EV-1 both physically and psychologically. Most recently, GM made another horrible mistake by announcing the VOLT.

As a series hybrid the VOLT (and all other series hybrids) eliminate the intricate parallel hybrid fiddling these engineers are wringing their hands over. Compared to all parallel hybrid designs, series hybrids are extraordinarily simple. While not perfect, considering today's battery technology, series hybrids already fit very well into the mix of vehicles we could be driving.

It's wonderful that all these people are working so hard to get the bugs out of parallel hybrids. This work must be done. But all these attempts to convert hybrids into plug-in hybrids and to work out the nuances of parallel designs is coming at the expense of a simpler and in many ways a superior series hybrid layout. The GM volt did not employ "rocket science". GM and Ford and DaimlerChrysler could all produce series hybrid test cars in a very short time. We could all be evaluating these cars today. No battery breakthroughs or other technological hurdles need to be overcome.

All these halfway measures, EV crushings, jury rigged hybrids and calls to wait another decade or two for batteries to get better, stink on ice. Conspiracies be damned, I simply can't get the idea out of my mind that GM and Exxon and all their ilk would like nothing more than to stall the advent of electric (fuel) transportation as long as possible.



As an engineer myself I'd like to take a stab at your question. Automotive engineers sleep well at night because they are actually working on solutions to a very complicated set of problems -- ones with engineering, environmental, economic, marketing, and other aspects.

They are not free to redefine the problem in a way that suits their philosophy or world view. For example, they cannot raise the gasoline tax or impose carbon restrictions. They can only work to improve the vehicles their industry produces.


Re the GM Volt:

Lead acid will work, but it can't compete with $1 gasoline. It might be able to break even with $3 gasoline, but something new has to BETTER than the status quo to break in with anyone but hobbyists and those dedicated to the cause.

Of course, it would help if we'd internalize the externalities of ICE's (pollution, GW, etc), to make the market price of gasoline reflect it's real cost.

The first EV-1 used lead-acid, and got maybe 60 mile range. The 2nd EV-1 used NIMH, and got maybe 110 mile range. Again, useable, but not good enough for anyone but a relatively small group of early-adopters (though I don't mean to entirely let GM off the hook here: they should have persisted with a niche, and worked to expand it, while improving the vehicle).

No, we needed something better than the status-quo, and that didn't exist until the latest generation of li-ion. With economies of scale, and some fairly straightforward (though not easy) engineering, they can be the breakthrough we've all been waiting for.

Scott K.


Hi. I have a few questions regarding your post. I am not an engineer, so please bear with me.

1. Why are you saying that the current production Prius cannot be modified to become an effective PHEV? When I look at the chart in this article, I see the kits from Hymotion and Energy CS getting 200+ MPG. If, like me, your primary goal is petroleum replacement, why would these modifications not be considered to be effective PHEVs?

2. You note that the Matrix, because of its AWD, is more suitable candidate for modification. Would my AWD Highlander also be a good candidate?

3. I also don't understand why people say these things are not cost effective. Here is my thinking: I just bought my Highlander Hybrid. When my warranty runs out seven years from now, if such a kit were to be availabe, I am would strongly consider making it a PHEV. I think gasoline prices will be considerably higher then, so the payback time for the mod would be shorter than it is now. Also, although these kits are expensive, they are cheaper to buy than even the little Toyota Yaris. Is there a flaw in my logic?



Prof Frank

All interesting stuff that we have known for years. What we need is a proper certification procedure for PHEV's. There are many issues and a need for proper definitions to set the standards. Since I have been working on the concept for about 30 years when everyone else was off on H2, I have developed many of the concepts of the PHEV and can help set the certification rules. If I were invited i would be happy to contibute to EPA and California's CARB which is next door to me to help define and create the certification requirements needed to bring PHEV into the marke.

Prfo Andy Frank


Scott. Look at the savings per mile and then consider the cost to make these conversions. Whether you are considering rate of return on investment or breakeven point, you can never justify these mods from an economic standpoint.

The comeback might be, sure, but these mods are good for the environment and they cut back on carbon emissions. Sure, but what kind of savings in co2, etc. could you get for a 10 to 20,000 investment in solar panels, thermal solar, retrofits to your house, etc.


Lee's observations are true with regard to serial hybrid simplicity compared to the parallel PHEV issues. Auto makers will not throw out their business models to jump on the EV bandwagon. The parallel hybrid while overly complex compared to serial hybrid - provides a viable, marketable bridge to EV technology and clean liquid fuels.

Look, some engineers LOVE this kind of challenge - it employs a lot of people and if we were to equate unemployed engineers and autoworkers with inefficiencies of species - better to keep folks busy.


Professor Frank, it's an honor to see your posted comments. The PHEV engineers absolutely need your insight and ideas in developing the future PHEV's. You've already earned your place in the PHEV history books and can be an incredibly valuable resource for future development. What I think is wonderful is seeing the tremendous amount of support and talk about PHEV's that I didn't see even one year ago. It's clear that successful PHEV's will have to be designed and built from the ground up, using nano-tech batteries and, ideally, in the series-hybrid format, excepting large trucks, heavy equipment, which would probably work better with the paralel hybrid with the needed torque. Hopefully, engineers are studying how to make small (1-litre) blue-tech diesel engines for the series hybrids. Bio-diesel is cheap and easy to make and pretty darn clean. With tremendous gov't and industry support, these PHEV, series-hybrid, clean diesel backup cars can be on the road in a few years.


It is indeed an honor to see Professor Frank's words on these pages. It's hard to fathom how Governor Schwarzenegger has not tapped your expertise to this point. Whether it's a mistake, an omission or just politics, a few of us writing letters to the Governor about your many accomplishments would possibly be helpful.

Considering the fact that CARB is right next door to you and that an invite is all that's needed to bring you on board, your sense of humor on the matter is laudable. Whatever happens, with CARB or EPA, we thank you for the years of persistence, hard work and inspiration you've provided so many.

Stan Peterson

Prof Frank,

Thank you for posting. Sometimesa I grow weary with the untrained professional cranks and ever willing to beleive conspiricists, that would repeal the Law of Gravity if they could but get a majority.

It takes skull sweat and determination and lots of both to make progress. I am grateful that you have shown a rational, reasonable, engineering basis for the necessary electrification of ground transport.

I only want to add that you are reported to favor Solar Energy. I fear the fiddling with the Albedo is unappreciated risk, and is a direc, first order effect. Wide scale adoption of the capture of the inefficient solar flux will alter the climate. It is much more dangerous than the tiny consequences from the secondary or ttertiary order effects of a logarithmically diminishing GHG effect, that is well into saturation.

I urge you to investigate and consider the consequences of wide scale increased absorption of the solar flux.

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