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 #2||Energy CS|
|Fuel economy [mpg]||200||221||114||66|
|Grid Elec consumption
|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)