|Average fuel usage over the complete RTS dataset. Click to enlarge.|
An Extended-Range Electric Vehicle such as the Volt can reduce real-world fuel consumption compared to a comparable 40-mile all-electric range (urban cycle) Plug-in Hybrid Electric Vehicle by more than 50%, according to a set of simulations run by GM using the operational data from 621 drivers captured in the Southern California Association of Governments (SCAG) Regional Travel Survey (RTS).
Furthermore, while only 5% of the simulated PHEV drivers would achieve EV-only operation, fully 64% of the E-REV drivers would achieve 100% EV operation. The E-REV platform showed only one-third the total number of initial cold engine starts, compared to the cold starts required by the PHEV systems, according to Peter Savagian, Engineering Director of GM’s Hybrid Powertrain Systems Organization. Savagian presented the initial results of the study at the 2008 SAE Hybrid Vehicle Technology Symposium in San Diego (13-14 February).
GM has written a draft paper—The Electrification of the Automobile: From Conventional Hybrid, to Plug-in Hybrids, to Extended-Range Electric Vehicles—describing the study and results in more detail.
GM is defining an extended-range electric vehicle as:
A vehicle that functions as a full-performance battery electric vehicle when energy is available from an onboard RESS [Rechargeable Energy Storage System] and having an auxiliary energy supply that is only engaged when the RESS energy is not available.
By contrast, the two predominant PHEV operating strategies under discussion today (both for parallel-hybrid configurations) are either a blended strategy which is very similar to a conventional hybrid, but with a larger, rechargeable battery; or an initial EV strategy that allows electric-only operation over the complete power and speed range of a defined cycle, often the urban schedule. Both of these approaches require more usage of the engine than the series-hybrid E-REV, which runs all-electric, regardless of the cycle demands, until the battery pack—a larger pack than used in the PHEV configurations—is depleted to the defined threshold. (See diagram below.)
|Battery charge and engine use in the two PHEV operating strategies and the E-REV operating strategy. Click to enlarge.|
In the study, using the basic specifications of a Malibu-like sedan, GM simulated the performance of:
Hybrid Electric Vehicle (HEV) with a 40 kW electrical power constraint;
Conversion PHEV: a PHEV powertrain with a 35 mph (56.32 kph) speed constraint, a 40 kW electrical power constraint, 3.5 kWh of usable electrical energy (as opposed to total battery pack energy), and a blended operating strategy;
Urban-Capable PHEV: a PHEV powertrain with a 60 mph (96.56 kph) speed constraint, a 53 kW electrical power constraint, and 3.5 kWh of useable electrical energy; and
E-REV: a powertrain with 8 kWh of useable electrical energy (Volt pack is spec’d at 16 kWh total) and EV capability not limited by electric power or driving speed.
The key to the results of the simulation is the behavior of the real drivers represented in the data set. The data contains widespread and significant driving at power levels and speeds beyond that represented by the urban driving schedule.
|Net battery energy versus distance driven, compared to the requirements of the three different cycles. Click to enlarge.|
GM calculated the driving intensity—the net energy per mile (kWh/mile)—required by the urban cycle, the highway cycle, and the much more aggressive US06 cycle, then compared these to the RTS data. (See diagram at right.) They found that while only 3% of the real-world drivers fit within the urban cycle and 21% fit within the highway cycle, fully 97% fit within the requirements of the US06 cycle.
In the study, GM found that:
An E-REV is more than ten times as likely to finish the day as an EV than an urban-capable PHEV derived from an HEV, when operated in the actual application, as represented by the RTS data set.
Similarly, an E-REV will consume, on average, less than half of the petroleum of a PHEV in the real world, if overnight charging is assumed.
An E-REV will reduce regulated emissions that are due to initial trip starts by more than 70% when compared to a PHEV in the actual application.
“Electric range” when operating on the urban schedule is not a direct measure of a plug-in vehicle's ability to run with the engine off, ability to displace petroleum or ability to reduce regulated emissions in the actual application. Rather, the ability to run with full performance on electric power alone leads to improvements which would be realized in actual application.
We conclude that electrification that enables E-REVs may be well worth the effort. Specifically designed electric powertrains, incorporating higher power motors and thermal systems, higher energy batteries and integrating them into vehicle structures specifically designed for that purpose will be rewarded with societal benefits realized in real world use.—“The Electrification of the Automobile”
While PHEVs can make improvements when compared to HEVs, an E-REV appears to realize a much greater portion of societal benefits.
During the SAE presentation, Savagian noted that GM designed the Volt with the intention of delivering 40 all-electric miles on the urban schedule. Driven under the more aggressive US06 scenario, he said, the Volt would deliver about 32 all-electric miles.
GM plans to produce and sell HEVs, PHEVs and E-REVs.
E. D. Tate, Michael O. Harpster and Peter J. Savagian (2008) The Electrification of the Automobile: From Conventional Hybrid, to Plug-in Hybrids, to Extended-Range Electric Vehicles