A set of papers presented yesterday by engineers from Toyota, Argonne National Laboratory and GM at the SAE 2008 World Congress explored the impacts and requirements of different types of plug-in hybrid electric vehicles (PHEVs)—specifically extended range electric vehicles versus a blended power split approach.
The Toyota paper, Study on the Potential Benefits of Plug-in Hybrid Systems, started by noting that while plug-in hybrids can address the three big issues facing transportation—fuel consumption and energy diversification; greenhouse has reduction; and air quality—breakthroughs in battery energy density, reliability and cost must occur.
Toyota’s basic argument is that the costs and trade-offs of deploying an extended range electric vehicle architecture at this time outweigh the benefits, and that blended systems have greater benefit at this point in time.
Toyota considered a series hybrid with a small ICE as a range extender using an EV strategy (e.g., the Chevrolet Volt) and a parallel hybrid with a plug-in pack using a blended strategy (e.g., the Toyota PHV based on the Prius).
Using the US06 cycle to represent typical North American driving, Toyota concluded that up to 100 kW of output is required to drive that cycle in a mid-sized vehicle. The company then considered the operation of a blended system designed for charge depleting operation over the city cycle. Toyota concluded that maximum output required is approximately 40 kW, and the average was less than 20 kW.
An analysis of the impact of reducing battery power showed that while a 40 kW battery enabled all electric operation almost 100% of the time in the city cycle, using a 20 kW battery still enabled electric operation 95% of the time. An extended range vehicle, Toyota pointed out, would require a battery, motor and electrical system capable of providing maximum power (100 kW).
Toyota also concluded that while CO2 reduction increases for PHEVs with longer all-electric range, the benefit is not linear, and that as EV range increases, CO2 reduction levels off due to the high percentage of drivers with short daily driving distances.
Toyota argued that providing increased EV range increases vehicle cost due to higher battery cost; reduces luggage space; and increases fuel consumption in charge sustaining mode due to increased battery mass.
A paper from Argonne National Laboratory, Comparison of Powertrain Configuration for Plug-in HEVs from a Fuel Economy Perspective, analyzed several PHEV powertrain configurations, including series (e.g., Volt), pre-transmission parallel (e.g., the Sprinter van) and power split (e.g., Prius) with respect to component sizes and fuel economy for 10- and 40-mile all electric range (AER) applications.
Argonne sized the batteries in its evaluation (which used the Powertrain Systems Analysis Toolkit, PSAT) to follow the UDDS urban cycle while in all-electric mode, and to meet the AER targets for the vehicle. Argonne also specified that the PHEV would operate in electric-only mode at higher vehicle speed in comparison with regular hybrids.
|Component Sizes in Argonne Study|
|Parameter||10-mi AER||40-mi AER|
|Pre-trans Par.||Split||Series||Pre-trans Par.||Split||Series|
|Engine power (kW)||76||74||109||79||77||114|
|Propulsion motor (kW)||48||62||90||50||71||95|
|Generator power (kW)||NA||63||106||NA||65||111|
|Battery power (kW)||58||52||55||61||64||58|
|Battery capacity (A·h)||18||21||18||71||69||71|
|Total vehicle mass (kg)||1,675||1,667||1,700||1,764||1,800||1,794|
|Simulated fuel economy results under urban and highway test cycles for the Argonne study of three different configurations in 10AER and 40AER cases. Click to enlarge.|
In terms of fuel economy, Argonne found that the split configuration provided the best fuel economy. The series configuration suffered from dual power conversion—from mechanical (engine) to electrical (generator) and back to mechanical (electrical machine). Engine efficiency was higher for the series configuration that for other configurations, and electrical efficiency was practically identical for all three configurations.
The Argonne researchers concluded that:
Although both the power split and series configurations require two electric machines and an engine, the series configuration, as expected, requires significantly higher component power as a result of the many component efficiencies between the engine and the wheel.
In terms of efficiency, all of the configurations achieve similar characteristics when operated in electric mode. Both series and power split configurations do not use a multi-gear transmission, but the parallel configuration makes up for losses by operating the electric machine at higher efficiency points. In CD mode, the power split provides the best fuel economy as a result of its dual path of power form the engine to the wheel.
On the basis of the thermal and electrical consumption analysis, series configurations appear to be an appropriate choice for vehicles designed to provide long AER because of their simplicity in terms of control and their ability to operate in electric-only mode at high vehicle speed. The power-split configurations appear to be a valid choice for vehicles based on a CD approach.
The GM paper, The Electrification of the Automobile: From Conventional Hybrid, to Plug-in Hybrids, to Extended Range Electric Vehicles, was presented in an earlier form at the 2008 SAE Hybrid Vehicle Technology Symposium in San Diego (13-14 February). (Earlier post.)
GM is currently developing both a blended strategy PHEV (the Saturn VUE Green Line PHEV is a derivative of the conventional 2-Mode Hybrid) and the extended range electric vehicle (E-REV), the Volt.
For the paper, GM simulated the performance of a mid-size sedan with a conventional powertrain; an HEV with a 40 kW electrical power constraint; a converted PHEV 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; an urban-capable PHEV with a 60 mph (96.56 kph) speed constraint, a 53 kW electrical power constraint, and 3.5 kWh of useable electrical energy; and an E-REV with 8 kWh of useable electrical energy and EV capability not limited by electric power or driving speed.
The key to the results of the simulation is the use of the operational data from 621 drivers captured in the Southern California Association of Governments (SCAG) Regional Travel Survey (RTS).
|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.
GM concluded that:
The real-world RTS data set contains widespread and significant driving at power levels and speeds beyond that represented by the urban driving schedule.
An E-REV is more than ten times as likely to finish the day as an EV than as urban-capable PHEV derived from an HEV, when operated in the actual application, as represented by the RTS data set.
An E-REV will consumer, 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.
In the event of a petroleum disruption, an E-REV could support uncompromised vehicle operation for the majority of drivers.
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 in real world use. While PHEVs can make improvements compared to HEVs, an E-REV appears to realize a much greater portion of societal benefits.
Masayuki Komatsu (Toyota), et. al.; Study on the Potential Benefits of a Plug-in Hybrid System (SAE 2008-01-0456)
Aymeric Rousseau (Argonne); Comparison of Production Powertrain Configuration Options for Plug-in HEVs from Fuel Economy Perspective (SAE 2008-01-0461)
Edward Tate (GM) et. al.; The Electrification of the Automobile: From Conventional Hybrid, to Plug-in Hybrids, to Extended-Range Electric Vehicles (SAE 2008-01-0458)