Hybrids are vehicles that combine two energy sources (for example, an IC engine and a battery) in a single vehicle, and use electric motors to provide some or all of the vehicle’s motive force. The hybrid drivetrain offers several advantages: limited range becomes less of a problem, or no problem; a portion of inertia losses can be recovered through regenerative braking; and the engine can be operated near its optimum (most efficient) point.43 A key disadvantage can be the added weight, cost, and complexity of the hybrid’s multiple components.
A number of proponents have claimed that a hybrid configuration can yield fuel economy improvements of as much as 100 percent over an otherwise-identical conventional vehicle, and a number of experimental vehicles, including winners of DOE’s “Hybrid Challenge” college competition, have claimed very high levels of fuel economy, up to 80 mpg. An examination of the actual vehicle results indicates, however, that the conditions under which high fuel economies were achieved are conditions that typically lead to high levels of fuel economy with conventional vehicles, and the test vehicles typically had limited performance capability. In OTA’s view, the results reveal little about the long-term fuel economy potential of hybrids that could compete with conventional vehicles in the marketplace.
There are numerous powertrain and energy management strategy combinations for hybrid drivetrains, though many are ill-suited for high fuel economy or for the flexible service characteristic of current vehicles. OTA examined a limited set of hybrids designed to achieve a close performance match with conventional vehicles, combining IC engines with battery, flywheel, and ultracapacitor storage (see box 1-4) in series and parallel combinations (see box 1-5). OTA found that hybrids of this sort could achieve 25 to 35 percent fuel economy
improvement over an otherwise-identical vehicle with conventional drivetrain and similar performance if very good performance could be achieved from the storage devices and other electric drivetrain components. The importance of improving electric drivetrain components is paramount here. For example, a series hybrid without improved storage, that is, using an ordinary lead acid battery, would achieve lower fuel economy than the conventional vehicle, because the battery’s lower specific power (power per unit weight) requires a larger,
heavier battery for adequate performance, and because more energy is lost in charging and discharging this battery than would be lost with a more advanced battery. This latter result agrees with results obtained by several current experimental vehicles built by European manufacturers.
Perfecting high power density/high specific power44 batteries or other storage devices is critical to developing successful hybrids. Because the hybrid’s fuel provides its energy storage, attaining high specific power and power density would allow the storage device to be much smaller and lighter--critical factors in maintaining usable space onboard the vehicle and improving fuel economy.
As noted, there are numerous strongly held views about the fuel economy potential of hybrids,
ranging from the view that hybrids offer limited (if any) potential to a view that hybrids can yield
100 percent or higher fuel economy improvement with equal performance. European and
Japanese automakers are particularly skeptical about hybrids. Those who are optimistic appear to
be basing their position on the likelihood of radical improvements in the weights and efficiencies
of batteries, motors and controllers, and other electric drivetrain components. OTA’s analysis
assumes that substantial improvements in these components will occur, but there clearly is room
for argument about how much improvement is feasible.
According to OTA’s analysis, in 2005, a mid-size series hybrid combining a small 50 HP (37
kW) engine with a bipolar lead acid battery, with an optimized steel body, could achieve 49 mpg
at an increased price of $4,900 over the baseline (30 mpg) vehicle. If the energy storage device
were a flywheel and the body were aluminum-intensive, the hybrid could achieve 61 mpg, but at a
substantially higher price, and the engine would have to be turned on and off several times during
all but the shortest trips45 —raising some concerns about emissions performance, because
immediately after an engine is started emissions generally are higher than during steady
operation. 46 By 2015, a series hybrid with an improved bipolar lead acid battery (assuming this type of
battery can be perfected) and an optimized aluminum body could be considerably more
attractive---attaining 65 mpg at an estimated additional cost of about $4,600 to the vehicle
purchaser. A similar vehicle with ultracapacitor or flywheel could achieve still higher fuel
economies-71 and 73 mpg, respectively—but the earlier problems with turning the engine on
and off would persist, and the price would likely be substantially higher than with the battery. The
need to turn the engine on and off is a function of the limited energy storage and high cost/kwh of
storage of the ultracapacitor and flywheel, so that improving these factors would reduce this need
and improve emissions performance for these vehicles.
The projected fuel economy values for these hybrids is strongly dependent on improvements in
the component efficiencies of the electrical drive system. Although the values projected by OTA
are higher than those attainable today, PNGV and others hope to do still better—which would, in
turn, yield higher vehicle fuel economy. For example, in 2015, an additional 4 percent increase in
motor/generator efficiency would raise the lead acid-based hybrid’s fuel economy from about 65
mpg to nearly 69 mpg; the same increase would raise the ultracapacitor-based hybrid’s fuel
economy from about 71 mpg to approximately 75 mpg. Similar improvements in other
components, such as the energy storage devices, could allow the ultracapacitor-based hybrid (and
the flywheel hybrid) to achieve PNGV’s goal of 82 mpg, which is triple the fuel efficiency of
current mid-size cars.
An intriguing feature of many of these hybrids-specially those using batteries for
energy storage is that they can operate in battery-only mode for some distance. For
example, the 2005 and 2015 battery hybrids in tables 1-1 and 1-2 have battery-only ranges of 28
and 33 miles, respectively. This would allow them to enter and operate in areas (e.g., inner cities)
restricted to EV operation. In addition, although these vehicles are designed to be independent of
the electric grid, they could have the capacity to be recharged, allowing them to operate as
limited-capability/limited-range EVs in case of an oil emergency—an attractive feature if the
future brings more volatile oil supplies.
Although most U.S. developers appear to be focusing their efforts on series hybrids, OTA
estimates that parallel hybrids that used their engines for peak loads and electric motors for low
loads could achieve fuel economy gains similar to those of the series hybrids examined by
OTA—25 to 35 percent. The development challenges of parallel hybrids appear to be more severe
than those of series hybrids, however, because of this type of hybrid’s unique driveability
problems 47 and its requirements for stopping and restarting the engine when going back and forth
between low and high power requirements.48
The hybrids discussed above are designed to compete directly with conventional autos—that is,
they would perform as well and, being disconnected from the grid, have unlimited range as long as
fuel is available. There are other configurations, or other balances between engine and energy
storage, that could serve a different, narrower market. For example, vehicle designers could use a
smaller engine and larger energy storage that would be recharged by an external source (e.g., the
electricity grid) to achieve a vehicle that could serve as an EV in cities49 and would have relatively
long range. This design would not perform quite as well as the hybrids discussed above, however,
and would have to be recharged after a moderately long trip.
California is considering allowing hybrids to obtain ZEV credits, if these vehicles meet a
minimum EV range requirement. This would tend to push hybrid designs in the direction
discussed above (small engine, large energy storage), and reduce the likelihood that those energy
storage devices with low specific energy—such as ultracapacitors and possibly flywheels-will be
attractive candidates for commercialization.
Advanced Automotive Technology: Visions
of a Super-Efficient Family Car
OTA-ETI-638
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