4-2 and table 4-11, shows that urban fuel economy for the HEV “Taurus” is 32.7 mpg,
highway fuel economy is 41.2 mpg, and composite fuel economy is 36.1 mpg, which is about
30 percent better than the current Taurus. Most of the improvement is in the urban cycle, with
only a small (8.4 percent) percentage improvement on the highway cycle--not a surprising result
because engine efficiency is quite high at highway speeds.
The 30 percent improvement is an optimistic value for current technology, since the efficiencies
of every one of the components have been selected to be at 2005 expected values, which are
higher than the actual observed range for 1995. It also assumes the availability of a semi-bipolar
battery that can produce high-peak power for acceleration. In the absence of such high-peak
power capability, fuel economy drops precipitously. If a normal lead acid battery with a peak-
power capability of 125 W/kg is used, composite fuel economy is only 24.5 mpg, which is
almost 12 percent lower than the conventional Taurus. These findings are in good agreement
with the observed fuel efficiency of some HEVs with conventional lead-acid batteries. As noted,
both Nissan and BMW reported lower fuel economy for their series hybrid vehicles, which used
nickel-cadmium batteries with specific peak power of 125 to 150 W/kg.^59
Table 4-12 presents detailed assumptions and results for analyses of several series hybrid
vehicles that might be ready for introduction by the years 2005 and 2015. For these vehicles, ICES
were combined with bipolar lead acid batteries, ultracapacitors, or flywheels using the same
flexible operating regime evaluated above. The main focus of the results should be on the last five
rows in the table, which lists urban, highway, and composite fuel economy, range as a pure EV
with the engine off, and the amount of time the storage mechanism can put out maximum power if
it begins with a full charge.
In 2005, improvements to engine peak efficiency, higher battery peak-power, and body-weight
reductions are expected to provide significant improvements to the fuel efficiency of an HEV with
battery storage (using a bipolar lead acid battery); fuel economy increases to 48.5 mpg. This
however, is only a 25 percent improvement in fuel economy over the 2005(m) scenario vehicle
using the same body, aerodynamic, and rolling resistance improvements. The reduction in fuel
economy benefit relative to the advanced conventional car--the benefit in 1995 was 30 percent--
occurs primarily because engine technologies such as variable valve timing (VVT) and lean-bum
help part-load fuel efficiency more than peak efficiency. Hence, a crucial advantage of the series
hybrid--maintaining engine efficiency close to the highest point--is steadily eroded as part-load
efficiencies of the IC engine are improved in the future.
Several of the HEVs evaluated in table 4-12 can, if necessary, operate for a while as an EV,
though with reduced performance and limited range. With a bipolar lead acid battery, for example,
the 2005 series hybrid has a range of about 28 miles maximum, or 22 miles realistically. The use
of an ultracapacitor, if it is sized only to provide peak power requirements for acceleration,
reduces the range to less than one mile, owing to the ultracapacitor’s high power-to-energy ratio.
In fact, if sized this way, the ultracapacitor stores only 0.1 kWh, so that it can deliver the required
peak acceleration power of 40 kw for only eight seconds, which clearly is impractical. In OTA’s
(^59) S. Friedman and K. Scheurer, "On The Way to Clean(er) Vehicles,” SAE paper 94C052, 1994; and Nissan, personalcommunication with
Energy and Environmental Analysis, Inc., June 16, 1994.