BOX 4-2: Calculating the Fuel Economy Effects of Converting a Taurus
to a Series Hybrid with Flexible Engine OperationShifting the drivetrain to a series hybrid configuration saves energy in several areas. First, because there is no
idling of the engine, the 16 percent of fuel consumed during idling on the city cycle and 2.0 percent on the highway
cycle is saved. Second, accessory power demand is not likely to be reduced in a hybrid, as an engine running at or
near its optimal brake-specific fuel consumption point rejects much more heat to the coolant, and, hence, cooling
fan and water pump requirements will increase, but the engine itself is much smaller. Accessory fuel consumption
will be reduced by the improvement in efficiency. Third, the use of regenerative braking will reduce tractive energy
requirements by an amount similar to that for an EV.^1 Fourth, the use of an electric motor drive eliminates the
transmission and improves drivetrain efficiency. Finally, by operating at or near its optimal point, the engine brake
specific fuel consumption is greatly reduced.
On the negative side, a small engine (with smaller cylinders) is inherently less efficient owing to the higher
surface/volume ratio of its combustion chambers. In the Taurus example, the engine would be a 1.0 Iitre four-valve
four-cylinder engine, rather than the 3.0-litre two-valve V-6 used. Although some have discussed using one-or two-
cylinder engines, their noise and vibration characteristics are so poor that only a four-cylinder engine is thought to
be acceptable in a mid-size car (even the three-cylinder Geo Metro engine is considered quite rough in automotive
circles). Hence, peak efficiency is reduced by 2 to 3 percent relative to a two-litre four-cylinder or three-litre 6-
cylinder engine. The generator also must be sized for peak continuous output of 45 kW (e. g., for long hill climbs)
while operating most of the time at 19 kW, making it heavier and less efficient under the standard operating mode.
Detailed analysis of the efficiency without a comprehensive simulation model requires some assumptions
regarding average generator and motor efficiency. To provide an optimistic view of hybrid potential, we chose a set
of “2005 best” values for component efficiencies, as follows:
. Generator efficiency at 19 kW
at 45 kW
. Motor efficiency
urban cycle
highway cycle
. Drivetrain gear efficiency
urban
highway
91 percent
94 percent82 percent
90 percent94 percent
96 percentThe motor and generator efficiency values are 3 to 4 percent higher than those of the most efficient current
motors and generators. Engine efficiency was assumed at slightly off-peak value of 33 percent (in reality, this is
higher than the peak efficiency of small engines today). A cold-start related fuel economy loss of 5 percent was also
used on the urban cycle. The calculation is detailed in table 4-11.
lThe battery for a hybrid vehicle will be designed to emphasize high power capability rather than high energy storage, in contrast to an EV battery.Therefore, even though the hybrid’s battery will be substantially smaller than an EV battery, it should have relatively good capability to absorb the
energy pulse from regenerative braking.