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OCR for page 195
Appendix E
Duty Cycles and Fuel Economy of Hybrid Vehicles
The section on Hybrid Vehicles in Chapter 4 raises the issue of whether a hybrid
combat vehicle would have better fuel economy than a vehicle with a conventional
mechanical power plant (e.g., a diesel or turbine engine without the electrical conversion
and storage subsystem of a hybrid) for "AAN-like" duty cycles. This appendix cannot
answer the question, but it does explain in more technical detail the reasons why the
committee believes the Army should make complete engineering assessments before
making design decisions.
A hybrid-electric power plant reduces fuel consumption principally because the
engine runs more of the time at or near peak efficiency, rather than at lower engine
speeds where the engine is less efficient at converting stored energy in the fuel to
mechanical energy in the driveshaft. In addition, if the hybrid system uses electrical
motors at the drive wheels as electrical generators during vehicle braking, some of the
kinetic energy of the vehicle's motion can be recaptured and stored (regenerative
braking). By running the engine at near peak efficiency and storing the extra energy
during off-peak demand phases of the duty cycle (and by storing the energy captured
during regenerative braking), the electrical subsystem can provide the energy required
during peak power demand phases of the duty cycle (acceleration, climbing hills, etch. If
the duty cycle includes enough off-peak (and braking) time to keep the electrical storage
component energized, then a smaller engine can be installed, which is how fuel economy
is improved. Ideally, the engine would be sized so that it runs constantly near its
maximum rated continuous speed, where it is most efficient, because storage capacity for
excess engine power output over vehicle power demand is always available. The mean
power demand of the duty cycle, however, must be much Tower than the rating of the
engine for maximum sustained power output.
Figure E-! shows the duty cycle for a vehicle weighing 3.4 metric tons (7,500
Ib.) with an engine rated at 235 brake horsepower and a maximum torque of 440 ft-Ib.
The figure shows the percentage of time during the duty cycle that the engine operates
within nine ranges of engine speed and seven ranges of torque. (Power equals engine
speed times torque.) This duty cycle approximates that of a city bus making frequent
stops along its route. Note that torques greater than 250 ft-Ib. (the back row of celIs) only
occur during a very small percentage of the duty cycle. The cells representing most of
the duty cycle are clustered toward the front left of the diagram at low-to-medium engine
195
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196 REDUCING THE LOGISTICS BURDEN FOR THE ARMYAFTER NEXT
speeds and Tow torque. The average power, for two different driving conditions, ranges
from 14.5 to 15.4 brake horsepower, or about 15 percent of the engine's rated power.
This duty cycle is a good candidate for a hybrid vehicle. Applications like this one are
the source of the rule of thumb that, to consider a hybrid-electric power plant for reasons
of fuel economy, the average power demand in the duty cycle should be one-fifth or less
of the peak demand.
an
. _ _
cle Ave. Powe = P
As r 145Hp(w a nng)
Ave.Power=754Hp(w/on ng
Peak Torque = 440 ft-lbf
~d'4'~
SPe~(rPm, ` ~nib
7°~150
5~7
_ =50 ,C
F CO
FIGURE E-! Duty cycle for a 3.4 metric-ton vehicle with an engine rated at 235 brake
horsepower and a maximum torque of 440 ft-Ib.
Representative terms from entire chapter:
power demand
