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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.
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