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The Three Fundamental Efficiencies of Hybrid Technology - Why Are Hybrid Cars More Fuel Efficient than Regular Cars

March 12th 2006

The Three Fundamental Efficiencies of Hybrid Technology - Why Are Hybrid Cars More Fuel Efficient than Regular Cars

Lexus RX330

        In transportation based hybrid power production ­ How the power is produced takes a back seat to how the power is processed. The three fundamental efficiencies of refined wheeled vehicle hybrid power processes are "Prime Mover Power Averaging", "Regenerative Braking," and "Peaking Power."

Power Averaging:

        "Power Averaging", also referred to as "load averaging or load leveling", allows maximum power output of the prime mover to be reduced to that slightly greater than that needed to cruise on level terrain at maximum sustained velocity. This reduces prime mover (engine) mass and volume; offsetting the mass and volume of the required temporary storage device.


        Peak process demand in an automobile or other wheeled vehicle occurs during acceleration and grading (climbing hills). Existing hybrid systems translate this demand real time to the prime mover.  In the next generation of hybrid automobiles the prime mover will operate at a near constant output from start up to shut down. The prime mover will feed near constant power into a temporary storage device (battery, capacitor, flywheel, or hydraulic accumulator) and the variable real time demand power will be pulled off that storage device. This will allow for a very significant reduction in maximum power requirements of the prime mover, optimize prime mover power production, and allow the introduction of prime movers not well suited to rapidly variable demand.

        With Power Averaging as a primary efficiency of hybrid technology, the fueling algorithm of the next generation of hybrid automobiles will be a bit more complex than a simple iterative program. The coming generations of automotive control computers will "remember" power use based on both "routes" and "habits", calculate average demand, and de-couple operator control from prime mover demand with the ability to handle unanticipated variability of daily commuting. How that will be accomplished is beautiful in its simplicity.


        The reciprocating engine dominates automotive technology because of its ability to respond rapidly through a broad power range. Power Averaging will allow the adaptation of prime movers that, although may not respond rapidly to power variation, operate at very high efficiently when operated at a near constant output. Given modern materials, the economics of scaled production, and the primary fundamental hybrid efficiency of Power Averaging, the choice of prime mover technology will be expanded to include Turbine and Sterling technology as well as existing Clean Diesel Combustion,conventional gasoline powered reciprocating engines, fuel cells, or any other primary power technology.

Regenerative Braking:

        Regenerative Braking can be described as the recovery of kinetic energy (energy of the vehicle mass moving) to stored energy through numerous regenerative braking methods. Prior to regenerative capable hybrids, highway driving was more fuel-efficient than city driving even though aerodynamic losses of high speed highway driving far exceeded that of low speed driving in stop and go traffic. 


        In stop and go driving the energy conversion process can be described as the conversion of chemical potential energy of the fuel source by the prime mover, to kinetic energy of moving vehicle mass, to friction braking heat loss. The energy loss is directly proportional to the number of stop and go cycles. The multiple cycles of this conversion loss in stop and go driving can greatly exceed that of aerodynamic drag in constant state highway driving. Existing hybrid systems now regenerate significant amounts of energy lost during braking and this fundamental efficiency is expected to radically improve as the power and energy density of the temporary storage devices continue to improve.

Peaking Power:

        "Peaking Power," when described as a fundamental efficiency of hybrid technology, refers to short term/ high power supply of energy to the power train from a temporary storage device during the peak demand periods of acceleration and grading (climbing hills.) This allows the drive train to be supplied power at rates much higher than the maximum power rating of the prime mover for short periods. The prime mover must still be capable of producing a sustained power slightly greater than the power required to cruise at maximum sustained velocity on level grade, but the cruising (or average) demand is much less than peak demand.

        The coming generations of hybrid vehicles will have the operator controlled capability to rapidly accelerate to velocities significantly greater than maximum sustained velocity. This will be accomplished by summing the energy available in storage with prime mover output. As the storage device discharges, the sustained operating velocity will fall back to where prime mover power is balanced against drag and grade. The drive train will be the limiting factor in accelerative performance but the accelerative performance of yesterdays "muscle cars" may pale in comparison to the accelerative performance of a hybrid with a “muscled” drive train.

        I am sure there will be more variations in storage and conversion methods but "Prime Mover Power Averaging", "Regenerative Braking," and "Peaking Power" will remain fundamental efficiencies in hybrid technologies whether it be wheeled vehicles, small unit electrical power production, or the myriad of other applications.

        In closing I want to say the potential for significant enhancement of transportation efficiency still exists in both automotive and rail applications. Energy saving returns can come a lot quicker and cheaper than reinventing the technology. We have begun by sequencing traffic lights to traffic patterns and may soon expand that concept out to a 12,000-ton train that may never have to stop for a meet.

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By Mark J. Carter
I have been a student of Transportation Hybrid Technolgy for over 25 years. I authored a paper titled "Flywheel Storage and Regenerative Braking as Applied to Wheeled Vehicle Transporation Processes" as a term paper at the University of Northern Iowa in the Fall of 1978. I followed on with a "1984 Rolex Award" entry in 1983, proposing an automotive application of hybrid technology.
I am currently employed as a Safety Inspector with the Federal Railroad Administration, United States Department of Transportation. Mark J. Carter  P.O. Box 765 Jefferson, TX 75657  MJCarter01@aol.com

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