Apr 17, 2012
"Bicycling Science" supports part of your idea. 10^3 mpg is possible; 10^4 is highly dubious. Steel on steel produces the lowest rolling resistance (if noise is acceptable). Above 20 kph, aerodynamic resistance is bigger than rolling resistance for a bike or HPV. The lighter the vehicle, the lower the speed at which aerodynamics dominates. The most aerodynamically efficient vehicle is tear-dropped shaped with minimal frontal area. For ultra-efficiency, the rider would have about as much space as in a coffin.
Apr 18, 2012
Tylerfolsom you have excellent points. The supine vehicle position shouldn't be a problem as long as the seat (more like a recliner) is comfortable. Wilson (3rd ed, p. 166) shows that a road bike at 4 mph has about 2,400 mpg equivalent. If the drag coefficient is cut by 80%, and the rolling resistance cut by a similar amount, this would mean that somewhere between 80-90% of power is saved in the locomotion, since at 4 mph the rolling resistance accounts for the majority of apparent friction. If we can assume for convenience the power is saved by a factor of 8 then the speed is doubled to around 8 mph, or 2,500 mpg at 8 mph. The value at 4 mph would certainly be greater than this, but alas it is not a practical speed. I have calculated my own bicycling trips to be somewhere in the range of 800-1300 mpg based on food consumption. Thus, if a vehicle can be made that can reach far beyond this efficiency, then 10^3 is easily achieved already as a starting point. Keep in mind that this solely relies on the efficiency of the human body, which thermally (correct me if I'm wrong) is app. in the 10-20% range (I can only recall Campbell-Reese-Mitchell and Bicycling Magazine). Thus if electric assists, regenerative braking, or wholly-electric systems (it is possible to combine the three as one) are used, the efficiency may be reaching the upper 10^3 mpg equivalent ranges, close to 10^4 mpg. Suppose a fossil fuel plant generates electricity with a thermal efficiency of roughly 40%. The electric transmission losses amount to 10%, and the battery storage another 20%, and the motor yet another 10%. The product of complements gives about 24%, which represents a figure better than typical human efficiency. Analogously the implementation of electric technologies has enabled road-based gas vehicle efficiency to jump from the 10^1 to 10^2 mpg orders. The electric technologies for ultra-light vehicles range from relatively developed, for e-bikes, to virtually infeasible, such as in regenerative braking. However with the efficiency of this system's vehicles the latter may be re-examined. Basically to sum it up, the technology starts with 10^3 just with raw human efficiency. With electrical technology integrated and developed, the vehicles should get closer to 10^4 mpg. That is where my rationale stems from. However, I am always open to comments and independent analyses which can show otherwise. If the case is compelling the description should be changed to reflect the better estimate.
Jul 24, 2013
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