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Regeneration on flat terrain

tommiboy said:
now that i think about the terrain, my drive between dallas and fort worth is very flat. wouldn't that mean that i won't be able to take much advantage of the regenerative feature?
You don't really get any regeneration at highway speeds unless it's a very steep downhill. Otherwise, the engine and/or motor is required to do some pushing at all times to overcome friction and drag.

Regeneration is mostly hype. The actual amount of energy saved is not significant--most electrical power comes from the engine spinning the motor as a generator. For example, a typical 5-minute block of city driving may have a single "gold star", representing 50 Watt-hours of electricity created by regenerative braking. This equals 3 kW-minutes, which is 180 kW-seconds. The main drive motor draws a maximum of 33 kW, so this one "gold star" will power the motor for about 5.5 seconds at full power, or 11 seconds at half power. I usually get much more than 11 seconds of stealth mode in 5 minutes of city driving, so the rest of the electricity comes from the generator.

For highway driving, the Prius' great mpg comes from a low drag design, a small gas engine, and not going too much over 60 mph. (Those who understand have claimed a theoretical "sweet spot" of approximately 60 mph for the Prius' gas engine.) I typically drive 69 mph in a 65 mph zone, and I typically see 48 mpg. I have noticed this is heavily affected by wind (despite the Prius' low drag); a headwind or tailwind can subtract or add 5-10 mpg to a trip. In fact, those who drive in hilly or mountainous areas claim terrain has little effect on mileage.

Douglas (2002 Silver, Wisconsin)
 

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maintaining speed

tommiboy said:
if there is no problem keeping 75mph while going up a grade, doesn't that mean that for my driving, once i get up to 80mph, i should be able to stay there for the duration of my trip (traffic allowing, of course)?

so if i understand correctly, the speed issue is not whether the car can get up to and stay at a specific speed (say 80mph), but rather the time for acceleration to get there?
Yes, there should be no problem maintaining 80 mph. It will take you longer to get there than other cars, and you will have less passing acceleration at that high of a speed, but you can stay there indefinitely.

Those who have studied and modeled the Prius have concluded that it has a maximum sustainable speed of 84 mph. The maximum short-term speed is 100 mph (assuming level terrain and no wind, either of which can push you faster or hold you back). However, at speeds above 84 mph, the supplemental electrical power required by the motor to hold the speed is greater than the amount of electrical power that can be generated by the engine while it is providing enough kinetic power to hold the speed.

These calculations were performed on the THS found in 2001-2003 models. Undoubtedly, the 2004 Prius will have a higher sustainable speed. It will also have better acceleration to 80mph and better passing capability at those speeds.

Douglas (2002 Silver, Wisconsin)
 

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Drafting

Setting aside the safety aspects of drafting, and just considering the physics, would the semi driver be a little perturbed?

Presumably, the energy to keep your car moving at a steady speed has to come from somewhere, and since it's not coming from your car, it must be coming from the semi. So, I expect any increase in your mileage would come at the expense of a resulting drop in the semi's mileage.

However, my knowledge of aerodynamics is shaky. Is it possible that the poor aerodynamics of a semi (flat rectangular back end) mean that the semi has already lost that mileage? And therefore, it would really be "free" energy to the drafter?

Douglas (2002 Silver, Wisconsin)
 

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Aerodynamics

OK, I accept your assessment of the aerodynamics. So, why don't semi drivers attach some kind of lightweight cone-shaped baffle to the back of their trailer to break up their low-pressure zone? Presumably, this would improve their mileage and save money.

Douglas (2002 Silver, Wisconsin)
 
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