http://www.evworld.com/view.cfm?sect...le&storyid=887

the information comes from what i can glean from the user comment section at the bottom, not the article itself. the discussion is lively and interesting however as i said, i'm not a mechanical engineer so i'd be interested in hearing your comments (presuming your a mechanical engineer)

here is the particular quote from the discussion that references this:

Chris Ellis (author of the article that makes the claim):
Roger, as I said in an earlier comment, what may be irrelevant at Japanese speeds and acceptable at American speeds can be a problem at (still legal) European speeds. My calcs show a Prius needing a NET 21+ kW at the wheels to sustain 80 miles per hour on the level, implying a requirement for a lot more kWs out of the engine and into the transmission. As you say, 20% losses probably grow to 30% at higher speeds. You've shown that the use of a continuous electrical path doesn't matter much at 60, but it certainly does at 80, which was my original point. However, I absolutely accept your excellent point that the generator needs to generate (and therefore spin) most of the time (because there is a heavier than normal ancillary electrical load), which makes the solution more challenging. So, if you accept there is clearly a significant problem at high speeds, I will concede that a simple lockup on the generator/sun gear probably isn't enough to fix it. Which could explain why Toyota didn't put it in, and why GM/DC and others are working on more efficient solutions for higher speeds.
29/Aug/2005
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Roger Pham (previously rebutting):
Chris, yes, we have perfect agreement that the loss due to the electrical path will be much higher at higher speeds. At higher speed, due to the higher drag involved, the engine will be in lower gear ratio (figurative speaking) meaning that the engine will have to turn more rpm for every turn at the wheels in comparison to cruising at lower speed, due to the continously variable feature of the transmission coming into play. This means that the generator will turn proportionally much faster than the engine, and proportionally, more of the total torque and horsepower at the driving wheels will have to go through the generator-motor pathway which will now be subjected to ~30% loss of efficiency and not 20% because of higher current going through the copper winding meeting more internal resistance instead of impedance. I would predict that the total efficiency loss at 85 miles per hour would be in the order of 15-20 %, and I'm sure that you have calculated a more accurate number based on actual testing. That's why technical review articles such as yours are important in order for the prospective buyer to be awared of problems not mentioned otherwise. The Prius of course was designed for the American and Japanese market, where the typical cruising speed would be a lot slower than in the European autobahn. Direct injection turbo diesel (TDI) would be more suitable for the European market, where diesel fuel costs half the cost of gasoline due to much less tax on diesel fuel. Daimler-Chryler has just revealed a four seat TDI capable of 70mpg freeway miles, having a cd (drag) of merely .16 (very slippery). Horses for different Courses, indeed. Roger
29/Aug/2005
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