10-11-2006, 07:07 PM
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Enthusiast
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Real Name: Greg Gunther
Hybrids: 2006 Prius
Posts: 2
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Prius Mileage, Theoretical Considerations.
I've read about people's getting very high mileage numbers on secondary highways while driving in the 50 mph speed range using the glide technique. In this condition, the car is powered by the battery and electric motor only, and the ICE (Internal Combustion Engine) is mostly off.
I have no doubts that those cars did reach high mileage numbers during that period, but something bothers me.
The only energy source for the Prius (at this writing) is gasoline. On the average over time, the battery will be charged to 80% full. If the car is running on stored electricity for some period, then the battery's energy debt must eventually be repaid. That repayment will come as a result of burning gasoline.
Okay, but can't we drive more efficiently? I'm not sure. I think Toyota has set things up to maximize efficiency and the driver doesn't have a lot of choice.
One thing is clear--over 20 mph or so, using the electrical-drive system to turn the wheels wastes energy. Here's why:
Every step involved in generating energy wastes energy.
How much? Here's an estimate:
Generator: 60%? I couldn't find Prius numbers and took a guess based on other generators.
Battery: 95%, maybe a little higher.
Electric motor: 95%. Another guess, but probably not far off.
Total electrical-system efficiency: about 54%.
This means approximately 46% is wasted.
Conclusion: For best mileage, use the ICE to drive the wheels directly and avoid using the electrical system to drive the car. Again, not that we have that much choice in the matter.
The ICE's efficiency is about 35% efficient over the upper 70% of its power output range. That's pretty good, partly as a result of the unique Atkins-stroke engine. In the lower 30% of its output range, efficiency is less, down to about 25%. (See http://techno-fandom.org/~hobbit/cars/prius-curves.gif.)
This is a simplistic theoretical analysis. How realistic is it?
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Greg Gunther
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