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Regenerative braking is grossly over-rated
Posted by Reality (more from Reality) on Thu, 14 Jun 2012 10:03:09
In Reply to: Re: The one thing I don't get:, Snowmobile , Thu, 14 Jun 2012 08:14:58
Regenerative braking came up before in another thread, I let it go. Instead of hand waiving, let's put some numbers into perspective:

65mph is a little less than 29 meters per second. Therefore a 1500kg car (3300lbs) going at 65mph only has about half a MegaJoule of kinetic energy. Gasoline energy density is 47MJ/kg, and a gallon of gas is about 3kg. So the entire kinetic energy of the car only represents 1/300 of a gallon of gas. How many times does one haul the car to stop while driving on highway for a 40mile stretch (burning one gallon of fuel)? Remember, only a small percentage of the car's kinetic energy can be recovered in regenerative braking. Most of the energy spent at highway speed is fighting air and ground/drive train friction; none of that is recoverable.

Regenerative braking does not account for much of the fuel saving of hybrids. Hybrids save fuel by shutting off the engine when the car is stopped. Gasoline engine has to burn significant amount of fuel even when idling in order to prevent detonation. There isn't much fuel saving to be had running at highway speed. The hybrid car probably would do better mpg-wise at highway speed by stripping out the batteries and electrical drive components and keep the same small ICE engine. Going through a round-trip energy conversion at highway speed would probably lose more than gaining. That's why even the Volt has the ICE driving the wheels directly at highway speed.

posted by 75.67.1...
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