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Old 04-26-2009, 11:56 AM   #4 (permalink)
Ernie Rogers
Ernie Rogers
Join Date: Feb 2008
Location: Pleasant Grove, Utah
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Originally Posted by Ryland View Post
When I get my self better organized and a kill-a-watt meter on the charger for my comuta-car, I plan to keep track of power going in to the charger, it sounds like your formula is assuming that people are tracking the watt hours as it comes out of the batteries, not what it takes to charge the vehicle and that charging is what people tend to keep track of, energy in and miles driven.
Hello, Ryland,

I'm curious about your photo--that's not one of your vehicles, is it?

About the expression, "energy in and miles driven," I would be quite happy to work in those terms, but few people do. In this kind arrangement, you would report----

kWh (at the wall socket) per mile for the electric car

BTU (at the pump) per mile for the fuel burner.

The common term for this kind of vehicle efficiency is "pump-to-wheels." The problem with it is there is no satisfactory way to relate the two types of vehicles because the electricity is fundamentally different than the fuel.

Let's say you burn a gallon of gasoline and get 116,000 BTU of heat.

If you run 34 kWh of electricity through a resistor, you also get 116,000 BTU of heat. Electricity is "work." According to thermodynamics, you are allowed to convert work to heat with 100% conversion, like through a resistor.

But, the second law of thermodynamics says that you are not allowed to convert heat to work at 100%--it's impossible. The best you are allowed to do is the theoretical maximum, generally called the "Carnot limit." The machine that converts heat to work is what we call a heat engine. Fuel-burning cars use engines that always convert less than 50% of the heat of the gasoline to work. Work can move a car, but heat can not.

So, is it fair for the EV driver to assert that his car would get 120 miles per gallon running on gasoline? Not if he assumes the gasoline can produce 34 kWh of electricity--that's impossible. But, it would be fair to say that he might be able to get half of that, 60 mpg (or 17 kWh of electricity per gallon)because it is at least theoretically possible that at some future time a heat engine may reach 50% efficiency.

Ernie Rogers
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