Thursday, August 20, 2009

Hybrid Automobile Efficiency and the Truth about MPG

Recently there have been a few product announcements for plug-in hybrid drive automobiles. These announcements have contained mythical claims for very high "fuel economy." Please do not be fooled.

General Motors announced that their new Chevy Volt would acheive a fuel efficiency of something to the tune of 230 miles per gallon. Bull-caca. Chevy based this on data that says 8 out of 10 people typically commute 40 miles a day or less and that the Volt can go 40 miles on a full charge until it needs to use gasoline. Additionally they claimed that the Volt could be charged daily for about 41 cents a day. A few days later, Nissan annouced that their new all electric vehicle would get 367 MPG... It doesn't even use gasoline. Let's break down the numbers.


Background
While exact specifications are not available yet, the Volt which is a series hybrid and will probably incorporate regenerative braking like the Prius and other electric or hybrid vehicles. Regenerative braking is the primary reason as to why hybrid drive vehicles are more efficient (the other reason being that electric motors are more efficient than internal combustion engines (ICE) like a typical gasoline motor).


Numbers Game
The MPG rating for the Volt is going to be largely variable depending on how you look at the use of the vehicle. Compared to the Toyota Prius which has a curb weight of 3042 and EPA rating of 51/48 MPG (city/hwy), the Volt at an estimated 3500 lbs will probably be sligtly worse if you never plugged it in at home. Let's assume an efficiency 45/43 MPG.

So how did GM come up with 230 MPG?!!? Well, assuming a driving schedule of less than 40 miles a day, the Volt would never use any fuel and would have infinite MPG. That said, if one drives 40 miles on a full charge, and then drove an additional 10 highway miles on gasoline, you would have used 0.22 gallons of fuel, over a total of 50 miles and this results in 227 MPG. If you bump that up to driving an additional 20 miles (60 miles total) your fuel efficiency will drop to 135 MPG. If you drove an additional 100 miles (140miles total) it drops further to 63 MPG. If you ran the tank dry (assuming a 12 gallon tank) you are left with about 48 MPG.

So as you can see, you can work those numbers any way you want really to come up with some more realistic numbers, but more likely some fanciful ones that will be used in advertisements and by car dealers to get you hook line an sinker.

Now, how about the claim that you can charge the vehicle for about 41 cents a day? I think it's bunk as well. A gallon of gasoline contains about 33 Kilowatt-hours (kWh) of energy. To give you an idea of how much energy that is, your typical clothes dryer uses about 6000 Watts or 6 kW when it's in use. The dryer runs for about 45 minutes or so to dry a typical load of laundry. Therefore it uses about 4.5 kWh of energy.

A current generation gasoline powered car is about 25% efficient with respect to the energy contained in the fuel, gasoline. Electric motors have efficiencies in the 80 to 90% range, but let's assume a safe 85% efficiency for the battery and electric motor in the Volt. That means that the Volt can go about 3.4 times further on the same energy in a gallon of gasoline than a car actually fueled by gasoline. Another way of looking at it is that to go the same distance, the Volt would only need about 30% of the energy of the gasoline powered car. So, with Chevy's stated 40MPG range on a full charge, and the claim that 8 out of 10 people only drive 40 miles, lets's see how that works out. For a standard 30 MPG car to go 40 miles it would use 1.33 gallons of gas or 44 kWh. The Volt would use 30% of that for the first 40 miles (in purely electric mode), or 13.2 kWh. At current electric utility prices of 9-10 cents per kWh, the Volt would cost between $1.20 and a $1.30 to recharge. Three times the cost of what Chevy claims.

The moral of the story folks is of course, don't believe everything you hear. The Volt, while efficient and maybe a step in the right direction is far from 230MPG. In some following articles I'll cover why some reasons why electric cars are going to suck in the winters of the North Eastern U.S. and why anything over about 120 MPG is pretty much impossible to achieve.