By Ray Iannuzzelli PE

 

I am a mechanical engineer and have performed thermo-mechanical analyses of the lithium-ion batteries used in some of today’s most sophisticated electric and hybrids cars. Since purchasing my 2012 Volt over four months ago I’ve followed the perplexing battle of words between Left & Right involving the car I drive daily. Many of the political pundits and talk-show hosts know little or nothing about the Volt or plug-ins, in general. They seem to be guided by some divinely inspired reasoning to attack this American made vehicle. Perhaps they dislike our president so much that anything he touches or associates with is fair game for criticism, or perhaps they feel that while subsidizing an oil company is OK cars and real people are a different story.

Whatever their reasoning for attacking the Volt, I wanted to write an article dispelling some of the hype and comparing the Volt with the best cars available in a way that most folks could understand. In that way I felt we could strip the invective and political atmosphere from a debate that should be about what makes the Volt a special vehicle, i.e. quantifiable metrics.

In order to make sense to drivers of conventional cars, I believe the comparison must be based on economics as well as energy use, i.e. how much energy does it use and how much does it cost? The EPA has shied away from using the cost metrics due to the vast differences in gasoline prices across the U.S. This is understandable. Additionally, any comparison involving the Volt must also consider the national variation in the cost of electricity to charge its batteries.

Recently, Toyota announced its new Prius PHV (Plug-in Hybrid Vehicle) soon to be available in the US. With the success of the standard Prius it is quite apparent that both Prii would be interesting competitors of the Volt.



2012 Volt EREV                                                            2013 Prius PHV

How Do They Compare?

I used EPA data for both vehicles wherever available to develop a spreadsheet comparison of the cost per mile to drive both cars a certain distance (see assumptions in table 1). I tried to use national averages wherever available. The cost per mile is presented in terms of $/mile and KWh/mile, where Kilowatt-hours (KWh) is a measure of energy.



 

Figure 1 Cost per Mile Comparison Map
Figure 1 presents the cost per mile ratio. It is the ratio of the Volt’s Cost per Mile to the Prius PHV’s Cost per Mile. A number greater than 1 indicates that the Prius PHV is less expensive while a number less 1 indicates that the Volt is less expensive. What is interesting about figure 1 is that it shows three distinct regions where the advantage switches between the Volt & Prius PHV. For low mileage trips (less than 16 miles), the Prius PHV is less expensive most likely due to its weight advantage over the Volt. For intermediate distance trips (between 16 and 65 miles), the Volt is the clear winner due to its bigger capacity electric battery. For long distances (greater than 65 to 75 miles), the Prius PHV again gains the advantage because of its inherently better gasoline/hybrid mileage (50 mpg) compared with the Volt’s internal combustion engine (~37 mpg).

Another observation can be made from figure 1 which shows two distinct long-distance cross-over points depending on whether we use $/mile or KWh/mile.  As gasoline prices rise and the price of gasoline reflects its energy content, the $/mile cross-over (65 miles) will approach the KWh/mile cross-over (75 miles). Note: for this comparison the Volt uses premium while the Prius uses regular fuel.

The underlying cost per mile comparison charts used in figure 1 are presented in figures 2 and 3.



Figure 2 $ per Mile Comparison of Volt & Prius PHV


 

Figure 3 KWh per Mile Comparison of Volt & Prius PHV



The assumptions used to generate figures 1 are listed below in table 1.



 


1)     Average EPA rated gasoline mileage

2)     EPA electric range

3)     Volt owner’s manual

4)     Battery Size/Electric Range

5)     Based on average driving data from my volt between 11/18/11 and 3/25/12

6)     Average Prius mileage between 9/18/10 and 3/5/12 ~ 49.5 mpg

7)     Reported by various media sources & Toyota

8)     Average price of gasoline reported by EIA

9)     Average price of electricity reported by EIA

10)   Data averaged from several websites

11)   "Best" electric range (reported in HybridCars.com)

12)   "Best" electric mileage = battery size/’best’ electric range = 4.4/15 = 0.293

13)   Electric mileage reported by EPA