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CMU/Ford study assesses optimal mix of conventional, hybrid, plug-in hybrid and electric vehicles for minimizing GHG and cost

Breakdown of (a) equivalent annualized life cycle cost and (b) life cycle GHG emissions for four independently cost-optimized vehicle designs. Traut et al. Click to enlarge.

In a new study, a team from Carnegie Mellon University (CMU) and Ford Research and Advanced Engineering set out to determine the optimal mix for the fleet of mid-size personal vehicles in the US—while maintaining current driving patterns—with the goals of minimizing greenhouse gas emissions (GHG) or cost. They also addressed the question of GHG or cost reduction with and without a workplace charging infrastructure.

Their study, they suggested in a paper analyzing the best possible outcomes and published in the journal Energy Policy, is a step towards understanding what should be incentivized by policy makers.

They developed an optimization problem to minimize life cycle cost or GHG emissions over the personal vehicle fleet by jointly determining (1) the optimal design of each CV (conventional vehicle), HEV (hybrid electric vehicle), PHEV (plug-in hybrid electric vehicle), and BEV (battery-electric vehicle); (2) the optimal allocation of each vehicle design in the fleet based on annual vehicle miles traveled (VMT); and (3) the optimal allocation of workplace charging infrastructure to PEVs in the fleet. Within the fleet, they considered only vehicles of similar size and acceleration performance to the Toyota Prius.

They also incorporated vehicle design constraints to ensure comparable acceleration performance and vehicle allocation constraints to ensure BEVs are assigned only if they have sufficient range to accommodate the vehicle’s driving distance on most days (base case 95% of days).

Prior studies compare and select among a small set of fixed vehicle configurations based on selected commercially available vehicles or a small set of simulated vehicle alternatives. However, interactions among engine sizing, motor sizing, and battery sizing can be important in comparing vehicle characteristics, and optimal battery sizing represents a compromise among drivers with different travel patterns. We follow Shiau et al. (2010) [earlier post] and pose a mixed-integer nonlinear programming (MINLP) formulation to determine the best configuration of vehicles in the design space in order to compare the best design of each conventional vehicle (CV), HEV, PHEV, and BEV model under acceleration performance constraints that ensure vehicles are comparable. We further incorporate charging infrastructure decisions that determine which of the PEVs should be only charged at home vs. charged both at home and at the workplace, given charging infrastructure costs and production emissions, and we use driving pattern data to model required BEV ranges and PHEV electricity and gasoline usage.

—Traut et al.

Optimal vehicle allocations for minimizing life cycle GHG emissions (top) and cost (bottom) in selected scenarios. From Traut et al. Click to enlarge.

Among their findings were:

  • In agreement with prior studies, without sufficient grid decarbonization, plug-in vehicles do not offer substantial GHG emissions reductions compared to HEVs. GHG reductions improve with low-carbon electricity. Thus, grid decarbonization is needed to make plug-in vehicles a relevant means of reducing GHG emissions beyond grid-independent HEVs.

  • Compared to CVs, HEVs offer cost and emissions reductions in almost all scenarios and are an optimal or near-optimal solution for minimizing cost across many scenarios.

  • Under the current US electricity generation mix, workplace charging availability provides no GHG emissions benefit in the optimized solution, but workplace charging does provide additional benefits of optimistically up to 21% in combination with low-carbon electricity.

  • Workplace charging availability changes the GHG-minimized vehicle allocation slightly, allocating smaller capacity PHEVs and BEVs.

  • Gas prices above $3.25/gal (plus 5.2% per year) cause PHEVs to appear in the minimum cost solution, but for plug-in vehicles to dominate over HEVs, either gas prices of $7/gal (plus 5.2% per year) or gas prices of $4.5/gal (plus 5.2% per year) in combination with low vehicle and battery costs (DOE 2030 program goal levels, including battery costs under $200/kWh) are needed. High carbon prices (over $100/tCO2e) do little to drive plug-in vehicles to appear in the cost-minimizing solution.

  • BEVs are restricted by range requirements from being a significant part of the minimum cost or GHG solutions. Even when range requirements are dramatically reduced, requiring BEV range adequate for only the average trip rather than the 95th percentile trip, a fleet of entirely BEVs is much more expensive and GHG-intensive than the other vehicle types, and BEVs are not GHG-minimizers for the full fleet even when charged with zero-emissions electricity. BEVs enter the GHG-optimal fleet only for short-range vehicles and only in cases with grid decarbonization.

This formulation represents a best-case scenario for minimizing cost or GHG emissions with these vehicle technologies; market outcomes would likely deviate, and we do not attempt to predict firm or consumer behavior.

—Traut et al.

This research was supported by National Science Foundation grants from the Foundation’s Material Use, Science, Engineering and Society program, the CAREER program, and the Graduate Research Fellowship program. Support was also provided by Ford Motor Company, Toyota Motors of America, and the Steinbrenner Graduate Fellowship.


  • Traut, E., et al. (2012) Optimal design and allocation of electrified vehicles and dedicated charging infrastructure for minimum life cycle greenhouse gas emissions and cost. Energy Policy doi: 10.1016/j.enpol.2012.08.061



Harvey isn't going to like this study, especially the portion on BEV,s. I don't understand the graph to the right of the article but it appears to be a well thought out study.


You are right..I don't like it because it does not account for major near term changes in batteries performance (3X to 10X) and much lower price (-70% to -80%) and possible increase in liquid fuel price (up to $10/gal)

I paid $59.95 for a spare 750 mAh Li-On camera battery a few years ago and $4.95 last week for a similar but much more powerful 1200 mAh Li-On unit for my new digicam. That's about a 92% reduction in price for 60% more.

I guess that study is OK for a quick current day window look but rather meaningless for mid and longer term outlooks. It would be much better with an extended time line?


I forgot to add that our electricity is from clean 100% Hydro/Wind at a uniformed 24/7 very low $0.06/kWh domestic rate. Public charging stations could get the same clean electricity at a uniformed 24/7 Industrial rate of $0.03/kWh and even less during off peak periods. However, our gasoline price is already between $5.15 to 5.75/gal. It normally goes up about $0.45/gal on weekends.

That would change their (current day) tables considerably.

To fill-up a large ICEV car/Pick-up can cost over $100, specially on weekends.


Another useful info. Our sole nationalized Hydro, even with those very low uniformed rates, makes $3.5B net profit every year. About $1.5B/year is re-invested in new hydro/wind plants and about $2.0B/year is given to the local government (the people @ $950+ per family) as dividends.

The Hydro dividends would be enough to give FREE e-energy for two BEVs per family?

That would change the end results of the above study?

Dave R

Nothing really new here - BEVs on average grid electricity are not significantly different than HEVs or PHEVs from a cost or GHG perspective. But:

1. GHG emissions of current power plants is not sustainable and need to be drastically reduced regardless.
2. The "long tailpipe" of plug-ins greatly reduces pollution concentrations near population centers.
3. Oil is not getting any cheaper and it's not physically possible to get anywhere close to self-sufficient in the USA despite what drill-baby-drill proponents suggest.
4. BEVs are a joy to drive without the vibration, noise of an ICE.

Efforts should be made to estimate the benefits of these items as well.


It's the way the US generates power, not the cars which are the problem.
France with 75% plus of its electricity from nuclear can run its BEVs just fine, with massive carbon savings.


When will we get those batteries? 2113?


Vaporware is vaporware
and wackie tobacco is enlightening
Any new battery technology will be a least a decade if not longer to get into a car. Too many lawyers. No big car company is going put anything into all of our stupid hands until they have tested it out every possible stupid thing we do like, what happens if i drive my electric car in flooded new york, stupid or not the lawyers will sue

Plus it is not in the car companies benefit to pursue a BEV when they get more profit from HEV and HEV SUV, these are there new SUV's

and reality is reality and that means
lithium will come down until it becomes an in demand and it wll go back up in price as all limited minerals do


Peter XXX....decent EV batteries should start to hit the market place around 2020/2022? Meanwhile, HEVs may be the best interim solution. Toyota's HEVs are probably the best.

Nano-technologies will reduce the amount of lithium and/or similar material required.

Much smaller, lower cost, more efficient FCs may become a worthwhile alternative?


Peter...improved batteries could hit the market place before 2020?

I forgot to add that our electricity is from clean 100% Hydro/Wind
Except for Gentilly, you mean.
The Hydro dividends would be enough to give FREE e-energy for two BEVs per family?
Harvey seems to think that the whole world is like eastern Canada.  I've tried breaking the bad news to him, but he goes blithely on.

This article highlights yet another study that show that plain hybrid vehicles emit less CO2/mile than partial or full electric vehicles charged on grid mix average CO2/kWh.

The key takeaways from the study are:

BEVs enter the GHG-optimal fleet only for short-range vehicles and only in cases with grid decarbonization.

Even when range requirements are dramatically reduced, requiring BEV range adequate for only the average trip rather than the 95th percentile trip, a fleet of entirely BEVs is much more expensive and GHG-intensive than the other vehicle types, and BEVs are not GHG-minimizers for the full fleet even when charged with zero-emissions electricity.

But HarveyD keeps soldiering on about his electric car dreams, even though electric cars are actually WORSE for all of us than designing and using best-of-breed hybrid vehicles. And NO, this is not a matter of improving battery technology.

Such misconceptions are really unfortunate, because politicians are very susceptible to magical thinking, and are prone to instituting laws that look good to the uninformed, but actually make things worse for all of us.

Should I say it again? Ok, I will: The best solution to minimize GHG emissions AND fossile fuel consumption is diesel-hybrid cars. They can easily get 70mpg (and higher as technology and design improve) for a Prius-sized vehicle and are wastly superior to an electric car fleet running on any kind of realistic grid mix.


I originally drank the koolaid
I dreamed of solar panels on my roof charging my BEV

Nobody is going to get rich from that so it isnt going to happen


I have come to believe that a 200 mile range EV under $30,000 could change things a bit.

Bob Wallace

Why would anyone bet that EV batteries will not improve?

We can argue over whether it will take 2, 5 or 10 years before a much better/more affordable battery comes to market, but never?

Why would anyone bet that the "worldwide" grid will not get cleaner?

We can argue over whether nuclear will play a larger or smaller role or over how fast wind, solar, geothermal, hydro, and tidal will get installed, but a continued dirty grid?

We can even argue whether there's a role for hydrogen in our transportation system.

But to assume that today's reality is tomorrow's reality? I just don't see how that makes sense.

Right now PHEVs might make the most sense due to mature ICE technology and the current state of battery technology, but batteries are almost certain to improve and drop in price. As SJC says, a 200 mile EV for under $30 that was capable of at least 90% recharge in <20 minutes will shake things up.

Anyone remember electric typewriters than had built in memory so that you could type a draft, make corrections, and then print out a finished copy? Those were the pre-desktop computer "PHEVs" of word processing.

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