Getting Realistic Fuel Economy Rating for Plug-In Hybrids No Easy Chore

National Laboratories Team Up to Tackle the 'PHEV Conundrum,' Devise Realistic MPG 

By John O'Dell, Senior Editor

You might have noticed that we have a hard time swallowing GM's claim that its upcoming Chevrolet Volt extended-range plug-in hybrid will be rated at 230-miles per gallon in city driving.

Actually, it is possible that the car can be rated at that phenomenal level of fuel efficiency - what we have trouble with is the idea that it will actually deliver.

The problem is in determining how to handle blended power-source vehicles such as the Volt when determining fuel efficiency.

It's easy with conventional internal combustion engine (ICE) vehicles and standard hybrid-electric vehicles (HEVs) - they use only one fuel, so however much is used over however many miles are driven is the fuel economy, or MPG.

And battery-electric cars (EVs) are -relatively - easy as well.

Variables Galore

One method is to compute the BTU value of the electricity they consume, adjust for energy loses in the charging process, and compare the resulting total BTU consumption with the BTUs in a comparable amount of gas (1 gallon equals 33.44 kilowatt hours of electricity) and you can come up with an MPG equivalent (there are other wrinkles, but this isn't a science class so we're just giving you the short-and-sweet version).

But plug-ins (PHEVs), especially those with extended all-electric range, bring so many variables to the table that finding a meaningful MPG-equivalent figure is daunting.

If the Volt's claimed 40 miles of all-electric travel holds up in the real world, then a person who owns one and never drives more than 40 miles before recharging could claim infinite MPG - plus the value of the electricity used.

But GM says the Volt's range extender consumes a gallon of gas every 30 miles after the battery is depleted, so a driver with a 50-mile commute would get 75 MPG on gasoline, plus the electrical consumption, while someone driving 100 miles a day would be getting around 50 MPG from the gas engine-generator, pus the energy value of the electricity used for the first 40 miles.

Getting the picture?  A fuzzy one, isn't it.

National Labs Tackle Problem

Now, though, the national research laboratories are bringing the combined brainpower of scientists and engineers from the National Renewable Energy Laboratory (Golden, Colo.), the Argonne National Laboratory (Argonne, Ill.), and the Idaho National Laboratory (Idaho Falls, Idaho) to bear on the problem.

Jeff Gonder (left), a research engineer at NREL in Colorado and leader of the team, spent a long hour on the phone the other day trying to explain to Green Car Advisor the system they are coming up with.

There's a formula- and acronym-dense version of their work available, if you are that deeply into its, but what we're presenting here is, well, let's call it Gonder light. And it is our boil down of what he told us, so we apologize in advance if, in an effort to make it easy to comprehend, we made it too simple and left out something important.

To check their findings, Gonder said, the researchers used real-world data from the only large fleet of extended-range plug-in now available, 100 plug-in Priuses using extended-range battery systems from Hymotion. The fleet has been studied and the data compiled by researchers at the Idaho National Laboratory.

Unconventional

Conventional fuel economy testing doesn't work for extended-range plug-ins because they use two power sources and operate in two different modes, determined by the distance they are traveling.

In the first mode, Gonder explained, the battery's energy is depleted over about 40 miles or travel. Then the engine-generator comes on, consuming gasoline and sustaining the battery charge for the rest of the driving until the car is parked and the batteries recharged form the grid.

The trick is to come up with a reasonable way of measuring the energy consumption in both charge-depleting and charge-sustaining modes, adjust for different driving distances and wind up with an average gasoline MPG equivalent that is somewhat representative of what the "average" driver would experience.

That's the figure that would be slapped onto the window sticker so shoppers could see at a glance how the three powertrain systems - ICE, HEV and PHEV - compare.

The standard test for an internal combustion engine MPG rating "is done on a chassis dynamometer and it is repeatable but the results aren't what the average driver gets so there are a number of adjustments factored in to get a real world prediction." Gonder said.

Success

"We have developed a way to take the test results from a plug-in hybrid vehicle on the dyno and accurately predict end-use fuel and electricity use," he said. The team checked its predictions against real-world data from the Hymotion Prius fleet.

The dynamometer testing, done at the Argonne lab, involved separately testing the plug-ins during battery charge depleting and sustaining modes, applying the adjustments used for ICE ratings "and then combining them for an average prediction," Gonder said.

In an earlier paper on the plug-in hybrid MPG conundrum, Gonder and co-author Andrew Simpson (a former NREL researcher, now an energy storage engineer at Tesla Motors) argued that "the measurement technique ultimately selected must capture specific standardized performance aspects to accurately evaluate the tested vehicle with respect to annual operating costs, national petroleum impact, and CO2 production."

In their report on the joint study by all three labs, Gonder and his teammates point out the importance of listing both the gasoline (or other combustible fuel) use and the electrical energy use - adjusted for inefficiencies such as losses during the charging process - in an MPG-equivalency rating for PHEVs.

They also suggest that  and suggest that an overall fuel economy number for PHEVs be based on a long term average.

Fuel Cost, Not MPG

Their scenario of a potential window sticker for a Hymotion-converted Prius also shows consumption and fuel cost under an energy intensive driving scenario - without battery recharging - and an energy-conscious driving mode with batteries recharged every 30 miles.

We think it might providing monthly or annual fuel cost might give consumers the best way to compare gas, diesel, conventional and plug-in hybrids when seeking out fuel economy data for cars and tucks without having to get a degree in electric engineering.

The long-term average for the Hymotion Prius under the team's system is fuel use of 4.2 liters per 100 kilometers,or 55 MPG, electricity use of 5.5 kilowatt-hours per 100 kilometers, of 89 watt-hours per mile, and a combined annual cost for the fuel and electricity of $789 a year.

One one extreme, the energy-intensive driving rating puts the cost at  $987 a year and fuel consumption average at 38 mpg; at the other end the energy-conscious rating shows an annual combined energy cost of $478, with a fuel rating of 144 MPG and electrical consumption of 181 watt-hours per mile.
 
The sticker accounts for the wild "mileage may vary" swings a PHEV driver would experience and, the authors suggest, would encourage energy-conscious driving by clearly showing the financial savings that could result.

More to Do

Gonder says that there's more work to be done before the test the labs are developing can be considered useful for all PHEVS - it needs to be checked against real-world data for cars, such as the Volt, that haven't even hit the streets yet.

But their work to date will be presented to the Society of Automotive Engineers (SAE) which is developing a test protocol for PHEVs (Gonder is o the committee doing the work) and might have some influence on the National Highway Traffic Safety Administration, which is in the process of developing an official government protocol for determining PHEV fuel economy for the EPA ratings assigned to every new passenger vehicle.

Its complex, but we think such a process would go a long way to giving consumers a reasonable method to compare fuel efficiency of the three types of vehicles that soon will be sharing the road and vying for consumers' attention in manufacturers' an dealers' advertising campaigns.

Posted by

John O'Dell October 9, 2009, 2:19 PM

Permalink

Categories:

Fuel Economy, Plug-ins and Electric

Technorati Tags:

Finding Plug In Hybrid Fuel Economy, Fuel Economy for PHEVs, MPG, PHEV

• 
• 
• 
•