Car jargon: What is MPGe?


As the international automotive industry rolls out its new cars and trucks for 2011, there will be an astounding array of new alternative-fuel transportation technologies. A consumer will need to wade through a lot of new technical jargon to try to compare these platforms. So, how do buyers make sure they are not comparing apples to oranges when making their purchasing decisions?

A good place to start is the Environmental Protection Agency website at www.fueleconomy.gov. The EPA has assembled a comprehensive database of all popular models manufactured during the past 25 years. Available on the site are EPA-approved miles-per-gallon ratings for city, highway and combined usage, as well as estimated annual fuel cost, barrels of petroleum used per year and carbon footprint information.

However, comparing fuel efficiency and overall energy usage between hybrids and conventional gasoline-powered vehicles is tricky, since plug-in electric hybrids refuel from both electric and gasoline sources. To address this, the EPA now measures the distance traveled by each new vehicle relative to each unit of energy consumed. The new energy-consumption standard is called "miles per gallon equivalent," or MPGe.

MPGe allows competing vehicle platforms to be compared by defining distance traveled in miles, divided by the amount of energy units consumed. This performance is then compared to the equivalent energy available in a gallon of gasoline.

As an example, the Nissan Leaf is a car with an electric motor that is propelled by a battery pack. The equivalent energy usage for miles traveled compared to a gasoline vehicle is 99 MPGe, even though the Leaf will never consume an ounce of gasoline. Unlike gasoline vehicles, the electric Nissan Leaf is recharged from an electrical outlet. It will perform better in the city (106 MPGe) than on the highway (92 MPGe) because it consumes no energy when stopped at traffic stoplights. This EPA information will be featured on the Monroney window sticker for the Nissan Leaf when it is rolled out in 2011.

MPGe compares the energy in one gallon of gasoline to an equivalent 33.7 kilowatt-hours of electrical energy stored in the vehicle's battery pack. A "watt" is a measurement unit that shows how much electric power can be delivered by a battery pack to drive an electric motor. A "kilowatt" measures electric power in 1,000-watt increments. A "kilowatt-hour" measures electrical energy in the battery pack by showing how long it can deliver this power continuously over time.

The kilowatt-hour is used to show the amount of electrical energy stored in a battery pack, in the same way that gallons of gasoline are used as a unit of measurement for the relative volume of fuel stored in a gasoline tank. For example, the Nissan Leaf will employ a battery pack with a 24 kilowatt-hour capacity that can deliver 24 kilowatts of power to the electric motor over one hour, 12 kilowatts of power over two hours, six kilowatts of power over four hours, or three kilowatts of power over eight hours depending on driving conditions, acceleration, temperature and speed.

Even though the potential energy in the Nissan Leaf battery pack is less than one gallon of gas (24 kilowatt-hours compared to 33.7 kilowatt-hours), an electric-motor drive train converts this energy to propulsion power at the wheels with an efficiency that is three to four times greater than a gasoline engine. Much of the potential energy consumed by an internal combustion engine is lost as waste heat through the exhaust pipes and engine block.

Nevertheless, a 10-gallon tank of gasoline with the equivalent of 337 kilowatt-hours of potential energy can propel most of today's internal combustion engine cars about 300 miles between fill-ups. By comparison, the Monroney sticker on the Nissan Leaf shows that it will need to be recharged about every 73 miles under practical driving conditions.

Another factor influencing a purchasing decision is cost of ownership over time, including the price of the energy to replenish the vehicle's propulsion system. Electricity costs significantly less than gasoline to propel a vehicle and this gap will widen if the price of gasoline continues to rise. The annual cost to purchase the energy needed to recharge the Nissan Leaf, if driving it 15,000 miles per year, is forecast to be about $561 at 12 cents per kilowatt-hour.

Part Two of this series will look at how the EPA applies the MPGe standard to the plug-in hybrid Chevrolet Volt. I will also suggest how consumers might use information on Monroney stickers to choose their next ride.

Stan Hanel has worked in the electronics industry for more than 30 years and is a long-time member of the Electric Auto Association and the Las Vegas Electric Vehicle Association. Hanel writes and edits for EAA's "Current Events" and LVEVA's "Watts Happening" newsletters. Contact him at stanhanel@aol.com.

 

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