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With the growing number and popularity of hybrid and the imminent arrival of plug-in hybrid-electric vehicles, measuring fuel economy and exhaust emissions has become a difficult process.

The problem is the difficulty of ensuring repeatable, accurate results when subjecting hybrids and plug-in electric hybrids to the same laboratory procedures as conventional vehicles, driving schedules that involve a number of starts and stops, top speed, duration of drive et cetera.

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Hybrids are more sensitive to cold weather and the use of heating and air conditioning systems. The amount of time the internal combustion engine is in use depends on the state of charge of the battery.

Consumer complaints about actual mileage compared to the ratings led to "adjustments" to the ratings for hybrids a couple of years ago. Now, on the eve of a new batch of hybrids and plug-in hybrids, we are seeing claims of hundreds of miles per gallon or the metric equivalent. Of course, these assume the vehicle is running in electric power the majority of the time - which is only possible under ideal versus real world conditions.

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One example of the need for a new standard is the upcoming Chevrolet Volt. This car operates primarily on electric power, but at a certain point a small gas engine leaps into operation to recharge the batteries - not power the car. The Volt can go a long way on electric power alone, enough to use no fuel at all during the current EPA test cycle - on a full battery. But what if the battery is low and the gas engine is in use generating the electricity needed to keep the vehicle running?

SAE (Society of Automobile Engineers) International has recently published a new standard to measure the exhaust emissions and fuel economy of hybrid vehicles. The new standard is expected to be used for government-mandated laboratory tests conducted by the U.S. Environmental Protection Agency. The same numbers are used by Natural Resources Canada (NRCan).

SAE Standard J1711 - Recommended Practice for Measuring the Exhaust Emissions and Fuel Economy of Hybrid Electric Vehicles, including Plug-in Hybrids, provides test procedures that take into account the diversity of hybrid vehicle designs. It was developed to more accurately evaluate vehicles under a wider range of cycles and offers "a technology-neutral way, allowing potential fuel savings of these vehicles to be evaluated in a realistic manner."

The man in charge of developing the new standard was Michael Duoba, an automotive engineer with the U.S. Department of Energy's Argonne National Laboratory. The chairman of the SAE International Hybrid J1711 Task Force says the new procedure provides more consistent information for the consumer. "Until now, the fuel economy claims for plug-in hybrids were not calculated according to similar procedures, making car-to-car comparisons virtually impossible."

The new recommended practice for procedure for hybrid-electric vehicles designed to be driven on public roads, involves uniform chassis dynamometer test procedures, instructions for measuring and calculating exhaust emissions and fuel economy on both the urban and highway driving schedules. SAE says the procedures are structured so that other specific driving schedules can be substituted "provided that the corresponding preparatory procedures, test lengths, and weighting factors are modified accordingly."

Because different standards are used around the world, SAE J1711 does not specify which emissions to measure, whether they are hydrocarbons, carbon monoxide or dioxide or nitrogen oxides. For the purposes of the new test procedure, a hybrid electric vehicle is described as "a road vehicle that can draw propulsion energy from both of the following sources of stored energy: 1) a consumable fuel and 2) a rechargeable energy storage system (RESS) that is recharged by an electric motor-generator system, an off-vehicle electric energy source, or both.


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