Recommended Practice for Measuring Emissions and Fuel Economy of Hybrid Vehicles


The automobile has a rich history shaping how it is constructed and used today. Around the year 1900, cars could be powered by steam, gasoline, or electric engines. With steam being impractical for automobiles, the main competition existed between gasoline and electric. Gas-powered combustion-engine vehicles arose as the primary engine used for cars, due to a more-affordable price. Automobiles powered entirely by combustion engines have been suitable for automobiles ever since then. However, in the past few decades, concerns have been raised by the atmospheric pollution derived from burning gasoline and the possibility of running out of petroleum oil. This has prompted the car-makers to increase the fuel efficiency of their vehicles.

One way to lower gas emissions is by designing hybrid cars, which have both electric- and gas-powered engines. While hybrid cars lower gas emissions, they are still using some gasoline to generate power. SAE J 1711-2010: Recommended Practice for Measuring the Exhaust Emissions and Fuel Economy of Hybrid-Electric Vehicles, Including Plug-In Hybrid Vehicles sets guidelines to calculate emissions from hybrid-electric vehicles (HEVs) before they are put on the market.

In the past year, the average fuel economy of a light-duty car was 23.4 mpg, with new light-duty cars having 36 mpg. The average fuel economy for a hybrid car is much higher, generally not being less than 30 mpg. SAE J 1711-2010 lays out the methods needed to understand the fuel economy of hybrids and measure the emissions of a HEV that is meant to be used on public roads. It does not specify which kind of emission to measure, such as carbon dioxide or nitrous oxides, leaving that decision to the tester.

SAE J 1711-2010 establishes several uniform chassis dynamometer test procedures that should be used to determine the fuel economy of a HEV. These are similar to the tests needed for estimating fuel economy of conventional cars, with some additions meant to understand the energy produced for the car by the electric engine. The standard provides several equations that should use the data from the tests to calculate the fuel economy.  

An important consideration that is not covered by the scope of this standard is an understanding of the entire atmospheric pollution related to the use of hybrid cars. Since a significant amount of the total energy used to power a HEV comes from electricity, a much smaller amount of gas is needed than for a conventional vehicle of a similar size. Depending on the location in which the hybrid driver chooses to charge his or her vehicle, there can be vastly different sources of energy producing the electricity. If it is from renewable forms of energy generation, such as wind or solar power, the overall emissions from the HEV will be incredibly low. However, the electricity from the utility is more likely generated by burning methane or coal, which pollutes the atmosphere with carbon and other greenhouse gases. These emissions must be taken into account when thinking about the carbon footprint of electric cars. Simply evaluating the emissions of the vehicles underrepresents the carbon footprint. A more accurate analysis is possible because the emissions that come from the production of electricity by the utility can easily be measured.

The Society of Automotive Engineers (SAE) is a nonprofit that develops and publishes standards for automotive, commercial vehicle, and aerospace industries. SAE standards are maintained by over 9,000 engineers and other professionals, being used by 120,000 in locations around the globe. Another standard by the SAE, which provides a background on the vocabulary for automobile emissions, is SAE J 1145-2011: Emissions Terminology and Nomenclature
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