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Starting in 2007, diesel engines used in highway vehicles are required to meet a NOx standard of 0.20 g/bhp-hr. Until 2010, this standard could be met through the use of averaging, banking, and trading with a family emission limit not to exceed 2.00 g/bhp-hr. Practically, this means that heavy duty on-highway engines have been equipped with Selective Catalytic Reduction (SCR) to control NOx since the 2010 model year. Similar standards have applied to non-road diesel engines since 2014.
SCR systems use Diesel Exhaust Fluid (DEF) to reduce NOx emissions. DEF is stored in a separate tank on the vehicle and injected into the exhaust when the SCR is active. DEF is not intended to be added to the fuel.
In the last three years there have been three incidents in which DEF was inadvertently added to aviation fuel that was then loaded onto planes at small airports. These incidents were the result of human error where DEF was accidentally added to the fuel system icing inhibitor (FSII) storage tank on the aviation refueling truck at small airports. FSII is mixed into aviation fuel, particularly for smaller aircraft, to prevent water in fuel from freezing. If aviation fuel is contaminated with DEF, the fuel will crystallize and can clog jet engine fuel filters. This has caused multiple in-flight engine failures, with all aircraft landing safely. The FAA has issued service bulletins and notices but that has not prevented the potential of future occurrences.
To reduce the likelihood that DEF is inadvertently added to aviation fuel in place of FSII, EPA is initiating an action to exempt model year 2010 and later heavy-duty on-highway diesel engines installed in aviation fuel delivery trucks operating at certain airports from the requirement to comply with the most recent tier of NOx emissions standards.