The association says the decline is most likely due to the decrease in federal funding for clean diesel projects, as well as the recent trend of funding being spent more on projects that use engine repowers and/or vehicle replacements rather than retrofit devices.
Of the total sold in 2011, 57% were diesel particulate filters, both passively regenerated and actively regenerated filters; 23% were diesel oxidation catalysts, and 4% were flow-through filters.
In California, 7,558 diesel retrofit devices were sold, of which 89% were DPFs and 11% were FTFs.
DPF sales, although they were up slightly, were expected to be much higher in 2011, especially in California due to the requirements of California Air Resources Board's in-use truck and bus regulation. CARB projected that up to 100,000 retrofit DPFs could be installed over the 2011-2014 timeframe to comply with the regulation.
In addition, CARB's in-use off-road diesel vehicle regulation was expected to generate additional demand for DPFs, but amendments to the regulation approved in December 2010 meant to give fleets more time to comply due to the economic recession continue to depress the retrofit market opportunity for off-road diesel engines in the state.
Overall, these annual retrofit sales numbers are relatively small compared to the total number of diesel engines currently operating in the U.S. (up to 20 million based on EPA estimates), the association says.