NRDC: No Breathing in the Aisles: Diesel Exhaust Inside School ...

technology coupled with low-sulfur diesel fuel has demonstrated the potential to reduce
particulate emissions overall, further testing is necessary to ensure that the particle traps are
durable over a school bus’s full useful life. Testing of particulate traps on school buses is also
essential to determine whether the school bus driving cycle (which is different from other
applications like refuse haulers and transit buses) generates the heat required to properly
regenerate the trap. Currently, the South Coast Air Quality Management District (SCAQMD)
in Southern California is testing school bus engines from three school districts to determine
whether this technology will perform properly under a school bus operating cycle.
Preliminary results are promising, but also indicate that school bus models prior to 1991 may
experience calibration problems or failures. 121
Repowers. Repowering a school bus refers to the removal of the aging bus engine and
replacement with a new engine that meets or surpasses current emission standards. As part of
this process, an aging diesel engine can be replaced with a new alternative fuel engine and
fuel system at a reasonable price. The SCAQMD is currently considering the repowering of
existing diesel engines to run on alternative fuels as a strategy for school districts that traditionally
purchase used buses from other school districts to reduce their emissions. Whether
this option is viable, however, will depend upon the cost of the repowering and the availability
of grant funding in each region.
Rebuilds. Rebuilding an existing bus engine, using modern equipment, is another option to
reduce excessive emissions from an existing school bus. However, it may be harder to receive
grant funding for rebuilds due to the uncertainty over emissions benefits. The benefits of
rebuilds may turn out to be marginal and subject to durability issues, given that modern
equipment may not always work properly with an engine built several years earlier. This
strategy is probably the least likely to improve school bus emissions performance.
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