Retrofit
A retrofitted school bus is a cleaner school bus. It
is cleaner because it either has been fitted with a device designed to
reduce pollution and/or it uses a cleaner fuel. There are a variety of
ways to retrofit a school bus. Retrofit options range in cost as well
as effectiveness and some come with special requirements. EPA verifies
retrofit technology and certifies cleaner fuels to ensure that they
produce the emission reductions advertised by their manufacturer.
Technology Options Chart
Pollution Control Devices
Cleaner Fuels
Calculating Emission Reductions
- EPA's Emissions Calculator
For More Information
Pollution Control Devices
Diesel Oxidation Catalysts
Diesel oxidation catalysts are devices that use a chemical
process to break down pollutants in the exhaust stream into
less harmful components. Diesel oxidation catalysts can
reduce emissions of particulate
matter (PM) by 20 percent and hydrocarbons
(HC) by 50 percent and carbon
monoxide (CO) by approximately 40 percent. Oxidation
catalysts cost about $600 to $2,000, can be installed
on any new or used bus, and run on regular diesel fuel.
Although installation time can vary, field experience suggests
it takes about 1 to 3 hours to install an oxidation catalyst.
Fact sheet: "Questions and Answers on Using a Diesel Oxidation Catalyst in Heavy-Duty Diesel Trucks and Buses", (June 2003, EPA420-F-03-016) [ 100K PDF ],
.
Diesel Particulate Matter Filters
Diesel particulate matter filters are ceramic devices that
collect particulate
matter in the exhaust stream. The high temperature of
the exhaust heats the ceramic structure and allows the particles
inside to break down (or oxidize) into less harmful components.
They can be installed on new and used buses, but must be
used in conjunction with ultra-low sulfur
diesel (ULSD) - fuel with a sulfur content of less than
15 parts per million. The combination of PM filters and
ULSD can reduce emissions of PM, HC, and CO by 60 to 90
percent.
PM filters come in a kit that includes mounting brackets
and an electronic monitoring device. The cost of the kit
can range from $5,000 to $10,000. Some buses may need the
more expensive filters to compensate for lower exhaust temperatures.
PM filters work best on engines built after 1995. As a result,
knowing the age and type of each engine in the fleet as
well as the drive cycles of the buses is an important part
of any retrofit project. These factors determine whether
or not buses are candidates for filters. Although installation
time can vary, field experience suggests it takes about
6 to 8 hours to install the filter.
Fact sheet: "Questions and Answers on Using a Diesel Particulate Matter Filter in Heavy-Duty Diesel Trucks and Buses", (June 2003, EPA420-F-03-017) [ 102K PDF ]
Cleaner Fuels
Ultra-Low Sulfur Diesel (ULSD)
ULSD will be available nationwide in June 2006, but currently is available
in certain parts of the country. EPA has a map that shows the current availability of ultra-low sulfur diesel fuel around the nation. The
primary purpose of ULSD is to enable or improve the performance of aftertreatment
technologies such as a PM filter. The quantity of emissions reductions
from the use of ULSD alone will vary depending on the application, level
of sulfur reduction, and other fuel characteristics of the replacement
fuel (e.g., cetane number, aromatics, PNA). Some case studies suggest
that the use of ULSD alone can reduce emissions of PM between 5 and
9 percent. While ULSD-only emission reductions for PM are relatively
modest on a per-vehicle basis compared to aftertreatment retrofit, the
emission reductions can be significant if an entire fleet is fueled
with ULSD. The price differential between ULSD and regular diesel fuel
varies by location but ranges between 8 and 25 cents per gallon. In
2006, when ULSD is available nationwide, the cost differential will
be much less.
Biodiesel
Biodiesel is a domestically produced, renewable fuel that can be manufactured
from new and used vegetable oils and animal fats. Biodiesel is safe,
biodegradable, and reduces air pollutants such as PM, CO, HC and air
toxics. However, emissions of NOx increase with the concentration of
biodiesel in the fuel. Some biodiesel produces more NOx than others,
and some additives have shown promise in modifying the increases.
Blends of 20% biodiesel with 80% petroleum diesel (B20) can be used
in unmodified diesel engines. Biodiesel can be used in its pure form
(B100), but may require certain engine modifications to avoid maintenance
and performance problems. Pure blends of biodiesel may not be suitable
for cold climates. B20 reduces emissions of PM by about 10 percent.However,
B20 also increases NOx emissions by approximately 2%. The B20 blend
costs about 15 to 30 cents per gallon more than regular diesel fuel.
B100 reduces emissions of PM by roughly 40 percent and costs about 75
cents to $1.50 more than regular diesel fuel.
Emulsified Diesel
Emulsified diesel is a blended mixture of diesel fuel, water, and other
additives that reduces emissions of PM as well as NOx. Emulsified diesel
can be used in any diesel engine, but the addition of water reduces
the energy content of the fuel, so some reduction in power and fuel
economy can be expected. Emulsified fuel will stay mixed for a fairly
long time. However, if a vehicle sits dormant for months at a time the
water can settle out of the fuel and possibly cause problems. Therefore
school buses may want to phase out the use of the fuel towards the end
of the school year and then reintroduce it in the fall. Case studies
suggest that emulsified diesel can reduce emissions of smog-causing
NOx by about 10 - 20 percent and PM by about 50 - 60 percent. Emulsified
diesel costs roughly 20 cents more per gallon than regular diesel fuel.
Calculating Emissions Reductions
EPA has developed a tool (known as the retrofit
calculator) that helps characterize a fleet and calculate the emission
reductions that a retrofit project will generate.
For More Information
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