United States Air and Radiation EPA420-F-03-015
Environmental Protection June 2003
Agency
Office of Transportation and Air Quality
&EPA Technical
Highlights
Clean Fuel Options for Heavy-Duty
Diesel Trucks and Buses
The pollution emitted by diesel engines contributes greatly to our
nation's air quality problems. Even with more stringent heavy-duty
highway engine standards set to take effect in 2004 and 2007, existing
trucks and buses will continue to emit large amounts of nitrogen
oxides (NOx) and particulate matter (PM), both of which contribute to
serious public health problems in the United States. Fortunately, there
are several techniques and technologies designed to reduce diesel
pollution from existing engines. Using cleaner fuels is one way existing
trucks and buses can be upgraded (or "retrofitted") to pollute less. In
addition to using cleaner fuel, a retrofitted engine can be fitted with a
device designed to reduce pollution for increased effectiveness. This
fact sheet discusses some clean fuel options.
Pollution from Engines
Heavy-duty trucks and buses account for about one-third of NOx emis-
sions and one-quarter of PM emissions from mobile sources. In some
urban areas, the contribution is even greater. The fine particles in diesel
exhaust (known as particulate matter) can penetrate deep into the lungs
and pose serious health risks including aggravated asthma, lung damage,
and other serious health problems. In addition, diesel exhaust is a likely
human carcinogen. Children are more susceptible to air pollution than
healthy adults because their respiratory systems are still developing and
they have a faster breathing rate.
> Printed on Recycled Paper
-------
Diesel exhaust also has environmental impacts. PM from diesel engines
contribute to haze, which restricts visibility. In addition, diesel exhaust
contributes to ozone formation (a component of smog), acid rain, and
global climate change.
UStra-Low
Sulfur
Diesel
Compressed
Natural
Gas
Major Clean Fuel Options
The primary purpose of Ultra-Low Sulfur Diesel (ULSD) is to enable or
improve the performance of aftertreatment technologies such as a particu-
late matter (PM) filter. The combination of a PM filter and ULSD can
reduce emissions of PM by 90 percent. The quantity of emissions reduc-
tions 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.
ULSD will be available nationwide in June 2006, but currently is available
in certain parts of the country. The price differential between ULSD and
regular diesel fuel varies by location but currently ranges between 8 and
25 cents more per gallon. In 2006, when ULSD is available nationwide,
the cost differential will be much less.
Compressed Natural Gas (CNG) is a gaseous fuel that is a mixture of
hydrocarbons, mainly methane, and is produced either from gas wells or in
conjunction with crude oil production. The majority of natural gas con-
sumed in the United States comes from sources within North America.
Increased use of fuel produced in North America helps reduce our reliance
on oil from overseas, which is good for our nation's energy security.
When purchasing new school buses, one option to consider is buses that
run on CNG and have been fitted with an oxidation catalyst to reduce
pollution. Vehicles powered by natural gas perform just like vehicles
powered by diesel fuel, but natural gas buses emit 70 to 90 percent less
PM than an unmodified diesel bus. CNG buses may have higher formalde-
hyde, hydrocarbon, and ultra-fine PM emissions than diesel engines using
ULSD and a PM filter. Adding a catalyst to a CNG bus will help further
reduce emissions.
-------
Overall the cost of natural gas and diesel fuel are very similar, but in
some areas of the country, natural gas fuel may be more economical.
However, a new compressed natural gas school bus costs at least
$30,000 more than a new diesel school bus (currently, only the largest
school buses - Type D - are available with CNG engines). Since it is a
gaseous fuel, it must be stored and handled differently than more tradi-
tional liquid fuels like diesel or gasoline. Therefore, CNG buses require
special refueling facilities as well as special maintenance facilities.
Other Clean Fuel Options
BiodieseS Biodiesel is a domestically produced, renewable fuel that can be manu-
factured from new and used vegetable oils and animal fats. Biodiesel is
safe, biodegradable, and reduces air pollutants such as PM, carbon
monoxide (CO), hydrocarbons (HC) and air toxics. However, NOx
emissions 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 percent biodiesel with 80 percent petroleum diesel (B20)
can be used in unmodified diesel engines. B20 reduces emissions of PM
by about 10 percent. However, B20 also increases NOx emissions by
approximately 2 percent. The B20 blend costs about 15 to 30 cents per
gallon more than regular diesel fuel.
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. B100
reduces emissions of PM by roughly 40 percent and costs about 75 cents
to $1.50 more than regular diesel fuel.
Emulsified Emulsified diesel is a blended mixture of diesel fuel, water, and other
Diesel 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. EPA data demonstrate a reduction in NOx
emissions of 5 to 30 percent and a reduction in PM of 20 to 50 percent.
Emulsified diesel costs roughly 20 cents more per gallon than regular
dieselfuel.
For More information
For more information on these topics, please visit these EPA Web sites:
• Voluntary Diesel Retrofit Program: www.epa.gov/otaq/retrofit
• Clean School Bus USA - Basic Information on Retrofit Options:
www.epa.gov/otaq/schoolbus/retrofit.htm
• Alternative Fuels: www.epa.gov/otaq/consumer/fuels/altfuels/
altfuels.htm
• Clean Diesel Independent Review Panel: www.epa.gov/air/caaac/
clean_diesel.html
* Health Assessment Document for Diesel Engine Exhaust: http://
cfpub. epa. gov/ncea/cfm/recordi splay. cfm?deid=29060
You can also contact the Office of Transportation and Air Quality library
for document information at:
U.S. Environmental Protection Agency
OTAQ Library
2000 Traverwood Drive
Ann Arbor, MI 48105
Phone: 734-214-4311
Email: GroupAALibrary@epa.gov
------- |