*»EPA
United States
Environmental Protection
Agency
Office of Transportation and Air Quality
E PA-420- F-95-009a
October 1995
Emission Control Potential for
Heavy-Duty Diesel Engines
Diesel engines are typically used to power trucks, buses and nonroad equipment (for farming,
construction, mining, etc.) because of their exceptional fuel economy and durability advantages.
However, visible exhaust ("smoke") from older diesel engines has raised public awareness of diesel
engine emissions.
Diesel engines use compression instead of spark plugs to ignite the fuel. The high temperatures
typical of diesel compression ignition cause oxygen and nitrogen from the intake air to combine as
oxides of nitrogen (NOx). NOx reacts with hydrocarbons (HC) and sunlight to form ground-level ozone
(smog); NOx also combines with other atmospheric constituents to form fine particulate matter. Ozone
and particulate matter are associated with many adverse health and welfare effects, including respiratory
illness, acid rain, eutrophication, and visibility problems (haze).
Despite previous design improvements, diesel engines contribute a substantial portion of the NOx,
PM, and, to a lesser extent, the HC emissions from mobile sources. Manufacturers have begun a
comprehensive review of diesel engine design to move toward more effective controls for NOx, PM and
HC. One strategy may be to better manage the process of air and fuel delivery to the cylinder, reducing
emissions production. Another strategy may be to use "aftertreatment" (post-combustion) technologies
to break down or capture emissions. Diesel engines of the future may use a combination of strategies,
possibly incorporating fuel changes as well. The following is a brief description of several potential
diesel emission control options:
•	Fuel Delivery: Designing electronic controls and improving fuel injectors to deliver fuel at the
best combination of injection pressure, injection timing and spray location to burn its fuel more
efficiently without causing the temperature spikes that increase NOx emissions.
•	Air Intake: Redesigning turbochargers, aftercoolers and intake valving to provide optimum
pressure, temperature and routing of the intake air is important for managing the physical and
chemical processes needed to achieve good air-fuel combustion. Exhaust gas recirculation
(mixing some exhaust gas with the intake air) is an established diesel engine technology that
could be used more extensively in future diesel engines.
•	Aftertreatment Technologies: Catalysts and particulate traps can be used to convert or capture
emissions prior to exhaust. Traps are used to remove and eventually burn particulate emissions.
Catalysts for diesel engines are more complex than similar technologies used in cars, but hold
promise for reducing NOx and particulate emissions by converting them to less-harmful
compounds.
•	Diesel Fuel Parameters: Employing fuel additives and improving fuel properties such as raising

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reduced NOx and PM emissions and may also provide engine manufacturers with greater
flexibility to use new emission control technologies.
As part of its NOx/PM Heavy-Duty Engine Initiative, EPA, in conjunction with industry and other
concerned groups, will research and compare the costs and benefits of these and other potential engine
and fuel changes to determine the most feasible, cost-effective, durable and safe emission-reduction
program for heavy-duty diesel engines.
For further information, please call the NOx/PM Heavy-Duty Engine voice mailbox at (313) 741-
7887, or write to:
U.S. Environmental Protection Agency
National Vehicle and Fuel Emissions Laboratory
NOx/PM Initiative Team
2565 Plymouth Road

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