EPA 420-F-95-009b
Office of Mobile Sources
June 1996


     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 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
operation cause oxygen and nitrogen from the intake air to
combine as oxides of nitrogen (NOx).  NOx is an invisible, toxic
gas that can form fine aerosol particles or salts which
contribute to acidic precipitation (commonly known as "acid"
rain, snow or fog).  If engine temperature is decreased to reduce
NOx, this tends to increase the amount of uncombusted fuel that
may be emitted as particulate matter (PM) or gaseous hydrocarbons
(HC).  HC reacts with NOx and other pollutants to form
ground-level ozone (smog).  Ozone and PM are associated with many
adverse health and welfare effects, including respiratory
illness, environmental damage 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
is to better manage the process of air and fuel delivery to the
cylinder, reducing emissions production.  Another strategy is 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 important
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.
     This allows the engine to efficiently burn the fuel 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.  This 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
     technology on cars that may be effective in heavy-duty
     diesel engines.

-Aftertreatment Technologies: Using catalysts or particulate
     traps to convert or capture engine-out emissions before they
     leave the exhaust pipe.  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 conversion to less-harmful compounds.

-Diesel Fuel Parameters: Employing fuel additives and improving
     fuel properties such as raising the cetane number, lowering
     the aromatics content or decreasing sulfur levels can
     contribute to reduced NOx and PM emissions and may also
     provide engine manufacturers with greater flexibility to use
     new emission control technologies.  


     EPA's Office of Mobile Sources is pursuing requirements for
a greater degree of emission control from heavy-duty diesel
engines by proposing new emission standards for engines from
heavy-duty highway and nonroad applications.  

     The Office of Mobile sources is the national center for
policy on air pollution from highway vehicles and nonroad
equipment.  You can write to us at the EPA National Vehicle and
Fuel Emissions Laboratory, NOx/PM Initiative Team, 2565 Plymouth
Road, Ann Arbor, MI 48105.  Our phone number is (313) 668-4333.