 |
A Heavy Duty Diesel Engine Developed for Extremely Low
Emissions
ABSTRACT
A diesel engine emits less carbon dioxide (CO2) compared to
the other types of internal combustion engines and is considered an engine
that will contribute to reduce global warming. However, the relatively
high particulate matter (PM) and nitrogen oxides (NOx) emitted in the
exhaust gas are problematic. If these emissions are successfully reduced,
solving diesel engine emissions problems will be accelerated and
contribute to the reduction of global warming effects. In order to reduce
PM and NOx simultaneously, Diesel Particulate active Reduction system
(DPR) was developed and combined with an electronically controlled exhaust
gas recirculation (EGR) system. As a result of the development, a
production diesel engine equipped with DPR and EGR could achieve extremely
low emissions.
CONTENTS of TECHNOLOGY
DPR-Cleaner that consists of a cordierite
wall-flow filter made with a plug alternatively placed at the rear end of
the cell inlet and at the front end of the cell outlet effectively removed
PM in diesel exhaust gas and significantly reduced PM at a tail pipe (Fig.1).
However, under such city driving
conditions as low load at low speed, passive regeneration of the filer
with catalytic reaction dose not occur, and the filter only accumulates PM
as the vehicle is operated. Therefore, DPR required active
regeneration of the filter in addition to passive regeneration for
maintaining healthy operation of the DPR regardless of vehicle operating
conditions. For active regeneration of the filter, a common-rail
fuel injection system was modified to have a capability of multiple
injections (Fig.2). When exhaust
gas temperature is low, after injection takes place to increase exhaust
gas temperature. In addition, post injection is used to supply
unburned hydrocarbons to the DPR-Cleaner. Oxidation of unburned
hydrocarbons produces heat, and PM accumulated on the filter is combusted.
With the multiple-injection capability, therefore, the DPR successfully
reduced PM under a variety of vehicle driving conditions.
A key technology to reduce NOx is a cooled-EGR system. A large
quantity of hot EGR gas causes fuel consumption and smoke to increase,
which prevents the NOx reduction with EGR from achieving an extremely low
level. As a solution to this problem, cooling EGR gas is effective.
However, the cooled-EGR system increases heat dissipation to engine
coolant. Consequently, a combination of cooled EGR and internal EGR was
employed to reduce NOx emissions so that heat dissipation to engine
coolant could be reduced especially under high load (Fig.3,4).
Conclusion
Reducing diesel exhaust emissions has been
difficult. However, the successful development of the technologies
reported herein seems to indicate high potential of achieving further
reductions of exhaust emissions required for future diesel
engines.
|
