A Heavy Duty Diesel Engine Developed for Extremely Low Emissions

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.

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).

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.


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