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This paper describes the results of a combination of EGR and supercharging applied to a diesel dual fuel (DDF) engine using natural gas (CNG) introduced from an intake pipe. The oxygen concentration in the intake charge were varied from 21% (without EGR) to 17% (26% EGR) with a cooled EGR technique. The boost pressures were set at two conditions, 100 kPa (naturally aspirated operation) and 120 kPa (supercharged operation) with a Roots blower supercharger driven by an inverter controlled motor. The influence of combining EGR and supercharging on the engine performance, combustion characteristics, and emissions were investigated with coconut oil methyl ester (CME) and gas oil under ordinary diesel and DDF operation modes. The results showed that the trade-off relation between the NOx and smoke emissions under the DDF operation combined with EGR and supercharging improved remarkably while also maintaining a relatively high brake thermal efficiency. Regardless of the engine operation mode, both the brake thermal efficiency and smoke emissions with the CME with supercharging improved more than with gas oil operation over the tested EGR regions. more...