A Diesel Dual Fuel (DDF) engine is an adapted diesel engine that uses natural gas and diesel fuel as the energy source at the same time. Natural gas is mixed with air at the intake manifold while diesel fuel is injected into the combustion chamber directly to initiate the combustion process. Based on the past DDF literatures, they are indicated that Carbon Monoxide (CO) emissions were more substantial at low load conditions than those when running in diesel engine modes. The Diesel Oxidation

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... alyst (DOC) that is installed to this diesel engine is, therefore, not capable to reduce CO emissions abide by to the emission regulation. Literatures also indicate that the exhaust temperature, mass flow rate, Oxygen (O2) concentration, CO concentration, as well as Propane (C3H8) concentration may affect CO conversion efficiency of the catalytic converter. In the present work, Design of Experiments (DOE) is employed to explore the behavior of various factors that affect CO reductions in the catalytic converter. Once the knowledge is founded, the optimization of CO reductions in the catalytic converter at 90% is studied extensively. Using Fractional Factorial Design for screening factors on CO conversions, it is found that the exhaust temperature, mass flow rate, O2 concentration, and CO concentration affect CO conversions of the catalytic converter significantly. Optimization of these factors, by using Box-Behnken Design, for reducing CO concentration of 6200 ppm which is the maximum CO amount emitted from the tested engine shows that 90% of CO conversion can be reached at the exhaust temperature of 200 0 C, the mass flow rate of 25 kg/h, and the oxygen concentration of 16%.