Experimental Investigation on Effects of Diesel Oxidation Catalysts on Emission Characteristics of a Methanol-Diesel Dual-Fuel Engine

Methanol fuel is a promising hydrogen carrier for engine carbon neutrality. However, the high unburned hydrocarbon (UHC) and carbon monoxide (CO) emission levels at low loads partly restrain its application in diesel engines. To identify the underlying causes of pollutants formation and the effects of diesel oxidation catalyst (DOC) on the emission characteristics in methanol-diesel reactivity controlled compression ignition (RCCI) engines and provide a theoretical basis for the development of methanol engines, several parametric experiments were performed by changing the methanol substitution rate (MSR) and engine load at 1,600 revolutions/min (rpm). The variation of total hydrocarbons (THC), CO, oxides of nitrogen (NOx), unburned methanol (MeOH), and formaldehyde (HCHO) emissions in methanol-diesel RCCI mode and the conventional diesel combustion (CDC) mode before and after DOC were also experimentally investigated. The results revealed that as the MSR increased in RCCI combustion, the THC, CO, MeOH, and HCHO emissions increased, the DOC inlet temperature reduced gradually, and the NO and NOx emissions decreased dramatically, but the NO2 emissions and NO2/NOx rate increased. As the engine load level increased, the THC, CO, MeOH, and HCHO emissions decreased, and the DOC inlet temperature increased. In RCCI mode, the conversion efficiencies of DOC for THC, MeOH, and HCHO at 30% load were quite low due to the lower DOC inlet temperature; the HCHO emissions at the DOC outlet were even higher than at the DOC inlet. With the increase of load, the conversion efficiency of DOC for THC, CO, MeOH, and HCHO emissions was continuously improved. At 90% load, the conversion efficiency of the DOC for THC, CO, MeOH, and HCHO emissions was greater than 98%. DOC can effectively reduce THC, CO, MeOH, and HCHO emissions from methanol-diesel dual-fuel engines. The NO2/NOx rate at the DOC outlet increased in CDC mode, but it decreased in RCCI mode. Exhaust thermal management strategies should be applied to improve the exhaust gas temperature to realize highly efficient conversion for DOC at medium and low loads in RCCI operation.