Experimental and Numerical Investigations of Ultra-High Compressed Natural Gas Substitution Cases in a Heavy-Duty Diesel Engine

The dual-fuel combustion strategy using compressed natural gas (CNG) and diesel exhibits immense potential for improving conventional diesel engines. To reduce CO2 emissions, diesel should be substituted with CNG to the maximum extent possible. In this study, a numerical investigation was conducted using a three-dimensional computational model for cases with 97 % CNG substitution (CNG97), which has not been explored through experiments previously owing to the technical limitations of diesel injection systems. The simulation model was validated through experiments conducted under operating conditions similar to a diesel engine; the results revealed that ignition was initiated by the diesel fuel regardless of the CNG substitution rate. In the CNG97 case, ignition occurred near the center of the cylinder owing to short spray penetration. Consequently, the CNG remained inside the piston bowl and top crevice area. In addition, the small quantity of diesel weakened the initial stage of combustion. These two effects retarded combustion significantly, which caused substantial phasing loss in CNG97. Based on these findings, it was concluded that the diesel injector should be optimized for a high CNG substitution rate to reduce CO2 emissions effectively.