Novel Diesel combustion concepts such as premixed charge compression ignition (PCCI) and reactivity controlled compression ignition (RCCI) promise lower NOx and PM emissions than those of conventional Diesel combustion. RCCI, which can be implemented using low-reactivity fuels such as gasoline or gases and high-reactivity fuels such as Diesel, has the potential to achieve extremely low emissions and improved thermal efficiency. However, to achieve RCCI combustion, a higher boost pressure than that of a conventional engine is required because a high EGR rate and a lean mixture are necessary to achieve a low combustion temperature. However, higher boost pressures can cause damage to intake systems.In this research, the addition of gaseous fuel to a CI engine is investigated to reduce engine emissions, mainly NOx and PM emissions, with the same IMEP level. Two different methods were evaluated. One was the substitution of propane for Diesel while maintaining the same CA 50 and IMEP levels under conventional operating conditions. The other method was using early Diesel injection at 30° CA BTDC for the implementation of dual-fuel PCCI combustion with no EGR.A HORIBA-7100DEGR exhaust gas analyzer was used for detecting the NOx, CO and THC emissions; an AVL 415S smoke meter was used for measuring the filtered smoke number (FSN); and a Cambustion DMS-500 was used for measuring the PM size distribution.The results of the first experiment, which used propane to reduce emissions with conventional boost pressures, suggest the possibility of avoiding additional boosting systems to increase the boost pressure. The second experiment showed that emissions could be reduced using a novel combustion concept, dual-fuel PCCI, without the dilution gas, thus preventing an increase in fuel consumption and low combustion efficiency due to the supplemental EGR.The two suggested methods of adding propane to a CI engine yielded low PM emissions levels (under 0.2 FSN versus 1.5 FSN in the conventional case) while maintaining low NOx emissions (40∼50 ppm). Furthermore, the number of large particles (> 100 nm) was drastically reduced by the addition of propane, and the total number of particles was also reduced.