The effects of different Research Octane Number [RON] fuels on a multi-cylinder light-duty compression ignition [CI] engine were investigated at light load conditions. Experiments were conducted on a GM 1.9L 4-cylinder diesel engine at Argonne National Laboratory, using two different fuels, i.e., 75 RON and 93 RON.Emphasis was placed on 5 bar BMEP load, 2000 rev/min engine operation using two different RON fuels, and 2 bar BMEP load operating at 1500 rev/min using 75 RON gasoline fuel. The experiments reveal difficulty in controlling combustion at low load points using the higher RON fuel. In order to explain the experimental trends, simulations were carried out using the KIVA3V-Chemkin Computational Fluid Dynamics [CFD] Code. The numerical results were validated with the experimental results and provided insights about the engine combustion characteristics at different speeds and low load conditions using different fuels. It was observed that cycle-to-cycle and cylinder-to-cylinder variability issues complicate the multi-cylinder engine operation to a significant extent. Effective compression ratios [CR] of all 4 cylinders were found to be different, which indicates the variability in injected fuel amount as well. With all these differences, validating the experimental emission trends with the simulations appeared to be somewhat difficult. Experiments indicated that to operate the engine using a higher RON fuel more premixing is required. Also 93 RON fuel injection at 5 bar BMEP load suggested that multi-cylinder engine operation at a higher injection pressure needed the injection strategy to be altered.