It is well known that the operation of dual-fuel engine at lower loads suffers from lower thermal efficiency and higher unburned percentages of fuel. To study this problem, tests have been conducted on a special single-cylinder, compression-ignition research engine (Ricardo E6) to investigate the effect of pilot fuel quantity on the performance of an indirect diesel engine fueled with gaseous fuel. Diesel fuel was used as the pilot fuel and methane or propane were used as the main fuel which inducted in the intake manifold to mixed with the intake air. Through the experimental investigations, it is shown that the low thermal efficiency and poor emissions at light loads can be improved significantly by increasing the amount of pilot fuel. While increasing the amount of pilot fuel at high loads led to early knocking. ABD RABBO, M. F. 1976 Aerodynamic drag of ridge arrays in adverse pressure gradients. Ph.D