In this experimental study, Hydroxy (HHO; Oxy-hydrogen) - CNG (Compressed Natural Gas) fuel mixtures were used as an additional fuels in a non-modified diesel engine via intake manifold by substitution of diesel fuel. Three cases of experiment performed in a 3.6 L, four cylinders, four stroke diesel engine; first case is non-substitute liquid fuel, neat diesel operation; second case is HHO (5.1 lpm) addition to air mixture and operated under substituted diesel engine and third case is 25HHO-CNG mixture (25% Hydroxy-75% CNG-(5.1-15.3 lpm)) inducted the intake manifold and running with substituted diesel engine. During the tests stepping motor and driver used on fuel pump plunger to substitute diesel fuel as similar to pilot diesel injection. The effects of HHO and HHOCNG fuels relative to neat diesel operations were investigated in between 1200 and 2600 rpm engine speeds and expressed on both engine performance values like; brake torque, brake power, brake specific fuel consumption and brake thermal efficiency; and exhaust emissions; NOx, CO2, CO and exhaust gas temperature (EGT). Results showed that the addition of HHO-CNG mixture is most effective in improving the engine performance and providing more environmentally friendly attitude on exhaust emissions compared to neat diesel operations. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.