ENGINE WITH THE HOMOGENEOUS CHARGE COMPRESSION IGNITION, HCCI, OFFERS A NUMBER OF BENEFITS OVER CONVENTIONAL SI AND CI, SUCH AS MUCH LOWER NOX EMISSION AND PARTICULATE MATTER EMISSION. IN THIS PAPER, THE EFFECTS OF INTAKE TEMPERATURE AND PRESSURE, EQUIVALENCE RATIO, ENGINE SPEED AND COMPRESSION RATIO ON THE AUTO IGNITION OF THE NATURAL-GAS HCCI ENGINE HAVE BEEN INVESTIGATED. THE CHEMICAL KINETIC MECHANISM INCORPORATED THE GRI-3. 0 MECHANISM THAT CONSIDERS 53 SPECIES AND 325 REACTIONS TOGETHER. TO SIMULATE HCCI ENGINE CYCLES, A VARIABLE VOLUME COMPUTATION HAS BEEN PERFORMED BY INCLUDING THE PISTON MOTION INTO THE CHEMICAL REACTION DESIGN, CHEMKIN, CODE. THE SENKIN CODE USED IN ORDER TO SIMULATE THE IN-CYLINDER COMBUSTION. IT WAS FOUND THAT INCREASING THE INTAKE TEMPERATURE AND PRESSURE WOULD RESULT IN ADVANCE IGNITION AND HIGH PEAK TEMPERATURE. IT WAS ALSO FOUND THAT ADDITION A SMALL PERCENT OF ETHAN AND INCREASING THE EQUIVALENCE RATIO WOULD RESULT IN ADVANCE IGNITION, IN CONTRAST, INCREASING THE EQUIVALENCE RATIO AND ENGINE SPEED RETARDED THE IGNITION TIMING.