The aim of the study was to investigate the effect of ignition timing and hydrogen fraction on the performance and exhaust emissions of an engine. The engine was converted from port injection system to direct injection with enrichment hydrogen to natural gas at WOT condition and engine operation at 4000 rpm. The results showed that when hydrogen volume fraction was raised from 0 up to 50% at 20 bar injection pressure and at the ignition timing from 19 to 28 °CA BTDC, a good stratified effect was achieved. Also, performance results demonstrated that fuel conversion efficiency, power output, and torque were increased up to 3.7% and the brake thermal efficiency first rose and then fell with an increment of hydrogen enrichment to CNG. The peak cylinder pressure and temperature with the advance of ignition timing increased. In addition, the advance of the ignition timing caused NOx, HC, and CO emissions to decrease but with the increase in hydrogen fraction in the blend of fuel, HC, and CO dropped and NOx emission rose. This study concluded that the 30% blends of hydrogen and 21° BTDC ignition timing can optimize engine performance and emission without any engine

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