<div class="htmlview paragraph">A study was conducted to investigate combustion variability and exhaust emissions from high-speed, natural gas fueled engines. Two types of fuel systems were used in the investigation: a mixer and a port fuel injection.</div> <div class="htmlview paragraph">The overall engine performances were not much different at stoichiometric fuel-air ratio. But as the equivalence ratio was reduced the engine with the mixer produced higher levels of hydrocarbons and larger coefficient of variations in imep. The same engine exhibited longer flame development angle and rapid burn duration in comparison to the fuel injected engine. The differences in burn durations increased as the equivalence ratio decreased and the mixer system produced larger variations in their values at these operating points. The investigation showed the performance of the engine was better with natural gas injection system than with the mixer, particularly at lean equivalence ratios.</div>