The impact of minutely oxygen-enriched air on spark-ignition (SI) engine combustion was studied by obtaining engine performance measurements and investigating in-cylinder reactions. This study was initiated to determine if development of a new air-cleaner method, which may employ molecular sieve or membrane technology to slightly increase the oxygen concentration in the inducted air, is beneficial for engine operations.The air introduced into a single-cylinder SI engine was added with oxygen to produce oxygen concentrations of 21, 22 and 23%. Some results from engine tests performed with the oxygen enrichment are: The heat release lag, cycle variation and combustion period decreased; substantial reduction of emissions of unburned hydrocarbon emission and noticeable decrease of carbon monoxide were observed; and the brake thermal efficiency and engine output increased.In order to gain a better understanding of in-cylinder reactions and to help explain the findings obtained from the performance/emission test with oxygen enrichment, high-speed spectral IR digital imaging was performed in an SI engine. It was found that addition of small amounts of oxygen into the fuel/air mixture noticeably increased thermal radiation (due to higher temperature) from the reaction zone throughout the combustion period. This is considered to play a role in reducing UHC and CO emissions as well as increasing thermal efficiency.