This study investigated the effect of varying the spark advance timing and excess air ratio (air excessive ratio; lambda) on the combustion and emission of nitrogen oxide (NOx) in a hydrogen-fueled spark ignition engine under part load conditions. The engine test speed was fixed at 2,000 rpm and the torque condition was 60 Nm. Excess air ratio was varied from the stoichiometric (lambda = 1) to the lean mixture condition (lambda = 2.2) by throttling. The spark advance timing was controlled to determine the maximum brake torque timing (MBT) for each excess air ratio value. Subsequent to the determination of the spark advance timing for MBT, the spark timing was varied from MBT timing to top dead center. Based on the results, it is concluded that the leanest mixture condition (lambda = 2.2) with MBT spark timing exhibited the highest brake thermal efficiency of 34.17% and the NOx emissions were as low as 14 ppm. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.