To reduce the fuel consumption and emissions of spark-ignited (SI) engines, hydrogen enrichment was used to improve the performance of a lean burn SI engine operating at low speed and load conditions. A hydrogen port-injection system was mounted on the intake manifolds to introduce hydrogen into the intake ports sequentially while keeping the original gasoline injection system unchanged. A hybrid electronic control unit (HECU) was adopted to control injection timings and durations of gasoline and hydrogen, accomplishing four excess air ratios of 1.00, 1.18, 1.43 and 1.67 and three hydrogen volume fractions in the intake of 3%, 5%, 8%. The experimental results showed that engine brake thermal efficiency and torque output were increased, combustion durations were shortened, cyclic variation and HC emissions were reduced, but NO(x) emissions were increased with the increase of hydrogen addition. CO emission was also reduced under lean conditions with hydrogen enrichment. (C) 2010 Elsevier Ltd. All rights reserved.