Hydrogen with low spark-energy requirement, wide flammability range and high burning velocity is an important candidate for being used as fuel in spark-ignition engines. It also offers CO2 and HC free combustion and lean operation resulting in lower NO, emissions. However, well examined external mixing of hydrogen with intake air causes backfire and knock especially at higher engine loads. In addition, low heating value per unit of volume of hydrogen limits the maximum output power. In this study, attention was paid to full usage of hydrogen advantage employing internal mixing method. Hydrogen was directly injected into cylinder of a single-cylinder test engine using a high-pressure gas injector and effects of injection timing and spark timing on engine performance and NO, emission were investigated under wide engine loads. The results indicate that direct injection of hydrogen prevents backfire, and that high thermal efficiency and output power can be achieved by hydrogen injection during late compression stroke. Moreover, by further optimization of the injection timing for each engine load, NO, emission can be reduced under the high engine output conditions. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.