During the last decade the use of alternative fuels for diesel engine has received renewed attention. The interdependence and uncertainty of petroleum based fuel availability and environmental issues, most notably air pollution, are among the principal forces behind the movement towards alternative source of energy. Hydrogen is expected to be one of the most important fuels in the near future for solving greenhouse problem for protecting environment and saving conventional fuels. In this experimental investigation, a diesel engine using hydrogen as fuel was investigated with diesel as an ignition source for hydrogen. Hydrogen was injected into the intake port, while diesel was injected directly inside the cylinder. The parameters such as injection timing and injection duration of hydrogen were varied for a wider range at a constant injection timing of 23 degrees before injection top dead center (BITDC) for diesel. The hydrogen flow rate was kept constant at 101pm for varied load conditions. The maximum brake thermal efficiency of 29.4% was obtained at full load for the optimized injection timing at top dead center (TDC) with injection duration of 90 degrees crank angle (CA). The oxides of nitrogen (NOX) emission tends to reduce to a lower value of 705 ppm at full load condition for the optimized injection duration at TDC and with an injection timing of 60 degrees CA compared to neat diesel fuel operation. The smoke emission reduces by three fold for the hydrogen operated engine at optimized conditions. Using port-injected hydrogen there is an increase in brake thermal efficiency of the engine with a greater reduction in emissions. (c) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.