In this paper, the hydrogen production characteristics of a purpose-designed and built small scale plasma converter for small engines was investigated. Hydrogen was produced through the reformation of ionised hydrocarbon fuel and air mixture by means of a spark discharge The experimental results showed that a suitable size of the reaction chamber can increase the concentration of the produced hydrogen and that under a given methane supply rate, a low O-2/C ratio resulted in high hydrogen production concentration as well as high hydrogen volume flow rate. As the methane supply rate was increased, however, the hydrogen concentration reduced while the hydrogen volume flow rate increased. Hydrogen concentration was also improved when the intake gas mixture resided longer in the reaction channel, although the hydrogen volume flow rate revealed an inversed trend. It was also shown that by combining an intake air swirl with an appropriate O-2/C ratio and methane supply rate, both the methane conversion efficiency and the hydrogen production concentration were improved. In general, under the optimal operating condition, the plasma converter produced a maximum hydrogen concentration of 48% and a hydrogen volume flow rate of 70 mL/min. (c) 2006 Elsevier Ltd. All rights reserved.