Due to the renewability of hydrogen (H-2), it has been considered as a substitute for gasoline in spark-ignition engines. However, the low stoichiometric energy content and the low density of H-2 suggest that it is not an ideal sole fuel. In this work, a study of an alternative use of H-2 in SI engines has been conducted experimentally, i.e. as a supplement at idle operations. In the experiments, idle operation is firstly defined by using engine speed and cycle-to-cycle variation. Then, the engine performance and emissions at the idle condition with gasoline only and with different level of H-2 supplementation are studied. For accuracy, the extremely small flow rate of H-2 at idle operation is measured by using the choked flow effect. For comparability, H-2 supplementation is on an energy basis, that is, the total energy flow at a given condition are the same for both gasoline only and gasoline with H-2 supplementation. From the experiments, it is found that cycle-to-cycle variation is linearly related to the burning period and hydrogen fraction, which is independent on the relative air-fuel ratio. From a performance perspective, the idle efficiency is significantly improved with H-2 supplementation. The relative efficiency improvement with 25% H-2 supplementation is approximately 21%. From an emissions perspective, carbon dioxide, carbon monoxide, and unburned hydrocarbon emissions are all significantly reduced with H-2 supplementation. Due to the high combustion temperature, nitrogen oxides emissions are increased with H-2 supplementation. However, this increment is of less important as nitrogen oxides emissions at the idle condition are small. (C) 2016 Elsevier Ltd. All rights reserved.