Exhaust gas assisted fuel reforming is an attractive on-board hydrogen production method, which can open new frontiers in diesel engines. Apart from hydrogen, and depending on the reactions promoted, the reformate typically contains a significant amount of carbon monoxide, which is produced as a by-product. Moreover, admission of reformed gas into the engine, through the inlet pipe, leads to an increase of intake air nitrogen to oxygen ratio. It is therefore necessary to study how a mixture of syngas and nitrogen affects the performance and emissions of a diesel engine, in order to gain a better understanding of the effects of supplying fuel reformer products into the engine. In the current research work, a bottled gas mixture with H-2 and CO contents resembling those of typical diesel reformer product gas was injected into the inlet pipe of an HSDI diesel engine. Nitrogen (drawn from a separate bottle) at the same volumetric fraction to syngas was simultaneously admitted into the inlet pipe. Exhaust analysis and performance calculation was carried out and compared to a neat diesel operation. Introduction of syngas + N-2 gas mixture resulted in simultaneous reduction of the formation of NOx and smoke emissions over a broad range of the engine operating window. Estimation of the bottled carbon monoxide utilisation showed that by increasing either the load or the speed the admitted carbon monoxide is utilised more efficiently. As a general rule, CO2 emissions increase when the bottled carbon monoxide utilisation is approximately over 88%. Isolation of the H-2 and N-2 effect revealed that a CO diluted flame promotes the formation of smoke. When the intake air is enriched with syngas + N-2, an increase of engine speed results in reduction of maximum pressure rise rate (dp/da). The effect of load on dp/da varies depending on engine speed. Finally, the engine is more fuel efficient when running on neat diesel. Copyright (C) 2014, The Authors. Published by Elsevier Ltd on behalf of Hydrogen Energy Publications, LLC.