The paper presents results and discussion on syngas combustion in a spark ignited internal combustion engine driving a power generator (16 kW/1500 rpm). The syngas applied as the engine fuel is produced by gasifying municipal sewage sludge. Unlike fermentation biogas, the syngas obtained from sewage sludge gasification is found a fuel, which generates several difficulties (non-stable combustion, misfiring events) while burning in the SI engine. The analysis of cycle-by-cycle engine run shows that the engine working on syngas features will run unsteadily due to the occurrence of misfiring events caused by the relatively low calorific value of the syngas. Thus, combustion of such a syngas in the SI IC engine can generate malfunctions. Therefore, the syngas was enriched with methane at various ratios starting from 0% up to 60% by volume. The impact of methane addition on the engine performance and exhaust emission was also presented in the paper. As investigated, 40% methane enrichment in the syngas was found sufficient amounts to obtain satisfactory engine run. Thus, in practice, the syngas from sewage sludge gasification is a troublesome fuel, but can satisfactorily be applied to the SI engine when enriched with natural gas or biogas. (C) 2012 Elsevier B.V. All rights reserved.