Spark-ignition engines running on gaseous fuels are commonly used for combined heat and power (CHP) production. In the present work, an experimental investigation has been conducted in order to study combustion in a spark-ignition engine fueled with producer gas. The engine is a single cylinder one, coupled to a fluidized bed gasifier in a mobile combined heat and power (CHP) production unit. Three biomass feedstocks were evaluated, which are olive, peach and grape kernels. Mixtures of each producer gas separately with propane at various blending ratios were fed to the engine, operating at various engine speeds. Cylinder pressure measurements were performed, followed by heat release rate analysis. Loss in power output compared to neat propane operation was experienced, owing to the lower calorific value of the producer gas, independently of the feedstock. Additionally, lower cylinder pressures and heat release rates were observed, coupled to prolonged combustion duration. Finally, combustion stability was moderately affected by the introduction of producer gas. (C) 2014 Elsevier Ltd. All rights reserved.