Today, environmental concerns push for new technologies to reduce our energy dependence on fossil fuels. The use of treated biogas in dual-fuel diesel engines is common as it has no major effects on engine performance and it only requires minor engine modifications, consisting mostly on premixing and injection timing advance. However, biogas itself cannot be directly used in engines because of degradation caused by acid attacks and sulfidation, making sulfur compound filtration a necessity. In this case, biogas use is economically competitive for relatively big installations with electrical power above approximate to 100-200 kW. Simple, low-costs solutions are needed for small installations (below 100 kW). The objective of this paper is to assess the degradation of a 3.1 kW diesel engine in dual-fuel mode with preheated untreated biogas with phase separation. The modifications on the engine consist only in an exhaust/biogas heat exchanger with phase separation and lubricant replacement with a higher total base number. These changes are relatively costless compared to removing sulfur from the biogas. A test engine did function for 550 h with preheated untreated biogas containing 2000 ppmv of H2S. The load was maintained at 60% of maximum load with 50% of diesel replacement with biogas. Degradation was monitored by visual inspection, weight and size measurements of engine parts, in-cylinder pressure measurements, and oil analysis. Dismantling of the engine at the end of the experiment showed almost negligible signs of degradation. For comparison purposes, a similar test was conducted on the engine with the same operating conditions but without biogas preheating or phase separation. The degradation observed was noticeably more important in that experiment as the oil total base number decreased faster and cylinder head gasket wear was more important. Biogas preheating with phase separation seems a promising low-cost alternative for biogas use in small farms as it limits acid attacks and sulfidation in engines. (C) 2013 Elsevier Ltd. All rights reserved.