The use of Fast Pyrolysis Bio-Oil (FPBO) in diesel engines can be a valuable approach for CHP applications. However, the properties of FPBO make this application very challenging. The purpose of this work is to develop and demonstrate the use of FPBO in modified engines. Proper atomization of the fuel is of utmost importance and a. o. influenced by the fuel surface tension. The final equilibrium value of the surface tension is only slightly higher than for diesel, but large deviations in surface tension are observed at the short droplet lifetimes occurring in an engine's cylinder. Addition of 10-20 mass% of ethanol to the FPBO can significantly improve the atomization properties. Two diesel engines are available at BTG, a one-cylinder (1C) and a four-cylinder (4C). Both engines have been modified and operated on FPBO. Additional safety measures have been implemented for the 1C-unit to enable unattended operation. So far, the longest continuous and stable run has been 52 h. In total, this 1C engine has been operated for nearly 400 h on FPBO or FPBO blends. More recently, the four cylinder, 48 kW diesel engine has been modified to enable fueling with FPBO. This engine can be seen as a prototype for a commercial CHP unit (100 kW-1MW electrical output). The unit was successfully commissioned and initial tests are in line with the 1C experiments. Due to a lower calorific value, the volumetric fuel consumption is significantly higher in case of FPBO. However, the overall electrical efficiency is very similar to diesel.