Comparison between the energetic and environmental performance of a combined heat and power unit fueled with diesel and waste vegetable oil: An experimental and numerical study

A combined experimental and numerical characterization of a Compression Ignition (CI) engine is here presented under Diesel and Waste Vegetable Oil (WVO) fueling at different loads. Main objective is to prove the feasibility of preheated WVO as substitute of Diesel fuel in CI engine for cogeneration purposes without modifying the engine geometry. After a characterization of the WVO physical properties, an appropriate pre-heating system is mounted on the engine injection line in order to reduce viscosity. The in-cylinder pressure analysis reveals no significant differences under both fuelling modes, despite an increased fuel consumption measured for WVO. This last provides higher NOx, CO and CO2 emissions, with a strong reduction (50–80%) of the soot amount. The potential impact of employing WVO is then assessed through a Life Cycle Assessment (LCA) methodology with focus on the fuel production and use stages. A reduction in all the major impact categories is noticed with respect to Diesel fueling. At the same time, a 3D CFD model of the engine is developed and validated. The WVO injection process shows slower spray break-up and evaporation rates due to higher viscosity and density. Despite the higher penetration lengths and increased amount of fuel burnt closer to the cylinder walls, the soot at the exhausts remains low as a consequence of the absence of aromatic compounds and an enhanced oxidation process due to the presence of oxygen atoms in the WVO molecules. This aspect is also responsible of the increased NOx release.