EGR Strategy for NOx Emission Reduction in a CAI Engine Fuelled with Innovative Biogas

Gaseous fuel utilization reduces pollutant emissions and makes thermal energetic systems less dependent on oil. Their carbon content is low, which means low CO2 emissions, and a lower cost with respect to conventional fuels. If gaseous fuels have a non-fossil origin, their adoption would have a beneficial impact on global CO2 balance and on the dependence of the energy sector from fossil fuels. Biogas is produced from the anaerobic digestion of organic materials, with the composition depending on the feedstock and on the production processes. The use of biogas in internal combustion engines (ICE) is very attractive. However, the presence in the biogas of an inert gas like CO2 has adverse effects on combustion, reducing combustion speed, narrowing flammability limits with harmful effects on combustion stability. A possible way for improving combustion performance of biogas is hydrogen addition. Innovative anaerobic digestion processes, which maximize the H2 yield, can end up with biogases made of CH4, CO2 and H2. The biogas, generally adopted as fuel in positive ignition ICEs, is also suitable for Controlled Auto Ignition (CAI) engines. Aim of this work is to investigate the biogas combustion in CAI ICE by means of numerical simulations. The effect of fuel composition and EGR on engine performance and exhaust emissions was evaluated. Authors compared conventional and innovative biogases composed by CH4, CO2 and H2 of different compositions. CAI systems are controlled with varying intake gas conditions: air-fuel ratio, boost pressure and charge temperature. The temperature depends on recycled exhaust gas content, which has an impact on the combustion process. Authors investigated the effect that EGR has on the in-cylinder gas temperature and reaction mechanism, focusing on NOx emission formations.