Numerical study on the use of ammonia/hydrogen fuel blends for automotive spark-ignition engines

Abstract

The importance of new alternative fuels has assumed great relevance in the last decades to face the issues of global warming and pollutant emissions from energy production. The scientific community is responsible for developing solutions to achieve the necessary environmental restriction policies. In this context, ammonia appears as a potential fuel candidate and energy vector that may solve the technological difficulties of using hydrogen (H2) directly in internal combustion engines. Its high hydrogen content per unit mass, higher energy density than liquid hydrogen, well-developed infrastructure and experience in handling and storage make it suitable to be implemented as a long-term solution. In this work, a virtual engine model was developed to perform prospective simulations of different operating conditions using ammonia and H2-enriched ammonia as fuel in a spark-ignition (SI) engine, integrating a chemical kinetics model and empirical correlations for combustion prediction. In addition, specific conditions were evaluated to consider and to understand the governing parameters of ammonia combustion using computational fluid dynamics (CFD) simulations. Results revealed similar thermal efficiency than methane fuel, with considerable improvements after appropriate H2-enrichment. Moreover, increasing the intake temperature and the turbulence intensity inside the cylinder evinced significant reductions in combustion duration. Finally, higher compression ratios ensure efficiency gains with no evidence of abnormal combustion (knocking), even at high compression ratios (above 16:1) and low engine speeds (800 rpm). Numerical simulations showed the direct influence of the flame front surface area and the turbulent combustion velocity on efficiency, reflecting the need for optimizing the SI engines design paradigm for ammonia applications.