Optical engine experiments on combustion and emission performance of n-dodecane/ammonia dual fuels

Abstract

Ammonia is regarded as a promising carbon-free fuel, but it suffers from poor ignition and combustion as well as heavy emissions. High-reactivity fuels are often adopted as ignition boosters to solve these issues. In this work, the combustion and emission characteristics of n-dodecane and ammonia dual fuels were studied in an optical engine, with emphasis on the role of n-dodecane addition and injection timing. Simultaneous pressure acquisition, high-speed photography, and FT-IR spectroscopy were adopted for measurements. The results demonstrate the significance of ammonia energy ratio and n-dodecane injection timing in improving engine performance. Specifically, engine combustion is suppressed as ammonia energy ratio is raised, manifesting decreased peak pressure, postponed combustion phasing, and enlarged cyclic variations. Visualizations show that such changes are closely related to the evolutions of flame initiation and development. Meanwhile, there are always unburned regions which become pronounced at higher ammonia energy ratios, answering for the massive emissions of NH3 and NOx. Furthermore, with the advance of n-dodecane injection timing, engine performance is first improved and then suppressed, featuring nonmonotonic variations in in-cylinder pressure and combustion evolutions. Particularly, ignition delay and late-stage combustion duration are largely prolonged, which is associated with the weakness of fuel stratification and combustion mode transition. The advance of n-dodecane injection timing facilitates unburned NH3 reduction while increasing NOx and N2O emissions. High thermal efficiency while reduced pollution emissions can be achieved under suitable injection timing conditions. This work can provide useful insights into the combustion control of diesel and ammonia dual-fuel engines.