Controlling fuel droplet size distribution: A key refinement for RCCI engines from energy, exergy, economic, and environmental perspectives

Abstract

The study delves into the aerosolization process of fuel flow in combustion engines with the aim of adjusting the droplet size distribution to optimize combustion. While smaller droplets offer advantages in better evaporation and combustion, the research nullifies their benefits in enclosed engine environments. Simulating the consecutive processes of fuel injection, droplet formation and breakup, combustion, and pollutants’ production and motion within a Reactivity Controlled Compression Ignition (RCCI) engine highlights that the production of excessively fine droplets (below 25 µm Sauter mean diameter) requires a sudden increase in injection pressure, raising the risk of some high-momentum droplets escaping combustion and adhering to engine surfaces. Moreover, the combustion initiates earlier while injection is still in progress, and upstream droplets combust at larger sizes without sufficient time to fragment. These phenomena diminish engine efficiency, escalate emissions of volatile organic compounds, and yield economic detriments. In light of these findings, the study postulates that neither excessively fine nor large droplets are ideal for combustion, and uniformity in fuel droplet size distribution is the key in efficient combustion. Regarding this, to generate more homogeneous and sufficiently small droplets, the study reveals two pivotal characteristics for injection pressure function: a continuous increase combined with specific number of vibrations, leading to an increasing oscillatory function. This discovery is evaluated through a comprehensive framework encompassing energy, exergy, environmental impact, and economic considerations. Not only does it enhance power efficiency and reduce VOC emissions, but it also yields more irreversibility, opening opportunities for more waste heat recovery. These advantages can lead to significant improvements in overall engine performance and efficiency.