Exergy-Based Evaluation of High-CO2 Biogas/Diesel RCCI Combustion Heat Flow for Enhanced Mixture Distribution, Power Output, and Fuel-Energy Performance

Abstract

Utilising high-CO2 biogas in compression-ignition engines poses significant challenges due to poor mixture reactivity, inefficient combustion, and increased energy degradation. This work addresses these difficulties by conducting experimental research on a port-injection at the valve reactivity-controlled compression ignition (PIVE-RCCI) strategy. This study addresses these concerns by conducting experiments on a PIVE-RCCI technique to improve mixture distribution and combustion efficiency in biogas-diesel engines. The engine was modified to provide biogas through the inlet valve, allowing for controlled variations of biogas injection pressure (BIP: 1-4 bar) and port swirl ratio (PSR: 0-80%) at 1600 rpm and 4.9-5.7 bar IMEP. Energy and exergy analyses were used to determine the effect of intake flow dynamics on temperature uniformity, heat transfer, and power generation during combustion. The results reveal that normal airflow conditions minimise accounted heat loss, indicating higher thermal efficiency (ITE) and increased output power across all BIPs. In contrast, introducing a strong intake swirl dramatically improves combustion performance. The 80% PSR configuration resulted in the lowest exergy destruction and the maximum energy recovery potential, with an ITE of 26.54% at 4 bar BIP. Increasing BIP increased power output, whereas the optimal combustion work was found at 1 bar BIP and 40% PSR. The optimal working conditions were 1 bar BIP, 80% PSR, and 5.45 bar IMEP, which resulted in 26.00% exergy destruction, 39.38% destruction-to-released exergy ratio, 86.00% exergy-energy ratio of heat transfer, and 63.78% exhaust exergy-energy ratio. This work's novelty lies in integrating biogas injection, intake swirl control, and exergy-based evaluation to measure mixture distribution and energy recovery in high-COQ biogas RCCI combustion. The findings offer useful operational guidance for increasing energy efficiency and advancing the commercialization of renewable gaseous fuels in RCCI engines. As a result, operating the engine at half load, 80% PSR, and atmospheric air pressure (1 bar) conditions significantly enhanced the combustion efficiency and energy utilisation.