Effects of port mixing and high carbon dioxide contents on power generation and emission characteristics of biogas-diesel RCCI combustion

Reactivity-controlled compression ignition (RCCI) combustion burns fuels of varying reaction rates to enhance combustion stratification. Biogas-diesel RCCI combustion requires energy assessment to improve power and reduce emissions. This study investigates the effects of different carbon dioxide (CO₂) contents (25, 35, and 45%) and port mixing distances (0, 28.75, 57.5, 86.25, and 115 mm) experimentally, at 6.5 bar IMEP and 1600 rpm in a premixed and port injection at the valve. An exergy analysis facilitates understanding high-CO₂ biogas stratification and the cause of emissions trade-off experimentally, unavailable in the literature. Based on energy analysis, a 35% CO₂ content demonstrates moderate combustion work and enhanced output power, while 45% CO₂ shows lower heat loss across the mixing distances. Injecting biogas at the valve increases combustion work by 9.11 – 27.33% and reduces power output by 0.33 – 4.90% due to increased exhaust loss. Based on exergy analysis, evenly distributed in-cylinder temperatures and sufficient fuel stratification were observed for 35% CO₂ and 50% distance. The 35% CO₂ increases the internal energy potential recovery by 24.26% over mixing spaces because of reduced destruction. Injection at the valve causes stratified CO₂ layers at the bowl centerline and diffused temperature at the cylinder wall, simultaneously decreasing CO₂, carbon monoxide, unburned hydrocarbon, nitrogen oxides, and particulate matter emissions by 4.46, 2.04, 3.05, 44.67, and 9.13%, respectively. Therefore, increased compression work allows more heat transfer through the exhaust, while heat outflow through the cylinder wall reduces combustion work. Furthermore, the reduced emissions due to increased CO₂ content at higher mixing distances compromised the output power of the biogas-diesel RCCI engine.