An experimental study on the coupling effect of swirl ratio and injection timing in reactivity-controlled compression ignition gas engines

Abstract

This study investigates the coupled effects of swirl ratio and start of injection on diesel–natural gas reactivity-controlled compression ignition, with particular focus on their unexplored interactions and their effects on combustion characteristics and emissions. Experiments were conducted on a single-cylinder research engine at a representative operating point, using four distinct intake port configurations to generate swirl. Diesel injection timing was swept between 34 and 47 crank angle degrees before top dead centre, constrained by misfire limits. In-cylinder pressure-based thermodynamic analysis, along with flow simulations, enabled the interpretation of how in-cylinder turbulence influences combustion. The results for the first time reveal that swirl has a limited effect on cylinder-bowl-initiated ignition but plays a dominant role in combustion development as it progresses into the squish region. A high swirl advanced combustion phasing by 5.4 degrees and increased the peak heat release rate by 20%, but reduced combustion efficiency by up to four percentage points compared with low-swirl configurations. This was attributed to excessive homogenisation of the mixture and greater wall heat losses, which also increased unburnt hydrocarbon and carbon monoxide emissions. Nitrogen oxides emissions correlated strongly with combustion timing and temperature, while remaining largely insensitive to swirl. There was no direct trade-off between nitrogen oxides and particulate emissions, with particulate mass remaining ultra-low. However, more premixed conditions (early injection and large swirl) produced more fine particles, with particulate number emissions double the EU Stage V limit. The neutral-port-only configuration, combining the lowest swirl with the highest average turbulent kinetic energy, yielded optimal performance, with the highest thermal efficiency, nearly 44%, and the lowest combined carbon monoxide and hydrocarbon emissions. The results demonstrate that reactivity-controlled compression ignition performance is governed by a balance between swirl- and tumble-driven flow structures rather than by swirl intensity alone, and that flow-injection optimisation can enable compliance with stringent off-road emission limits without exhaust aftertreatment.