Disparities in the constituents among biogases pose various control challenges during combustion, thus complicating its operability. Improving biogas by eliminating carbon dioxide (CO 2 ) remains difficult, and high CO 2 concentration is known to affect combustion. Still, the effect on Biogas/Diesel dual-fuel reactivity-controlled compression ignition (RCCI) combustion is not well explored. The influence of high diluent (CO 2 ) proportions (25 -45%) and Biogas fraction (40 -70%) on

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... dual-fuel RCCI combustion was evaluated experimentally, at full load and 1600 rpm. The results show distinct temperature trends across the combustion regimes for all the CO 2 ratios and Biogas fractions. The radiation absorption effect of high CO 2 reduces the cylinder temperature and delays the expansion to a 10 -20 o crank angle after the top dead centre (CA ATDC). The temperature and pressure decrease with the increase in CO 2 ratios and Biogas fractions, unlike heat released rate. Burning Diesel and 45% CO 2 indicate the fastest combustion, with combustion phasing (CA50) at 6.353 ms ATDC while 25 and 35% CO 2 were slowed by 0.258 and 0.578 ms, respectively. Inferentially, 35% CO 2 at a fraction of 50% could form the best reactivity.