Reduced graphene oxide nanoparticles and integrated DOC + SCR aftertreatment in a diesel engine fueled with sunflower biodiesel-n-butanol ternary blends: Combustion, performance, and aftertreatment response

Reducing gaseous emissions from compression ignition engines while maintaining engine efficiency remains a key challenge for biodiesel-alcohol fuel strategies, particularly when fuel modification and exhaust aftertreatment are considered together. This study experimentally investigated the effects of reduced graphene oxide (rGO) nanoparticles (30, 60, and 90 ppm by mass) added to a ternary B30D60BUT10 fuel, consisting of 30 vol% sunflower oil methyl ester, 60 vol% diesel, and 10 vol% n-butanol, in a single-cylinder direct-injection diesel engine equipped with an integrated diesel oxidation catalyst and selective catalytic reduction (DOC+SCR) system. Experiments were conducted at 1800 rpm under four steady-state loads (8.24–24.72 Nm), using D100 and B30D70 as reference fuels. Combustion was evaluated from 50 consecutive pressure cycles, while gaseous emissions were measured upstream (P 1 ) and downstream (P 3 ) of the aftertreatment system. The results demonstrate a strongly load- and concentration-dependent rGO response. At 19.22 Nm, the 90-ppm formulation achieved the highest brake thermal efficiency of the complete test matrix (34.81%), exceeding D100 by 3.22 percentage points, while at 24.72 Nm, the unmodified B30D60BUT10 blend provided the highest BTE (32.66%). At 24.72 Nm, 90 ppm rGO also produced the highest mean IMEP (6.378 bar) and the lowest COV-IMEP (2.10%), with an IMEP standard deviation of 0.130 bar, 22.2% lower than D100. Engine-out HC decreased systematically with increasing rGO concentration; the 90-ppm formulation reduced HC by up to 84.5% relative to D100. Across the DOC+SCR system, the 90-ppm formulation achieved an apparent HC reduction of 52.8% at maximum load, while the maximum apparent P 1 -P 3 NO x reduction was 52.2% for the unmodified B30D60BUT10 blend, reducing NO x to 680 ppm downstream despite its relatively high engine-out NO x concentration. In addition, rGO simultaneously reduced BSFC and engine-out NO x relative to D100 at 8.24 Nm with 60 ppm rGO and at 19.22 Nm with 90 ppm rGO, indicating that the conventional BSFC-NO x trade-off can be weakened under selected operating conditions. Overall, the results show that rGO does not provide a universal optimum concentration; instead, its effectiveness depends on engine load and the performance parameter considered. The combined fuel-modification and aftertreatment results therefore demonstrate the importance of matching rGO concentration with engine operating conditions when evaluating biodiesel-n-butanol-diesel fuels for low-emission compression ignition applications.