Cyclic variation analysis and RSM optimization of a diesel engine fuelled with GNP-doped waste cooking oil biodiesel–butanol–diesel ternary blends

The effects of adding graphene nanoplatelets (GNPs) to waste cooking oil biodiesel-n-butanol-Euro diesel blends on the in-cylinder combustion characteristics and cycle variations of a diesel engine were examined. The combustion stability was evaluated by calculating the coefficient of variation of the maximum pressure (COVPmax) and the coefficient of variation of the indicated mean effective pressure (COVimep) over 100 consecutive cycles. The effect of GNP on combustion stability has shown a heterogeneous characteristics depending on engine speed and load. Under low load at 1500 rpm, samples containing GNP achieved up to 66% improvement in COVimep, while at 2000 rpm, an increase in instability was observed due to the loss of homogeneity in GNP distribution. The developed 2nd degree Response Surface Methodology (RSM) models, fitted using only statistically estimable terms, had good statistical validity and reliability (R2=0.853 for COVPmax; R2=0.583 for COVimep; p < 0.001). The optimal conditions were 1500 rpm, 50 Nm load and the B20Bu20G fuel blend with 0.05 g/L GNP (desirability = 0.98). The optimization predicted COVPmax and COVimep values of 1.93% and 1.52%, respectively. Analysis of Variance (ANOVA) test results confirmed that all models were statistically significant and engine speed and load were identified as the main components determining cyclical differences.