Effect of inlet air temperature on thermodynamic combustion characteristics of a Calophyllum inophyllum biodiesel–diesel HCCI engine: BO-GPR prediction

Homogeneous charge compression ignition (HCCI) combustion of a Calophyllum inophyllum biodiesel–diesel blend (80D20CIBD) was characterised in a Kirloskar TV1 engine (5.2 kW, CR 17.5) across inlet air temperatures (IAT) of 50–110 °C, benchmarked against diesel, B20, and B100 in CI mode. Thermodynamic combustion parameters — peak pressure, combustion phasing (CA50), combustion duration, and rate of pressure rise — were extracted from in-cylinder pressure and heat-release-rate data. Among HCCI conditions, IAT 90 °C achieved the highest brake thermal efficiency (30.2%, 5.2% below diesel CI) with simultaneous NOx and smoke reductions of 8.0% and 33.3% relative to diesel CI at full load. All HCCI conditions occupied the simultaneous NOx–smoke reduction quadrant, and combustion duration shortened by up to 56% relative to diesel CI. A multi-criteria (TOPSIS) evaluation across all responses showed that the overall-preferred condition is weighting-dependent, motivating a continuous Bayesian-optimised Gaussian process regression (GPR) surrogate. The GPR model, validated by leave-one-out cross-validation (n = 28), achieved a mean R 2 of 0.9657, outperforming five benchmark models. Brake power was the most significant predictor of brake thermal efficiency, fuel composition was most influential for hydrocarbon and carbon monoxide emissions, and inlet air temperature was the dominant driver of NOx. The resulting GPR optimisation surface maps the IAT–brake-power domain, providing an interpretable, uncertainty-quantified tool for HCCI combustion management of biodiesel–diesel blends.