This study investigates the performance, exergy behaviour, and environmental impact of diesel–butanol blends in a Tempest four-stroke, water-cooled, direct-injection CI engine operating over a wide speed range (1200–3600 rpm). Engine simulations were performed for butanol concentrations from 0 to 100%, with constant injection timing and an equivalence ratio of 0.85. Previous research used second-law sustainability metrics sparingly and lacked data-driven multi-parameter analysis; this work integrates hierarchical clustering, RSM, and advanced exergy indicators, including SI, EPC, and DN. The models were statistically validated using ANOVA, demonstrating strong significance (p 0.94). This study demonstrates that low butanol substitutions (2.5–10% by mass) in diesel fuel offer a favourable compromise for compression-ignition engine performance, exergy-based sustainability, and emissions when operated at low-to-moderate speeds (∼1200–2400 rpm). Blends in this range achieve up to 11% higher useful work exergy output, improved thermal and exergetic efficiencies, substantially enhanced Sustainability Index (SI) and Exergy Performance Coefficient (EPC), and 38–70% lower Formula: see text emissions (especially pronounced at 1800 rpm) compared with neat diesel. Higher butanol fractions (>20–30%) degrade performance, markedly increasing HC emissions and exergy destruction primarily due to elevated latent heat of vaporization, prolonged ignition delay, and poorer fuel–air mixing. Engine speed emerges as the overwhelmingly dominant parameter governing power, efficiency, and sustainability metrics (confirmed by ANOVA F-values orders of magnitude larger than fuel-property terms), with maximum first-law efficiency and work output occurring near an intermediate speed (∼1800 rpm), while best second-law sustainability is consistently achieved at the lowest practical speeds.
Jehad Yamin (Tue,) studied this question.