Abstract
This study was conducted to elucidate the combined effects of intake-air temperature (IAT), excess air ratio (λ), and fuel blend composition on the combustion behavior of an HCCI engine. Three butanol/diethyl ether blends (B15, B30, and B45) were systematically evaluated at a constant engine speed of 1000 rpm and a compression ratio of 12. The IAT was varied between 35 °C and 65 °C in 15 °C increments, while different λ values were applied to each blend to capture a broader spectrum of operating conditions. The findings demonstrate clear differences in combustion behavior and performance among the blends. Specifically, increasing the diethyl ether content in the B15 blend, together with higher IAT, advanced in-cylinder pressure development, heat-release rate, start of combustion, and CA50. It also provided the widest stable λ operating range, although PRRₘₐₓ reached 14 bar/°CA at rich conditions, exceeding the knock safety limit. In contrast, relative to the B15 blend, a butanol fraction of 45% retarded ignition timing while achieving optimal combustion phasing between 7° and 11° after TDC. This shortened the combustion duration by approximately 59%, improved indicated thermal efficiency by nearly 20%, and reduced knocking by about 70%. The blend also achieved the highest IMEP of 6.27 bar, maintaining cyclic variability below 10% and ensuring stable combustion even at elevated IAT values. Additionally, the lowest emission levels were observed for the B15 blend at 65 °C, with CO and HC concentrations of 0.065% and 171 ppm, respectively, whereas CO₂ emissions showed the opposite trend, increasing as CO decreased. Overall, the results identify B45 as the most effective blend for maximizing efficiency and combustion stability, while B15 provides the broadest λ operating window, highlighting a measurable trade-off between efficiency optimization and operating flexibility in HCCI engines.