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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.

In the present study, performance of an HCCI engine powered with ethanol/toluene/n-heptane tri-fuel blend was optimized by using response surface method. The studied independent parameters were engine speed, lambda ratio, and fuel blends. The impact of these parameters on engine torque, COVimep, CA10, CA50, indicated thermal efficiency, IMEP along with emissions of NO X , CO, and HC comprehensively investigated. According to the results, the optimal HCCI engine operation condition was proposed as engine speed of 1343 rpm, lambda value of 2.29, and ethanol ratio of 22%. At this condition, the engine outputs, i.e., IMEP, COVimep, indicated thermal efficiency, CA10, and CA50, engine torque were estimated to be 4.21 bar, 4.28%, 0.37, 1.41 °CA, 4.62 °CA, and 8.2 Nm, respectively. The engine-out emissions, including HC, NO X , and CO emission, were predicted to be 243 ppm, 1.05 ppm, and 0.03%, respectively. The result indicates that using ethanol/toluene/n-heptane fuel mixture improved the HCCI combustion and NO X emission. The near-zero NO X emissions were recorded at all fuel mixture. However, enhancing ethanol ratio in the fuel blends showed an increase in CO and HC emissions. Overall, this study showed that response surface technique could be used as a promising method to model the HCCI engines.

The main disadvantages of HCCI engines are the knocking tendency at high engine loads, the challenge of the start of the combustion, control of the combustion phase, and the narrow operating range. In this study, we aimed to control the combustion processes in HCCI engines and to expand their working range by improving the fuel properties of fusel oil by the addition of diethyl ether. Thus, the variations in the in-cylinder pressure, rate of heat release, indicated mean effective pressure, start of combustion, combustion duration, CA50, indicated thermal efficiency, mean pressure rise rate, hydrocarbon and carbon monoxide emissions were investigated. It was observed that the in-cylinder pressure and rate of heat release were taken into advance and the test engine could be operated for a wider range by increasing the diethyl ether ratio in the blend. The indicated mean effective pressure increased by 67.5% with DEE40 fuel compared to the DEE80. Under the same operating conditions, HC and CO emissions decreased by 41.6% and 56.2%, in use of DEE40. Furthermore, the highest indicated thermal efficiency was obtained as 42.5% with DEE60 fuel. Maximum hydrocarbon and carbon monoxide emissions were observed with DEE80 fuel as 0.532% and 549 ppm, respectively.