Investigating the relationship between butanol molecular structure and combustion performance in an optical SIDI engine

Abstract

Butanol has a high potential as a renewable substitution for gasoline in spark-ignition direct-injection (SIDI) engines. Different butanol isomers showed various flame characteristics that are strongly related to their molecular structure. However, there has been limited research on the implications of the butanol isomer's molecular structure on SIDI engine combustion, performance, and emissions. This study investigated butanol isomers as gasoline substitutes in SIDI range extender engines, with a focus on the effects of isomer molecular structures. This work employed a single-cylinder optical SIDI research engine and a high-speed camera to examine engine performance, flame kernel stability, flame propagation, and particle number (PN) emissions. The investigated blends comprised 70% toluene reference fuel (TRF) and 30% butanol isomers (1-butanol (n), 2-butanol (s), isobutanol (i), and tertbutanol (t)). Experimental tests are carried out at a 1000 rpm engine speed and a load of 5.7 bar IMEP, while the fuel condition is kept stoichiometric.The results elucidate that adding 1-butanol makes the flame kernel more stable and reduces COVimep compared to pure TRF. On the other hand, TRF-i, TRF-s, and TRF-t decrease flame initiation stability and increase COVimep. 1-butanol exhibited the highest apparent flame speed, IMEP, and peak in-cylinder pressure, followed by TRF, TRF-i, TRF-s, and TRF-t. Over half of the TRF-t cycles exhibited either no or a delayed flame kernel. Flame circularity improved with 1-butanol and diminished with the other blends. Diffusion flame intensity and PN emissions were higher for TRF, TRF-n, and TRF-t compared to TRF-s and TRF-i. In conclusion, linear chain butanol isomers with more internal C-H bonds and terminal C-OH bonds provide a more stable flame kernel and superior engine performance than branched isomers featuring internal C-OH bonds. The results from this study can be useful for expanding the practical applications of butanol isomers as a renewable fuel replacement in SIDI engines.