Investigation on the autoignition characteristics of propanol and butanol isomers under diluted lean conditions for stratified low temperature combustion

Abstract Low-temperature combustion (LTC), which can be achieved through either fuel-lean mixtures or high dilution, is an effective way to reduce NOx and soot emission for an internal combustion engine. Alcohols are considered suitable fuels for LTC as they have the advantage of better fuel-air mixing, such as high autoignition resistance and fast vaporization. Propanol and butanol can be produced as bio- or electro-fuels, which are carbon-neutral and become candidates as transportation fuels. This work focuses on investigating the autoignition and emission behavior of these alcohols regarding the application in stratified LTC. The autoignition properties of propanol and butanol isomers have been investigated in a rapid compression machine (RCM) under 90% dilution, at temperatures from 800 K to 1100 K, pressures of 20 and 40 bar and equivalence ratios of 0.25, 0.5 and 0.9. Under all investigated conditions, the order of reactivity among butanol isomers is consistent, namely n-butanol > isobutanol ≅ 2-butanol > tert‑butanol, while n-propanol is always more reactive than isopropanol. The reactivity of propanol isomers is similar to that of iso and 2-butanol. It was observed in the measurement that the difference between the reactivity of different fuels was not constant in the investigated temperature range, which was explained by the autoignition chemistry transition between 800 K and 950 K. According to the modeling, emissions of all butanol and propanol isomers are similar. The non-diluted mixtures can only achieve low-temperature combustion (LTC) in extreme lean conditions, while the diluted mixtures can achieve LTC in a wider range of equivalence ratios and have both low NOx and hydrocarbon emissions.