Experimental Study on the Influence of n-butanol Blending on the Combustion, Autoignition and Knock Properties of Gasoline and its Surrogate in a Spark Ignition Engine

The impact of n-butanol blending on the combustion, autoignition and knock properties of gasoline has been investigated under supercharged spark ignition engine conditions for stoichiometric fuel/air mixtures at intake temperature and pressure conditions of 320 K and 1.6 bar, respectively, for a range of spark timings. A toluene reference fuel (TRF) surrogate for gasoline containing toluene, n-heptane and iso-octane has been tested experimentally in the Leeds University Ported Optical Engine (LUPOE) alongside a reference gasoline and their blends (20 % n-butanol and 80 % gasoline/TRF by volume). Although the gasoline/n-butanol blend displayed the highest burning rate, and consequently the highest peak pressures compared to gasoline, TRF and the TRF/n-butanol blend, it exhibited the least propensity to knock, indicating that addition of n-butanol provides an opportunity for enhancing the knock resistance of gasoline as well as improving engine efficiency via the use of higher compression ratios. The anti-knock enhancing quality of n-butanol on gasoline was however observed to weaken at later spark timings. Hence, whilst n-butanol has shown some promise based on the current study, its application as an octane enhancer for gasoline under real engine conditions may be somewhat limited at the studied blending ratio. As expected based on previous ignition delay studies, the TRF showed an earlier knocking boundary than the rest of the fuels, which may possibly be attributed to the absence of an oxygenate (ethanol or n-butanol) as present in the other fuels and a lower octane index. Overall, the TRF mixture gave a reasonable representation of the reference gasoline in terms of the produced knock onsets at the later spark timings for the pure fuels. However, on blending, the TRF did not reproduce the trend for the gasoline at later spark timings which can be linked to difficulties in capturing the temperature trends in ignition delays around the negative temperature coefficient region observed in previous work in a rapid compression machine (Agbro et al., Fuel, 2017, 187:211-219).