Experimental investigation on the knocking combustion characteristics of n-butanol gasoline blends in a DISI engine

Highlights: • N-butanol shows better knock resistance characterized by improved KLST. • Bu20 blend fuel slightly degrades the knock resistance compared with gasoline. • Knock oscillation frequency depends on combustion chamber resonance modes. • Probability distribution is applied to evaluate variation of knock intensity. - Abstract: n-Butanol is a very competitive alternative biofuel for spark ignition (SI) engines given its many advantages. Current researches are mainly concentrated on the overall combustion and emissions performance concerning the feasibility of n-butanol gasoline blends in SI engines. In this work, focus was given on the knocking combustion characteristics of operation with pure n-butanol as well as a blend fuel with 20% volume content of n-butanol (Bu20), which was investigated experimentally in a direct-injection spark ignition (DISI) single cylinder engine. Operation condition is fixed at a constant engine speed of 1500 r/min, using three throttle openings with stoichiometric air–fuel ratio. Spark timing was swept to achieve different knocking levels. The results of n-butanol and Bu20 were benchmarked against those obtained by the research octane number (RON) 92 commercial gasoline. Compared with the baseline fuel gasoline, neat n-butanol shows better anti-knock ability with more advanced knock limited spark timing, whereas slightly deteriorative knock resistance can be found for Bu20. It is hypothesized Bu20 has higher end gas temperature due to its higher brake mean effective pressure (BMEP) and faster burning rate compared with gasoline, which indicates the knock tendency depends not only on the fuel octane number, but also on the factors that affect the end gas thermodynamic state. The heavier knock propensity of Bu20 is furthermore confirmed by its more advanced knock onset and higher peak oscillation pressure. Results of fast fourier transform (FFT) indicate the knocking oscillation frequencies are mainly determined by the combustion chamber resonance modes. Statistics probability distribution and cumulative distribution are applied to evaluate the knock intensity variation for each fuel.