Auto-ignition and combustion characteristics of n-butanol triggered by low- and high-temperature reactions of premixed n-heptane

This paper investigates the auto-ignition mechanism, combustion process, and emissions characteristics of n-butanol in active-thermal atmospheres. On a single-cylinder engine, the active-thermal atmosphere created by low- and high-temperature reactions of premixed n-heptane from intake port was used to trigger and control the ignition and combustion of n-butanol, which is directly injected into the combustion chamber near the top dead center (TDC). The experimental results reveal that the autoignition of n-butanol can be classified into three modes, namely, thermal atmosphere combustion, active atmosphere combustion, and active-thermal atmosphere combustion, depending on in-cylinder gas temperature and radical concentrations just before injection. The ignition timing of the overall combustion event was primarily determined by the equivalence ratio of premixed n-heptane, but was lightly affected by n-butanol quenching and charge cooling. Then, it can be flexibly controlled by modulating the directly and port-injected fuel equivalence ratios. In one combustion cycle, n-butanol ignited and burned after n-heptane; this combustion event can be referred to as dual-fuel sequential combustion (DFSC). For n-heptane/n-butanol dual-fuel sequential combustion events, ultra-low NOx and almost smoke-free emissions were observed over a wide operating range. Even with a large premixed fuel equivalence ratio, smoke-free and low-NOx emissions can be achieved simultaneously by selecting an appropriate directly injected fuel equivalence ratio.