Study on the knock tendency and cyclical variations of a HCCI engine fueled with n-butanol/n-heptane blends

Abstract

The homogeneous charge compression ignition (HCCI) combustion operation is conducted in the 2nd cylinder of a natural-aspirated four-stroke diesel engine. In the HCCI combustion mode, the n-butanol, n-heptane and their blends are injected into the intake port to form a lean homogeneous air-fuel mixture, which is consumed by the autoignition after compression. The objective of this study is to investigate the knock tendency and the cyclical variations of the HCCI engine. Experimental results show that the volume fraction of n-butanol affects the knock tendency greatly, which obviously decreases as the n-butanol volume fraction increases. The knocking combustion in the HCCI combustion is characterized by the high heat release rate (HRR). Both elevating the engine speed and raising the intake temperature contributes to an obvious increase in HRR and the knock tendency. But the HRR and knock tendency may slightly decrease when the engine speed reaches to 1400rev/min and intake temperature reaches to 160°C. Furthermore, the knock tendency can be weakened by increasing the excess air-fuel ratio. Cyclical variations of the HCCI engine are quantified by the coefficient of variation for the peak pressure (COVPmax) and it exhibits an almost opposite trend to the knock tendency. The COVPmax may considerably increase along with either increasing the blending ratio of n-butanol or increasing the excess air-fuel ratio. Moreover, it is reveled that the COVPmax is sensitive to the relative position of peak HRR. The cyclical variations may dramatically reduce when the peak HRR timing gradually approaches to the combustion center (CA50).