Experimental Study on the Characteristics of Knock under DI-HCCI Combustion Mode with Ethanol/Gasoline Mixed Fuel 2013-01-0544

Gasoline homogeneous charge compression ignition (HCCI) can achieve high efficiency and extremely low NO_X emissions. However, the working condition range of HCCI is limited by knock occurring during engine operation. To achieve an expanded HCCI working condition range, it is necessary to explore a method predicting knock cases accurately to avoid knock occurring.

Based on a DI-HCCI engine with ethanol/gasoline mixed fuel, the knock cases under different conditions have been investigated. In-cylinder pressure signals are used to identify the knock cases and the knock oscillations are extracted with fast Fourier transform (FFT). The effects of the ethanol proportion in the fuel and air/fuel ratio on the characteristics of knock have been studied.

The results have shown that the knock parameters, such as maximum frequency, start point angle and the duration, have close relationship with the knock intensity. Stronger knock causes higher maximum frequency, earlier start point and longer duration, but will lead to faster attenuations of knock oscillation. Furthermore, air/fuel ratios and the ethanol fraction have effects on the knock intensity. As air/fuel ratio decreases, the knock intensity, frequency and knock probability will be strengthened. When the fraction of ethanol increases, the knock amplitude, frequency and tendency will be impaired due to the high octane number of ethanol, which can also make the knock boundary expand effectively. From the analysis results of combustion phasing CA10 and CA50, it shows that ethanol has significant effect on retarding combustion, which leads to impaired knock. This retarding effect of ethanol is more sensitive with lower intake temperature. In addition, a 0-dimension combustion predicting model was set up in this paper. With the relationships found among knock and combustion parameters, such as air/fuel ratio, ethanol fraction and intake air temperature, the combustion pressure and knock characteristics could be predicted with an error below 10%.