Gasoline homogeneous charge compression ignition (HCCI) can
achieve high efficiency and extremely low NOX 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%.