Detecting controlled end-gas autoignition for a pilot-ignited methane/hydrogen premixed blend

This paper aims to better understand the onset and intensity of controlled end-gas autoignition (c-EGAI), which can help extend the knock limit of pilot-injection-ignited combustion modes. A set of experiments was designed where the pilot injection pressure (P_Inj), pilot start of injection (SOI) thermodynamic conditions, and premixed equivalence ratio were varied to induce c-EGAI. Common knock detection techniques such as maximum amplitude of pressure oscillation (MAPO), ringing intensity (RI), and signal energy ratio (SER) were found lacking in their ability to adequately characterize c-EGAI, suggesting that the underlying mechanism differs from conventional knock. Short time Fourier Transform (STFT) analysis showed that c-EGAI excited frequencies in the vicinity of the lowest resonant mode, with amplitudes comparable or greater than the diesel pilot ignition. The number and magnitude of the peaks in the fundamental cylinder oscillation modes (found from STFT) was able to correctly quantify the c-EGAI onset and intensity. To substantiate the time-frequency analysis, a kinetic analysis was performed. The kinetic predictions of end gas autoignition had a 1:1 scaling with the c-EGAI onset determined using the STFT.