“…However, the phenomenological complexity of the combustion and the nonlinearities associated with it render purely data-driven vibroacoustic combustion estimation approaches, commonly proposed for CI engines, ineffective for LTC applications (refer to Table 1). Limited works that proposed such an approach for HCCI, using a full operational envelope test matrix, were bearly able to predict the combustion onset with errors below 4 CA [37,39]. Notably, peak HRR or PRR values that are relevant for establishing operational limits are extremely difficult to capture in HCCI due to their rapid and random nature.…”