estimation function has recently become a must for Engine Control Units (ECUs), since external disturbances can modify the engine knock tendency during on-board operation.
Amann and Alger  found that because of the differences in combustion duration and knock propensity, the knock intensity
was found to be a strong function of engine oil reactivity differences.
Whilst discussing spark-ignition knock, Heywood  points out that due to the highly variable nature of engine knock, fundamental definitions of knock intensity
is extremely difficult to make.
The system also has the ability to compute, in real-time, additional individual cycle combustion parameters such: as IMEP, PMEP and NMEP; misfires; peak pressure, maximum rate of pressure rise and locations; polytropic coefficients of expansion and compression; upper, lower and average pressure envelopes; injector start and duration, both crank angle and time; mass burn fraction and rate; engine averages of IMEP, NMEP, peak pressure, maximum rate of pressure rise and their location; and knock intensity
In general, the severity of the knock depends on the point in the combustion cycle at which autoignition occurs: an extended ignition delay should reduce knock intensity
and vice-versa .
The obtained raw cylinder pressure traces were band-pass filtered, rectified and the resulting traces then individually integrated to result in a single characteristic number per combustion cycle (knock intensity
Many index and methods for knock intensity
evaluation and knock onset determination have been proposed by means of in-cylinder pressure analysis.
The faster the flame speed, the less the end gas involved in the knock combustion, and the weaker the knock intensity
Analyses were made of end-gas autoignition and its development process during engine knock operation at different levels of knock intensity
In both tests the compression ratio at a standard knock intensity
for the test fuel is compared to results for a set of primary reference fuels (PRFs).
The outcomes of this analysis are reported in Figure 16, which shows the numerical knock intensity
, [[DELTA]p.sub.knock], represented by the circle size, as a function of engine speed and combustion phasing.
This forces to adopt a sub-optimum Spark Advance (SA) angle, thus reducing maximum temperature and knock intensity
, while correspondingly increasing exhaust temperatures, with respect to maximum efficiency conditions.