The controller task is to manage each cylinder spark advance
to achieve a pre-defined combustion phase, while guaranteeing that a pre-fixed knocking intensity level is not largely nor frequently overcome.
As already mentioned in the test condition paragraph the reference gasoline cases, in terms of air fuel ratios and spark advance
, were chosen according to the standard engine map set by the ECU.
A high value of the Dwell Integral index indicates combustion initiation before the Spark Advance
, so a threshold could be simply set (0.05 for example) to discriminate pre-ignition cycles.
The Figure also shows the spark timing at knock borderline, namely the Knock Limited Spark Advance
(KLSA), for each speed, connected by dashed and continuous lines, for dry and WI operation, respectively.
However, while spark advance
enables maximum power output, it also increases in-cylinder burnt and unburned gas temperatures, resulting into higher probability of auto-ignition in the unburned zone and higher NOx formation in the burnt zone.
Because of the exhaust temperature dependence on combustion phasing, water injection effect on such quantity should not be compared at fixed spark advance
. Supposing the operating point is not knock limited, exhaust temperature difference/reduction should be compared under maximum efficiency conditions (which can be obtained with about the same CA50%MFB), for every water mass quantity, and consequently with different spark advance
As direct injection is relatively a new technology, DI engines used were probably calibrated more conservatively (as seen by respective spark advance
plots) in the U.S.
The fittings of MFB10 and MFB50 scatters with normal distributions show a linear dependence of the average value on spark advance
, while standard deviation is substantially constant.
Single-Cylinder Engine Results Knock Limited Spark Advance
In order to extend the number of simulated conditions for which the PDF-Knock model is tested, two additional spark advances
are simulated, namely SA=18 CA bTDC and SA=21 CA bTDC (hereafter SA18 and SA21 conditions); these are reported in Figure 23.
As mentioned in , , this would require a spark advance
before the IVC in order to increase peak in-cylinder pressure and temperature for stable combustion.
A National Instruments CompactRio system was used as an engine control unit to allow control of relevant engine parameters such as spark advance
. A closed loop lambda control system allowed the air fuel ratio to be maintained at a user selected value.