Etude de la combustion de gaz de synthèse issus d'un processus de ...

Chapter 5
tel-00623090, version 1 - 13 Sep 2011
From figure 5.13 it is observed that the compression stroke is independent of the
ignition timing. The TDC position is reached at 137 ms and is kept during around 4 ms
to 141 ms. The measured compression stroke duration is about 44 ms. Expansion
stroke is indeed slightly influenced by the ignition timing, which become slower when
the ignition is made far from TDC. However, as seen in the figure 5.13 the ignition
timings of 5 ms BTDC and 7.5 ms BTDC could be considered to have the same piston
displacement along time. The 12.5 ms BTDC ignition timing is the one with lower
displacement for every fuel under study. Therefore, considering a criterion of 1.0 mm
for the final piston position the duration of the expansion stroke is around 49 ms for
ignition timing of 5 ms and 7.5 ms BTDC and 51 ms for 12.5 ms BTDC ignition timing.
This could be explained by the fact that when the combustion starts to earlier in relation
to TDC (12.5 ms BTDC) a large part of heat is released when the piston is still moving
up. Therefore, when the ignition is made close to TDC the heat released is available on
the expansion stroke to push the piston down. Thus, it is expected that some influence
of the fuel type on the expansion stroke exists (figure 5.14).
From figure 5.14 it is observed that both syngas compositions show similar time
evolution of the piston position. However, methane shows a different time evolution of
the piston position in the expansion stroke to the whole igniting timings. This is
explained by the combustion time of the mixtures that are lower for methane, followed
by the downdraft and finally by updraft. Thus, the heat released is not so much
available on the expansion stroke for methane as it is for updraft syngas. This also
indicates that the turbulent burning velocity of methane should be higher than the
syngas compositions ones.
5.2.1.2 Equivalent rotation speed
The compression stroke is imposed by the RCM hydraulic system; the expansion
stroke is a function of the heat release of the mixture being burned. Thus, the
equivalent rotation speed is not constant and its variation should be evaluated taking
into account the fuel and the ignition timing.
Figure 5.15 shows the average of the piston velocity during various experiments
without combustion and different spark times and after TDC synchronization.
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