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

Experimental study of engine-like turbulent combustion
25% higher are obtained with the downdraft syngas. As the turbulent burning velocity
could be considered proportional to the laminar one (Verhelst and Sierens, 2007), this
result could be endorsed to:
tel-00623090, version 1 - 13 Sep 2011
- the effect of pressure on the laminar burning velocity. The correlations of laminar
burning velocity of the typical syngas compositions developed on 4.1.2.3 show
that the effect of pressure is very significant (coefficient β for updraft is 40%
higher when compared to downdraft syngas β coefficient). For example, at the
same temperature an increase in pressure from 1.0 to 20.0 bar results in an
increasing difference on burning velocity from 12% to 35%.
- The higher pressure used on RCM also implies temperature to increase due to
compression but the effect of temperature on burning velocity of syngas typical
compositions is irrelevant since the coefficient α is of the same order (see
4.1.2.3).
5.2.3 Ignition timing
Timing advance is required because it takes time to burn the air-fuel mixture. Igniting
the mixture before the piston reaches TDC will allow the mixture to fully burn soon after
the piston reaches TDC. If the air-fuel mixture is ignited at the correct instant, maximum
pressure in the cylinder will occur sometime after the piston reaches TDC allowing the
ignited mixture to push the piston down the cylinder. Ideally, the time at which the
mixture should be fully burned is about 20º ATDC (Hartman, 2004). This will utilize the
engine power producing potential. If the ignition spark occurs at a position that is too
advanced relatively to piston position, the rapidly expanding air-fuel mixture can
actually push against the piston still moving up, causing detonation and lost power. If
the spark occurs too retarded relatively to the piston position, maximum cylinder
pressure will occur after the piston has already traveled too far down the cylinder. This
results in lost power, high emissions, and unburned fuel. For further analysis of these
experimental results, figure 5.20 synthesizes the peak pressure P max , and the position
of peak pressure θ max expressed in milliseconds ATDC for the variable ignition timing in
milliseconds BTDC.
160