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

Numerical simulation of a syngas-fuelled engine
⎛
VT ⎞
s r
v = ⎜2. 28Sp
+ 0. 00324 ( P −Pmot)
⎟
⎝
Pr
Vr
⎠
(6.25)
Where P mot =P r (V r /V) γ is the motored pressure. S p is mean piston speed (m/s), V s is
swept volume (m 3 ), V r , T r and P r are volume, temperature and pressure (m 3 , K, bar)
evaluated at any reference condition, such as inlet valve closure, V is instantaneous
cylinder volume (m 3 ) and γ is the specific heat ratio. The second term in the velocity
expression allows for movement of the gases as they are compressed by the
advancing flame.
6.1.4 Mass burning rate
tel-00623090, version 1 - 13 Sep 2011
In the combustion modeling studies, the main purpose is to specify the mass fraction of
burned gases at any time during the combustion process. This is achieved by using
several approaches. In general, two approaches have been widely used for
determining the mass fraction burned. In the first approach, the mass fraction burned at
any crank angle is specified by using empirical burning laws, such as the cosine burn
rate formula and Wiebe function (Heywood et al., 1979). This approach does not
necessitate detailed combustion modeling, hence modeling of combustion in this
manner is more practical, but it gives less reliable or less sensitive results about SI
engine combustion [Heywood et al., (1979); Bayraktar and Durgun, (2003)]. Empirical
burning equations include some constants that must be determined suitably at the
beginning of computation. In the case of using the Wiebe function, these are the
efficiency parameter, the form factor, the crank angle at the start of combustion and the
combustion duration. For the cosine burn rate formula, these are spark advance and
combustion duration. In such models, these parameters are generally determined
either by matching the experimental mass fraction burned curves obtained from the
cylinder pressure measurements with the calculated ones or by making an engineering
judgment [Zeleknik (1976); Heywood et al. (1979)]. If sufficient agreement is achieved
between the calculated and measured pressures, then the chosen parameters are
used for parametric studies. In the second approach, the combustion is modeled by
considering the turbulent flame propagation process (Heywood, 1988). This modeling
technique is generally called quasi-dimensional modeling because it accounts for the
details of engine geometry and the flame propagation process and therefore will be
followed in this work.
The role of in-cylinder air motion begins from the very start of the engine cycle. During
the intake stroke, the incoming air generates flow structures with large-scale turbulent
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