Page 376 - Advanced thermodynamics for engineers

16.6 SIMULATION OF COMBUSTION IN SPARK-IGNITION ENGINES 365

in these examples this aspect of the engine ‘cycles’ has been omitted. Those interested in the gas
exchange processes of internal combustion engines should refer to Winterbone and Pearson (1999).
Ignition is achieved in a spark-ignition engine by means of a spark plug, which adds about 30 mJ to
the charge in the vicinity of the spark gap. Once a reasonable sized flame kernel has been created the
flame will propagate through the charge. The speed of propagation is a function of the laminar flame
speed of the mixture and the level of turbulence in the combustion chamber. If the mean flow in the
chamber is negligible then the flame will attempt to traverse the charge in a hemispherical mode. The
latter is affected by the shape of the combustion chamber. A simple model of such a situation is
depicted in Fig. 16.11(a) and (b) for a disc chamber with a central spark plug. The hemispherical mode
of growth of the burned zone would mean that combustion would advance at an ever increasing rate as
the flame radius increased. However, this situation is affected by the geometry of the combustion
chamber, and in particular the piston. When the flame contacts the piston surface the rate of energy
release is decreased because the enflamed region does not grow as rapidly.
In many modern engines, the cylinder head is not flat, and often the design is like that in
Fig. 16.11(c) and (d), referred to as a pent-roof chamber. The shape of the cylinder head has an effect
on the flow in the chamber and it is possible to develop high levels of turbulence, which will enhance
the flame speed (see Section 15.4). In addition, the shape of the chamber will affect the volume of the
flame during its initial travel, simply because its volume will be reduced by the cylinder geometry. The
flame travel is depicted in Fig. 16.11(c) and (d), and the rate of entrainment of charge can be compared
to Fig. 16.11(a) and (b) – however, the reduction in volume is compensated by the increased turbulence
of the mixture, which increases the flame factor f f (discussed below, and in Chapter 15).
A computer program has been used to simulate combustion in spark-ignition engines. The
combustion process is based on a two-zone combustion model, in which a flame traverses the charge,
and has burned and unburned zones, in which the properties are calculated using equilibrium ther-
modynamics (see Chapter 13) – hence it will not calculate the levels of pollutants. The unburned zone
contains the reactants (fuel and air), and no reactions occur between the constituents. The burned zone
contains the products of combustion and dissociation.
The basic parameters of the simulated engine are listed in Table 16.1.
The program has been used to investigate the effect of the following engine parameters on the
pressure, temperature and flame radius inside the cylinder of the engine considered:
• compression ratio
• engine speed
• air–fuel ratio
• ignition timing
• flame speed factor
• exhaust gas recirculation (egr).
These effects are shown as the basic parameters plotted against crankangle, related to the angle
after tdc firing (atdc).

16.6.1 EFFECT OF COMPRESSION RATIO ON ENGINE COMBUSTION
The compression ratio, in this case for a realistic engine it is the effective one, has two effects on
combustion. The first, and obvious effect, is on the thermodynamic cycle. The pressure and temper-
ature at the end of compression will be affected by the compression ratio, with a higher compression