Page 384 - Advanced thermodynamics for engineers

16.6 SIMULATION OF COMBUSTION IN SPARK-IGNITION ENGINES 373

3000

2500
Temperature (K) 2000

1500
burned, 692
burned, 687
burned, 697
1000 burned, 702
unburned, 692
unburned, 687
500 unburned, 697
unburned, 702

-40 -20 0 20 40 60 80 100 120 140
Crankangle (deg atdc)
FIGURE 16.18
Effect of ignition timing on temperature–crank angle diagrams.

16.6.5 EFFECT OF FLAME SPEED FACTOR ON ENGINE COMBUSTION
The flame speed factor used for the base set of data was f f ¼ 4.00. This means that the turbulent flame
speed (see Section 15.4.3) is given by:
u t ¼ f f u [ : (16.31)
This value of f f ¼ 4.00 was obtained by comparing the measured and predicted pressure–crank
angle diagrams for this engine when running on a test bed. The flame speed factor was both increased
and decreased as shown in Table 16.6.
The results obtained are shown in Fig. 16.19. This shows that if f f is increased to 5.00 then the
combustion is more rapid, and the peak pressure moves closer to tdc, while its magnitude increases
significantly. This has a small effect on the variation of temperature in the combustion chamber,
although slightly higher peak temperatures are achieved in the burned zone, which will increase the
emissions of NO x . The time for the combustion to reach the full radius of the chamber reduces from
about 32 to 27 crank angle. A slower burning velocity, as achieved with the smaller flame speed

factor of 3.00, lengthens the burning period to about 47 . The slower burning velocity also reduces the

peak pressure, and moves it later in the cycle. More importantly, slower burning makes the conditions
in the end gas (the unburned gases) more susceptible to detonation (knock), because both the tem-
perature and pressure of the end gas is increased. Some of the major recent improvements in engine

Table 16.6 Flame Speed Factors
Lower flame speed factor 3.00
Baseline flame speed factor 4.00
Higher flame speed factor 5.00