Page 345 - Advanced thermodynamics for engineers

334 CHAPTER 15 COMBUSTION AND FLAMES

4.5
4.0
Laminar flame speed / (m/s) 3.0
3.5

2.5
2.0
1.5
1.0
0.5
0
300 400 500 600 700 800 900 1000
Reactants temperature / (K)
FIGURE 15.7
Variation of laminar flame speed with reactants temperature (predicted by Kuehl’s equation, with p ¼ 1 bar).

by an external source of energy – usually a spark plug. It can be shown that the minimum energy for
ignition, based on supplying sufficient energy to the volume of mixture in the vicinity of the spark-gap
k 3
to cause a stable flame, is Q p;min f . This means that the minimum energy is increased as the
2 3
p u [
inverse of the laminar flame speed, cubed. In other words, much more energy is required when u [ is
low. The typical level of energy required to ignite the mixture in a spark-ignition engine is around
30 mJ. As shown above, u [ is dependent on the initial temperature of the reactants, and the equivalence
ratio of the charge (because this affects the adiabatic temperature rise, and hence T b ). This means that
the strongest spark is needed when the engine is cold, and also if the engine is running lean. This
dependence also explains why it is attractive to develop stratified charge engines, with a rich mixture
zone around the spark plug. The typical variation of minimum energy to ignite a mixture is shown in
Fig. 15.8.

Minimum ignition energy Rich Lean

Air-fuel ratio
FIGURE 15.8
Minimum ignition energy against air–fuel ratio.