Page 377 - Advanced thermodynamics for engineers

366 CHAPTER 16 RECIPROCATING INTERNAL COMBUSTION ENGINES

(a) (b)

(c) (d)

FIGURE 16.11
Schematic diagram of flame propagation in a spark-ignition engine with central spark plug and flat cylinder
head and piston crown (the so-called disc chamber) (a) hemispherical flame has not reached piston (b) flame
has reached piston and is truncated (c) and (d) similar flame size in pent-roof chamber.

ratio increasing both these parameters. Compression ratio will also have a significant effect on the
geometry of the combustion chamber, and a higher compression ratio will often result in a combustion
chamber of narrower aspect ratio. This means that the flame will contact the piston earlier (see
Fig. 16.11(b)), and this will tend to reduce the rate of heat release. The compression ratio of the base
engine was both increased and decreased as shown in Table 16.2.
The effect of compression ratio on the engine cycle is shown in Figs 16.12 and 16.13. It can be seen
that a higher compression ratio gives a higher peak pressure, but has little effect on the release