
7
CHAPTER
Ignition
7.1 Ignition system fundamentals
7.1.1 Functional requirements
The fundamental purpose of the ignition system is to supply a spark inside
the cylinder, near the end of the compression stroke, to ignite the compressed
charge of air–fuel vapour.
For a spark to jump across an air gap of 1 mm under normal atmospheric
conditions (1 bar), a voltage of 3–4 kV is required. For a spark to jump across
a similar gap in an engine cylinder, having a compression ratio of 8 : 1,
approximately 8–10 kV is required. For higher compression ratios and weaker
mixtures, a voltage up to 30 kV may be necessary. The ignition system has to
transform the ...