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The electronic combustion controller utilises the physi-
cal correlation between the level of the ionisation cur-
rent and the air ratio
λ
. The maximum ionisation cur-
rent is achieved at an air ratio of 1 for all gas qualities.
The ionisation signal is evaluated by the combustion
controller and the air ratio is adjusted to a value
between
λ
=1.24 and 1.44. This range provides for an
optimum combustion quality. Thereafter, the electronic
gas valve regulates the required gas volume subject to
the prevailing gas quality.
To check the combustion quality, the CO
2
content or
the O
2
content of the flue gas is measured. The pre-
vailing air ratio is determined with the measured val-
ues. The relationship between the CO
2
or O
2
content
and air ratio
λ
is illustrated in the following table.
Air ratio
λ–
CO
2
/O
2
content
Air ratio
λ
O
2
content (%) CO
2
content (%) for
natural gas E
CO
2
content (%) for
natural gas LL
CO
2
content (%) for
LPG P
1.20
3.8
9.6
9.2
11.3
1.24
4.4
9.2
9.1
10.9
1.27
4.9
9.0
8.9
10.6
1.30
5.3
8.7
8.6
10.3
1.34
5.7
8.5
8.4
10.0
1.37
6.1
8.3
8.2
9.8
1.40
6.5
8.1
8.0
9.6
1.44
6.9
7.8
7.7
9.3
1.48
7.3
7.6
7.5
9.0
To achieve an optimum combustion control, the system
regularly carries out an automatic self-calibration; also
after a power failure (shutdown). For this, the combus-
tion is briefly regulated to max. ionisation current (cor-
responding to air ratio
λ
=1). Automatic calibration is
carried out shortly after the burner start and lasts
approx. 5 s. During calibration, higher than normal CO
emissions may occur briefly.
Function description
Electronic combustion control unit
5773 238 GB
Functions
