Volvo B 230 FT User Manual

Group 28 Igni r ion sysrems
Design
and function - Compensation functions
Compensation functions
c
F-::::
I?
h
Il
"'"
~
©
1/
B
A
Theory of knock
Knock is caused by de t onation of the fuel/air mixture in the combustion chamber.
What is knock 7
Detonation may occur either aher las in A lar before (as in 51 the plug has delivered a spark. In either case, the mix-
ture is ignited at two or more points in the chamber and combustion progresses from these po i nts Ie). Th is results in
the
simultaneous propagat io n of two or more flame fronts , causing abnormally rap id and uncontrolled combustion,
accompanied by an explosive rise in pressure which produces the typical, metallic sound known as 'pinking' or
'knock'.
' Compression ' knock
A. In the case in wh ich knock occurs aher the plug has delivered a spark. detonation is caused by the fact that the
flame front ini tiated by the spark. compresses and heats the unignited m ix ture to the point at which detonation is
unavo i dable. A relatively high eng ine load is required to produce this type of knock.
' Glow ignition ' knock
B. In the type of knock which occurs before the plug has delivered a spark, detonation of the mixture may be due to
incandescent
carbon part icles or gasket edges, fouled spark plugs or other areas of the combustion chamber
which absorb heat more easily. IThis phenomenon is sometimes known as 'g low ignition'.)
Effects of knock
C. The abnormal pressure rise which accompanies knock subjects the piston, gudgeon pin. co nnecting rod, cr ank-
shah and bear ings to abnormally high mechanical stresses. Furthe rmo re, the t empera t ure ri se caused by th e phe-
nomenon is so high that the cylinder wall. piston and spark plug may easily suffer damage , while the energy con-
tent of the fuel is poorly utilized. The unusual engine resonance caused by knock is detected by a knock sensor
mounted on the cylinder block . The sensor signal is transmitted to the control unit .
28

Volvo B 230 FT Specifications

General

Engine Type	Inline 4-cylinder
Fuel System	Electronic Fuel Injection
Cylinder Bore	96 mm
Piston Stroke	80 mm
Turbocharger	Yes
Displacement	2.3 L (2316 cc)
Valvetrain	SOHC, 8-valve