Volvo 819 E - Page 31

Group 28 Ignir ion sys r ems
Design and function - Compensa t ion functions
Knock ' threshold '
Under certain condi tions, however, the combination of high perlormance and lo w fuel con sumption demands that
the engine runs as close as possible to the knock threshold . The closer the timmg curve to the threshold , Ihe higher
the efficiency of fuel utilization. In other words . the knock threshold is not a fixed limit, but varies depending on run -
ning
conditions.
Methods of preventing knock
Knock may be prevented by retarding the ignition. Since detonation of the mixtu re occurs when the piston is closer
to TOC, the pressure and temperature in the cylinder are thereby lowered, reducing the risk of detonation . Knock is
also pr
evented by enriching the mixture, which has the effect of reducing the combus t io n t em per ature .
• Knock may be caused by the fo llo wing :
- Use of a fuel with an octane rat in g which is too low for t he compression rat io of Ihe engine .
- Incorrect fuel/air ratio . If the mixture is too lean,the temperature In the combustion chamber will be h igh despite a conslant com·
pression ratio. A lean mixture may be due 10 a fac1 0r such as tow fuel pressure or air lea ka ge into the induction system.
-
Excessively high combustion temperature. Carbon deposits in Ihe combustion chamber WIll reduct! the rate of heat dissipation,
resulting in high temperature land cre at ing the risk of glow IgnitIon) . DefiCient cooling (due, for example, to a fau lty water pump,
radiator or thermostat , or to rustnime deposits m the cooling passages). Will have the $time effect.
- Incorrect or worn spark plugs . The fisk of detonation will be greatly mcreased if the plug IS of the wrong type ILe. if the heat rating
is incorrectl or is it IS fouled .
- Faulty timing. The resistance to knock Will be im pa ired if ignition takes place too early. since the cylinder pressure and tem-
perature w ill be higher under these conditions.
- A sudden increase in engine speed. What is known as 'transitional" kno ck w ill occur if the engine speed and load are suddenly in·
creased. If the engine is running under normal con ditions of speed and load {with the timing fairly weI! advanced), sudden de -
pression
of the accelerator will produce an ins tantaneous increase in the quantity of mixture admitted before the system has had
time to adjust the t iming accordingly. For a brief period, therefore. the timing will be too advanced for the amount of mixture in
the cylinders. lO ne of the funct ions with which the control unit is programmed limits the scope of timing adjustment to a spec-
ified number of degrees per crankshaft revolUhon.1
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