Diesel knocking and detonation
We
already know that if the delay period is long, a large amount of fuel will be
injected and accumulated in the chamber. The auto ignition of this large amount
of fuel may cause high rate of pressure rise and high maximum pressure which
may cause knocking in diesel engines. A long delay period not only increases
the amount of fuel injected by the moment of ignition, but also improve the
homogeneity of the fuel air mixture and its chemical preparedness for explosion
type self ignition similar to detonation in SI engines. It is very instructive
to compare the phenomenon of detonation is SI ensues with that of knocking in
CI engines. There is no doubt that these two phenomena are fundamentally
similar. Both are processes of auto ignition subject to the ignition time lag
characteristic of the fuel air mixture. However, differences in the knocking
phenomena of the SI engine and the CI engine should also be care fully be
noted: 1. In the SI engine, the detonation occurs near the end of combustion
where as in the CI engine detonation occurs near the beginning of combustion as
shown in fig. 6.10. 2. The detonation in the SI engine is of a homogeneous
charge causing very high rate of pressure rise and very high maximum pressure.
In the CI engine the fuel and air are in perfectly
mixed and hence the rate of pressure rise is normally lower than that in the
detonating part of the charge in the SI engine. 3. Since in the CI engine the
fuel is injected in to the cylinder only at the end of the compression stroke
there is no question of pre ignition or pre mature ignition as in the SI
engine. 4. In the SI engine it is relatively easy to distinguish between
knocking and non- knocking operation as the human ear easily find the
distinction. However, in the case of the CI engine the normal ignition is
itself by auto ignition and hence no CI engines have a sufficiently high rate
of pressure rise per degree crank angle to cause audible noise. When such noise
becomes excessive or there is excessive vibration in engine structure, in the
opinion of the observer, the engine is sending to knock. It is clear that
personal judgment is involved here. Thus in the CI engine there is no definite
distinction between normal and knocking combustion. The maximum rate of
pressure rise in the CI engine may reach as high as 10bar per crank degree
angle.
It is most important to note that factors that tend to reduce
detonation in the SI engine increase knocking in CI engine and vice versa
because of the following reason. The detonation of knocking in the SI engine is
due to simultaneous auto ignition of the last part of the charge. To eliminate
detonation in the SI engine we want to prevent all together the auto ignition
of the last part of the charge and therefore desire a long delay period and
high self ignition temperature of the fuel. To eliminate knocking the CI engine
we want to achieve auto ignitions early as possible therefore desire a short
delay period and low self ignition temperature of the fuel. Table 6.2 gives the
factors which reduce knocking in the SI and CI engines
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