Page 49 - E BOOK ENGINE MECHANICAL M2

1. GENERAL

6. DIESEL KNOCKING
(1) Occurrence of Diesel Knocking
Air-fuel mixture accumulated during the ignition delay period is combusted at a stretch to increase
the pressure rapidly in the flame propagation period. Diesel knocking occurs if the pressure in-
creases too rapidly. The knocking in the diesel engine is physically the same as that in the gasoline
engine.
At (A) in the figure 1-5, the ignition delay is short and
(A) X little change is made in the pressure increase after igni-
tion.
At (B) in the same figure, the ignition delay period is
long with great change in the pressure increase after
ignition.
In other words, the degree of diesel knocking changes
according to angle X formed by tangent T at a point
where the flame propagation starts and only compres-
sion is made and pressure tangent T after ignition.
The greater the angle X, the greater the diesel knocking.
In addition, some degree of the diesel knocking is inevi-
table, since the diesel engine uses self-ignition type
combustion system.
X
(B)

Fig. 1-5

(2) Prevention of Diesel Knocking
Judging from the cause of how diesel knocking occurs, the ignition delay period should be shortened
to prevent the diesel knocking. The nozzle is generally designed so as to decrease the injection
quantity during the ignition delay period. In addition to this method, the followings are taken into con-
sideration.
◇ To use high cetane numbered fuel
◇ To Increase the temperature in cylinder (increase of compression pressure)
◇ To keep the engine coolant at appropriate temperature
◇ To adjust the injection timing adequately
◇ To provide appropriate fuel injection pressure and atomization

1 - 6 Pub. No. PTAE1228