Knocking is a phenomenon of generating unwanted pressure
wave which create unpleasant sound and can damage engine wall during combustion
in engines. Knocking is largely associated with SI engines. It occurs due to
self-ignition of unburned charge into combustion chamber.
When two flame fronts in SI engines collide, they produce an
unwanted pressure wave which generates unpleasant sound (knock-knock) and can
damage engine wall. Learn more about how Engine knocking works in this article.
Engine Knocking :
Process:
Knocking is mainly associated with SI engines. It depends on
auto ignition quality of fuel. The higher auto ignition temperature causes lower
down knocking tendency. To understand whole concept of knocking let’s look out
the combustion process.
When the piston reaches at TDC after compression, the spark
plug produces spark which ignite the compressed mixture and starts the
combustion process. It ignites only those part of mixture witch is in contact
with spark plug and generates a main flame front which further ignites the
whole mixture. This will generate a high temperature and pressure force inside
the cylinder. This burnt part of mixture (combustion products) separate the
fresh mixture from spark plug to the other end of the cylinder. As this flame
front expands, it compressed the unburned part of the charge. This compression
increases the temperature and pressure of unburned part of mixture. If the
temperature of this part reaches its auto ignition temperature, it will ignite
from other end and a new flame front starts moving towards opposite direction
of main flame. When both of these flame fronts collide, it will generate a high
pressure wave which produces an unpleasant sound and also damage the cylinder
wall.
To clear out image of flame front, lets look at the figure
given below.
Figure 1 (a) shows the top view of cylinder or combustion
chamber for normal condition and 1 (b) shows the pressure distribution during
power stroke for ideal condition without knocking and 2 (a) & 2 (b) shows
actual condition with knocking.
It can clearly see that the normal condition generates a
uniform pressure wave which is required to drive vehicle. But when the flame
front compressed the fresh charge and the charge auto ignited, will create rapid
change in pressure wave. This rapid change in pressure wave creates unwanted
sound until the whole charge of opposite flame ignited. If the compressed
mixture does not reach its auto ignition temperature during compression, the
whole charge ignited by main flame front and the normal combustion takes place.
Factor affecting knocking:
Any factor which tends to decrease auto ignition temperature
of charge or tends to ignite the charge except spark plug will affect knocking.
Main affecting factor are given below.
Density factor:
The lower dense charge generates low energy and lower rise
in temperature thus avoid knocking. If the density of charge decrease, will
decreases the knocking tendency. The factors which affect the density of charge
are as follow.
Compression Ratio
Increase in compression ratio will increase density of
charge thus increase knocking tendency.
Mass of the charge induced:
The mass of charge depends on throttling or supercharging. A
reduction in mass of induced charge, reduce the knocking tendency.
Time Factor:
The lower the time available to auto ignition will decrease
the knocking tendency. The most affecting time factors are
Engine Turbulence:
Turbulence reduces the ignition lag or time available for
auto ignition. So increase in turbulence will increase the main flame speed and
reduce ignition lag, thus reduces knocking tendency.
Spark Plug Location:
To reduce time available for auto ignition, spark plug
should locate in order to have minimum flame travel for proper combustion. Generally
spark plug located at center to reduce the tendency of knocking.
Engine Size:
As the engine size increase will increase the flame travel
thus increases knocking tendency.
Combustion Chamber:
Combustion chamber design play main role in flame travel.
The more compact the combustion chamber has minimum flame travel distance thus
reduce knocking tendency. Also, the hot spot in combustion chamber should be
avoided which can ignite the charge.
Temperature Factor:
Any factor which increases the temperature of inside gases
will increase the knocking tendency. These are
Inlet Temperature of Mixture:
More the inlet temperature of mixture, increase tendency of
auto ignition thus increase knocking tendency.
Supercharging:
Supercharging increase both mass and temperature of inside
the cylinder thus increases tendency of knocking.
Temperature of Combustion Wall
It will play main role in knocking. The higher the temperature
of wall, more chance of auto ignition, thus more chance of knocking.
How to fix knocking:
We have known about knocking phenomenon and factor affecting
it. The most methods are used to avoid knock are as follow.
- Spark plug location should select to minimize the flame travel distance.
- By proper cooling of Engine wall.
- Combustion chamber is so design which has less hot spot and higher turbulence.
- Uses higher auto ignition temperature fuels or add additive in gasoline to increase its knocking temperature.
- Higher engine speed will reduce knocking.
- Reduce compression ratio.
- Reduce supercharging to avoid knocking.
This is all about what is engine knocking, its affecting factors and how to fix it. If you have any query regarding this article, ask by commenting. If you like this article, don't forget to share it on social networks. Subscribe our website for more interesting articles.