Some of the major drawbacks of battery
ignition system are listed below:
a.
There
are chances of discharging of battery,
b.
There
are chances of misfiring at higher speed of the engine,
c.
It
requires complicated wiring,
d.
There
are chances of failure of system,
e.
There
are many mechanical complications in the operation of the system.
These difficulties experienced with
battery ignition system can be avoided by using magneto ignition system.
Magneto
Ignition System:
It is exactly same like battery ignition
except the source of generation is magnet instead of the battery. There are
three types of magneto ignition system, namely:
a.
Rotating
magnet system,
b.
Rotating
armature type system,
c.
Polar
inductor magneto.
Advantages:
a.
It
is more reliable compared to coil ignition system, because there is no
maintenance problem in magneto ignition system,
b.
It
is used for medium to high speeds. Also it is very popularly used in racing
cars,
c.
Space
required is less as compared to coil ignition system,
d.
By
providing suitable shunts on magnet, the danger of burning of spark plug is
minimised,
e.
Very
light in weight and compact in size,
f.
Automatic
time adjusting of ignition can be affected.
Disadvantages:
a.
Initial
cost is very high as compared to coil ignition system,
b.
To
start with, 75 RPM is necessary,
c.
For
high power engines, some other devices are necessary to start an ignition.
Problems
in Aero Engine Ignition System:
Problems are met in providing ignition
for aircraft engines which do not arise in connection with automobile engines
and certain features are incorporated in aeroengine ignition systems to meet
the special requirements.
Long periods of continuous operation
impose severe mechanical strain on the rotating parts of the magnet and
distributor units and these are designed to combine maximum strength with
minimum weight in order to insure almost reliability under working conditions.
Contact breakers and sparking plugs are sources of radio interference because
they transmit electromagnetic waves and a method known as screening is used to
prevent the waves interfering with radio reception. Their effect of screening
and the use of weaker fuel mixtures in aero engines make it necessary for the
ignition system to produce a higher H.T voltage than with other types of
engines.
It is usual duplicate the ignition
system by using two magnets, each cylinder being provided with two sparking
plugs. Each system is quite independent of the other, but in most cases they
are synchronized so as to give simultaneous sparks in the cylinder.
This arrangement gives a slight increase
in power and if one system fails, the engine will operate quite well on the
other.
The effects of high working temperatures
tend to cause leakage of the H.T currents as stated previously. The resistance
of insulating materials decreases with the rise in temperatures. Special
consideration is given to the kinds of insulation materials employed in aero
ignition system.
Variation of spark timing in accordance
with engine speed, load and mixture conditions is a necessity and an
arrangement whereby this is carried out automatically is essential.
Magneto ignition system is the general
rule in aircraft and since the H.T voltage of a magnet is largely dependent on
the speed at which the magnetic spindle is rotated, there is difficulty in
starting an aero engine without special apparatus to ensure an efficient spark.
No comments:
Post a Comment