Introduction:
Ignition is of fuel is only concerned with
starting the combustion and not with the behaviour of the combustion wave or
flame. It is necessary to ignite the beginning of the combustion process in the
mixture of air fuel taken in during the suction stroke of the engine. The
ignition must add sufficient energy for starting and sustaining the burning of
air fuel mixture.
 |
| Ignition System |
The
basic requirement of the ignition system is to supply the minimum required
energy within a small volume in a very short period of time to ensure that
minimum energy is lost other than needed to establish the flame. Therefore the
rate of energy supply is very important to generate and sustain the flame.
The
intensity of the spark should be sufficiently high to raise the temperature of
the surrounding mixture above its ignition temperature to initiate and sustain
the combustion process. If the spark is weak, the energy from the point of
spark will be quickly dissipated to the surrounding mass of mixture and ignition
will not be initiated as the released energy is not sufficient to raise the
temperature of the surrounding mixture above its ignition temperature.
Requirements of
Ignition System:
The
important requirements of the spark ignition system are listed below:
1. The
voltage across the spark plugs electrodes should be sufficiently large to
produce an arc required to initiate the combustion. The voltage necessary to
overcome the resistance of the spark gap and to release the enough energy to
initiate the self propagating flame front in the combustible mixture is about
10,000 to 20,000 volts.
2. The
intensity of the spark should lie in the specified limits because too high
intensity may burn the electrodes and too low intensity may not ignite the
fresh mixture properly.
3. The
volume of the mixture at the end of compression should not be too large,
otherwise the spark produced may not be sufficient to ignite the whole charge.
There is definite relation between the size of the produced spark and the
clearance volume.
4. There
should be no missing cycle due to failure of spark.
5. In
a multi-cylinder engine, there must be arrangement to carry this voltage to the
right cylinder at the right time.
Battery ignition
system:
 |
| Spark Plugs |
a. Battery
of 6-12 volts,
b. Induction
coil,
c. Contact
breaker,
d. Condenser,
e. Distributor,
f. Spark
plugs.
Advantages:
a. Its
initial cost is low compared to magneto. This is the main reason for the
adoption of the coil ignition on cars and commercial automobiles.
b. It
provides better spark at low speeds of the engine during the starting and
idling. This is because maximum current is available throughout the engine
speed range including starting.
c. The
maintenance cost is negligible except for the battery.
d. The
spark intensity remains unaffected by advance and retard position of the timing
control mechanism.
e. The
simplicity of the distributor drive is another factor in favour of the coil
ignition system.
Disadvantages:
a. The
engine cannot be started once the batter is dead.
b. The
overall weight of the battery ignition system is greater than magneto which is
major consideration in adopting the system in aero-engines.
c. The
wiring involved in the coil ignition is more complicated than used in magneto
ignition and therefore there is more likelihood of defects occurring in the
system.
No comments:
Post a Comment