23 March 2018

Battery ignition system | Components | Working | Limitations

Nowadays most of the spark-ignition engines use a battery ignition system. In this system, the energy required for producing a spark is obtained from a 6 or 12 Volt battery. Construction of a battery ignition system depends on the type of ignition energy storage as well as on the ignition performance. 

Components of battery ignition system :

The essential components of a battery ignition system are :
  1. Battery 
  2. Ignition switch 
  3. Ballast resistor 
  4. Ignition coil 
  5. Contact breaker 
  6. Capacitor 
  7. Distributor 
  8. Spark plug 
Now we can check the details of the various components used in battery ignition system :
  • Battery :
The battery provides electrical energy for ignition. It is charged by a dynamo driven by the engine. A battery converts the chemical energy into electrical energy because of electrochemical reactions. The battery must be mechanically strong to withstand the strains. For doing reasonable care and attention two years or more free life to be obtained from the battery. 

There are two types of batteries are used for spark-ignition engines :
  1. The lead-acid battery 
  2. The alkaline battery  
Mainly lead-acid battery are used in light-duty commercial vehicles and alkaline battery are used for heavy-duty commercial vehicles.

  • Ignition switch :
By the use of the ignition switch and ballast, resistor battery is connected to the primary winding of the ignition coil. By using this ignition switch the ignition system can be turned ON or OFF.

  • Ballast resistor :
A ballast resistor is provided in series with the primary winding to regulate the primary current. The function of this is to provide injury to the spark coil by overheating if the engine should be operated for a long time at low speed.

This coil is made of iron wire, and iron has the property that its electrical resistance increases very rapidly if a certain temperature is exceeded. 

  • Ignition coil :
The ignition coil is the source of ignition energy. The coil stores the energy in its magnetic field and delivers it at the appropriate time in the form of an ignition pulse through the high-tension ignition cables to the respective spark plug. 

The ignition coil consists of a magnetic core of soft iron wire or sheet and two insulated conducting coils, called primary and the secondary winding.

  • Contact breaker :
Contact breaker is a mechanical device for making and breaking the primary circuit of the ignition coil. 

  • Capacitor :
The capacitor is two metal plates separated by an insulating material are placed face to face. The insulation is often only air but in most cases, it consists of some high-quality insulating material suitable for the particular technical requirements, material which because of space limitation must be as thin as possible but nevertheless capable of withstanding electrostatic stresses without suffering damage.

  • Distributor :
The function of a distributor is to distribute the ignition pulses to the individual spark plug in the correct sequence and at the correct instants in time. Depending on whether a particular engine has 4, 6 or 8 cylinders, there are 4, 6 or 8 ignition pulses generated for every rotation of the distributor shaft. The use of distributor represents a considerable simplification in a battery ignition system because in most cases we want to use only a single ignition circuit.

There are two types of distributor :
  1. The brush type 
  2. The gap type 
  • Spark plug :
The spark plug provides the two electrodes with a proper gap across which the high potential discharges to generate a spark and ignite the combustible mixture within the combustion chamber. 

The spark plug consists of a steel shell, an insulator and two electrodes. The central electrode to which the high tension supply from the ignition coil is connected is well insulated with porcelain or other ceramic materials. The electrodes are usually made of high nickel alloy to withstand the severe erosion and corrosion to which they are subjected in use. 

There are two types of spark plug used :
  1. Hot spark plug
  2. Cold spark plug 
Working of a battery ignition system :

In the ignition system, the source of the ignition energy is the ignition coil. This coil stores the energy in its magnetic field and delivers it at the instant of ignition in the form of a pulse of high voltage current through the high tension ignition cables to the correct spark plug. 

Working of a battery ignition system

As we explained above in the ignition coil it is consist of two coils of wire. One wire wound around the other and another is insulated from each other. The primary wining with few turns of heavy copper wire and secondary winding with many turns of fine copper wire. 

One end of the primary winding is connected through the ignition switch to the positive terminal post of the storage battery, and another end is grounded through the contact breaker. The capacitor is connected in parallel with the contact breaker. One end of secondary wining is also grounded through the contact breaker and another end is connected through the distributor and the high tension ignition cables to the centre electrode of the spark plug. 

The primary winding of the coil is connected to the positive terminal post of the stored energy when the ignition switch is closed. If the primary circuit is closed through the breaker contacts, a current flows and its called as primary current. It is flowing through the primary coil, which is wound on a soft iron core produces a magnetic field in the core. A cam driven by the engine shaft is arranged to open the breaker points whenever an ignition discharge is required. When the breaker points open, the current flowing through the points now flows into the condenser. As the condenser becomes charged, the primary current falls and the magnetic field collapses. The condenser then discharges into the battery, reversing the direction of both the primary current and the magnetic field. The secondary winding consists of a large number of turns of very fine wire wound with the primary winding. The high secondary voltage is led to the proper spark plug by means of a rotating switch called the distributor, which is located in the secondary or high tension circuit of the ignition system.  

If a condenser were not used in the primary circuit, the high primary voltage caused by the collapse of the magnetic field around the primary winding would cause an arc across the breaker points. Spark timing is controlled by the crank angle at which the breaker points open, while the distributor merely determines the firing sequence of the spark plug. 

Limitations :
  • The current switching capability of the breaker system: As the engine speed increases the primary voltage decreases.
  • Dwell period becoming shorter: Time available for the build-up of the current in the primary coil and the stored energy decreases as the engine speed increases. 
  • High source impedance: The system is sensitive to side-tracking across the spark plug insulator. 
  • The breaker points are continuously subjected to electrical as well as mechanical wear which results in short maintenance intervals. 
  • As a current increases a rapid reduction in breaker point life and system reliability. 
  • Acceptable life for these systems is obtained with a primary current limited to about 4 amperes.