Battery ignition system
Most of the modern spark-ignition engines use battery ignition system. In this system, the energy required for producing spark is obtained from a 6 or 12 volt battery.
The construction of a battery ignition system is extremely varied. It depends on the type of ignition energy storage as well as on the ignition performance which is required by the particular engine.
The reason for this is that an ignition system is not an autonomous machine, that is, it does not operate completely by itself, but instead it is but one part of the internal combustion engine, the heart of the engine.
Passenger cars, light trucks, some motorcycles and large stationary engines are fitted with battery ignition systems.
The components of the system are:
WORKING OF A BATTERY IGNITION SYSTEM:
The source of the ignition energy in the battery ignition system 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 charge of high voltage current(ignition pulse) through the high tension ignition cables to the correct spark plug.
Storage of energy in the magnetic field is based on an inductive process,as a result of which we also designate the ignition coil as an inductive storage device.
The ignition coil consists of two coils of wire, one wound around the other, insulated from each other; the primary winding, L1 with few turns of heavy copper wire and the secondary winding, L2 with many turns of fine copper wire. The primary and secondary winding are wound around a laminated iron core which has the effect of increasing the strength of the magnetic field and thus of the amount of energy stored.
One end of the primary winding is connected through the ignition switch to the positive terminal post of the storage battery, and the other end is grounded through the contact breaker. The ignition capacitor is connected in parallel with the contact breaker. One end of the secondary winding is also grounded through the contact breaker, and the other end is connected through the distributor and the high-tension ignition cables to the center electrode of the spark plug.
When the ignition switch is closed, the primary winding of the coil is connected to the positive terminal post of the storage battery. If the primary circuit is closed through the breaker contacts, a current flows, this is called primary current.
This current, 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 which had been flowing through the points now flows into the condenser, which is connected across the points. As the condenser becomes charged, the primary current falls and the magnetic field collapses. The collapse of the field induces a voltage in the primary winding, which charges the condenser to a voltage much higher than battery voltage. The condenser then discharges into the battery, reversing the direction of both the primary current and the magnetic field. The rapid collapse and reversal of the magnetic field in the core induce a very high voltage in the secondary winding of the ignition coil. The secondary winding consists of a large number of turns of very fine wire wound on the same core with the primary. 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.
The 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 plugs. Changes in ignition timing may be affected by rotating the plate which holds the breaker points, relative to the cam. Be-cause of this ignition will be delayed if the plate is displaced in the direction in which the camshaft rotates.
What happens when the condenser is not placed in the primary circuit of the battery ignition?
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. The arc would burn and destroy the points and would also prevent the rapid drop in primary current and magnetic field which is necessary for the production of the high secondary voltage.
LIMITATIONS OF BATTERY IGNITION SYSTEM:
(i) The primary voltage decreases as the engine speed increases due to the limitations in the current switching capability of the breaker system.
(ii) Time available for build-up of the current in the primary coil and the stored energy decrease as the engine speed increases due to the dwell period becoming shorter.
(iii) Because of the high source impedance (about 500 kΩ) the system is sensitive to side-tracking across the spark plug insulator.
(iv) The breaker points are continuously subjected to electrical as well as mechanical wear which results in short maintenance intervals. Increased currents cause a rapid reduction in breaker point life and system re-liability. Acceptable life for these systems is obtained with a primary current limited to about 4 amperes.
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