Ignition System

Ignition system is a device used to create high voltage electrical supply necessary for the spark plugs to create a spark. The spark produced is used to ignite air-fuel mixture in Spark Ignition engines.

Functions of an Ignition System:

·         It must create a high voltage (greater than 20000 V) from a 12 V battery supply.

·         It must control the timing so that spark is created at exactly the right time in all the spark plugs and also in the right cylinder based on the firing order. For a 4 cylinder engine, the typical firing order is 1-3-2-4.

Types of Ignition System:

The basic principle of working of ignition systems hasn’t changed much over the years. Modern day automobiles have one of the 3 types of ignition system:

·         Distributor or Mechanical Ignition System

·         Electronic Ignition System

·         Distributorless Ignition System

In this article, we will talk about the working of Distributor type ignition system.

Distributor type Ignition System:

As the name suggests, it consists of a distributor located at the nerve centre of the ignition system. The distributor triggers the ignition coil to generate the required high voltage. A distributor cap with a rotor is provided which directs the high voltage supply to the spark plug lines in firing order.

How is the high voltage supply generated?

When the ignition key is switched ON, a 12 V electrical energy is supplied from battery to the ignition coil via a primary resistor. The ignition coil has primary copper windings of more than 100 turns before exiting the negative terminal of the coil.

From the ignition coil, the current is supplied to the distributor which has two contact breaker points. One of the contact breaker points is fixed, while the other is movable and mounted on a spring loaded arm. The spring loaded arm rides on a distributor cam with lobes (number of lobes = number of cylinders in an engine). The speed of the cam is half the speed of the engine crankshaft.

When the two contact points are in contact, the current goes directly to the ground and this builds a strong magnetic field inside the ignition coil. The rotation of cam intermittently opens and closes the contact points. When the points are open, the current supply to the ground is interrupted and the magnetic field suddenly collapses in the coil. This collapse in magnetic field results in a surge of electrical voltage in the secondary winding. The high voltage supply is then supplied to the distributor cap via high tension wire.

Ignition Switch:

There are two wires leading to the primary winding of the ignition coil from the ignition switch. Primary wire is connected to the primary winding via a resistor and the secondary wire is directly connected to the primary winding.

When the engine is cranked, the secondary wire is used in order to supply more voltage for easy cranking of the engine. Rest of the time, primary wire is used to step down the voltage using the resistor to protect the contact points from premature wear.

Ignition Coil:

The ignition coil acts as a transformer, stepping up the voltage whenever there is a drop in the magnetic field. It consists of two windings. Primary windings are made of 100 to 200 turns of copper wire. The copper wires must be insulated in order to prevent shorting within the primary coil. The primary winding coils start at the positive terminal and loop their way around and exit out of the negative terminal. The negative terminal supply is given to the distributor.

Secondary windings contain 20,000 to 30,000 turns of copper wire. These wires also must be insulated to prevent internal shorting. The secondary winding sits inside the loop of the primary winding without any contact between them. To increase the magnetic field of the coil, the secondary winding is wound around a soft iron core.

Distributor:

Distributor consists of a cam at the center with lobes equal to the number of cylinders in an engine. The cam controls the opening and closing of the contact breaker points. A condenser is provided which acts like a capacitor to store current for a brief period of time.

The inclusion of a condenser is of prime importance to prevent premature wearing of the breaker points. When the points are open, the current supply to the ground is blocked. Therefore, the current looks to go to an alternative route. Without a condenser, the current would try to jump across the small gap between the points to produce arcing and this would burn up the points quickly. Condenser helps in storing the current and thereby preventing point arcing.

The gap between the points is crucial to run the engine at higher efficiency. The gap can be adjusted by adjusting a screw. The points are continuously subjected to wearing due to the rotation of cam. The points have to be replaced every 15000 to 20000 km depending on the amount of wear.

Distributor Cap:

The spark timing is taken care by the rotor spinning at the top. The secondary winding HT coil is connected to the rotor which spins inside the cap. The rotor spins past a number of contacts provided along the circumference of the cap. These contacts are arranged in firing order along the circumference. The contacts have a HT coil that supply high voltage electricity to the respective spark plugs.

Once the rotor comes in proximity to the contacts, a high voltage current generated in the ignition coil is supplied to the rotor. As the rotor and contact are in proximity, the high voltage current jumps across the gap and passes on to the respective spark plug. The high voltage current then generates spark at the tip of the spark plug, thereby initiating ignition in the respective cylinder. The rotor then spins away and moves to the next contact in firing order.

The contact points in the cap can wear out in time due to the repeated arcing between rotor and contacts. The electrical insulation of the HT coil can also wear out in time and needs to be replaced.

Ignition Timing:

Ignition timing is an important parameter to make sure that the engine is running efficiently under all conditions and loads. The ignition timing needs to be advanced when the speed of the engine is higher, so that even before the piston reached TDC, air-fuel mixture is burned and ready to push the piston down with greater energy and hence producing greater power.

The timing can be advanced either by spinning the plate on which the contact breaker points are attached or by rotating the cam. Ignition timing can be adjusted with two known mechanisms, namely:

·         Centrifugal advance

·         Vacuum advance

Centrifugal Advance Mechanism:

It consists of two centrifugal weights attached to a shaft hinged to the lower part of the distributor cam shaft. The weights are held to the lower shaft due to a spring force. Each weight has a spring attached to it.

As the engine speed increases, the speed of the distributor shaft also increases. The weights overcome the spring tension and start pulling out due to centrifugal force. The weights change the alignment due to centrifugal force and also force the cam to rotate and alter the ignition timing.

Vacuum Advance Mechanism:

Vacuum advance mechanism controls the movement of the distributor plate to which the points are attached. A vacuum diaphragm is connected to the plate on which the contact points are mounted. Vacuum is created by the engine inlet manifold. When the throttle pedal is not pressed, no vacuum is created. When the throttle pedal is pressed, vacuum is created which advances the timing.