A system that is used for starting the vehicle, by igniting (firing) the engines at a correct sequence is called the ignition system. An engine is a single or collection of many cylinders, where the fuel is burnt inside each cylinder by introducing a spark. This causes the piston present inside the cylinder to move up and down, thereby producing rotational energy.
To know how an engine works please visit my previous blog:
https://sciencetopic03.blogspot.com/2021/12/how-engine-works-petrol-and-diesel.html
The spark introduced in the cylinder should be at the right time, when the piston compresses the air-fuel mixture. This is done by the ignition system which introduces a high voltage spark at the spark plug, through an ingenious mechanism. The below diagram shows the operation of the ignition system in vehicles,
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Ignition System |
The main objective of the ignition system is to introduce a high voltage spark at a right time, to ignite the air-fuel mixture in the cylinder. To do this, various parts are included in the system, namely
- Rechargeable battery (Lead Acid)
- Ignition Switch
- Ballast resistor
- Ignition coil
- Contact breaker
- Distributor
- Spark Plug
Let’s look at all of these parts and understand the operation of the ignition system.
In a nutshell; to introduce a high voltage spark at the spark plug, a low voltage supply is passed from the battery, which is then converted into a high voltage supply and applied to the respective cylinders at the specified time through the spark plug. This high voltage will be in the order of 20000 to 30000 volts. Now let’s understand the operation briefly.
First of all, to begin the system let’s start with the ignition switch, which is connected between the ballast and the battery. The ignition switch is used to turn the system ON and OFF manually by the user, which in turn opens and closes the circuit, and conducts the current from the battery. The battery used here is the rechargeable lead-acid battery, which has a high output current to induce a strong magnetic field across the ignition coils, to produce a high voltage spark. The battery is recharged again, by generating the power through the produced rotational energy of the engine (dynamo effect).
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Credit (Lead Acid battery): By Emilian Robert Vicol - Flickr: VRLA_Valve-Regulated-Lead-Acid-Batteries__82645, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=22336061 |
Now when the system is turned ON by closing the ignition switch, the power from the battery passes through the ballast and reaches the ignition coil. The purpose of the ballast is to protect the ignition coil from the high increase in temperature due to prolonged use. The ballast resistor increases its resistance if the temperature increases. This resistance increment causes the current passing through the line to down to a safe value. Therefore the coil is protected from the heavy rise of temperature by the increased resistance of the ballast.
The ignition coil which is considered as the heart of the ignition system is used to transform the low output voltage from the battery into a high output voltage at the spark plug. It is generally a transformer that consists of both primary and secondary coil and a soft iron core. The secondary coil is made of thin wire and off many turns and is made to wound on the soft iron core. Upon that secondary winding, the primary winding is made to wound. This primary winding will be in the order of low turns and of thick wire; because the primary winding will be made to carry a high current from the battery. So the primary winding is made of thick wire to hold on to such a heavy current. As a high voltage is to be produced at the secondary end, it is made of thin wire and many turns. The construction of the ignition coil will be as, that one end of the primary coil is connected with the incoming supply and the other end is connected to the contact breaker. Similarly, one end of the secondary coil is connected to one end of the primary coil, and it is grounded. The other end of the secondary coil is connected to the distributor to supply the high voltage.
The purpose of the contact breaker is to produce a variable magnetic field in the ignition coil by making and breaking the primary current. Because to produce a supply at the secondary terminal, a varying magnetic field must be set upon by the primary coil according to Faraday’s law of electromagnetic Induction. But the primary winding in the ignition coil produces a steady magnetic field as it gets its power from the steady supply (DC). That is why normal power transformers are used in AC supply and not in DC supply. Because AC supply is a time-varying supply and produces a variable magnetic field. So a contact breaker is used to set up the varying magnetic field in the primary coil by making and breaking the primary current and disrupting the steady magnetic field. This contact breaker has a fixed contact and a movable contact, which are held together by a spring default. The movable contact is connected to the ground through a pivot arm causing the circuit to be closed. As shown in the below figure; by moving the pivot arm, the moving contact is made to move and the primary current is made to break by disconnecting the circuit. This induces a high voltage in the secondary winding, to make a spark at the spark plug. This making and breaking of contact is achieved by a rotating cam, which is rotated by the camshaft.
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Operation of the contact breaker and the condenser |
A capacitor is also connected in parallel with the contact breaker, to absorb the voltage appearing at the contacts terminal, when the contacts are opened. Thus the contacts are protected from burning. The condenser (capacitor) also provides a high change in power to the primary winding through the charging and discharging operation. When the varying magnetic field comes in contact with the secondary coil, an EMF gets induced in the secondary coil according to Faraday’s law of electromagnetic induction. Due to its huge number of turns of wires, a high voltage is developed across the secondary coil. By this arrangement of condenser, contact breaker, and the ignition coil, a high voltage is produced at the output of the ignition coil.
After this circuit, the high voltage is then supplied to the distributor, which consists of a rotor and several metallic electrodes (Ignition harness) according to the number of cylinders. The high voltage from the ignition coil is passed to the rotor in the distributor. When the rotor comes in contact with any of the metallic electrodes, a high spark is produced at the gap between the rotor and the electrode due to the ionization effect. This high voltage spark is then carried out to the spark plug which has one central electrode and a ground terminal. The spark is also provided to the spark plug through an electronic configuration method, which is considered as another type of ignition system. When the high voltage spark is received at the spark plug, a huge spark is produced between the central electrode and the ground terminal causing the closure of the circuit. This high voltage spark causes the combustion of the air-fuel mixture inside the cylinder. As the rotor rotates, spark plugs are energized continuously in a sequential order to produce the high voltage sparks in the cylinder, to ignite the air-fuel mixture. The rotor is made to rotate at a specified speed to provide the spark at the right time in the cylinder.
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Distributor |
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