Hello and welcome to MAE Learning. In this blog, we will explore the Ignition System in a petrol Engine.
The ignition system is the heart of any petrol (spark ignition) engine. It creates the spark that burns the air–fuel mixture. Without a proper ignition system, engine will not start, will misfire, give low power and high fuel consumption.
What is ignition system?
The ignition system is a group of electrical parts in a petrol engine. These parts produce a high‑voltage spark at the right time in each cylinder. This spark jumps across the gap of the spark plug and ignites the compressed air–fuel mixture in the combustion chamber.
Main jobs of the ignition system are:
- Change low battery voltage (12 V) into very high voltage (20,000–40,000 V or more).
- Supply this high‑voltage current to each spark plug in correct firing order.
- Control the exact timing of spark for smooth power, good fuel economy and low emissions.
Main components of ignition system
A typical conventional or basic electronic ignition system has the following major parts.
- Battery
- Ignition switch
- Ignition coil
- Distributor (in older systems)
- Contact breaker / electronic module
- Condenser (capacitor) in contact‑point systems
- Spark plugs
- High tension (HT) leads / cables
Battery
Battery stores electrical energy. It supplies low‑voltage DC power (usually 12 V) to the starter motor. It also supplies power to the ignition system. If the battery is weak, the starter will turn slowly. The coil will not get enough power, causing the spark to become weak. As a result, starting becomes difficult.
Ignition switch
Ignition switch is the key‑operated main switch on the steering column or dashboard. Turning the key to “ON” closes the circuit from the battery to the ignition coil. It also powers other vehicle electrical systems. When turned to “OFF”, it cuts the power and the engine stops.
Ignition coil
Ignition coil is the “voltage step‑up transformer” of the ignition system. It has a soft iron core and two windings: primary (few hundred turns) and secondary (many thousand turns). Low‑voltage current flows in the primary winding. When this current is suddenly interrupted, the magnetic field collapses. This collapse induces a very high voltage in the secondary winding. The high voltage is then sent to the spark plug.
Distributor (in conventional systems)
Distributor is used in many older and basic petrol engines. It has a rotor that rotates inside a distributor cap. The high‑voltage from the coil enters the rotor. It is then sent to each spark plug lead one by one in the correct firing order. Distributor also contains the contact breaker points and may have mechanical and vacuum advance mechanisms to adjust ignition timing.
Contact breaker and condenser
In older mechanical systems, the contact breaker consists of a pair of points. These points open and close with the help of a cam driven by the engine. When points close, current flows in the primary coil winding. When they open, current stops suddenly. This causes magnetic field collapse and generates high‑voltage in the secondary winding. A condenser (capacitor) is connected across the points. It prevents sparking at the contacts and improves the speed of field collapse. This gives a stronger spark.
Electronic ignition module / sensor (modern systems)
In modern electronic ignition, mechanical points are replaced by electronic sensors and an ignition control module. Sensors, like magnetic pickup, Hall sensor, or crankshaft position sensor, send engine speed and position signals to the module. The module switches the primary coil current on and off electronically at the correct time. This gives more accurate timing, less wear and better reliability compared to mechanical points.
Spark plugs
Spark plug is fitted in the cylinder head and its tip is inside the combustion chamber. High‑voltage current from the coil jumps across the small gap between the centre electrode and earth electrode of the plug. This creates a strong spark. The spark ignites the air–fuel mixture. Proper heat range, correct gap and condition of electrodes are very important for good performance and long engine life.
High tension (HT) leads
These are thick insulated cables. They carry high-voltage current from the coil to the distributor. Then they conduct the current from the distributor to each spark plug. Insulation must be strong; if it cracks, high-voltage can leak to engine body causing misfire, especially in wet conditions.
How ignition system works (step by step)
Here is the basic working sequence of a conventional battery–coil ignition system in a 4‑stroke petrol engine.
- Key ON, engine cranking
When driver turns the ignition key to “START”, battery current flows through the ignition switch. It reaches the primary coil winding and contact breaker (or module). Starter motor turns the crankshaft, so distributor cam and rotor also rotate. - Current builds in primary winding
Contact points closed (or transistor ON). This results in a steady current flow through the primary winding. This builds a strong magnetic field around the coil core. This field stores energy that will later be used to create high voltage. - Contact breaker opens / module cuts current
As distributor cam rotates, it pushes the contact breaker. This action makes the points open at a set angle. In electronic systems, module switches OFF the primary current on the basis of sensor signal. - Magnetic field collapses
A high voltage is induced. When the current is suddenly cut off, the magnetic field collapses quickly. This rapid change induces very high voltage in the secondary winding of the coil. Voltage can easily exceed 20,000–40,000 volts, enough to jump across the plug gap under high compression. - Voltage distributed to correct spark plug
High‑voltage leaves the coil via the HT lead. It enters the centre terminal of the distributor cap. The voltage reaches the rotor. Then it jumps to the terminal of the plug wire. This plug wire is for the cylinder that must fire at that instant. From there, it travels through the HT cable to the spark plug of that cylinder. - Spark at plug and combustion
At the plug tip, voltage jumps across the gap forming a spark. This spark ignites the compressed air–fuel mixture. Ignition occurs just before the piston reaches top dead centre (TDC) on the compression stroke. Rapid burning of mixture increases pressure and pushes piston down on power stroke, producing useful work. - Cycle repeats.
The engine continues to rotate. The distributor rotor sends high-voltage to the next cylinder in the firing order. The process continues smoothly at high speed.
Types of ignition systems
There are several types of ignition systems used in petrol engines.
1. Battery‑coil ignition system
This is the most common conventional system in cars, where a battery supplies power to the coil and distributor. It uses contact breaker points (or replacement electronic module) and a single ignition coil for all cylinders.
2. Magneto ignition system
In magneto system, instead of a battery, a rotating magneto unit produces the required electrical power for ignition. It is widely used in small engines, motorcycles, and scooters. It is also used in some stationary or aviation engines because it can work without a battery.
3. Electronic ignition system
Electronic ignition replaces mechanical points with solid‑state electronics. It uses sensors like crankshaft or camshaft position sensors and an electronic control module. Often, multiple coils or a coil pack are used to provide precise timing. This setup offers a higher energy spark, better fuel economy, and lower maintenance.
4. Distributor‑less ignition system (DIS)
In DIS, there is no mechanical distributor. Instead, there are coil packs. Each coil serves two cylinders in a waste-spark system. Alternatively, there is one coil per cylinder. All are controlled by the engine control unit (ECU). This design reduces moving parts, improves reliability and allows very accurate ignition timing control.
Ignition timing and advance
Ignition timing means the exact crankshaft angle at which spark occurs in each cylinder. Normally, the spark is given a little before TDC. For example, this could be 5–35 degrees before TDC. This timing ensures the mixture burns fully. As a result, maximum pressure is reached just after TDC for best power.
Because engine speed and load change, ignition timing must also change. Main methods are:
- Centrifugal (mechanical) advance: Small weights inside distributor move outward with speed and advance timing as RPM increases.
- Vacuum advance: Diaphragm unit on distributor changes timing depending on intake manifold vacuum (engine load).
- Electronic control: In modern engines, ECU uses many sensors (speed, load, temperature, knock) to set the best timing map.
Wrong timing can cause problems:
- Too advanced: knocking, overheating, possible engine damage.
- Too retarded: low power, high fuel use, hot exhaust, poor drivability.
Common ignition system problems
Ignition system faults are a very common reason for poor engine running or no‑start.
Some typical problems are:
- Weak or dead battery causing slow cranking and weak spark.
- Worn or burnt spark plug electrodes, wrong gap or oil deposits causing misfire and hard starting.
- Cracked distributor cap, worn rotor or damaged HT leads causing leakage of high‑voltage and misfire, especially in rain.
- Faulty ignition coil leading to weak or no spark when hot.
- Failed electronic ignition module or crankshaft sensor causing sudden engine cut‑off and no restart.
Regular maintenance like checking plugs, leads, battery condition and following replacement intervals helps keep ignition system healthy and engine smooth.
Ignition system in modern engines
Modern petrol engines use fully electronic, ECU‑controlled ignition, often integrated with fuel injection system. These systems use crank and cam sensors. They include knock sensors and many maps. These components control spark timing for power, economy, emissions, and knock control.
Common modern designs include:
- Coil‑on‑plug (COP): One coil directly mounted on each spark plug, no HT leads and usually no distributor.
- Coil‑near‑plug: Coil is near plug with a short lead, still reducing losses compared to long cables.
These systems provide a strong and consistent spark. They require less maintenance. They are easier to control using software, which is crucial for today’s emission norms. This is important for meeting performance demands.
