Spark Plug for Internal Combustion Engine

 Author: Ijaz Sadid

Spark plugs

Spark

The electrical discharge phenomenon that occurs when high voltage from ignition coil; is applied between ground electrode and center electrode of spark plug. This produces what we call a spark. This energy starts ignition and then combustion of air fuel mixture. Spark occurs for a very brief duration of about 1/1000 of a second. This phenomenon is quite complex.

Spark Plug

It is a device for delivering electric current from an ignition coil to the combustion chamber of an internal spark-ignition engine for ignition of the compressed air-fuel mixture by an electric spark plug.

 Spark plug produces a strong spark between its electrodes at a specified time for better ignition and combustion of air fuel mixture.

Inventor

Étienne Lenoir, in 1860, used an electric spark plug in gas engine, the first internal combustion piston engine. He is generally credited the inventor of the spark plug.

Spark Plug Requirements

·        Electrical

 It must have reliable high-voltage transmission, even at ignition voltages of up to 40,000 volts. It must have good insulation capability, even at temperatures of 1,000 °C, prevention of arcing and flashover.

·        Mechanical

It must be pressure-tight and gas-tight sealing of the combustion chamber, resistance to oscillating pressures up to approx. 100 bar. It must have high mechanical strength for reliable installation.

·        Thermal

It must be resistance to thermal shock (hot exhaust gases – cold intake mixture). It must have good thermal conduction by insulator tip and electrodes.

·        Electro-chemical

It must have resistance to spark erosion, combustion gases and residues. It must prevention of build-up of deposits on the insulator

Parts of a spark plug

1.       Connector for the spark plug. Feeds the ignition voltage to the central electrode.

2.      The steel pin (ignition pin) enclosed in a gas-tight fit within the conductive glass melt, forming a link with the central electrode.

3.       The insulator is made of an aluminium oxide ceramic and insulates the central electrode against earth at up to 40,000 Volts. The insulator can be produced in a plain form or with profiles to prevent flash-over.

4.       Nickel-plated spark plug body forms a gas-tight connection with the insulator through a heat shrinkage method, which is why the shrinkage zone shows the blue oxidation color. Thread serves to secure the spark plug in the engine block.

5.       Captive outer sealing washer, for sealing and heat dissipation.

6.       Electrical connection of spark pin and central electrode. On suppressed (R types) glass melt resistors. By means of appropriate additives the glass melt can be given a defined degree of resistance in order to ensure the required erosion resistance and suppression characteristics.

7.       The inner sealing ring forms the gas-tight junction between the insulator and the metallic body and provides heat conduction.

8.       The center electrode consists of a copper core enclosed in nickel. Depending on the type, the central electrode can be in platinum or iridium.

9.       The insulator tip extends into the combustion chamber. It has a fundamental influence on the thermal rating.

10.   The lead-in chamfer makes it easier to screw the spark plug into engine block.

11.   The clearance volume influences self cleaning action.

12.   One or more earth electrode is welded onto the body of the spark plug, and with the central electrode form the spark path. Specially developed nickel-based alloys (or platinum or titanium reinforcements) increase the erosion resistance of the electrodes.