Spark plug is a device mainly used in internal combustion engines of automobiles. It ignites the compressed fuel mixture in the combustion chamber of IC engine. Spark plug is used only in spark ignition engine (Petrol engine) not in compression ignition engines (Diesel engines). The function of the spark plug is to delivering the electric charge from ignition system to the compressed fuel in combustion chamber and also provide proper gap in the combustion. Spark plug ignites the fuel just before the power stork in IC engine. For proper functioning of spark plug following are the necessity conditions:
- The spark plug must be fitted air/gas tight. The joint between the lower part of spark plug and combustion chamber as well as central electrode and insulator must be air tight.
- Spark plug must be located at proper position in the engine for proper functioning of engine at idling as well as throttling for maximum fuel efficiency.
- Spark plug should not cause pre-ignition (ignition of fuel before the spark occurs). So the temperature of spark plug must not be so high to cause pre ignition. Pre ignition occurs at 900-1000 ℃.
- The spark plug used must have quality to withstand at highly fluctuating temperature and pressure in the combustion chamber. The pressure can be raise into the combustion chamber up to 50 bar. So that the insulator material must have high electric resistance, good thermal conductivity and high mechanical strength.
- The electrodes which are more efficient are made up of platinum or platinum tungsten alloy but not used in common due to high cost so in that place nickel alloy having manganese, tungsten, silicon and chromium elements are widely used electrode in spark plugs.
- To minimize the lead and carbon fouling at the firing end of spark plug a long insulator tip and large clearance volume should be necessary because the temperature more than 400℃ which burn of the carbon deposit on the spark plug.
A spark plug have various parts like rust resistant shell, central electrode which is enclosed in a ceramic insulator, side electrode which is attached to the metallic shell(rust resistant shell). The lower part of the spark plug is having threads which is fit and sealed into the combustion chamber. The upper part of spark plug is attached to ignition coil having shell body. The insulator and central electrode are subjected to high combustion temperature this requires that heat must flow from insulator to metallic shell and after that goes into the combustion chamber. Spark plug has different parts some of the major parts of it are as follows:
It is made up of hard ceramic materials with high dielectric strength. It provides electrical insulation for the central electrode and mechanical strength to the spark plug.
Spark plug terminals:
It is upper part of the spark plug which is outside the engine it is connected with ignition coil from the top and attached to central electrode to conduct high voltage.
It is connected to the spark plug terminals. Central electrode is made up of nickel alloy having elements of manganese, tungsten, silicon and chromium. High voltage is supplied through the terminals to the central electrode.
Side/ Ground electrode:
It is made up of nickel steel and hot forged to the side of the metal shell. Side electrode is embedded into the combustion chamber so it sustains a very high temperature. Multiple side electrodes may be used.
Spark plug gap:
Spark plug is designed with some gap between the central and side electrode. Side electrode little bends to maintain a proper gap for ionizing the fuel into the gap when high voltage is supplied through electrodes.
Outer metallic body:
It is steel shell having threads on the surface for easily removal and installing into the cylinder head of the engine. It helps to cool the spark plug by transfer heat to engine body through conduction.
Spark plug ignites the fuel in combustion chamber. One side of the spark plug is connected to the ignition coil which supply high voltage to spark plug and other side of it is sealed in the combustion chamber of engine. There are two electrodes in the spark plug one is at the center and other one is on periphery called as side electrode. Voltage develops in between the electrodes when current starts flow from the ignition coil. In earlier strokes there is no current flow but when the voltage increases the fuel in between the gap of electrodes start ignite. The fuel is gaseous mixture and it starts ionizing, now current starts flow between electrodes because the gas between electrodes is ionized which results bursting of fuel mixture into the combustion chamber which pushes the piston and gives power stroke. At this time very high temperature is generated. During discharge process high current is supplied which results longer duration spark. A spark plug conducts high current with potential difference of nearly 25 KV.
Type of spark plugs:
Spark plug may be classified on the bases of spark plug heat range. A spark plug must not work on too high temperature or too low temperature because at high temperature it may cause pre-ignition and at low temperature it results in rooting and oiling up. Spark plug must works at suitable range of heat.
The heat rang of the spark plug is defined as the ability of a spark plug to transfer heat from firing end to the combustion chamber of the engine. Insulator tip length plays important role in the controlling of heat range of spark plug and it is varies in different spark plugs. According to heat range spark plugs are divided into three categories i.e. hot running, cold running, and medium running spark plugs.
Cold running spark plug:
A cold running spark plug transfer heat very rapidly from firing end to combustion chamber because it has short small insulator tip and small heat rejection path. It is used to avoid overheating where combustion chamber temperature is very high like in racing or sports cars and in heavy duty engines.
Hot running spark plug:
It has long insulator tip and therefore it has high heat rejection path which is used to avoid cold fouling where combustion chamber temperature low which is generally seen in case of idling or starting of engine and in low power operations.
Medium running spark plug:
It lies in between cold and hot range of spark plugs. The length of the insulator tip is not too small and not to large. It is best suitable in economically running.
Spark plug used in spark ignition engines i.e. in petrol engines of automobiles like cars, scooters and bikes to ignite the fuel into the combustion chamber.
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