Saturday, July 15, 2017

Spark plug working and its construction

Spark plug is used in all automobile engines whether it’s two wheeler, three wheeler, four wheeler or even bigger vehicles. Spark plug is an electrical device which is mounted on cylinder head of internal combustion engines, this electrical spark plug generated a spark which is used to ignites the compressed fuel uses in engines

Spark plug consists of an insulated central electrode which in turn connected through an insulated wire to the ignition coil where ignition is required. This insulated wire may also be connected to the magneto circuit on the outside and form a grounded terminal on the base of the plug. A spark gap in cylinders of and Internal combustion engines is usually divided into:-

(i)            Spark-ignition engines:-
These engines required spark plugs to begin combustion,
(ii)           Compression-ignition engines Known as diesel engines:-
           In these engines air is compressed and then diesel fuel injection is done into the heated compressed air mixture where fuel gets auto ignition.

Compression-ignition engines may use glow plugs to improve cold start characteristics.
There are following functions of Spark plugs:-

There are two primary functions of Spark Plugs
(i)            To ignite the air/fuel mixture.
As discussed earlier electrical spark generated through spark plug , this spark jumps the air gap when voltage applied to spark plug is high enough for crossover of air gap. This spark is used to ignite the air/ fuel mixture in combustion chamber.

(ii)          Removal of heat from Combustion chamber:-
There is misconception that spark plugs create heat but spark plugs are used to remove hear instead. Temperature at the end of spark plug i.e. firing end must be kept low to such levels so as to prevent pre-ignition. Also it should  be kept in  mind that temperature must not be so high so as to prevent unwanted firing.
The heat range of the spark plug must be carefully selected  as if heat range is not correct we will face of troubles. Spark plug firing end temperature is between 900-1,450 degrees. If <900 degrees, carbon polluting is possible. If Temperature is >1500 degree than overheating becomes an issue.
Spark plug acts as a heat exchanged i.e. removes non-useful thermal energy from combustion
chamber and transfers that heat into engine cooling system.
Ability of spark plug to remove heat from spark plug tip and is called as heat range.

Operation of Spark plug :

The spark plug is connected to ignition coil or magneto high voltage generation end. Due to this high voltage at one end there will be potential difference between center of electrode and side electrode. Now due to air gap ignition doesn’t happen now when voltage level reaches to high level then air fuel mixture get ionized and at certain level whole air fuel mixture get ionized and this ionized gas becomes good conductor of electricity and this ignites spark across gap.
Voltage required to generated spark is usually of the level of 20KV-100KV.

When air gap get ionized such that current starts flowing across gap then , temperature level attained will be around 60000 K. This extreme heat causes the expansion of air and fuel mixture to expand more quickly and there will be small explosion.

There are two type of spark plus:-
(i)            Cold Spark plugs
(ii)           Hot spark plugs
Cold Spark Plugs:-
 In these spark plugs heat flow path is short. It is shortened so that there will be faster heat transfer and also you can find small insulator nose on these spark plugs and surface area of this insulator nose is smaller. Due to smaller surface area high heat absorption could not occur in these spark plugs.

Hot Spark Plugs:-
These spark plugs have longer insulator nose so longer heat transfer path. As there is longer heat transfer path so which leads to slower heat transfer to the surroundings cylinder heads.

There are following parts of spark plugs:-
(i)            Insulator
(ii)           Hexagonal
(iii)          Shell & Plating
(iv)         Gasket
(v)          Threads
(vi)         Ground electrode
(vii)        Central electrode
(viii)       Spark plug electrode gap
(ix)         Insulator nose or end tip of insulator
(x)     Terminal nut
(xi)     Resistance

Spark plug parts; functioning of different parts of spark plug

(i)    Insulator: Insulator body is usually made from Aluminum oxide ceramic. Then high pressure dry molding technique is used for construction of insulator. When molding is done then it is fired in kiln to temperature that even exceeds the melting point of steel, by doing this insulator will properties such as high dielectric strength, excellent resistance to shock and very high thermal conductivity. On outer surface of spark plug circles so as to provide grip of spark plug.

(ii)  Hexagonal:-This is provided as to provide grip of fixing the spark plug at required place and tightening of spark plug is done from that point only.

(iii) Shell and Plating:- Shell is just below hexagonal and is made using special cold extrusion process. Now this shell is plated to provide protection against the rust and corrosion.

(iv) Gasket:- These may or may not be used in spark plugs. Where gasket is used in Spark plugs there gasket is having folded steel design that provides a smooth surface for sealing purposes. Spark plugs without gaskets have a tapered seat shell which seals through close tolerance.

(v)  Threads:- Threads are used to mount the spark plug at specified location and these threads are made though rolling process instead of cutting the same.

(vi) Ground electrode: Ground electrodes are made from nickel alloy steel and made into different shapes. Property of ground electrode is that under extreme temperature and pressure conditions ground electrode be resistant to both spark erosion and chemical erosion.

(vii)  Center electrode:- Central electrode also should have same properties as that of ground electrode i.e. it must sustain extreme temperature and pressure conditions.

(viii)  Electrode gap:-The gap between ground electrode and center electrode is known as spark plug electrode gap. Gap should be kept as minimum as possible but could not touch the tip of insulator as required spark will not produce in that case.

(ix)  Insulator nose or end tip of Insulator:-There are carbon deposits with passage of time on insulator tip due to sparks generated during period of operation so spark plug nose shape and size is such that it should be capable of removing carbon content, oil and fuel deposits.

(x)  Terminal Nut:- There are two options available in spark plug for connections i.e. spark plug comes with terminal nut or without terminal nut i.e. with threaded nut and connection type or even both. Terminal nut can be detached from spark plug.

(xi) Resistance:- Resistance is provided to high voltage provided to spark plugs so that current could not be very high. Resistance is also kept in thousands. Resistance vary from 1000- 7000 ohms. 5000 ohm is usually cut of point for because it provides a value between where spark plugs are manufactured and where problems seem to appear with high resistance. Most reports Tempest receives about rough running engines associated with high spark plug resistance involve values of 7000 ohms or more. 

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