Spark plug
is a device used commonly in Spark Ignition (S.I) engines to ignite the
air-fuel mixture in the combustion chamber. It produces electric spark when
current is passed by the ignition system.
Gottlob
Honold of Bosch developed a spark plug that helped in the development of spark
ignition engines in 1902. Later in 1903, Oliver Lodge developed a more reliable
version of spark plug to fit in S.I engines.
When
does a spark plug fire?
We all are
aware of the four strokes of S.I engines. At the end of compression stroke,
the pressure build up in the combustion chamber becomes intense. The fuel
breaks into finer particles due to the compression of charge. This is the easiest
time to ignite the fuel and hence spark plug is excited by the ignition system
to generate an electric spark that generates enough temperature to ignite the
air-fuel mixture.
Spark plug
is installed at the top of the cylinder head and ignites when the piston reaches
top dead centre (TDC) in the compression stroke.
Functions of a Spark
Plug:
A Spark plug basically has two main functions:
·
To ignite the air-fuel mixture. The electric
energy passes through the central electrode and jumps the gap at the tip to
generate spark, which then ignites the air-fuel mixture.
·
To dissipate the heat from combustion chamber. We
might be at a misconception that spark plug creates heat, but no. Spark plugs
act as a heat exchanger remove the heat from combustion chamber and
transferring it to the cooling system. The temperature at the tip should be
maintained such that it should not be high enough to lead to pre-ignition and
it should not be low enough to result in fouling.
CONSTRUCTION
OF A SPARK PLUG:
Insulator
Body:
Insulator
body is made of aluminium oxide ceramic. The shape of the body is moulded in a
dry moulding system. After it is dry moulded, it is heated to a temperature
higher than the melting point of steel. This provides high thermal conductivity
and shock resistance. The surface is ribbed for better grip and protection from
secondary voltage.
Terminal:
It is also known as connector. This is where the spark plug
wire from the ignition coil is connected to the central electrode.
Hex Head:
This part is to fit the socket wrench in order to tighten
and loosen the spark plug in its hole in the cylinder head.
Shell:
Shell is made
of steel and is fabricated with the help of cold extrusion process. The shell
is plated for corrosion resistance.
Gasket:
Gasket is
used for sealing purpose. Threads are provided to screw the spark plug in the
cylinder head.
Central
Electrode:
The central
electrode is connected to the outer terminal. It is an internal wire that
conducts current from outer terminal to its tip. The tip is usually made of
copper to carry away the heat of combustion.
Ground or
Side Electrode:
They are
manufactured from nickel alloy steel. They have an edge shaped area at the
bottom to create the gap between central and side electrode. The high voltage
jumps from the central electrode to the side electrode to create the required
spark.
Spark Plug Working:
The high voltage required to generate a spark is supplied by
an ignition coil. As the electrical energy passes through the central
electrode, a voltage difference is created between the central electrode and
the ground electrode. We all know that spark plug is fitted at the top of the
cylinder head. Hence, air-fuel mixture will be present in the gap between the
two electrodes.
Air-fuel mixture acts as an insulator, therefore no current
can flow between the electrodes. Once the voltage is high enough to exceed the
dielectric strength of the air-fuel mixture, the mixture is ionized. Ionized
gases act as a conductor and allow the electrons to flow through them, thereby
creating a spark. Different spark plugs require different voltages to generate
the spark. It usually ranges between 20,000
V to 1,00,000 V.
A small flame is created at the tip of the spark plug which
then flows as a flame front at the top of the piston depending on the
composition of air-fuel mixture.
Types of Spark
Plugs Based on Temperature:
Spark plugs can be divided into two types: Cold plug and Hot plug.
·
Cold
Plug: Cold plugs are used in engines that generate high horse power and
high compression pressure and temperature. It has less insulation; therefore it
can transfer more heat from combustion chamber to the cooling system. It is of
prime importance to transfer the excess heat from the combustion chamber to the
outside in order to prevent pre-ignition and engine damage.
·
Hot Plug:
Hot plugs have more insulation compared to cold plugs. These plugs are
available in most standard engines. These spark plugs retain more heat to burn
off the excess carbon deposits.
What is Spark Plug
Fouling?
Fouling is known as coating of the insulator tip with
foreign particles such as oil, carbon or fuel. This allows the high voltage to
leach back down to the metal shell and to the ground instead of jumping the
electrodes gap and creating spark.